REGULAR ARTICLE Genotypic differences in nitrogen efficiency of white cabbage (Brassica oleracea L.) Gunda Schulte auf ’m Erley & Elsa Rakhmi Dewi & Olani Nikus & Walter J. Horst Received: 10 March 2009 / Accepted: 14 July 2009 / Published online: 8 August 2009 # Springer Science + Business Media B.V. 2009 Abstract In vegetable production, N balance sur- pluses are especially high which increases the risk of environmental pollution. The cultivation of N- efficient cultivars may contribute to alleviate the problem. A 2-year field experiment was conducted with eight white cabbage cultivars of three different maturity groups at two N fertilization levels. Geno- types differed both in N efficiency (head fresh weight at low N supply) and in yield at high N supply. These differences were not related to N uptake but to N utilization efficiency. At low N supply, harvest index was the main determining factor for genotypic yield differences. For earlier maturing cultivars a slower leaf emergence was responsible for the low harvest index. The response of the cultivars to low N supply was dependent on the weather conditions, particularly temperature, (highly significant year × cultivar × N supply interaction) at early growing stages. This suggests that breeding of cultivars with generally low-temperature tolerance could contribute to enhanc- ing N utilization. Especially at high N supply, a high N harvest index was important for yield formation due to its effect on head water accumulation. For late cultivars, a high N retranslocation from leaves to the heads was related to yield both at low and high N supply. The study suggests that breeding of N- efficient cultivars may reduce N release to the environment by reducing the necessary N input and reducing the N content remaining in the crop residues. Keywords N limitation . N uptake . N utilization efficiency . Head fresh weight . Head water accumulation . Brassica Introduction In vegetable production environmental pollution by nitrogen (N) losses into the atmosphere and hydro- sphere is especially high (Greenwood 1990). This is caused by the high N fertilization levels which are common in commercial production and often exceed official recommendations (Booij et al. 1996). Espe- cially for Brassica cabbage species, high fertilizer rates are recommended; for white cabbage in a range of 250 kg N ha -1 to 350 kg N ha -1 , depending on maturity group (Scharpf and Weier 1994). During the past years, efforts have been made to decrease fertilization levels without affecting yield (head fresh weight per unit surface area). Experiments with varying N fertilization techniques, e.g. by split application or band placement, revealed only a limited reduction potential of fertilizer N for different Plant Soil (2010) 328:313–325 DOI 10.1007/s11104-009-0111-1 Responsible Editor: Hans Lambers. G. Schulte auf ’m Erley (*) : E. R. Dewi : O. Nikus : W. J. Horst Institute for Plant Nutrition, Leibniz University of Hannover, Herrenhäuser Str. 2, 30419 Hannover, Germany e-mail: [email protected]
Transcript
REGULAR ARTICLE
Genotypic differences in nitrogen efficiency of white cabbage(Brassica oleracea L.)
Gunda Schulte auf’m Erley & Elsa Rakhmi Dewi &Olani Nikus & Walter J. Horst
Received: 10 March 2009 /Accepted: 14 July 2009 /Published online: 8 August 2009# Springer Science + Business Media B.V. 2009
Abstract In vegetable production, N balance sur-pluses are especially high which increases the risk ofenvironmental pollution. The cultivation of N-efficient cultivars may contribute to alleviate theproblem. A 2-year field experiment was conductedwith eight white cabbage cultivars of three differentmaturity groups at two N fertilization levels. Geno-types differed both in N efficiency (head fresh weightat low N supply) and in yield at high N supply. Thesedifferences were not related to N uptake but to Nutilization efficiency. At low N supply, harvest indexwas the main determining factor for genotypic yielddifferences. For earlier maturing cultivars a slowerleaf emergence was responsible for the low harvestindex. The response of the cultivars to low N supplywas dependent on the weather conditions, particularlytemperature, (highly significant year × cultivar × Nsupply interaction) at early growing stages. Thissuggests that breeding of cultivars with generallylow-temperature tolerance could contribute to enhanc-ing N utilization. Especially at high N supply, a highN harvest index was important for yield formation
due to its effect on head water accumulation. For latecultivars, a high N retranslocation from leaves to theheads was related to yield both at low and high Nsupply. The study suggests that breeding of N-efficient cultivars may reduce N release to theenvironment by reducing the necessary N input andreducing the N content remaining in the crop residues.
Keywords N limitation . N uptake .
N utilization efficiency . Head fresh weight .
Head water accumulation . Brassica
Introduction
In vegetable production environmental pollution bynitrogen (N) losses into the atmosphere and hydro-sphere is especially high (Greenwood 1990). This iscaused by the high N fertilization levels which arecommon in commercial production and often exceedofficial recommendations (Booij et al. 1996). Espe-cially for Brassica cabbage species, high fertilizerrates are recommended; for white cabbage in a rangeof 250 kg N ha−1 to 350 kg N ha−1, depending onmaturity group (Scharpf and Weier 1994). During thepast years, efforts have been made to decreasefertilization levels without affecting yield (head freshweight per unit surface area). Experiments withvarying N fertilization techniques, e.g. by splitapplication or band placement, revealed only a limitedreduction potential of fertilizer N for different
G. Schulte auf’m Erley (*) : E. R. Dewi :O. Nikus :W. J. HorstInstitute for Plant Nutrition,Leibniz University of Hannover,Herrenhäuser Str. 2,30419 Hannover, Germanye-mail: [email protected]
Brassica vegetables (Everaarts et al. 1996; Everaartsand De Moel 1998). Another possibility to save Nfertilizer is a more precise prediction of the Ndemand. For this purpose, models predicting the timecourse of cauliflower growth (Alt et al. 2000), soil Navailability for cauliflower N uptake (Kage et al.2003) or the time course of N uptake of whitecabbage and Brussel sprouts (Fink and Feller 1998,2001) have been developed. A further approachwhich was hardly investigated yet is the breeding ofN-efficient genotypes, which are characterized by alow susceptibility in yield to reduced N fertilizationlevels (Schenk 2006).
A high N efficiency can be achieved either by ahigh N uptake or by an efficient N utilization for yieldformation (Sattelmacher et al. 1994). Nitrogen recov-ery from the soil is generally high for Brassicavegetables (Everaarts 1993). At harvest, soil Nmin
contents below white cabbage are normally less than40 kg N ha−1 at 0 cm and 90 cm soil depth withalmost no N leaching during the vegetation period(Everaarts and Booij 2000 and references therein).The efficient N uptake of white cabbage is caused bythe large and evenly distributed root system, reachingdown to a soil depth of 2–2.5 m, and by a high abilityof the roots to absorb N in all soil layers (Kristensenand Thorup-Kristensen 2004).
Total dry matter production and head yields as wellas plant N contents are strongly influenced by thelevel of N supply (Peck 1981). Ontogenesis, incontrast, does not seem to be influenced by N rate(Peck 1981), which was also found for cauliflower(Everaarts and De Moel 1995). The number of leavesdeveloped until heading and the harvest index (Haraet al. 1982; von Brandis and Scharpf 1987) were notfound to be affected by the N rate. However, sucroseaccumulated in outer leaves under N-limiting con-ditions together with enhanced sucrose concentrationsin the heads (Hara 1989) indicating a limitation forthe utilization of C for growth.
Head quality is impaired under excess N supplybecause of burst heads, tipburn (Peck 1981), lowersucrose concentration in the heads and inferior taste(Nilsson 1988; Hara 1989). Although uniformity ofthe heads was slightly improved and head shape wasnot affected by increasing N fertilization, relative corelength was increased leading to a decline in headquality (Everaarts and De Moel 1998). Dry mattercontent of the heads usually declines with increasing N
fertilization (Peck 1981; Everaarts and De Moel 1998).However, the ratio of marketable heads and storagelosses are only slightly affected by N fertilization anddo not counterbalance the yield increase (von Brandisand Scharpf 1987; Freyman et al. 1991).
Summarizing, the potential for an improvement ofN efficiency by increasing N uptake seems to berather limited, but there might be potential to improveN utilization efficiency. Due to the small effects of Nrate on harvest index it is difficult to predict if ahigher total biomass production or an enhanced headgrowth might be more effective. Severe yield limi-tations by impaired head quality are not to beexpected. The objective of this study was, therefore,to explore genotypic differences in N efficiency forcabbage cultivars of differing maturity groups and toinvestigate the main factors contributing to yield atlimiting and optimum N supply. This is expected tohelp finding selection criteria for the breeding of N-efficient cultivars.
Materials and methods
Field experiments were conducted in 2004 and 2005on a loamy silt soil (FAO 2006) at the experimentalstation of the Faculty of Natural Sciences, LeibnizUniversity of Hannover, Germany in Ruthe located20 km south of Hannover. The experiments weredesigned as split plots with N fertilization rates asmain plots and cultivars as sub-plots with fourreplicates. N rates comprised no N fertilization andan N supply of 300 kg N ha−1 (fertilizer plus soilmineral N content at transplanting). Eight whitecabbage cultivars from different maturity groups wereused in the study: Parel (Bejo, Warmenhuizen, TheNetherlands) as an early cultivar, Toughma (RijkZwaan, Welver, Germany), Castello (Nickerson-Zwaan, Edemissen, Germany) and Perfecta (Bejo,Warmenhuizen, The Netherlands) as mid-early culti-vars and Lennox, Bartolo (Bejo, Warmenhuizen, TheNetherlands), Bloktor and Novator (Syngenta, Enkui-zen, The Netherlands) as late cultivars. Castello andBartolo are used as reference cultivars in varietytesting for the registration of new varieties by theBundessortenamt, Hannover, Germany, for mid-earlyand late white cabbage cultivars, respectively. The othercultivars were chosen because of recommendations ofplant breeders and were all considered as N-efficient.
Seeds were sown in peat cubes and raised in agreenhouse for the first 3 weeks. Thereafter, theseedlings were placed in an open greenhouse to adaptthe plants to the natural climatic conditions. After5 weeks, plants were transplanted into the field onMay 13 and May 19 in 2004 and 2005, respectively.Irrigation was supplied to ensure growth of the youngplants. Plant spacing was 0.60 m by 0.48 m in 2004and 0.55 m by 0.48 m in 2005 giving a plant densityof 3.3 and 3.8 plants m−2 in 2004 and 2005,respectively. Individual plots were 2 m wide and8 m long. Soil mineral N contents (0–0.9 m) attransplanting were 83 kg N ha−1 and 73 kg N ha−1 in2004 and 2005, respectively. The high-N plots werefertilized with 150 kg N ha−1 as calcium ammoniumnitrate prior to planting, and with 70 kg N ha−1 and80 kg N ha−1 on 10 June 2004 and 22 June 2005,respectively. Weeds were controlled by hand.Oxydemeton-methyl (Metasystox) and Bacillus thur-ingiensis (Dipal/Turex) were sprayed for pest controlin both years.
Weather conditions varied between the two grow-ing seasons studied (Fig. 1). In comparison to thelong-term average, 2004 was cool and wet betweenMay and July and warm and dry in September andOctober. 2005 was warmer than usual except inAugust, while precipitation was high in May andJuly, very low in June and close to normal fromAugust to October.
Harvests were performed at the beginning ofheading (from mid-June to end of July, according tomaturity group) and at maturity (from end of July to
mid-October). Head formation was defined to startwhen the newly developing leaves had a curved shapeso that they covered the shoot apex. The harvest atmaturity was performed when the heads reached theirfinal size and maximum density. Replicate plotswere harvested at the same day. Six plants per plotwere randomly chosen for each harvest. Plants wereseparated into outer leaves and head including thestem. In 2005 the stem and head were separatelyharvested at maturity. Leaves were counted for eachplant individually and then bulked for fresh weightdetermination. Sub-samples were taken for dry weightdetermination. The samples were dried at 70°C untilthey attained a constant weight. Nitrogen concentra-tions of the dried and ground plant fractions weredetermined using a CNS analyzer (Vario EL, Elemen-tar Analysensysteme, Hanau, Germany). At heading,stems and heads were pooled for six plants, weighed,dried and ground for N analysis. At maturity, headswere weighed individually for each plant. Three headsper plot were sub-divided for leaf counting, thenpooled together before sub-samples were taken for dryweight determination and N analysis.
Water-soluble carbohydrate concentrations of outerleaves and heads were analysed using dried plantmaterial. One hundred mg dry matter was extractedfor 1 h in 20 ml distilled water and analysed by theanthrone method following the description of Yemmand Willis (1954).
For nitrate analysis 100 mg dry matter was extractedfor 1 h in 20 ml distilled water and nitrate concentrationwas determined according to Cataldo et al. (1975).
May June July Aug Sept Oct
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Fig. 1 Total monthly pre-cipitation (bars) andmonthly temperature(symbols) at the experimen-tal station Ruthe during the2004 and 2005 growingseasons and as long-termaverage
In addition to the leaf counting during harvests,leaf numbers were determined non-destructively in2005. Six plants per plot were chosen at heading andall outer leaves were numbered using a water proofedmarker. In weekly intervals from heading to maturity,the number of the oldest leaf still attached to the plantand of the youngest unfolded leaf was recorded. Fromthese values the ratio of lost leaves was calculated bydividing the number of leaves shed from the plant bythe total number of outer leaves formed until maturity.
Nitrogen efficiency was defined in this study asyield (head fresh weight) at limiting N supply(Craswell and Godwin 1984). Factors contributing toN efficiency are N uptake efficiency (shoot-N uptakeat limiting N supply) and N utilization efficiency(yield per unit shoot-N taken up). N utilizationefficiency was subdivided into biomass productionefficiency (Ortiz-Monasterio et al. 1997), harvestindex and head fresh weight/dry weight ratio. Bio-mass production efficiency was defined as the ratio ofshoot dry weight and shoot-N uptake at maturity.Harvest index was calculated as head dry weightdivided by total shoot dry weight at maturity. Sinceyield of white cabbage is given by head fresh weight,the ratio between head fresh and dry weight wasintroduced as a third factor of N utilization efficiency.Nitrogen harvest index was calculated as headnitrogen at maturity divided by total shoot-N uptake.
Data were analysed using SAS version 9. Analysisof variance was performed by PROC MIXED. Years,N rate and cultivars were considered as fixed factorswith years as main-plot factor, N rates as sub-plot
factor and cultivars as sub-sub-plot factor. Multiplecomparisons of means were made by the LSMEANS/PDIFF statement for N rate by cultivar interactions.Regression lines were calculated using PROC REG inSAS.
Results
Maturity times differed between cultivars and N rates(Table 1). Cultivar differences were according to theirmaturity group. Within mid-early cultivars, cvToughma matured earlier than cv Castello, while cvPerfecta matured later. Low N supply delayedmaturity, especially for the early cultivars. This effectwas stronger in 2004 than in 2005. Heading dates didnot differ between N rates.
Cultivars differed in yield (head fresh weight) atboth N rates (Table 2). At low N supply, cv Perfectahad the highest yield of all cultivars. Within the latematuring cultivars, cv Lennox and cv Novator had asignificantly higher yield than cv Bartolo. The earlycultivar Parel could not be compared to anothercultivar of the same maturity group. However, yieldwas not decreased by low N supply. The cultivarsParel, Perfecta, Lennox and Novator were classifiedas N-efficient, while cvs Toughma, Castello andBartolo as N-inefficient. Cultivar Bloktor had anintermediate N efficiency. At high N supply, yielddifferences were more pronounced between cultivarswith different maturity times. Again, cv Perfecta hadthe highest yield of all cultivars. Mid-early cultivar
Table 1 Heading and maturity dates (days after planting) of eight cabbage cultivars grown in Ruthe at two N supplies (N1: nofertilization and N2: 300 kg N ha−1) in 2004 + 2005
Castello had a higher yield than cv Toughma. Withinthe late cultivars, cv Lennox followed by cv Bloktorhad significantly higher yields than cv Bartolo.Cultivar Novator had a medium yield, mainly due toits poor performance in 2005. There was no signifi-cant year effect on yield; however, mid-early cultivarshad lower yields only at low N supply in 2004compared to 2005 which was mainly responsible forthe significant Y × Cv and Y × N × Cv interactions.
Among the factors contributing to N efficiency,total shoot-N uptake differed between cultivarsaccording to their maturity times. Within the maturitygroups N uptake did not differ much. Nitrogen uptakewas significantly greater and varied more betweencultivars under high N compared to low N supply.Late cultivars accumulated more N than earliermaturing cultivars explaining most of the highlysignificant effect of the cultivar and the N × Cvinteraction. Total shoot-N uptake did not generallydiffer between years; however, similar to what wasobserved for yield, the early and mid-early cultivarshad a lower N uptake under low N supply in 2004than in 2005.
Biomass production efficiency was higher for latethan for early maturing cultivars. Low N supply clearlyincreased biomass production efficiency of all culti-vars. Within mid-early and late maturing cultivars,respectively, cvs Perfecta and Bartolo had highbiomass production efficiency both at low and at highN supply. There was no N × Cv interaction for this trait.
Mean harvest index ranging from 0.39–0.65clearly differed between cultivars, also within matu-rity groups. At low N supply, cv Perfecta had asignificantly higher harvest index than the other mid-early cultivars. Within late maturing cultivars, cvBartolo had a significantly lower harvest index thanthe other cultivars. Harvest index was not influencedby N supply; however, there was a highly significantN × Cv interaction. In 2004, the harvest index waslower than in 2005, especially for the earlier maturingcultivars at low N supply. The late cultivars had ahigher harvest index in 2004 than in 2005 (highlysignificant Y × Cv interaction).
The ratio between head fresh and dry weightreflects the water content of the heads and rangedfrom 46.1 to 9.7. Cultivars differed significantly inhead fresh weight/dry weight ratio with the earlycultivars showing higher values than the late maturingones. There was not much variation within maturity
groups. The N rate did not generally affect the headfresh weight/dry weight ratio. However, cultivarranking within the earlier maturing cultivars wasaffected by N rate, at least in 2004 (highly significantY × N × Cv interaction). The early cultivars Parel andToughma had a significantly higher head freshweight/dry weight ratio under low N than under highN supply, while cv Perfecta had a higher head wateraccumulation under high N.
Cultivar differences in N harvest index weregenerally related to those in harvest index but withexceptions. At low N supply, cv Perfecta had thehighest N harvest index and cv Bartolo had the lowestN harvest index. At high N supply, cv Perfecta stillhad a high N harvest index despite a comparativelylow harvest index, while cv Parel had a low N harvestindex despite a high harvest index.
Total shoot dry weights at heading and maturitydiffered between cultivars according to their growingtimes (Table 3). Within the late maturing cultivars cvsLennox and Bartolo had higher shoot dry weights atmaturity than cvs Bloktor and Novator. At low Nsupply shoot dry weight was significantly lower thanat high N supply, except for the early cultivars Pareland Toughma. Shoot dry weight was lower in 2004than 2005. This effect was greater for the early andmid-early than for the late cultivars, particularly underlow N supply (significant Y × N × Cv interaction).
The outer leaves are important for the C assimila-tion and thus growth of the plants, and leaf numbersof outer leaves may also be decisive for the biomassdistribution between leaves and head and thus harvestindex. Late maturing cultivars developed more leavesuntil heading than early maturing cultivars. CultivarBartolo had the highest leaf number at heading underboth N rates. N rate had no major effect on leafnumber. A significantly decreased leaf number underlow N supply was found for cvs Parel, Toughma andCastello, but not for the other cultivars. Leaf numberbetween heading and maturity partly increased for theearlier maturing cultivars and it decreased for the latecultivars. Overall, leaf number at maturity was higherat low N that at high N supply, suggesting higher leaflosses at high N supply. Leaf number at maturitydiffered between but also within different maturitygroups. Within maturity groups at low N supply, itwas highest for those cultivars with a low harvestindex. The cultivars Toughma, Castello and Bartolohad the highest leaf numbers at maturity. At high N
318 Plant Soil (2010) 328:313–325
supply, the mid-early maturing cultivars and cvsBartolo and Bloktor had high leaf numbers. Thus,the relationship to harvest index was less clear at highN supply.
The ratio between head fresh and dry weight washighly significantly related to the N concentration inthe heads (Fig. 2a, c). Cultivars with a high head freshweight/dry weight ratio also had a high head N
concentration. This relationship was true for both Nrates, however, under high N supply all cultivarsshowed higher head N concentrations without con-comitant changes in the head fresh weight/dry weightratio. In addition, cultivar differences in head freshweight/dry weight ratio under high N supply wereaccompanied by greater differences in head Nconcentration than this was the case under low N
Table 3 Shoot dry weight and leaf number of the outer leaves at heading and maturity of eight cabbage cultivars grown in Ruthe attwo N supplies (N1: no fertilization and N2: 300 kg N ha−1) in 2004 + 2005
N supply Cultivar Shoot dry weightat heading(g m−2)
Shoot dry weightat maturity(g m−2)
Leaf numberat heading
Leaf numberat maturity
2004 2005 2004 2005 2004 2005 2004 2005
N1 Parel 18 71 d 310 460 e 12.0 13.5 e 12.7 14.2 e
Toughma 14 124 d 516 925 d 10.0 13.3 f 17.3 17.5 b
Castello 30 202 c 602 933 d 14.3 18.8 d 16.1 18.0 b
Perfecta 161 330 b 896 1025 c 20.4 21.3 c 14.7 15.3 d
Lennox 521 524 a 1366 1425 a 21.7 21.9 ab 15.0 14.9 d
Bartolo 501 513 a 1303 1442 a 22.0 22.9 a 19.8 20.0 a
Bloktor 445 519 a 1345 1156 b 20.4 21.5 bc 16.4 15.8 c
Novator 484 533 a 1298 1204 b 20.9 20.9 c 14.8 15.7 d
N2 Parel 42 92 D 349 509 G 15.3 13.7 E 15.3 11.6 D
Toughma 30 171 D 675 894 F 13.0 13.9 F 14.5 17.0 B
Castello 59 263 C 633 1131 E 16.7 18.9 D 12.8 17.8 BC
Perfecta 281 394 B 885 1432 D 18.5 21.5 C 15.1 15.9 BC
Lennox 751 661 A 1764 1660 A 21.4 21.5 B 14.3 15.2 C
Bartolo 626 711 A 1774 1721 A 22.4 23.5 A 16.0 19.3 A
Bloktor 666 686 A 1690 1474 B 21.5 23.0 AB 15.3 15.7 BC
Novator 728 623 A 1624 1323 C 19.3 21.0 C 12.8 14.6 D
Year (Y) ** ** ** **
N rate (N) *** *** ns *
Cult. (Cv) *** *** *** ***
N × Cv *** *** *** ***
Y × N ns ns ns ns
Y × Cv *** *** *** ***
Y × N × Cv ** *** *** ***
Different letters (lower and upper case letters for low and high N supply, respectively) denote significant differences (means over theyears) between cultivars within columns at P<0.05
ns non significant
*, ** and *** = significant at P<0.05, 0.01 and 0.001, respectively
Plant Soil (2010) 328:313–325 319
supply, resulting in a steeper regression line for highN supply.
The head nitrate concentrations did not differbetween the N rates (Fig. 2b, d). The relationshipbetween head nitrate concentration and head freshweight/dry weight ratio was significant in 2004 butless close than between head fresh weight/dry weightratio and head N concentration. In 2005 no significantrelationship was observed.
The comparison of head-N accumulation and totalshoot-N uptake between heading and maturity (Fig. 3)allows an estimation of the N retranslocation from theouter leaves to the heads during that period. Net Nretranslocation is thus given by head-N accumulationbetween heading and maturity minus total shoot-Nuptake between heading and maturity. Early and mid-early cultivars took up more N between heading andmaturity than they accumulated in the heads withinthe same period. For the late cultivars net N
retranslocation from the outer leaves occurred, espe-cially under high N and in 2004. At low N supply,across all cultivars more N was taken up than wasallocated to the heads, while at high N supply, therewas a net N retranslocation from the outer leaves tothe heads (P<0.01 for N rate comparison). Within themid-early cultivars, Toughma and Castello allocatedsignificantly lower N amounts from shoot-N uptake tothe heads than Perfecta at both N rates. For Perfecta,shoot-N uptake and head-N accumulation were equal.The late cultivars Lennox and Novator retranslocatedmore N from the outer leaves than cv Bartolo at lowN supply (P<0.01 in both cases). At high N supply cvNovator had a higher N retranslocation than cvBartolo (P<0.05).
Leaf losses demonstrate a high N retranslocationfrom the outer leaves since N is withdrawn from theleaves before they are shed. Averaged over cultivars,the ratio between lost leaves and total number of outer
c
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r2 = 0.91***
N1: y = -38.4 + 2.77x
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r2 = 0.89***
N1: y = -5.4 + 0.98xr2 = 0.84**
y = 14.0 + 11.7x
r2 = 0.45**
Fig. 2 Relationship be-tween head N concentrationand the ratio of head freshweight to dry weight in2004 (a) and 2005 (c) andbetween head nitrate-concentration and the ratioof head fresh weight to dryweight in 2004 (b) and 2005(d) for eight cabbage culti-vars grown in Ruthe at twoN supplies (N1: no fertil-ization, open symbols andN2: 300 kg N ha-1, filledsymbols). ** and *** =significant at P<0.01 and0.001, respectively
320 Plant Soil (2010) 328:313–325
leaves was higher under high N than low N supply(Fig. 4), but there was a highly significant N × Cvinteraction. Generally, earlier maturing cultivars hadlower ratios of lost leaves than late maturing cultivars.However, cv Parel lost comparatively many leaves athigh N supply. Among the mid-early cultivars cvPerfecta was close to the late maturing cultivars.Among the late maturing cultivars, cv Bartolo had asignificantly lower ratio of lost leaves than all othercultivars at low N supply and than cvs Bloktor andNovator at high N supply.
Soluble carbohydrate concentrations in outer leavesand head leaves may give indications about possible Climitations for head growth. Late maturing cultivars hadhigher leaf carbohydrate-concentrations than the earlyand mid-early cultivars (Table 4). Within maturitygroups, cv Bartolo had higher leaf carbohydrate-concentrations than the other cultivars, but no signif-icant cultivar difference was observed within mid-earlycultivars. Low N supply increased carbohydrate con-centrations of the outer leaves. Averaged over culti-
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Fig. 4 Ratio between lost leaves and total number of outerleaves (means with standard deviation) of eight cabbagecultivars grown in Ruthe without N fertilization (N1) and at300 kg ha−1 N supply (N2) in 2005. Values denoted by differentletters (lower and upper case letters for low and high N supply,respectively) are significantly different between cultivars at P<0.05. ** and *** = P<0.01 and 0.001, respectively
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Fig. 3 Shoot (head + outer leaves) N uptake and head-Nuptake between heading and maturity (means with standarddeviation) of eight cabbage cultivars grown in Ruthe without N
fertilization in 2004 (a) and 2005 (b) and at 300 kg ha−1 Nsupply in 2004 (c) and 2005 (d)
Plant Soil (2010) 328:313–325 321
vars, carbohydrate concentrations in outer leaves werehigher in 2005 than in 2004. However, this wasrestricted mainly to the mid-early cultivars.
Carbohydrate concentrations in the heads showed ahighly significant Y × N × Cv interaction. They were
generally higher for late maturing cultivars than for theearly and mid-early cultivars. Cultivar Toughma had asignificantly higher head carbohydrate-concentrationthan the other mid-early cultivars under high N supply.N rate and year had no significant effect on headcarbohydrate-concentrations.
Discussion
The cultivars investigated differed in N efficiency, i.e.in yield at low N supply. Based on the results of thisstudy the cultivars Parel, Perfecta, Lennox andNovator can be classified as N-efficient, while cvsToughma, Castello and Bartolo were N-inefficient(Table 2). Cultivar Bloktor had an intermediate Nefficiency. The early maturing cv Parel cannot becompared to another cultivar of the same maturitygroup. However, a rough estimation of N efficiencywas performed by comparing yield at low N supplywith high N supply. Yield of this cultivar was notdecreased at low N supply, but tended to be evenhigher than at high N supply. This was not due to thefact that N supply was not limiting for this cultivar atthe lower N rate. Shoot growth (Table 3) and Nuptake (Table 2) were decreased to a similar extent asfor the other cultivars under low N compared to highN supply.
The cultivar differences in N efficiency were notdue to differences in N uptake at low N supply, sincethis trait hardly differed between cultivars of the samematurity group (Table 2). This was probably due tothe generally very efficient N uptake of white cabbage(Kristensen and Thorup-Kristensen 2004) with lowsoil N losses during the growing period (Everaartsand Booij 2000). Since N uptake efficiency did notdiffer between N-efficient and -inefficient cultivars,the cultivar differences in N efficiency can beattributed to differences in N utilization efficiency.The separate listing of the three factors determining Nutilization efficiency revealed that N-efficient and-inefficient cultivars differed in harvest index, but notin biomass production efficiency and head freshweight/dry weight ratio (Table 2).
Biomass production efficiency reflects the efficien-cy of N use in plant carbon (C) assimilation.Genotypic differences in this trait were found withinlate maturing cultivars (Table 2). However, this wasof low importance for N efficiency. Obviously, total
Table 4 Water-soluble carbohydrate concentrations at maturityin outer leaves and heads of eight cabbage cultivars grown inRuthe at two N rates (N1: no fertilization and N2: 300 kg N ha−1)in 2004 + 2005
Cultivar Leaves(mg g−1)
Heads(mg g−1)
2004 2005 2004 2005
N1 Parel 27 36 f 149 234 d
Toughma 36 88 e 169 252 cd
Castello 66 106 cd 183 303 bcd
Perfecta 62 80 de 235 157 d
Lennox 121 105 b 318 224 abc
Bartolo 144 131 a 286 289 ab
Bloktor 109 76 c 363 286 a
Novator 115 76 c 238 233 bcd
N2 Parel 22 27 D 129 113 C
Toughma 32 87 C 297 238 A
Castello 33 100 C 95 208 C
Perfecta 31 69 C 80 261 BC
Lennox 92 87 B 244 330 A
Bartolo 103 124 A 190 267 AB
Bloktor 111 72 B 250 231 A
Novator 103 79 B 199 275 A
Year (Y) * ns
N rate (N) ** ns
Cult. (Cv) *** ***
N × Cv ns **
Y × N m ns
Y × Cv *** *
Y × N × Cv ns ***
Different letters (lower and upper case letters for low and highN supply, respectively) denote significant differences (meansover the years) between cultivars within columns at P<0.05
ns non significant
*, ** and *** = significant at P<0.05, 0.01 and 0.001,respectively
shoot dry matter production, although greatly de-creased at low N, is not the limiting factor for headyield in cabbage under N limitation. This conclusionis corroborated by the fact that carbohydrates accu-mulated in the outer leaves at low N supply (Table 4),suggesting a limitation for the utilization of C forgrowth.
The partitioning of dry matter to the heads, i.e. theharvest index, was the main determining factor for Nefficiency. The high harvest index of the N-efficientcultivars was accompanied by a low number of outerleaves at maturity (Table 3). Two different mecha-nisms seem to be responsible for high harvest indicesof early and late maturing cultivars. The N-inefficientmid-early cultivars Toughma and Castello kept onforming outer leaves after heading, especially at lowN and in 2004 (Table 3). This decreased theproportion of shoot dry matter that was allocated tothe head and thus head dry weight. In addition, theproportion of shoot-N uptake after heading that wasallocated to the heads was low for cvs Toughma andCastello (Fig. 3), which might also have decreasedhead growth. The ongoing leaf formation of these twocultivars was partly due to their low leaf numbers atheading (Table 3). This was not a pure genotypeeffect, but appeared only during the relatively cooltemperature conditions in 2004 (Fig. 1) and thenespecially at low N supply. While reduced leaf growthis a common response of crops to N limitation, slowerleaf emergence and increased duration of leaf expan-sion appear to be crop-specific responses (Gastal andLemaire 2002). Strong interactions between tempera-ture and N rate have also been found for leaf growthof wheat (Lawlor et al. 1988). However, not much isknown about the underlying mechanisms. This studyshows that there are also genotypic effects addition-ally modifying the temperature and N-rate effects andthat these interactions influence head yield of cabbageunder low N supply. Beside the lower rate of leafformation, also reduced rates of leaf-area expansionmight contribute to a continuous leaf unfolding. Alow leaf area of the surrounding leaves may preventshading of the shoot apex which seems to beimportant for head formation (Hara et al. 1982).
Apart from leaf emergence between heading andmaturity also the number of leaves shed from theplant until maturity determined total leaf number atmaturity. This played a role especially for latematuring cultivars (Table 3, Fig. 4). High leaf losses
can increase harvest index by an improved retrans-location of dry matter and nitrogen of the senescingleaves to the heads. Especially an improved nitrogensupply to the heads will promote sink strength for Cassimilates and thus growth. Major cause for leaflosses was most probably the shading of lower leaves,because leaf losses were mostly higher under highthan under low N supply. Cultivar differences in leaforientation could be observed in the field, whichprobably caused the genotypic differences in leaflosses. Cultivars Bloktor, Novator and also cv Parelhad more horizontally orientated leaves than the othercultivars. The significantly higher leaf losses of cvsCastello and Lennox under low compared to high Nsupply (Fig. 4) might reflect a higher susceptibility ofleaf senescence to N limitation in these cultivars.
Head fresh weight/dry weight ratio was higher forearlier maturing cultivars, but hardly varied withinmaturity groups (Table 2). Therefore, this parameterhad no major influence on genotypic differences in Nefficiency. However, the comparatively high yield ofcv Parel under low compared to high N supply couldbe attributed to an increased head fresh weight/dryweight ratio. A high head water accumulation isusually found under high N supply (Peck 1981;Everaarts and Booij 2000). However, in the presentstudy the N-rate effect clearly depended upon cultivar(Table 2). Generally, cultivar differences in head freshweight/dry weight ratio were related to high head Nconcentrations (Fig. 2a, c). A high N availability canlead to a high water accumulation in a tissue becauseit increases the size of individual cells (McDonald1989; Kano et al. 2007). The N effect on cell sizeseems to be mediated by an improved cell wallextensibility and not by an increased turgor pressuredue to high amounts of osmotica like nitrate (Palmeret al. 1996). Therefore, it is not surprising that nitrate-N concentrations were hardly related to head freshweight/dry weight ratio (Fig. 2b, d). However, head Nconcentration alone does not explain the differencesin head water accumulation between N rates, sincehead fresh weight/dry weight ratios were lower athigh N supply than could be expected from their headN concentrations (Fig. 2a, c). This may be due tohigher protein storage in head-leaf cells at high Nsupply. High amounts of stored proteins will increasethe N concentration of the heads, but it will not affectthe water accumulation of cells that are already fullyexpanded. Relatively high protein amounts may be
stored instead of being used for current growth whenN import into the head is generally high and at latehead growth, when growth rates are already decreas-ing. During this developmental stage soil-N availabil-ity will be still comparatively high at high N supplyand additionally N will be retranslocated from outersenescing leaves (Fig. 3). Therefore, a high N supplyto the heads during late head growth might becomparatively less effective in enhancing yield.
Apart from the parameters determining head yieldat maturity, it also has to be considered that maturitytime was delayed for the earlier maturing cultivars atlow N supply especially in 2004 (Table 1). Maturitydelay under N limitation was also reported in aCanadian study using a mid-early cabbage cultivar(Westerveld et al. 2003). In that study a strong yeareffect and year by N rate interaction were observed.The delay in maturity might have contributed to therelatively high shoot dry weight at maturity of theearly cultivars (Table 3). Maturity delay was quitesimilar among the earlier maturing cultivars andseemed to be governed by the decreased growth ratesunder low N and at the lower temperatures in 2004and not by a delay in ontogeny. Therefore, althoughmaturity delay might have had an impact on yield atlimiting N supply, it may not play a role for genotypicdifferences in N efficiency.
Selection of N-efficient cultivars under limiting Nsupply is necessary only if the decisive traits for yielddiffer from those determining yield under optimum Nsupply. The most important factor for genotypicdifferences in yield under low N supply was harvestindex. This trait also played a role for yield underhigh N supply, however, the relationship was lessclear (Table 2). For the mid-early cultivars head freshweight/dry weight ratio was more important for yielddifferences than harvest index. This was related tocultivar differences in N harvest index (Table 2).Similarly to the conditions under low N supply, a highN harvest index was related to a high proportion ofshoot-N uptake after heading that was allocated to thehead (Fig. 3). This was also related to the formationof outer leaves between heading andmaturity (Table 3).However, cultivar differences in leaf formation weregreater under low than under high N supply, indicat-ing differential susceptibility in this trait to Nlimitation. Therefore, selecting for cultivars withoutdelayed head formation will be more beneficial for Nefficiency than for high yield under optimum N
supply. For late maturing cultivars, differences inharvest index were more similar between low andhigh N supply, indicating a similar mechanismresponsible for this trait under both N rates.
In conclusion, it appears that a major effect of theN supply response lies in an alleviation of a decline inoverall growth due to low temperatures, especially forearly and mid-early cultivars. Therefore, breeding ofcultivars with a low susceptibility in leaf appearanceto low temperature might be effective in enhancing Nutilization. An additional trait for adaptation to low Nconditions is an increase of direct N allocation to thehead leading to increased head-water accumulation.Since a high head water content might reduce thestorage capability of the heads, this is a usefulapproach only for cultivars that are produced for thefresh market. Although no specific adaptations of latematuring cultivars to low N conditions were found,the genotypic variation in N harvest index shows thatthere is potential to decrease N amounts in cropresidues by cultivar selection. High leaf losses due toshading are of special importance here. High Nretranslocation to the head might also improve headgrowth and thus N utilization efficiency. A selectionfor cultivars with high harvest indices and N harvestindices will be advantageous without compromisingstorage ability of the heads by an increased watercontent. Testing the cultivars under stronger N-limiting conditions than in these experiments mightreveal further genotypic strategies of N efficiency.
Acknowledgements Cultivar recommendations and supply ofseeds by Sjaak van der Ploeg (Syngenta Seeds) are gratefullyacknowledged.
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