REVIEWARTICLE

The significance of magnesium for crop quality

Jóska Gerendás & Hendrik Führs

Received: 9 June 2012 /Accepted: 3 December 2012# The Author(s) 2013. This article is published with open access at Springerlink.com

AbstractBackground The quality of agricultural and horticul-tural products and its modulation by fertilization hasincreasingly received attention. However, whereas theimportance of magnesium (Mg) as an essential plantnutrient is well established, the impact of Mg nutritionon quality parameters has only been rarely addressed.Scope This review aims at evaluating the availableknowledge on the influence of Mg on produce quality.A short discussion on the term quality as used in thisreview is followed by an overview of the various func-tions of Mg in plant metabolism in relation to qualityaspects. Finally, the available literature onMg-associatedeffects on crop quality is critically surveyed. The ques-tion whether Mg application beyond yield optimumfurther improves crop quality is specifically addressed.Conclusion IncreasingMg supply onMg-deficient sitestends to increase the quality of agricultural crops, par-ticularly when the formation of quality traits is depen-dent on Mg-driven photosynthesis and assimilatetranslocation within the plant. In fruits and vegetables,ratios ofMg to other nutrients like Ca and Kwere shownto be a more reliable indicator of the quality responsethan the Mg status alone. Moreover, it is concluded thatMg doses beyond those required for maximum yieldrarely induce a further improvement of produce quality.

Keywords Carbon partitioning . Cation balance .

Nutrient function . Physiology . External quality .

Internal quality . Storage quality

The term ‘quality’ and quality traits

In the past food security and yield stability were themain challenges of agricultural and horticultural re-search, focussing on efficiency of crop production sys-tems and its intensification. Quality aspects hardlyreceived any attention. However, in the last decadesthe topic ‘quality’ moved into the focus of research.Reasons may be raising awareness of stakeholdersand, at least in the developed countries, the high degreeof self-sufficiency, reached today. Particularly in fruitsand vegetables quality aspects attracted interest in recentyears. Concerns related to quality are thereby not onlydependent on the production measures available, butalso on national and international developments in agri-cultural policy and increased competition in the marketsdue to globalisation processes (Huyskens-Keil andSchreiner 2003). Consumers often claim that the qualityof food items plays a major role in their preference anddecisionmaking. At the same time theymay have only alimited understanding of the traits underlying the de-sired quality parameters (van Rijswijk and Frewer2008). In fact, along the entire supply chain stakeholdersmay have a different understanding of the term ‘quality’(Huyskens-Keil and Schreiner 2003).

Therefore, to address the impact of production factorson the quality of agricultural and horticultural produce a

Plant SoilDOI 10.1007/s11104-012-1555-2

Responsible Editor: Ismail Cakmak.

J. Gerendás (*) :H. FührsK+S KALI GmbH,Bertha-von-Suttner-Str. 7,34131 Kassel, Germanye-mail: joska.gerendas@kali-gmbh.com

common understanding and definition of quality termsand traits is required (Shewfelt 1999). Schuphan (1961)already defined the term ‘quality’ as the sum of allsubjective and objective quality traits of food cropsand proposed a classification into market quality, uti-lisation quality and biological quality. This and similarearly definitions of food or product quality focused onproduct parameters and were widely used or taken as abasis by food chemists (Shewfelt 1999). Using suchdefinitions the term quality is interpreted in a product-oriented way. However, the term ‘quality’ is a multifac-eted one. Consumers may relate the term ‘quality’ tofeatures that cannot be determined on the product itself.Later, Kader (1998) defined quality as “the degree ofexcellence or superiority, which is a combination ofattributes, properties, or characteristics that give eachcommodity value of its intended use”. This definitionalready puts a stronger focus on the user perspective,even though it is still a quite product-oriented approach.In line with definitions given by Schuphan (1961) andKader (1998) Huyskens-Keil and Schreiner (2003) cat-egorised the product quality into (1) the market value,(2) the utilisation value, (3) the sensory value, (4) thenutritional and health value, (5) the ecological value,and (6) the imaginary value. Over the years the under-standing of the term quality has changed from the for-mer more product- and process-oriented evaluation ofproduct quality to a more consumer-oriented one(Kramer and Twigg 1970; Shewfelt and Brückner2000). Of course this has strong implications for newmarketing strategies including the development of qual-ity management systems (Huyskens-Keil and Schreiner2003). Nowadays, a general definition of the term ‘qual-ity’ is stated by DIN ISO 9,000 as “the totality offeatures and characteristics relevant to the ability of aproduct or service to fulfil its requirements” (InternationalOrganization for Standardization 2005). This points tothe important consideration that any judgment on thequality of an item requires a scale that is based on pre-defined requirements, needs or expectations (user-orient-ed definition). According to this definition the quality of afinal product is determined by the intended use andrelated expectations. For example, wheat could be usedfor manufacturing bread, biscuits, starch, beer and alco-hol. All these intended uses have specific, often evenconflicting quality criteria. Moreover, a holistic approachwould even need to take the way of production andprocessing (e.g. organic farming) as well as religiousconsiderations (e.g. halal, kosher) into account, even

though the production process may not affect the analyt-ical parameters of the product itself.

In a recent review Giusti et al. (2008), in view ofmarket globalization, introduced the term ‘total foodquality’, covering a wide range of aspects. Based onthe fact that these quality aspects also extend to animalfeed and industrial applications (e.g. cotton for fibre),the ‘total crop quality’ may be grouped and classifiedas follows:

& Organoleptic/sensory properties (appearance, col-our, texture, juiciness, taste, and aroma)

& Safety (toxic compounds naturally present, inorgan-ic, organic, and biological, hazardous contaminants)

& Nutritional value (available energy, concentrationand composition of proteins, fatty acids, vitamins,antioxidants, minerals, the digestibility andbioavailability)

& Functional properties (suitability for processingand transformation e.g. physical and chemicalmeasures like solubility, absorption, firmness,mainly of industrial interest)

& Service and stability (suitability for transport andstorage, shelf-life)

& Healthiness (constituents with additional benefi-cial health effects, mostly referred to as bioactivecompounds: prebiotic and probiotic compounds,flavonoids, carotenoids, vitamins, peptides, etc.)

& Psychological factors (convenience, price, easeof use, novelty, prestige, religious and ethicalconsiderations)

Depending on the intended use of a product and itsposition in a supply chain different quality aspects mayappear important or negligible or their importance maychange along the supply chain (Shewfelt and Brückner2000; Huyskens-Keil and Schreiner 2003). Good exam-ples are industrial crops like sugar beet and sugar cane.In such crops, the target constituent, in this case thesugar concentration (equivalent to the nutritional value),may be hardly affected by a specific productionmeasurebut constituents interfering with the recovery of thesugar (equivalent to functional properties), namelyalpha-amino N, K or Na (Buchholz et al. 1995; Barloget al. 2002) are likely to be affected. Hence, in thisreview the concentration of target and interfering con-stituents is treated as a quality trait. In contrast, in cropscience the concentration of a target constituent (sugar,oil) is typically regarded a yield component.

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A given production factor may influence severalparameters of yield and quality, partly with conflictingoutcome (Marschner 2012). For example, with increas-ing N supply crop yield may respond in a similar way asdoes the crop quality, which particularly applies to thesize of the harvested organ in planted vegetables bearingone target organ per plant (Wiesler et al. 2003). On theother hand the best quality may be achieved at a lower Ninput as required for maximum yield, as observed re-garding the concentration of ascorbic acid (e.g. in broc-coli, Brassica oleracea L. var. botrytis L., Xu et al.2010), and nitrate in vegetables (Greenwood and Hunt1986), while a N input beyond the level required formaximum yield improves the baking quality of wheat(McKenzie et al. 2006), and the concentration of Bvitamins and carotenoids in vegetables (Mozafar1993). The conflicting response of yield and qualitytraits of potato (Solanum tuberosum L.) to increasingMg supply may serve as an example and is illustrated inFig. 1. Whereas the yield maximum is reached at mod-erate Mg supply, most quality traits respond favourablyto higher Mg levels. However, in the underlying studiesused here the concentration of toxic glycoalkaloids isalso increased by increasing Mg supply.

Magnesium as such represents an important nutri-ent in the human and animal diet, and food and feedoften do not provide sufficient quantity. However, thesignificance of Mg as a nutritional component in foodand feed is not covered in this article, and the inter-ested reader should consult recent reviews on thesubject (White and Broadley 2009; Broadley andWhite 2010; Lean et al. 2006; Rosanoff 2012).

Functions of Mg in plant metabolism

Magnesium in carbohydrate formation

Photosynthesis as the central process for crop productiondepends on the plant’s Mg status in several respect.Efficient carbohydrate formation requires high light in-terception by the light harvesting complexes (LHCs)attached to the photosystems I and II (PSI, PSII). Mag-nesium as the central atom is an essential constituent ofchlorophyll a/b, which in turn is part of these LHCs. Dueto the complex roles of Mg in chlorophyll and proteinbiosynthesis severe Mg deficiency results in interveinalchlorosis of older and fully mature leaves asMg is highlymobile within the plant (Marschner 2012). Negativeimpacts of an impaired primary (energy) metabolismare to be expected not only on crop yield (Grzebisz2013), but also on crop quality parameters.

The share of the total Mg bound to chlorophylldepends on the Mg status and ranges from about 6 to25 % and highest values are found in Mg-deficientplants (Marschner 2012) indicating that it’s not the Mgbound to chlorophyll that is limiting its synthesisunder Mg-deficient conditions. Another 5–10 % isfirmly bound to cell wall pectins and sparingly solublesalts in the vacuole. The remaining fraction fulfilsfurther roles in plant physiology, some of which arealso associated with photosynthesis: In contrast to thefunction of Mg in light interception these additionalaspects are related to the charge and high transmem-brane mobility of Mg2+. After charge separation at theLHC-photosystem complexes and the subsequentelectron transport through the PS the development ofa significant pH gradient across the thylacoids (aroundpH 8 in the stroma, around pH 5 in the thylakoidlumen) is associated with dissipation of the electricalpotential by channel-mediated transmembrane trans-port of Mg2+ from the lumen to the stroma. Thisallows establishing the proton motive force facilitatingthe ATPase-mediated H+ influx into the lumen result-ing in ATP synthesis (Marschner 2012; Shaul 2002and literature cited therein). The pH rise in the stromaupon illumination is also required for efficient CO2

assimilation in the light-independent part of the pho-tosynthesis as the CO2-fixing enzyme (Ribulose-1,5-bisphosphate Carboxylase/Oxygenase, RuBisCO)requires a basic pH for optimal function (Portis Jr. etal. 1986). In addition, the high Mg concentration accu-mulating in the stroma due to trans-thylakoid Mg

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Fig. 1 Yield and quality parameters as affected by the Mgsupply to potato (schematically). For each parameter displayedin the graph the values were calculated relative to their maximalexpression (= 100 %) to Mg supply (based on: Evans andMondy 1984; Klein et al. 1982; Klein et al. 1981)

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transport for reasons of charge-balance is also essentialas cofactor for RuBisCO, as binding of Mg2+ to theenzyme is required for its full activation (Sugiyama etal. 1969; Taylor and Andersson 1996). As all thesefunctions of Mg are more sensitive to the Mg status asthe chlorophyll formation itself, the light energy thatcontinues to be absorbed under conditions of latentMg deficiency is not consumed in a safe way, givingraise to a higher sensitivity of Mg-deficient tissue tohigh light intensities (Cakmak and Kirkby 2008).

Magnesium in carbohydrate partitioning

As several plant organs (roots, fruits, buds) are not able tomeet their demand for assimilates (energy) directly pho-tosynthates and metabolites synthesised thereof (sugars,amino acids etc.) are translocated by a directed, demand-driven source-to-sink transport. This is particularly criti-cal for those crops storing substantial fractions of the totalplant biomass as carbohydrates or oils in tubers, bulbs,and grains. Particularly for these crops efficient formationof high quality produce strongly depends on unrestrictedand sufficient Mg availability (Grzebisz 2013). Currentlythree phloem loading types are discussed with two active,energy consuming ones and one passive loading type(Turgeon 2010): Phloem loading as an energy consumingprocess involves formation of electrochemical gradientsby H+-ATPases and subsequent H+-sucrose co-transportinto the phloem. Energy is provided by Mg-ATP, thecomplex formed from Mg2+ and ATP under physiologi-cal conditions, which improves the binding of ATP to theenzymes requiring ATP. Indeed, it was shown for mungbean that about 90 % of the total cytoplasmic Mg con-centration is complexedwith ATP, whereas the remainingMg is regarded as free Mg (Yazaki et al. 1988; Shaul2002). However, even though not yet fully understood, itappears that apart from its role inMg-ATP formation,Mgmay play additional direct or indirect crucial roles inphloem loading (Cakmak and Kirkby 2008 and literaturecited therein).

The role of magnesium for functional nucleic acidand protein biosynthesis

Nucleic acids are of pivotal importance for any livingorganism and play diverse roles in plant metabolism.Magnesium is involved in stabilizing conformationalstructures of the nucleic acids needed for properfunctionality. Moreover, nucleic acid-synthesizing

polymerases and degrading nucleases require Mg aswell (Sreedhara and Cowan 2002).

Ribosomes are macromolecular structures formedfrom protein and ribonucleic acids responsible for pro-tein biosynthesis. The active form of ribosomes requiresaggregation of two subunits, necessitating Mg to form abridge between the subunits. Hence, protein biosynthe-sis is strongly reduced under Mg deficiency leading toincreased concentrations of the precursor amino acids(Marschner 2012; Fischer et al. 1998).

Magnesium—its function as mobile ion

The majority (60–90 %) of the Mg in well-suppliedplants is extractable with water (Finck 1992). The ‘met-abolic Mg pool’ is mainly located in the cytoplasm andthe chloroplasts. This pool is subject to strict regulatoryprocesses specifically adapted to the actual metabolicneeds, while using the vacuole as a storage compartmentfor Mg. The demand for ‘metabolic’Mg can thereby besatisfied and maintained through import/export of Mgfrom the vacuole (Marschner 2012). Evidence was pre-sented that the transport across the tonoplast is facilitat-ed byMg2+/H+ exchangers (AtMHX, Shaul et al. 1999).In addition, Mg serves (together with K) as cation in theregulation of the cation-anion balance and as osmoti-cally active ion in turgor regulation of cells (Marschner2012). Especially the vacuole as storage compartmentfor myriads of minerals and metabolites is responsiblefor various quality aspects of sugar crops, fruits andvegetables.

Mg deficiency and Mg excess

As described, a plant well-supplied with Mg and sub-jected to deficiency conditions can initially count on aconsiderable pool of mobile Mg. Under proceedingdeficiency, however, all physiological processes de-scribed here are impaired. Hence, it is obvious thatthe Mg status not only affects the mineral but also themetabolite pools, thereby influencing various qualityparameters as discussed in the section below.

There is no evidence available on the direct effectof excessive Mg supply on plant metabolism. Howev-er, since direct Mg toxicity has not been observed (andwhich might not be expected due to its roles in plantphysiology) impairments in the plant’s physiologymight be merely related to nutrient imbalances inplanta, e.g. K, Ca andMn, particularly when considering

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the antagonistic effects of imbalanced supply of cationicnutrients (K, NH4, Ca, Mg) (see Review by Gransee andFührs 2013).

Mg effects on crop quality

General comments

Wherever possible, only experiments using fully sol-uble Mg sources are included in order to avoid mis-leading interpretations resulting from Mg sources oflimited availability (dolomite and magnesite) and toavoid secondary liming effects on nutrient dynamicsand disease resistance.

Agricultural crops

Cereals

Surprisingly few studies on the significance of Mg forproduct quality of cereals grown under field condi-tions were published, whereas a positive impact of Mgon grain yield of field-grown cereals was frequentlyreported (Beringer and Forster 1981; Grzebisz 2013).But what are quality traits in cereals? On the one handthe single grain mass, often referred to as the 1,000-grain weight, represents an important yield componentthat heavily depends on carbohydrate translocationduring grain filling (Marschner 2012; Grzebisz2013). On the other hand the single grain mass repre-sents an important technological quality parameter(functional property) as the milling efficiency whenproducing superfine flour depends on the endospermto bran ratio and thus on grain size (Greffeuille et al.2006). In brewing barley the volume weight is alsoconsidered as quality parameter (Verma et al. 2008).

Early indications of marked increases in the starchconcentration of rye (Secale cereale L.) grain resultingfrom Mg addition to acidic soddy-podzolic, sandy-loamy soils were observed by Magnitskii et al.(1970). Moreover, Al’shevskii and Derebon (1982)reported that in winter wheat (Triticum aestivum L.)grown on Mg-deficient soil with basal NPK dressingincreasing Mg-sulphate application in spring increasedthe 1,000-grain weight. Also Beringer and Forster(1981) documented in pot-grown barley an increasein the 1,000-grain weight with increasing seed Mgstatus up to 0.1 % Mg in the grain. The increase in

the 1,000-grain weight was accompanied by a strongeraccumulation of phytates, the Ca and Mg salts ofphytic acid. This accumulation of phytates was partic-ularly obvious at high Mg status beyond the levelrequired for maximum yield. Phytates are known fortheir negative effects on the intestinal absorption ofMg (Bohn et al. 2004) and trace nutrients (Gibson etal. 2010). However, recent reports indicate beneficialeffects of dietary phytates such as lowering bloodglucose and lipid levels, and their antioxidative andanticancerogenic activities (Schlemmer et al. 2009).

The impact of Mg on quality traits may also occurindirectly, e.g. through improving the nutritional statusof the crop. Ding et al. (2006) described the signifi-cance for Mg in N nutrition, and it may be hypothe-sized that an improved N metabolism results inenhanced N assimilation with improved Mg status(Grzebisz 2013). Indeed, Al’shevskii and Derebon(1982) reported increased concentrations of crude pro-tein and raw gluten in the grains in response to Mgsulphate application in field-grown winter wheat. Anincrease in the protein concentration of rye grainresulting from Mg addition to acidic soddy-podzolic,sandy-loamy soils was also reported by Magnitskii etal. (1970). This agrees well with pot experimentsinvestigating spring barley as reported by Fecenkoand Francakova (1980). In these experiments Mg fer-tilization had no clear effect on total N concentration,but reduced the share of soluble N on the total Nconcentration in the grains particularly when suppliedas MgSO4. In more recent studies the interaction of Nand Mg supply on yield and quality parameters ofwinter wheat grown in K-rich, Mg-deficient soil wasspecifically addressed (Chwil 2001, 2009). Whenpooled over the two N rates tested, the addition ofMgSO4 significantly increased the grain concentrationof crude protein and gluten (Fig. 2). Even though apositive effect regarding grain yield and protein con-centration was realized by high N supply, highestyields were only achieved when Mg was applied inthe highest rates at the respective N rates.

Potato (Solanum tuberosum L.)

As compared to cereals effects of Mg on potato qualitywere more frequently reported, and the well-knowninteraction of Mg and K (Marschner 2012) was par-ticularly frequently addressed (Cepl 1994; Allison etal. 2001; Zengin et al. 2008). However, the quality of

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potatoes is a complex parameter and the desired qual-ity traits depend on the intended usage (Hiltrop 1999;Talburt and Smith 1987). For warehouse potatoes(fresh consumption) among external quality parame-ters also the cooking type (floury or mealy, medium,waxy or hard-boiling) are important. The cooking typeis merely controlled by genetic factors, but manipulat-ing the growth conditions can have an important im-pact as well. For example, conditions enhancingassimilation and translocation of carbohydrates tendto increase the starch content and thus the mealiness ofthe cooked potato. Both starch content and mealinessare positively correlated with the specific gravity andthe dry matter content (Smith 1977; Talburt and Smith1987; Feltran et al. 2004), an important measure forpotato quality. For starch production the starch con-centration in the tubers obviously is the most impor-tant quality criterion. The dry matter content itselfrepresents an important quality criterion when produc-ing potatoes for further processing such as Frenchfries, crisps or dry instant products. High dry mattercontents ensure lower oil absorption (resulting in moreproduct per unit oil) and improve texture and shape ofthe product (Lulai and Orr 1979; Hiltrop 1999; Feltranet al. 2004). Therefore, due to the importance of Mgfor photosynthesis and assimilate translocation, astrong effect on the quality of potatoes particularlyfor industrial use is anticipated.

Another important quality trait is tuber firmness orresistance against mechanical stresses occurring duringharvest, transport and storage. In early studies it wasdemonstrated that mechanical resistance of potato tuberssignificantly correlates with specific gravity and texture

(Lujan and Smith 1964). Consequently it was concludedthat a firmer potato should exhibit a mealier texture (asrealized by higher starch contents) associated with betterbaking and processing qualities. Increased firmness alsoreduces the risk of bruising and various forms of dis-colourations (see section below). Indeed, Klein et al.(1982) reported that increasing Mg supply (0–115 kgMgSO4ha

−1) increased the firmness of Katahdin pota-toes grown in soil that was not nutrient-deficient. Later,Cepl (1994) observed a consistent response of the yieldand starch concentration of a late industrial variety toMg application (40 and 80 kgMgha−1) as compared toMg control treatment, whereas an early-maturing ware-house variety exhibited less consistent responses. In 3-year field experiments using two mid-early cultivarsPoberezny and Wszelaczynska (2011) observed thatincreasing Mg supply (0–100 kgMgOha−1 as MgSO4)consistently increased dry matter and starch concentra-tion. Zengin et al. (2008) investigated the effects of Kand Mg supply on potato quality over 2 years at twodifferent locations. However, in six out of eight treat-ment combinations suitable to assess the effect of Mgsupply no significant response of the dry matter contentwas apparent. The remaining two treatment combina-tions even showed significantly contrasting responsesmaking a proper evaluation difficult. From these studies,at least in tendency, an overall positive effect of Mg onmechanical resistance due to starch and therefore drymatter accumulation can be expected.

Potatoes for both fresh consumption and for indus-trial processing are stored for considerable durations.Hence, storage losses due to respiration and pathogenattack often cause severe economic losses, and hencethe storage properties of potato tubers received muchattention (Affleck et al. 2012). The effect of N and Ksupply on storage properties of potato tubers wasstudied intensively (e.g. Kolbe et al. 1995), whereasthis holds not true for the nutrient Mg. However, in arecent study Wszelaczynska and Poberezny (2011)showed that intermediate Mg doses ranging from 0to 100 kgMgOha−1 (optimum 60 kg MgO ha−1) re-duced fresh weight losses during storage for 6 monthsin two mid-early cultivars. For K a similar responsewas observed (range 0–240 kgK2Oha−1, optimum160 kgK2Oha−1), whereas N supply negatively affectedfresh weight losses during storage. Interestingly, freshweight losses were accompanied by dry matter andstarch losses (Poberezny and Wszelacrzynska 2011,Wszelacrzynska and Poberezny 2011) indicating that

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the storage losses observed in that experiment includedlosses due to respiration. However, the underlying phys-iological mechanisms remain to be elucidated.

Discolourations of various origins represent impor-tant quality criteria in potato. The so-called black spotincidence and crude pulp discolouration are a result ofenzymatic processes after damaging (e.g. harvest,transport) or processing (crude pulp) of potatoes.Damaging and processing of potato tubers allowspolyphenol oxidases to act on free phenolic com-pounds (e.g. phenolic amino acids) leading to theformation of dark melanins (Muneta 1981). Earlyreports on the effect of Mg fertilization on enzymaticdiscolouration have been provided by Mulder (1949).Unfortunately, available results are not consistent. Forexample, Klein et al. (1981) found that fertilizationwith MgSO4 reduced enzymatic discoloration andconcentration of phenolics, whereas the crude lipidand phospholipid contents of potato tubers were in-creased. A study by Mondy et al. (1967) indicated thatespecially the concentration of phenolic compounds oftubers positively correlated with enzymatic discolor-ation. However, enzymatic discoloration of potato wasshown to be inversely related to the lipid content(Mondy et al. 1965; Mondy and Koch. 1978). Thecontradictory results indicate that interactions withother production measures and storage time may havemasked the effect of Mg. Based on these reports it isdifficult to draw conclusions as to the involvement ofMg in the quality response. The enzymatic cascadefinally leading to melanin formation is inhibited bylow pH and antioxidants. Consequently, the additionof citric and/or ascorbic acid can reduce or preventsuch enzymatic discolourations. Indeed, particularlythe content of ascorbic acid represents a quality crite-rion in its own right (Delgado et al. 2001). Synthesisof ascorbic acid originates from glucose (Marschner2012) and a positive influence of favourable environ-mental conditions for photosynthesis (high light inten-sity) on ascorbic acid concentrations in various cropshas frequently been reported (e.g. Noctor and Foyer1998). In view of the significance of Mg for assimila-tion and carbohydrate translocation a positive effectof improved Mg supply may be expected. Indeed,Rubanov and Voitova (1970) observed an enhancedaccumulation of ascorbic acid in response to increasedMg supply. However, contrasting results on the impactof Mg on ascorbic acid contents were reported. Mondyand Ponnampalam (1986) did not observe significant

effects of increasing Mg supply on the concentrationof ascorbic acid, which agrees with early reports byKarikha et al. (1944). It may be concluded that thecontrasting results on phenol and ascorbic acid con-tents in combination with the occurrence of black spotincidence are at least somewhat interrelated, as allthese parameters are associated with several environ-mental stress factors that were not controlled in thefield experiments referred to.

In deep-fried potato products another non-enzymatic type of discolouration occurs that is closelyrelated to the presence of reducing sugars. Under dryheat conditions the so-called Maillard reaction of re-ducing sugars with amino N compounds forms browndeposits (Mottram et al. 2002; Gerendás et al. 2007).Moreover, when reducing sugars react with asparagineintermediates may form acrylamide, which is beingconsidered ‘likely carcinogenic for humans’ (Mottramet al. 2002). To our knowledge no results were pub-lished yet on the effect of Mg nutrition of potatoes onthe content of reducing sugars, asparagine or acrylam-ide formation, even though an effect can be expectedconsidering the Mg function in protein biosynthesisand carbohydrate partitioning. Future studies are nec-essary to clarify this point.

Another group of compounds found in potatoes, theglycoalcoloids, are of particular interest due to theirnegative impact on human health. Hence, the concen-tration of glycoalkaloids (e.g. alpha-solanine andalpha-chaconine) in tubers as quality trait in potatohas received considerable attention. These alkaloidsimpart a bitter flavour to potatoes (Sinden et al.1976), and most importantly they are toxic to humans(McMillan and Thompson 1979). Furthermore, accu-mulation of glycoalkaloids was associated with thegreening of tubers (Maga and Fitzpatrick 1980), al-though a direct link between the two processes has notbeen proven (Gull and Isenberg 1960). However, ithas to be stated here that in recent years positivebioactive properties were also attributed to this groupof compounds (see Milner et al. 2011, for review). Thetoxicity of these alkaloids, the positive response oftheir concentration in stems and leaves to increasingN supply (Love et al. 1994; Rogozinska and Wojdyla1999), and the significance of Mg for N metabolismhas prompted several investigators to study the influ-ence of Mg supply on glycoalkaloid accumulation inpotato tubers. Mondy and Ponnampalam (1985) ob-served a significant increase of the glycoalkaloid

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concentration in tubers (17 vs. 9 mg 100 g−1FM) afterapplication of 56 kgMgO per ha. This principallyagrees with earlier reports of Evans and Mondy(1984) based on 2-year field experiments showing asignificant increase of total glycoalkaloid content oftubers at all levels of fertilization (0–112 kgha−1). Theauthors suggested that the increase in chlorophyll syn-thesis, the stimulation of sugar metabolism and/or theincrease in amino acid production may be involved inthis effect. Indeed, in reports referring to the same fieldexperiments it was reported that Mg application in-creased both the total N and protein concentration(Mondy and Ponnampalam 1985; Klein et al. 1982).The maximal total amino acid concentration was ob-served at 45 kgMgOha−1. At this Mg dose the totalglycoalkaloid concentration also approached maxi-mum levels (Evans and Mondy 1984). However, morerecent studies by Rogozinska and Wojdyla (1999)based on 3-year field experiments showed no influ-ence of Mg supply on glycoalkaloid concentration ofpotato tubers. These contrasting results show the needfor further studies of the modulation of glycoalkaloidformation by Mg supply.

Sugar beet (Beta vulgaris L. ssp. vulgaris)

For sugar beet the extraordinary role of Mg for pho-tosynthesis and assimilate translocation was highlight-ed by Hermans et al. (2004, 2005). Due to these roles asubstantial increase in the sugar concentration of beetroots by Mg fertilization may be anticipated. As al-ready described, in this review the term qualityincludes not only the concentration of a target ingre-dient but also the concentration of constituents inter-fering with its recovery. In sugar beet not only thesugar concentration is discussed but also the molassesforming substances K and Na, as well as alpha-aminoN compounds (Buchholz et al. 1995).

Results on the effects of Mg fertilization on the beetroot sugar concentration appear inconsistent. Villariaset al. (2000) observed an increase of the beet rootsugar concentration from 13.6 to 16.9 % in responseto increasing Mg soil application (0–40 kgMgha−1).In contrast, Allison et al. (1994) conducted a meta-analysis of 14 field experiments carried out at differentsites in the UK. From this evaluation they concludedthat “effects [of Mg soil application] on processingquality were inconsistent and unlikely to be of agro-nomic or economic significance”. In other earlier

studies Mg soil application only in tendency increasedthe sugar concentration of beet root (Draycott andFarley 1971). Strnad and Javurek (1991) observed a(non-significant) decrease of the sugar concentrationin response to MgSO4 • H2O (kieserite) application(120 kgMgha−1). More recently, Szczepaniak et al.(2002), Barlog et al. (2002) and Barlog and Grzebisz(2004) also observed contrasting responses of the sug-ar concentration and technological quality of beet rootto increasing Mg supply.

Foliar application of Mg was also shown to beeffective in improving quality traits of sugar beet.Moustafa and Omran (2006) observed that foliar spraywith Mg-sulphate solutions increased the sugar con-centration and improved several technological param-eters (Table 1). Similar observations were reported byOsman (2005) and El-Sayed (2005). Kristek et al.(2003) reported that foliar spray with 5 %MgSO4 •7H2O (Epsom salt) increased the total sugar concen-tration from 14.8 to 15.5 % and the recoverable sugarconcentration from 12.32 to 12.95 % (mean of3 years). In an earlier report Kristek et al. (2000)observed a small, but significant increase of the sugarconcentration of beet root upon foliar application of5 % Epsom salt solution. However, in absolute termsthe positive effect of Mg fertilization on sugar con-centration and recovery was quite small, which isfurther underlined by other investigators reporting nosignificant improvement of beet root quality uponfoliar Mg fertilisation (Gutmanski et al. 1998).

A comparison of different trials in different years isdifficult as a number of different factors predominant-ly influence the sugar concentration and extractability.It appears that whereas the importance of Mg for sugarformation and accumulation in the beet root is clari-fied, more information on the response of sugar beetunder varying field conditions, particularly with re-spect to radiation and periodic drought, is highlysought after.

Oil crops

The global demand for vegetable oil has substantiallyincreased during the last decades, which could princi-pally be accomplished by increasing the acreage of oilcrops, and increasing their yield. However, the possibil-ity to increase the oil concentration deserves specificattention, particularly in view of increased profitability.Hence the following oil crops are specifically addressed

Plant Soil

with respect to the impact of Mg fertilization on oilconcentration of the harvest organ.

Oil palm (Elaeis guineensis Jacq.) In oil palm theharvested organ is the inflorescence (bunch). The mostcommon quality measure is the so-called oil to bunchratio. In agreement with the role of Mg for providingenergy-rich carbohydrate compounds several studiesrevealed a significant increase of yield and oil to bunchratio in oil palm. Tayeb (2005) reported an increase inthe oil to bunch ratio by 1 and 1.8 % after application of0.4 and 0.8 kgMgOpalm−1 a−1, respectively, in form ofkieserite in long-term (6 years) experiments conductedin Malaysia. This effect was largely due to a significantincrease in the oil to dry mesocarp ratio and in the oil tofibre values. In tendency similar observations, alsobased on long-term experiments carried out inMalaysia,were reported by Tang et al. (2001) (Fig. 3).

Early experiments carried out in the Ivory Coast al-ready highlighted the well-known interaction of Mg withK supply. Field experiments running for 2 years with twoand four rates of Mg and K, respectively, revealed thatMg application (0.135 kgpalm−1a−1) decreased theoil to bunch ratio at K rates ≤0.9 kgK2Opalm−1 a−1,whereas this ratio increased at higher, more adequate K

application rates (Ochs and Ollagnier 1977). Unfortu-nately no information was given on the soil K and Mgcontents, so that it is not possible to ascribe the qualityreductions to K and/or Mg directly. In view of the factthat oil palm is typically grown on acidic soils inadequatebase cation contents might be expected. Hence, in thisstudy the oil to bunch ratio negatively reacted to a Mgdominated nutrition of the palm indicating the need for abalanced nutrition of this crop also addressing the Krequirement. Interestingly, in this study the fatty acidcomposition was also evaluated. However, as comparedto the control, Mg application did neither affect the fattyacid composition nor the associated iodine value.

Brassica oil crops The Brassica species represent animportant, genetically diverse group of oil crops usedboth in temperate and tropical regions. Given its glob-al significance the number of investigations on theimpact of Mg published in scientific journals is sur-prisingly limited. Jahangir et al. (2005) applied differ-ent rates of Mg (0, 7.5, 15 and 30 kgha−1) to Indianmustard (Brassica juncea (L.) Czern.) and evaluatedthe impact on yield and oil quality. Even though theauthors reported yield increases due to Mg supply,these differences were insignificant. However, the oil

Table 1 Influence of foliar Mgapplication on sugar concentra-tions and selected technologicalparameters of sugar beet (basedon Moustafa and Omran 2006)

Treatment Sucrose(%)

K Na Alpha-amino N Juice purity(%)

Extractable sugar(%)(mmol 100 g−1 beet root)

Control 15.2 5.06 1.68 1.93 91.3 12.6

0.25 % MgSO4 15.6 5.08 1.68 1.85 91.5 13.1

0.5 % MgSO4 16.0 5.14 1.65 1.73 91.8 13.6

LSD0.05 0.32 0.37 n.s. 0.17 0.27 0.3

16

18

20

22

24

26

28

0 1 2 3

Kieserite rate (kg palm-1 a-1) Kieserite rate (kg palm-1 a-1)

FFB yield (t ha-1) Oil/bunch (FFB)

30

40

50

60

70

80

90

0 1 2 3

Oil/wet mesocarp Oil/dry mesocarp

Fig. 3 Effect of applicationof kieserite (MgSO4 • H2O;27 %MgO) on yield of freshfruit bunches (FFB, left) andoil content indices (right).Treatment effects (F-test)were not significant (basedon Tang et al. 2001)

Plant Soil

concentration was significantly increased from 39.2 to40.8 %. Nevertheless, as described for oil palm spe-cific quality traits of the oil itself (specific gravity,refractive index, moisture and acid value) did notrespond to Mg application.

Detailed investigations regarding the importance ofthe soil Mg status for the Mg response of rapeseed(Brassica napus L.) were reported by Bogdevich andMishuk (2006). Magnesium application slightly in-creased the oil concentration in rapeseed only at lowsoil Mg status (<150 mg exchangeable MgO kg−1 soil)indicating that the plants suffered from Mg deficiency(Fig. 4). On the other hand excessive soil Mg levels(337 mg exchangeable MgO kg−1 soil) led to a signifi-cant reduction of yield. Yield reductions due to suchhigh Mg contents in the soil are not a consequence of‘Mg toxicity’ per se (which has, to our knowledge, notbeen observed in plants so far), but are due to nutrientimbalances in the soil and consequently the plant. In-deed, such high imbalances in the cation compositionmay induce deficiencies in other cations like K, and Ca,so that the observed yield reductions are more likely aresult of other nutrient’s deficiencies. However, on aglobal scale such conditions are rarely observed.

Soybean (Glycine max (L.) Merr.) Soybean not onlyrepresents an important oil crop, but is also grown for itsprotein. Early studies already reported on the impact ofMg soil application (0–67 kgMgOha−1) on yield andquality of soybean (Nelson et al. 1945). They found thatincreasing the Mg supply up to 40 kgMgOha−1 signifi-cantly increased soybean yield, whereas the oil concen-tration remained almost unchanged. Interestingly, the

protein concentration was substantially reduced. The rea-son for this phenomenon remains unclear as the vital roleof Mg for protein formation and amino acid transportwould suggest a positive impact of Mg supply on proteinyield. This assumption is supported by a recent investi-gation on the effects of irrigation, P and Mg supply tosoybean grown in southeast Anatolia region demonstrat-ing a significant increase of yield and oil concentration byMg application provided the water and P supply wasadequate (Deliboran et al. 2011). In contrast to the studyof Nelson et al. (1945) the protein concentration wasmoderately, but significantly, increased in 1 of 2 years(Fig. 5). Overall, it appears that the protein formation insoybean is moderately affected by Mg soil application.

In contrast, Vrataric et al. (2006) not only reported asignificant increase in seed yield upon a single foliarMg application of 5 % MgSO4 • 7H2O solution duringvegetative growth based on 4-year experiment, but alsoan increase in the protein and oil concentration. A sec-ond foliar application after flowering did not furtherincrease the investigated parameters. The results werein principle agreement with earlier results published byKovacevic et al. (1991). These results may suggest thatin soybean the plant availability of Mg at later vegeta-tive growth stages, just before flowering, is of particularimportance for protein and oil accumulation. It is tempt-ing to speculate that at later growth stage root activity(Mg uptake) is diminishing, so that foliar ‘boosting’ byMg of the leaves delivering the C- and N-containingcompounds to the seeds effectively increases the seedquality. On the other hand too late applications areapparently not effective as the Mg applied fails to affectassimilate formation and partitioning.

0.0

0.5

1.0

1.5

2.0

2.5

128 143 250 337

yied

l (t h

a-1)

Mg soil status (mg MgO kg-1)

Control +Mg

350

370

390

410

430

128 143 250 337

oil c

onc.

(g

kg-1

)

Mg soil status (mg MgO kg-1)

Control +Mg

Fig. 4 Influence of Mg application (8 kgMg ha−1) on seed yield(left) and oil concentration (right) in spring rapeseed as affected bysoil Mg status. The HSD0.05 for seed yield as affected by soil Mgstatus and fertilizer treatments were 0.13 and 0.12, respectively,

and the HSD0.05 for oil concentration as affected by soil Mg statusand fertilizer treatments were 22 and 21, respectively (based onBogdevich and Mishuk 2006)

Plant Soil

Other oil crops

In groundnut (Arachis hypogaea L.) the significanceof Mg nutrition received limited attention. In a fieldexperiment carried out in India on a slightly alkalinesandy loam for two seasons application of MgSO4 didneither increase yield nor the oil concentration (Rajanet al. 1984).

In sunflower (Helianthus annuus L.) field experi-ments carried out in India only showed a significantyield response to 10 kgMgha−1, whereas the oil con-centration remained unchanged (Ahmedkhan et al.1990). However, the experiment was carried out at asite of high Mg availability (172 mg exchangeable Mgkg−1soil), so that a yield response beyond the high Mgavailability to the plants might not be expected. Inagreement, increasing Mg application (0–30 kgMgha−1) on a typic chromusterts soil developed frombasalt rock in India containing 56 mmolMgkg−1 alsodid not affect seed yield, but in tendency increased theseed oil concentration (Sagare et al. 1990). Whensunflower was grown on acidic and Mg-deficient soilsliming (CaCO3) and application of MgSO4 almostdoubled achene and oil yield (Csengeri and Kozak1985a, b). Such treatments also markedly increasedthe oil concentration in the achenes. Taken togetherthe results show that depending on the soil Mg avail-ability Mg application can increase yield and oil con-tents at Mg-deficient sites, indicating that furtherincreases beyond yield optimum do not have an effect.See below for a more elaborate discussion of this point.

Pulses

Azizi et al. (2011) evaluated the effect of different modesof MgSO4 application (soil, foliar and seed treatment,and their combinations) on yield and quality of two lentil(Lens culinaris L.) genotypes in Iran. Highest grain andbiological yield were obtained from soil fertilization,whereas the highest percentage of crude protein concen-trations in the seeds was obtained from foliar application,which generally agrees with findings in soybean (seeabove). The same was observed for other parameters like1,000-seed weight and pod number per plant. There wasa significant positive correlation between concentrationsof crude protein and Mg in the seeds. The authorsrecommend soil plus foliar application of Mg in orderto achieve highest magnesium and crude protein concen-trations, which principally agrees with observations incereals (Grzebisz 2013). As already proposed for soy-bean, foliar application appears to be effective withrespect to increasing the grain protein concentrationand in this case also Mg. However, whether the seedMg is directly related to protein formation in the seeds orwhether this is a consequence of enhanced translocationof amino acids from source leaves to the physiologicalsinks remains to be elucidated.

Fruits, vegetables and ornamentals

Compared to agricultural crops the various qualityparameters of horticultural crops as listed in the intro-ductory section play an extraordinary important roleparticularly for fresh market produce. Consequently,most investigations on the impact of nutrient supply onquality aspects, particularly organoleptic/sensory proper-ties, are referring to fresh market produce of horticulturalcrops. As shortly mentioned in the introductory section,purchasing decisions for horticultural produce are oftengoverned by psychological factors (e.g. convenience,price, ease of use, novelty, ethical considerations). How-ever, in developed countries other quality aspects likefood safety and the nutritional value and healthiness alsoaffect the final purchasing decisions of consumers.

Fruits

Pome fruits Mineral nutrients differentially affect in-ternal and external quality parameters of pome fruitslike apple (Malus domestica Borkh.). The optimal fruitnutrient concentration required for obtaining an

b ba

b

aab

aabb b ab a

0

50

100

150

200

250

300

350

400

450

500

0 40 80Mg supply (kg Mg ha-1)

Con

cent

ratio

n (g

kg-

1 )oil concentration 2006 oil concentration 2007protein concentration 2006 protein concentration 2007

Fig. 5 Influence of Mg supply on oil and protein concentrationof soybean seeds at highest P and irrigation level (means fol-lowed by the same letter are not significantly different; LSD0.05

was 34.95 gkg−1 and 19.23 gkg−1 in 2006, and 35.77 gkg−1 and18.64 gkg−1 in 2007 for the oil and protein concentration,respectively; based on Deliboran et al. 2011)

Plant Soil

envisaged quality characteristic depends on variousaspects including the genetic background, the environ-mental conditions, and the cultivation technique (e.g.fertilization practice). Marcelle (1995) provided anevaluation of favourable ‘concentration ranges’ ofthe macronutrients N, P, K, Ca, and Mg giving an ideaabout the complexity of the impact of mineral nutrientstatus on the various apple quality parameters. How-ever, for Mg it was stated that an impact on ‘goodeating quality’ is not well known except a negativeimpact on fruit firmness. Accordingly an optimal Mgconcentration ‘has to be relatively low’ for good stor-age properties. The physiological background under-lying this recommendation are discussed in a reviewby Bramlage et al. (1980) arguing that the commonlydescribed effects of various nutrients including N, P,K, Mg, and B on the quality of pome fruits are tocertain extent a consequence of their interaction withCa. The authors distinguished between the impact ofMg on fruits and the vegetative part of the plantsharbouring the fruits. They stated that there is onlyvery little evidence that Mg deficiency or Mg excessdirectly affects fruit quality, even though the fruit treessuffering from Mg deficiency appear weak and unpro-ductive. This view was underlined by Marcelle (1995)who suggested that most of the effects of Mg on fruitquality can be explained (1) by antagonism betweenMg and K and (2) by competition of Mg and Ca ‘forfixation sites during transport’. A proof of the state-ments given by Bramlage et al. (1980) and Marcelle(1995) is the well-known and often occurring bitter pitdisorder in apple. Bitter pit, in addition of giving anundesirable appearance, reduces texture and taste ofapple fruits. Bitter pit is a consequence of insufficientCa supply to the fruits. Even though Mg itself is notdirectly related to this nutritional disorder, the Ca/Mgratio can serve as an indicator for the incidence ofbitter pit (Askew et al. 1960). However, Marcelle(1995) states that in older literature the evaluation ofthe susceptibility to bitter pit was determined by form-ing two different ratios including Mg: K/(Ca + Mg)and (K + Mg)/Ca. The first ratio puts emphasis on thetransport antagonism between monovalent and diva-lent cations, whereas the latter ratio points to thespecific antagonistic effect of K and Mg in Ca accu-mulation. Based on the observation that bitter pitdevelopment was more closely related to (K + Mg)/Ca ratio than the K/Ca, Mg/Ca, K/Mg ratios alone andthe fact that the correlation was only slightly better for

(K + Mg)/Ca than for K/Ca (Garman and Mathis1956), Bramlage et al. (1980) suggest that Mg mayplay only a minor role in bitter pit development com-pared to K. More recently Burmeister and Dilley(1991, 1993) developed an artificial system for study-ing bitter pit development in apple. The authors in-duced bitter pit in harvested apple fruits by infiltrationwith Mg-containing solutions. This procedure wascapable of simulating naturally occurring bitter pitdisorder by changing the fruit internal Ca/Mg ratio(Burmeister and Dilley 1991).

The competition of the main base cations K, Ca andMg was also discussed as causal factor responsible forthe negative impact of high fruit Mg content on fruitfirmness, an important storage quality parameter, in areview by Marcelle (1995). In contrast, a 5-year soilfertilization experiment conducted by Noè et al. (1995)comparing different N, K andMg treatments on ‘GoldenDelicious’ apples showed that Mg fertilization im-proved fruit firmness. However, the results presentedin that study were quite variable. Also, even thoughthe authors stated that Mg deficiency is frequently ob-served in the experimental area the soil Mg contentseven exceeded the soil K contents. Therefore, a benefi-cial effect of Mg fertilization may not be expected.Instead, the observed increasing firmness by Mg appli-cation may be a secondary effect not directly related toMg. This is underlined by the fact that the fertilizationtreatments affected neither fruit Mg nor fruit Ca concen-tration and only slightly increased the K concentration.

Consumer acceptance of horticultural producestrongly depends on fruit appearance, for which—withrespect to pome fruits—the skin colour is most signif-icant. Reay et al. (1998) investigated the effect offoliar urea and MgSO4 sprays on chlorophyll, carot-enoid and anthocyanin concentrations in apple skin.All three constituents are not only thought to be healthpromoting agents but also play decisive roles in appleskin colouration. The Mg treatment of the tree canopyincreased the chlorophyll and carotenoid concentra-tion on the back side of the fruit (not facing the sun).Concerning a consumer-oriented approach to qualityone should remember that the chlorophyll and carot-enoid concentrations determine the ‘backcolour’ of thefruit ranging from light green to pale yellow. Whenusing this colour for grading food maturity in export-oriented regions like New Zealand a greener colourmeans that less fruits are suitable for export at a givenharvest date or the fruits need more time to reach the

Plant Soil

desired maturation state (Reay et al. 1998). The‘greening effect’ of Mg supply may be associated withincreased chlorophyll formation or, more likely, de-creased/delayed chlorophyll degradation. Enhancedassimilate translocation might also be involved in me-diating this effect at physiological level. A deeperunderstanding of the impact of Mg on the ripeningprocesses in pome fruits is required to design target-oriented fertilization strategies. Also, the response toMg supply in terms of ripening and colour formationand development may be cultivar-specific.

Taste and flavour related characteristics of pomefruits are dependant on the ripening stage as is thestorage quality. A number of indices incorporatingsome of the important traits determining taste, flavourand storage characteristics in apple like titratable acid-ity (TA), (total) soluble solid (TSS) concentrations,fruit firmness and starch degradation have been devel-oped, namely the Streif (Kaack and Pedersen 2011),and the Perlim and the Thiault index (e.g. Rabiei et al.2011). These are used for determining the ripeningstage, the ideal harvest period and changes upon stor-age and are to a great extent cultivar-specific. Accord-ing to the role of Mg in balancing ion charges and pHand in the formation and translocation of metabolitesan impact of Mg on each of these quality parameters isto be expected. Particularly the Thiault index (totalsugar (g L−1)+10×acidity (g/L), with total sugar =(Brix×10.6–20.6) can readily be modulated by fertil-ization. Even though the consumer preferences fortaste and flavour might differ between regions/countries, a high index is typically regarded as apositive flavour and, therefore, quality trait. Hence,the negative relationship between the Mg concentra-tion of the fruit and the Thiault index thus indicates anegative effect of Mg on apple quality (Marcelle1995). However, the author explicitly state that thisis not a direct consequence of high Mg but of animbalanced cation (e.g. Ca and K) supply to the fruitsunderlining the initially described importance of thecation ratios rather than the Mg concentration alone. In2 years (1994–1995) and seven different appleorchards growing six different apple cultivars Dris etal. (1999) investigated the relationship between thenutrient (e.g. N, P, K, Ca, and Mg) concentration ofleaves from fruit-bearing branches (BF) and non-bearing branches (BNF) and the apple quality param-eters including nutrient concentrations, fruit diameter,juice pH, TA, and TSS concentration. Even though

there was quite a close relationship between the Mgconcentrations in leaves of BF and BNF and fruits,none of the investigated quality parameters was corre-lated with the leaf Mg concentration. However, the Mgconcentrations may not have reached severe deficien-cy levels (never ≤0.125 mgMg g−1 d.m.). Apparently,in this study the investigated quality traits remainedstable within a certain leaf tissue Mg concentrationrange. It may be concluded that Mg levels in a moresevere deficiency range would have imposed qualitychanges as a consequence of disturbed metabolism.Also it can be speculated that Mg supply/concentra-tions beyond optimum would have negatively affectedthese quality traits as a consequence of imbalancedcation ratios (as described in the previous paragraph).However, in a 5-year study conducted by Noè et al.(1995) apples from Mg-treated trees (soil fertilization)showed higher TSS and acid concentrations as com-pared to N and K treatments after storage, whereasafter harvest the TSS was lower. In this study the Mgconcentration in the leaves did not fell below 0.25 mgMg g−1 d.m. In conclusion, it appears that flavourdetermining ingredients in the selected studies werenot or marginally affected by Mg nutrition verifyingthe statement given by Bramlage et al. (1980) that Mgdeficiency or excess have hardly any effect on applequality. Even though there are numerous other studiesavailable investigating the effects of mineral nutritionincluding Mg on apple fruit quality, the selected refer-ences were selected as appropriate examples.

Stone fruits As described for pome fruits Mg wasoften co-investigated with other mineral nutrients, par-ticularly Ca as the nutrient having the greatest impactat least on fruit appearance and texture. The fewresults reported considering Mg mostly dealt withquality traits related to storability.

Serrano et al. (2004) conducted a trial with peach(Prunus persica (L.) Batsch) and nectarine (Prunuspersica (L.) Batsch var. nucipersica (Suckow) C.K.Schneid.) trees, which were sprayed at three differentgrowth stages (after harvest in the previous year, 10 daysafter anthesis at the beginning of April, and right beforemanual thinning at the end of April) with a formulationcontaining Ca, Mg and Ti (evaluation per ion was notdone) to investigate quality parameters (1) at harvest, (2)after 7–28 days of cold storage and (3) after subsequentripening for 4 days at 20 °C. Fruits of treated plantsexhibited higher weight and pulp firmness than control

Plant Soil

fruits, whereas no effect was observed on colour, TSScontent, TA or the time required for ripening on the tree.Lower levels of weight loss, colour evolution, TSS/TAratio and ethylene production but higher pulp firmnesswere found in treated peaches and nectarines comparedto control fruits during the cold storage phase. In addi-tion, the storability of treated fruits could be prolongedby up to 14 days compared to fruits from control plantsas a consequence of improved parameters related toripening. However, in view of the co-application ofMg with Ca and Ti and the small effects of Mg on theinvestigated traits in other crops, the improvement of thefruits appears to be predominantly a consequence of Caand Ti supply. The same conclusion can be drawn froma 1-year study of Alcaraz-López et al. (2004). Theauthors used sprays containing Ca, Mg or Ti on peachtrees with special emphasis on improving mechanicalproperties of the fruits. Again the experimental designdoes not allow any conclusion on a singleMg effect, butthe combined treatments increased tree performance(branch elongation, flowering and fruit set intensities)and fruit size, improved resistance of the fruits to com-pression and penetration, and decreases weight lossduring postharvest storage. The authors concluded thattreatments seemed to delay the apparent ripening status.The same working group observed somewhat positiveeffects of foliar Mg spray on flesh firmness (Alcaraz-Lopez et al. 2003). Again, a combined treatment with Tisignificantly increased peel and flesh firmness, fruit sizeand weight, compression and impact resistance, theripening index and to some extent fruit colour. Table 2provides an impression on the inter-experimental varia-tion of the results.

Grape/grapevine (Vitis vinifera L.) Production ofquality grapes suitable for wine-making depends onnumerous factors. Moretti (2002) stated that amongintrinsic (degree of wood maturation, content of nutri-tive and nitrogenous compounds) and external factors(temperature, rain distribution, soil parameters) thequality of wine produced is also modulated by fertil-ization to a certain degree. The author reduced thiseffect mainly to fertilization-induced improved rootand shoot growth and the subsequent impact on vineperformance. Even though this statement is rathervague, Moretti (2002) in a 3-year trial found that foliarapplication of Mg (and Zn) enhanced shoot length andlignification and increased root number thereby layingthe cornerstone for high quality production. A more

direct relationship between Mg supply and yield andgrape quality was presented by Stefanini et al. (1994)based on trials conducted near Paolo del Colle. Twocultivars (Trebbiano toscan, Uva di Troia) receivedMgSO4 as foliar fertilizer (at low rates), via fertigationor applied to the soil at rates of ca. 36 and 72 kgMgha−1. They observed that Mg increased the weight ofprunings in Uva di Troia irrespective of the applicationmethod. In contrast Trebbiano toscano did not respond(fertigation, soil application) or even reduced weightof prunings after foliar Mg application. Also, the Mgtreatment reduced the fruit yield of cultivar Trebbianotoscano, whereas Mg application via fertigation in-creased fruit yield in Uva di Troia. The results furtherunderline the initial statement that grape yield andquality parameters are subject to various intrinsic andexperimental factors. Obviously, cultivar-intrinsic fac-tors are also responsible for the differences of thecultivars in the response to Mg application.

There are several mineral nutrition-related disordersin grape production, e.g. Fe deficiency chlorosis orstem necrosis. Even though not yet fully understood, itis thought that the Ca and Mg nutrition plays a keyrole in the development or avoidance of stem necrosis,a disorder periodically occurring in important grapevarieties (Rupp et al. 2002). Foliar spraying with Mgcontaining fertilizers is a common practice to correctnutrient imbalances in grape to reduce the risk of stemnecrosis. On the other hand it is felt that late applica-tions of agrochemicals including fertilisers may nega-tively affect wine processing and quality, and thisconcern is particularly strong for a compound knownas ‘bitter salt’. By conducting field trials over 3 yearsRupp et al. (2002) showed that foliar sprays withMgSO4 or MgO during the functional period of vérai-son (onset of ripening) increased the Mg content ofgrapes and must, but organoleptic testing of the result-ing vines did not reveal any impact of foliar Mgfertilization on wine taste. The authors concluded thatlate Mg application is safe regarding wine quality.

Red grape varieties are distinguished by their antho-cyanin content that are plant polyphenolic constituentsexhibiting antioxidative activities (Miguel 2011). It wasdemonstrated that the degradation of anthocyanins isreduced due to Mg supply (Shaked-Sachray et al.2002), and more recently the same group providedevidence that Mg appears not to increase the biosynthe-sis, but rather to decrease the catabolism of anthocya-nins in grape using cell suspension cultures (Sinilal et al.

Plant Soil

Tab

le2

Overview

oftheeffect

ofMgon

severalqu

ality

parametersof

differentagricultu

ralcrop

s,fruits,vegetables

andornamentals

Crop

Treatment

Applicationprocedure

Chan

gein

qua

litytrait

References

Agriculturalcrop

s

Rye

Mg

soil

+(starchconcentration),+proteinconcentration)

Magnitskiiet

al.19

70

Winterwheat

MgS

O4together

with

NPK

dressing

toMgdeficientsoil

+(1,000

-grain

weigh

t),

Al’shevshkiiandDerebon

(198

2)

Winterwheat

Mg(saltnotspecified)

sand/solutioncultu

re+(1,000-grain

weight),+(phytates,particularly

whenMgwas

appliedbeyo

ndyieldop

timum

)BeringerandForster

(198

1)

Winterwheat

MgS

O4

+(crude

protein,

raw

gluten)

Chw

il(200

1,20

09)

(Spring)

Barley

MgS

O4

soil

+(reductio

nof

solubleN

:totalN

ratio

)Fecnk

oandFrancakov

a(198

0)

Potato

MgS

O4

soil

+(firmness)

Klein

etal.(198

2)

Potato

Mg

soil

+(starchconcentration)

Cepl(199

4)

Potato

MgS

O4

soil

+(dry

matterandstarch

concentration)

Poberezny

andWszelaczynska

(2011)

Potato

MgS

O4,Kalim

agnesia(25%K,

6%Mg,

17%S)

soil

0(dry

mattercontent)

Zenginet

al.(200

8)

Potato

MgS

O4

soil

+(reductio

nin

freshweightlosses

during

storage

includinglosses

dueto

respiration)

Wszelaczynska

andPoberezny

(2011)

Potato

MgS

O4

soil

+(reductio

nin

enzymatic

discolouratio

ns),+

(increasein

phenol,lip

idandphospholipid

contentsof

tubers),comment:in

somestudies

enzymatic

discolouratio

nwereshow

nto

bepositiv

elycorrelated

with

phenol

andlip

idcontentsquestio

ning

theresults

ofKlein

etal.(198

1)

Klein

etal.(198

1),Mondy

etal.(196

7),Mondy

etal.,(196

5),

Mondy

andKoch(197

8)

Potato

Mg

soil

+(ascorbicacid

concentration)

Rubanov

andVoitova

(197

0)

Potato

MgS

O4•7H

2O,do

lomite

soil

0(ascorbicacid

concentration)

Mondy

andPon

nampalam

(198

6),

Karikha

etal.(194

4)

Potato

MgS

O4•7H

2O,do

lomite

soil

+(glycoalkaloid

concentrationin

tubers,

effect

stronger

with

Epsom

saltcompared

todo

lomite)

Mondy

andPon

nampalam

(198

5)

Potato

MgS

O4

soil

+(glycoalkaloid

concentrationin

tubers)

Evans

andMondy

(198

4)

Potato

MgS

O4

soil

+(protein

andtotalN

concentration,

correlartin

gwith

glycoalkaloidconcentatio

n)Mondy

andPon

nampalam

(198

5),

Klein

etal.(198

2),Evans

and

Mondy

(198

4)

Potato

MgS

O4

soil

0(glycoalkaloid

concentrationin

tubers)

RogozinskaandWojdy

la(199

9)

Sug

arbeet

Mg

soil

+/0/−

(sugar

concentration)

Villariaset

al.(200

0),Allisonet

al.

(199

4),DraycottandFarley

1971),StrnadandJavurek(199

1),

Szczepaniak

etal.(200

2),Barlog

etal.(200

2),BarlogandGrzebisz

(200

4)

Plant Soil

Tab

le2

(con

tinued)

Crop

Treatment

Applicationprocedure

Chan

gein

qualitytrait

References

Sug

arbeet

MgS

O4

foliar

+/0

([recoverable]

sugarconcentration,

seealso

Table1),

MoustafaandOmran(200

6),Osm

an(200

5),El-Sayed

(200

5),Kristek

(200

0;20

03),Gutmanskiet

al.

(199

8)

Oilpalm

MgS

O4

soil

+(oil:bunchratio

,seealso

Fig.3),−

(oil:bunchratio

whenMgisfertilizedin

anim

balanced

way)

Tayeb(200

5),Tang

etal.(200

1),

OchsandOllagnier19

77)

Indian

mustard

MgC

O3,MgS

O4

soil

+(oilconcentration)

Jahangiret

al.(200

5)

Oilseedrape

MgS

O4

soil

+(oilconcentration,

only

inMgdeficientsoils)

Bogdevich

andMishuk(200

6)

Soy

bean

MgS

O4

soil

0(oilconcentration),−(protein

concentration)

Nelsonet

al.(194

5)

Soy

bean

MgS

O4•7H

2O

soil

+(oilconcentration),+(protein

concentration)

Deliboran

etal.(2011)

Soy

bean

MgS

O4•7H

2O

foliar

+(oilconcentration),+(protein

concentration)

Vrataricet

al.(200

6),Koy

acevic

etal.(199

1)

Groun

dnut

MgS

O4

soil

0(oilconcentration)

Rajan

etal.(198

4)

Sunflow

erMgS

O4

soil

0(oilconcentration,

high

soilMgcontent)

Ahm

edkhan

etal.(199

0)

Sunflow

erMgC

l 2soil

+(oilconcentration,

high

soilMgcontent)

Sagareet

al.(199

0)

Sunflow

erdolomite,MgS

O4•7H

2O

soil

+(acheneandoilyield,

low

Mgsoilcontent)

Cseng

eriandKozak

(198

5a,b)

Lentil

MgS

O4

soil,

foliar,seed

treatm

ent

+(crude

proteinconcentrations,e.g.

with

foliarapplication),+(1,000

-seedweight),+

(pod

numberperplant)

Azizi

etal.(2011)

Tea

MgS

O4

soil

+(concentratio

nof

aminoacid,polyphenol,

catechin

concentrations,MgandSeffect

cannot

beclearlyseparated)

Jayang

aneshandVenkatesan(201

0)

Tea

MgS

O4

soil,

nutrient

solutio

n+(aminoacids,e.g.

theanine)

Ruanet

al.(199

8,20

12)

Tea

MgS

O4together

with

micronutirents

(not

specified)

foliar

+(aminoacidsin

madeblacktea),+(catchins

ingreenleaves

andmadeblacktea),comment:

foliarapplicatiuon

increasedmadeteayield

even

beyo

ndtheyieldof

soilapplication

treatm

ents

Jayang

aneshandVenkatesan(201

0)

Tea

MgS

O4

soil

+(secondary

metabolitesin

madeblack

tea)

Jayanganeshet

al.(2011)

Tea

MgS

O4•H2O

soil

−(decreasingpo

lyphenol

contentin

blacktea,

oolong

tea,greentea,except

undersevere

Mgdeficiency)

Ruanet

al.(199

9)

Fruits,vegetablesan

dornam

entals

Apple

N,P,K,Ca,Mg(saltsor

fertilizer

notspecified)

soil

0(noeffect

onjuicepH

),0(noeffect

onsoluble

solid

sanddrymatter),0(noeffect

ontitratable

acidity

)

Driset

al.19

99

Plant Soil

Tab

le2

(con

tinued)

Crop

Treatment

Applicationprocedure

Chan

gein

qualitytrait

References

Apple

N,P,K,Ca,Mg(saltsnotspecified)

notspecified

-(negativeeffect

onfirm

ness

andtexture,+

(positive

effect

onjuicepH

),-(negativeeftect

onsolublesolid

sanddrymatter)

Marcelle

1995

App

leK2SO4,MgS

O4

soil

+(increased

firm

ness

andtexture),+

,+b(positive

effecton

juicepH

),-,+b(solublesolid

sand

drymatter)

Noè

etal.19

95

App

leCaC

l 2,MgC

l 2,notspecified

infiltrationsolutio

n,notspecified

-(negativeeffect

onfirm

ness

andtexture,Mg

used

forinductionof

bitterpit)

Burmeister

andDilley

1991,19

93;

Bramlage

etal.19

80

App

leN

asurea,MgS

O4•H2O

foliar

-(greener,butfruitgreennessprolongs

timeto

harvest)

Reayet

al.19

98

App

leMgS

O4

soil

0(noeffect

onappearance,firm

ness

andtexture,

solublesolid

sanddrymatter)

Bennewitz

etal.(2011)

App

leMgS

O4

soil

except

increasedfruitweigh

tanddiam

eter

nofruitcharacteristicswereaffected

El-Gazzar(200

0)

Pineapp

leFullnu

trient

solutio

nwith

out

Mgsupp

lysand

cultu

resupplied

with

nutrient

solutio

n+(m

ild[yield

notaffected])Mgdeficiency

didno

taffect

sensoryprop

ertiesof

pineapple

Ram

oset

al.(201

0)

Pineapple

MgO

soil

+(average

fruitlength

andwidth

andcore

diam

eter)

Vélez-Ram

osandBorges(199

5)

Peach,nectarine

Ca,Mg,

Ti(saltform

notspecified)

foliar

0,−b

(app

earance),+,+

(firmness

andtexture),0,+

(juice

pH),0,−(soluble

solid

sanddrymatter)

Serrano

etal.20

04

Citrus

Dolom

itesoil

+(juice

pH,solublesolid

s,tirtatableacidity

Quaggio

etal.(199

2);Koo

(197

1)a

Citrus

Ca,K,MgS

O4

soil(sandcultu

reand

fieldexperiment

+(positive

effect

onjuicepH

,titratableacidity,

‘significant

predictorof

juiceacidity

whenMg

levelswerehigh’)

MossandHiggins

(197

4);

Citrus

MgS

O4,CuS

O4,ZnS

O4,

FeSO4,H3BO4

foliar

0(noeffecton

juicepH

,solublesolid

s,titratableacidity

)Ram

andBose(200

0)

Tomato

Ca,Mg(saltsnotspecified

fertigation(rockw

ool)

+(increased

firm

ness,texture)

Hao

andPapadopoulos(200

4)

Tomato

K,Mg(saltnotspecified)

soil

0(noeffect

onjuicepH

andtitratableacidity

)DaviesandWinsor(196

7)

Tomato

KCl,MgC

l 2substrate(perlitefertigation)

+(appearance),+(soluble

solid

sanddrymatter)

Chapagain

andWiesm

an(200

4)

Tomato

K2SO4MgS

O4

substrate(sphagnu

mpeat

fertigation)

+(intend

ency

fruitcolour

andsign

ificantly

drymatterwas

increasedat

highestcombined

KandMgfertilizatio

nlevelsperlsubstrate)

Borko

wskiandSzw

onek

(198

6)

Tomato

Establishm

entof

specific

K(K

2SO4):Mg(M

gSO4):

Ca(Ca(NO3) 2)ratio

s

sand/coirmedium

(fertig

ation)

high

Ca/Mgratio

sdecreasedfruitpH

,titratable

acidity,solublesolid

sanddrymatter,Ca:Mg

ratio

s<1caused

sign

ificantreductions

inyield

andfruitquality,low

Ca:Mgratio

sincreased

risk

ofblossom

endrot

Nzanza(200

6)

Cabbage

MgS

O4

nutrient

solutio

noptim

umrelatio

nshipbetweenhead

yield

andMgconcentrationin

theou

terleaves

HaraandSon

oda(198

1)

Plant Soil

2011). It would be interesting whether this phenomenonalso occurs in important other crops and how this phe-nomenon operates at the molecular level.

Tropical fruits/exotic fruits

Citrus fruits (Citrus sp.) Morton et al. (2008) statedthat an effect of Mg fertilization on citrus qualityparameters is only likely under severe Mg deficiencyconditions. Of course, severe Mg deficiency deeplydisrupts plants metabolism which will certainly affectquality and yield as well. Magnesium deficiency is atypical problem of acidic soils, which are often Mg-deficient (Gransee and Führs 2013, this issue). Over-laying the main citrus production sites with the distri-bution of acidic soils worldwide shows an increasedrisk of Mg deficiency in citrus production in manyparts of the world. Indeed, Quaggio et al. (1992)investigating the effect of liming of acidic and Mg-deficient Brazilian soils planted with citrus trees withdolomitic limestone found that the TSS and acid con-centrations in fruits linearly increased with dolomiticlimestone application for seven years. These increaseswere strongly correlated with the leaf Mg concentra-tions and already observed by Koo (1971). Moss andHiggins (1974) conducted a sand experiment undergreenhouse conditions for one season. They concludedthat particularly the increase in fruit acids (occurring athigh Mg treatment) might be related to an increasedCa/Mg rather than an increased K/Mg ratio as previ-ously suggested. This might explain the high acidconcentration, as an additional uptake of Mg at almostunchanged potassium uptake might have led to in-creased organic acid production to counterbalancethe increased cellular cation accumulation. Fruit acid-ity can be a critical factor as too high acidity, or lowbrix:acid ratio, delays maturation, particularly whenMg-accumulating rootstocks are planted (Moss andHiggins 1974). In another 2-year study Ram and Bose(2000) found that despite the high nutrient demand ofmandarin orange effects of foliar Mg and micronutri-ent (Cu, Zn, Fe, B) application on TSS, total sugar,reducing sugars and fruit acidity were not significant,even though the fruit yield was indeed higher (exceptwhen copper was applied together with the othernutrients). Table 2 summarises the variation in resultsof different investigations. At present it is uncertain towhat extent this variation is a consequence of thedifferent cultivars and soils/locations used.T

able

2(con

tinued)

Crop

Treatment

Applicationprocedure

Chan

gein

qualitytrait

References

Cabbage

MgS

O4Mgchelated

with

aminoacids,Mg-EDTA

nutrient

solutio

n+(chlorophy

ll,solubleprotein,

vitamin

Con

lyin

Mg-am

inoacid

treatm

ent,Mghascompared

toam

inoacidson

lyasm

alleffect.

Han

etal.(201

0)

Aster

Mgas

nitrateor

chloride

in-vitro,

greenhou

se→

nutrient

solutio

n+(improved

colour

characteristicsas

aconsequenceof

Mg-inducedreductionin

anthocyanindegradation)

Shaked-Sachray

etal.(200

2)

Belladonna,Chamom

ileMgS

O4

in-vitro

+(produ

ctionof

trop

anealkaloid

productio

n),+

(quantity

andqu

ality

ofessentialoilprod

uctio

nby

hairyroot

cultu

resystem

s)

Hanket

al.(200

3),Szöke

etal.(200

4)

n.d.

notdetected,+

positiv

eeffect,−

negativ

eeffect,0

noeffectob

served

(the

term

s‘positive’and‘negative’

referto

increasing

anddecreasing

effects,respectiv

ely,irrespectiv

eof

whether

thesetrends

areim

prov

ingor

redu

cing

thequ

ality

)aCon

clusionon

Mgeffectscanbe

draw

nas

dolomite

andcalcite

werecomprehensively

comparedin

thisstud

ybAtharvestandaftercold

storage,respectiv

ely

Plant Soil

Pineapple (Ananas comosus (L.) Merr.) Pineapple rep-resents an important and popular tropical fruit crop, butstudies on the impact of Mg on its quality are limited.Based on omission experiments carried out in Brazil for3 years Ramos et al. (2010) concluded that Mg deficien-cy does not alter the sensory properties (acidity, totalsoluble solids, vitamin C, pulp colour) of the pineapplefruit (cultivar ‘Imperial’). Yield data indicating a growthlimitation due to insufficient Mg supply where notreported pointing to a mild Mg deficiency level. Thismay explain the absence of effects on sensory propertiesas the deficiency level was not yet sufficient to markedlyaffect metabolic processes.

In pineapple nutrients are often applied as foliarspray. In a 1-year trial Velez-Ramos and Borges(1995) could indeed demonstrate a positive effect ofMg on pineapple yield as the average length and widthof the fruit and the core diameter increased significant-ly with increasing Mg applications at high K and Nsupply levels (cultivar ‘Red Spanish’). In general, inthe planted crop a higher fruit acidity was associatedwith higher doses of K and Mg, which agrees withprevious findings in this crop (Py et al. 1987) and withconsiderations given in the previous section on Citrus.However, a significant effect of Mg foliar applicationon the quality of the ratoon crop could not be demon-strated. Hence, for this fruit crop, too, it appears thatcomprehensive studies are needed to clarify the role ofMg in the formation of the different quality traitsunder various agroecological conditions and optionsfor Mg administration.

Vegetables

As mentioned in the previous section already, in veg-etables the ratios of the cations Mg, K, and Ca oftenare more closely correlated to the quality trait than theconcentration or supply of the nutrient underconsideration.

Tomato (Solanum lycopersicum L.) How does Mgsupply affect fruit appearance, texture andnutrient-dependent fruit disorders affecting thequality of tomatoes? A summary of the results ofdifferent experiments is given in Table 2. For1 year, Hao and Papadopoulos (2003) investigatedthe effect of differentiated Ca and Mg supply onthe physiological disorders blossom end rot (BER)and russeting, and on fruit firmness when grown

in rockwool. As expected at very narrow Ca/Mgratios BER was enhanced. Increasing the Ca/Mgratio reduced BER incidence. Unexpectedly, highMg supply together with low Ca supply increasedfruit firmness, whereas this effect of Mg was ob-served under high Ca supply only at later growthstages. Fruit russeting was lowest when Ca andMg were supplied in a ratio not lower than 6:1.A study published later by the same group showedthat at a given Ca supply increasing the Mg ap-plication enhanced the biomass allocation to thefruit, whereas the allocation to the leaves de-creased, pointing to the decisive role of Mg incarbohydrate partitioning (Hao and Papadopoulos2004). As a consequence of another cation inter-action in soil-grown processing tomatoes Hartz etal. (1999) found that K-nutritional disorders likeyellow shoulder and internal white tissue in pro-cessing tomatoes was more closely correlated withthe soil exchangeable K/√Mg than with soil Kavailability. In a non-repeated experiment Borkow-ski and Szwonek (1986) supplied tomato plantsgrown in peat with varying K (200–1,000 mgKl−1 substrate) and Mg (50–200 mgl−1) combina-tions. The highest marketable, high quality yieldwas observed when K and Mg were supplied inthe highest amounts at a ratio of 5:1. This clearlypoints to the fact that only balanced crop nutritioncan result in optimal quality.

With respect to fresh market tomatoes consumeracceptance is mainly determined by flavour e.g. bysweetness and sourness (Stevens et al. 1979;Malundo et al. 1995). An important measure forsweetness and sourness are sugars (TSS) and TA,respectively. Magnesium acts as (mobile) cation inmetabolite formation and translocation to fruits(Römheld and Kirkby 2007, see previous sections),and a quality-determining effect of Mg on theflavour characteristics of fresh market tomatoesmight therefore be expected. Even though hardlyany study investigated the effect of Mg nutritionon tomato quality specifically some informationcan be extracted from studies investigating theeffect of combined Mg, Ca, K, and N nutrition.However, these studies provide quite an inconsis-tent picture on the effect of Mg on tomato quality.Using fruits from a long-term field trial varying N,K, P, Ca, and Mg supply Davies and Winsor(1967) did not observe an effec t of Mg

Plant Soil

fertilization on tomato fruit composition, whileeffects of K (close relationship between K concen-trations and acidity of fruit juices) and, to a lesserextent, N supply could be demonstrated. In agreenhouse experiment conducted for 1 year usinga randomized block design with four replicationsChapagain and Wiesman (2004) investigated thepartial replacement of KCl in a fertigation solutionby KCl + MgCl2. Use of the combined K and Mgtreatment improved the quality of the fruit byincreasing its glucose, dry matter and Mg contents.The authors concluded that KCl treatment im-proved fruit appearance and quality, which waseven further enhanced by MgCl2 treatments. How-ever, they also state the need for more investiga-tions on the interaction of Mg in greenhousetomato fertigation. Hao and Papadopoulos (2004)investigated the effect of differentiated Ca and Mgsupply to tomato on various fruit quality parame-ters including dry matter and TSS when grown onrockwool. Neither the TSS nor the dry mattercontent were considerably affected by Mg supplyexcept that under high Ca/Mg ratio both parame-ters were reduced.

The inconsistency of the results reported stressthe need for additional investigations to improveour understanding of the importance of Mg nutri-tion on metabolite translocation and fruit qualityconsidering the various growing conditions, pro-ductions systems and usage relevant in commercialtomato production.

Cabbages (Brassica oleracea L. sp.) Effects of Mgsupply on cabbage quality have been described only tolimited extent. In the case of cabbage the definition ofquality is extended to head weight and size as qualitydetermining traits. In a non-repeated study investigatingthe contribution of Mg (and Ca and S) on cabbage (B.oleracea var. capitata L.) head-formation Hara andSonoda 1981 found an optimum relationship for Mg inthe outer leaves and cabbage head yield (determined byhead weight and size). In this study Mg concentrationsin the dry matter below 0.15 % (Mg deficiency) orabove 1.5 % (cation antagonismwith Ca and K) reducedhead yield. However, as this study was not repeated finalconclusions are difficult to derive. Another study inves-tigated the effect of MgSO4, amino-acid chelated Mg(Mg-AA), and Mg-EDTA on quality parameters of Chi-nese cabbage (Brassica rapa L., ssp. pekinensis, Han et

al. 2010). As concentrations of chlorophyll, solublesugar, soluble protein and vitamin C were increasedonly by Mg-AA, whereas the other Mg sources in-creased yield only, it is concluded that the improvementof quality due to Mg-AA supply in Chinese cabbageappears to be more related to the additional amino acidrather than to genuine Mg effects.

Flowering Chinese cabbage (Brassica campestrisL. ssp. chinensis var. utilis Tsen et Lee) and Pak Choi(Brassica rapa L., ssp. chinensis, Bai Cai) are impor-tant vegetables in China, Korea and Japan, and con-tribute valuable amounts of ascorbic acid (vitamin C)to the local diet. Liu et al. (2008) reported that Mgdeficiency reduced the vitamin C and soluble proteinconcentration in the stalk of flowering Chinese cab-bage indicating the importance of a sufficient Mgsupply for optimal healthiness of the product.

Not only in view of the importance of cabbage inthe human diet, additional studies are needed to clarifythe effects of Mg supply on cabbage quality.

Tea (Camellia sinensis (L.) Kuntze) In the context ofthis review the term ‘tea’ refers to fermented (black),semi-fermented (oolong) and non-fermented (green)Camellia teas (Ruan et al. 1999), and the focus ofattention is put on the economically most importantblack and green teas. The quality of made tea—thefinal product of processing—largely depends on thechemical composition of the raw material used—theharvested tea leaves—and their required quality char-acteristics largely depend on the intended type of teabeing produced.

The two most important chemical groups with re-spect to the liquor characteristics of a black tea infu-sion are theaflavins and thearubigins formed duringthe fermentation process (more precisely an enzymaticoxidation) from polyphenolic precursors and thereforethe quantitative and qualitative composition of poly-phenols (catechins) is most important for black teaprocessing (Ruan et al. 1999; Kumar et al. 2011). Inaddition, numerous volatile compounds primarilyformed during processing from precursors like prima-ry (fatty acids Okal et al. 2012, amino acids andproteins, Kottur et al. 2010) and secondary metabo-lites, are also important quality traits when producingblack tea.

On the other hand for green tea, the concentrationof catechins, known for their adstringency, in har-vested leaves should be moderately high as

Plant Soil

biochemical reactions are kept to a minimum duringgreen tea manufacture (Ruan et al. 1999). In green teathe adstringent and bitter catechins are balanced byhigh concentrations of free amino acids (typically 1–5 % d.w.) that not only contribute to the mellownessand freshness of the infusion, but together with solublesugars also act as precursors of volatile, flavour-determining constituents (Ruan et al. 2012). Hence,an optimum ratio of polyphenols (catechins) to freeamino acid is therefore critical for the production ofquality green and oolong tea (Ruan et al. 1999; Ma etal. 2005).

Due to the acidic soil conditions preferred bytea, its commercial production typically occurs onhighly withered, Mg-deprived soils (Ruan et al.2012), and Jayaganesh and Venkatesan (2010) con-sidered Mg the most important nutrient next to Nand K in commercial black tea cultivation. Ac-cordingly, based on a perennial trial with blacktea Jayanganesh and Venkatesan (2010) reportedthat soil application of 200–300 kgMg ha−1 insulphate form to a standard NPK basal dressingincreased amino acid, polyphenol concentrationand catechin concentrations by about 10–20 % ingreen leaves and made black tea. Consequently,the so-called made-tea yield was increased byabout 4 % as well. This correlated with an in-crease in foliar Mg concentrations. However, asN was applied as urea and K as KCl some ofthe quality and quantity improvements assignedto Mg application may have been, at least partial-ly, caused by the provision of sulphate. In fact,Ruan et al. (1998) provided evidence by compar-ing application of Mg (and K) either as oxide (andchloride) or as sulphate to plants grown for greentea production (3-fold replicated experiments) thatsulphate—being a component of the cysteine andmethionine—plays an important role in the forma-tion of amino acids in leaves of tea plants. Inaddition the authors found that nitrate reductaseactivity was enhanced under K but even more sounder combined K and Mg supply, indicating thatboth nutrients play an important role in the assim-ilation of N and thus in the formation of N-containing metabolites e.g. amino acids. Nitrogenassimilation, taking place ultimately in the chlor-oplasts, is a highly energy consuming process(eight mol electrons mol−1 nitrate). In view ofthe fundamental functions of Mg in photosynthesis

the increased N assimilation and subsequent accu-mulation of amino acids after Mg application indi-cates that in Mg omission treatments tea plantssuffered from Mg deficiency. Indeed, the authorsstated that the soil used in the experiment showeda poor nutritional status (Ruan et al. 1998). Innutrient solution experiments carried out withgreen tea it was recently shown that sufficientMg supply increased biomass production and con-centrations of quality-forming amino acids in tea,e.g. theanine, in young roots and shoots (Ruan etal. 2012, four replications, 1-year experiment).Moreover, from pot and field experiments theauthors concluded that adequate Mg supply led toincreased concentrations of amino acids and sugarsin vascular saps as a result of increased demand ofyounger growing tissues for metabolites. It wasspeculated that this mobilization of carbon-containing compounds was related to season-specific increased demands of young, growing tis-sues. This clearly demonstrates the decisive role ofMg in the translocation of metabolites (Cakmak etal. 1994) and, therefore, for quality formation intea.

Whereas a positive impact of Mg on amino acidproduction appears to be proven, the impact on thesecondary metabolism responsible for the forma-tion of quality- and flavour-determining com-pounds appears more complex. In the study ofJayanganesh and Venkatesan (2010) the effect offoliar sprays containing 1 % or 2 %Mg (represent-ing 2 or 4 kgMgha−1) including micronutrients onblack tea quality was investigated. The treatmentfurther increased the leaf Mg concentrations be-yond the level observed for soil application (200–300 kgMgha−1) without increasing, but even de-creasing the quality-determining amino acid con-centrations in green leaves, which remained almostat the unfertilized control level. In contrast, theamino acids in made black tea were increased bythis foliar application. Moreover, in both greenleaves as well as in made black tea, the secondarymetabolites polyphenols (catechins) were increasedby foliar application. These quality improvementsby foliar Mg and micronutrient application in-creased the made tee yield even beyond the yieldof the soil application treatments. Thus, foliarspraying of tea plants with Mg-containing solu-tions can be an effective tool for improving black

Plant Soil

tea yield and quality. Unfortunately, the authorsfailed to provide information on the micronutrientcontent of the green leaves so that the tremendouseffect of foliar spraying in terms of quality andyield increase can, despite the observed increasingeffect on green leaf Mg concentrations, not besolely ascribed to Mg. Jayanganesh et al. (2011)reported that MgSO4 supply consistently increasedsecondary metabolites and therefore black teaquality. In contrast, Ruan et al. (1999) found (afterconducting a 2-year field trial) for different typesof tea (black tea, oolong tea, green tea) that,generally, Mg had a decreasing effect on the poly-phenol content, and only under severe Mg-deficiency polyphenols contents slightly increased.It would be interesting to investigate the effect ofMg on both primary and secondary metabolism.This may be underlined by some observationsmade in biotechnological applications, where addi-tional Mg supply fuelled the production of second-ary metabolites (see next sections).

Ornamentals

Anthocyanins are important constituents for ornamen-tals as they typically accumulate in vacuoles givingspecies- and variety-specific colouration to flowers(Markham et al. 2000). Shaked-Sachray et al. (2002)reported that high temperature-enhanced degradationof anthocyanins that give aster (hybrid of Aster eri-coides × Aster pilosus) flowers their characteristiccolour is reduced by Mg application. A direct effectis suggested as the anthocyanin concentration is close-ly correlated with the Mg application-enhanced Mgconcentration in the petals. However, apparently thiseffect was not due to an increasing impact onanthocyanin-synthesizing enzymes like phenylalanineammonia-lyase or chalcone isomerase. Future researchshould clarify the role of Mg in reducing anthocyanindegradation as this could have strong implications forthe production of ornamentals with improved andprolonged colouration.

Magnesium in biotechnology

Hairy root cultures are common in research and areused as model system for transgenic studies. Such amodel system, however, could also serve as produc-tion system for specific compounds. The advantage of

such a growth system is its genetic stability, its stableuniversal and specific metabolism and its unlimitedgrowth on media. Hank et al. (2003) report that in-creased MgSO4 application to genetically modifiedroot cultures of the herbaceous plant species belladon-na (Atropa belladonna L.), which is produced for theproduction of tropane alkaloids for medicinal applica-tions, positively affected growth and tropane alkaloidproduction. Szöke et al. (2004) found that MgSO4

supply to chamomile (Matricaria recutita L., formerlyChamomilla recutita L.) root culture (widely known inclassical and folk medicine for its contents in essentialoils) positively affected not only growth of the rootcultures (genetically modified hairy root and wild-typecultures) but also improved the quality and quantity ofessential oil production. Both studies intended toimprove methodologies in biotechnology. Future re-search should also focus on the underlying mecha-nisms as the outcomes could (1) give new insightsinto the role of Mg in plant physiology, e.g. the phys-iology of secondary plant metabolites (underlined bythe interesting results on the effect of Mg supply onthe phenol metabolism of tea) and (2) enhance theefficiency of biotechnological approaches developedfor the production of essential and health promotingmetabolites, without impairing rare herbaceous spe-cies in their natural environment.

Mg response of yield versus quality: is the qualityimproved beyond Mg supply adequate formaximum yield

In the previous sections emphasis was given to theimpact of Mg supply on the quality of agricultural andhorticultural produce. However, for practical consider-ations the crucial question is whether the Mg require-ment for maximum quality is already met by applicationlevels necessary for obtainingmaximal yield, or whetherthe quality response to Mg deserves particular attentionin its own right as the quality is still improved beyondthe yield maximum. In the following table those articlesreporting both the yield and the quality response to Mgsupply have been analysed in this regard, provided thateither a yield plateau was reached, or that yield was stillpositively responding to increasing Mg supply whilequality attributes levelled off already.

This meta-analysis clearly proves that beneficialeffects of Mg application on crop quality beyond

Plant Soil

those rates required for obtaining maximum yieldsare rarely observed (Table 3). With respect to prac-tical consequences it is concluded that growersshould ensure adequate Mg supply required formaximum—or optimal economic—yield, which atthe same time will ensure optimal crop quality invirtually all cases.

Conclusions

Despite the well-known fundamental roles of Mg inplant metabolism, the number of studies addressingthe significance of Mg for the quality of agriculturaland horticultural produce appears very limited as com-pared to other major nutrients. Most published studieseither do not address Mg responses specifically, and/orremain rather descriptive not illuminating the bio-chemical and physiological mechanisms underlyingthe response of the various quality parameters. Thesegeneralisations do not apply to potato and tea, wheredue to the economic importance of their complexquality indices a number of detailed studies have beencarried out. Among fruits and vegetables apples andtomatoes appear to be the most intensively studied

crops. However, decisive quality parameters importantfor horticultural crops like TSS and acidity are oftenmore closely correlated with cation ratios, e.g. Ca/Mgand K/Mg rather than with Mg concentrations alone.Despite the rare studies on horticultural crops thefollowing conclusions can be drawn: (1) The Mg/Caratio mainly determines the functional properties, andservice and stability aspects as components of the totalfood quality, such as product firmness, texture andstorability that are mainly determined by the role ofCa in stabilizing cell walls. Since Mg is capable ofreplacing Ca from binding sites, imbalanced Ca/Mgratios in the tissue often negatively affect productquality. (2) The Mg/K ratio on the other hand appearsto influence primarily organoleptic properties throughthe role of both mobile cations in the regulation of (i)cellular cation/anion balances and organic acid (TA)accumulation, and internal pH and (ii) source-sinkrelationships. Juice acidity and TA may to a certaindegree be influenced by the cation/anion ratio, where-as TSS or the content of valuable amino acids inharvested organs is a consequence of metabolite-translocating processes. In conclusion, Mg is to beconsidered the forgotten element. This does not onlyapply to the nutrient management in the field and the

Table 3 A meta-analysis of theMg requirement for maximumyield as compared to the re-sponse of quality

aonly a few quality parameterspositively affected beyond thedose required for maximumyield

Crop, mode of Mgapplication

Quality is improvedat Mg doses beyondthe supply requiredfor maximum yield?

Reference

No Partlya Yes

Winter wheat, soil X Chwil (2009)

Sugar beet, soil X Barlog et al. (2002), Villarias et al. (2000)

Sugar beet, foliar X Barlog and Grzebisz (2001), Gutmanski etal. (1998)

Sugar beet, foliar X Kristek et al. (2000)

Potato - late industrial, soil X Cepl (1994)

Potato, soil X Klein et al. (1981)

Spring rapeseed, soil X Bogdevich and Mishuk (2006)

Oil mustard, soil X Jahangir et al. (2005)

Oil palm, soil X Tang et al. (2001)

Oil palm, soil X Tayeb (2005)

Groundnut, soil X Rajan et al. (1984)

Sunflower, soil X Ahmedkhan et al. (1990)

Cabbage, nutrient solution X Hara and Sonoda (1981)

Citrus, sand culture X Moss and Higgins (1974)

Apple, not specified X Bramlage et al. (1980), Marcelle (1995)

Plant Soil

number of studies addressing Mg-related responses ofcrop performance, but also to the intensity at whichscientific studies address issues of crop quality asaffected by Mg supply. There is still need for a compre-hensive evaluation of the role of Mg in quality formation.

Acknowledgments In this review the authors refer to pub-lished studies on the significance of Mg supply on productquality in agricultural and horticultural crops. Even though wetried to cover the existing literature in this area, we apologizeshould we have overlooked relevant publications in this reviewand are grateful for bringing those to our attention.

Open Access This article is distributed under the terms of theCreative Commons Attribution License which permits any use,distribution, and reproduction in any medium, provided theoriginal author(s) and the source are credited.

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