J PlantPhysiol. Vol. 134. pp. 308-315 (1989)

Introduction

High Light Intensity Enhances Chlorosis and Necrosis in Leaves of Zinc, Potassium, and Magnesium Deficient Bean (Phaseolus vulgaris) Plants

HORST MARSCHNER and ISMAIL CAKMAK 1

Institut fur Pflanzenernahrung, Universitat Hohenheim, Postfach 700562, D-7000 Stuttgart 70, F.R.G.

1 Present address: Department of Soil Science and Plant Nutrition, University of Cukurova, Faculty of Agriculture, Adana, Turkey

Received October 9, 1988 . Accepted December 12, 1988

Summary

The effect of varied light intensities (80/LEm- 2 s- 1 to 600/LEm- 2 s- 1) on chlorosis and necrosis in

leaves of zinc (Zn), potassium (K) and magnesium (Mg) deficient bean (Phaseolus vulgaris L. cv. Prelude) plants was studied in water culture experiments with different Zn, K and Mg supplies.

With increasing light intensity plant dry weight increased in plants with sufficient nutrient supply, but not in Zn and K deficient plants. Chlorophyll concentrations declined with increasing light intensity, particularly in plants deficient in either Zn, K or Mg. At high light intensity, severe symptoms of chloro­sis and necrosis occurred in the deficient plants, although light intensity was without significant effect on the concentrations of these mineral elements in the leaves. In the deficient plants exposed to high light in­tensity, partial shading of the leaf blades either prevented or at least drastically delayed development of chlorosis and necrosis in the shaded areas.

In leaves of Zn deficient plants the concentrations of carbohydrates (reducing sugars, sucrose, starch) in­creased with increasing light intensity, particularly in primary leaves. In contrast, root concentrations of carbohydrates were much lower in Zn deficient plants exposed to high light intensity. Resupply of Zn to Zn deficient plants for up to 96 h remarkably decreased concentrations of sucrose in the older leaves.

The effects of increasing light intensity on the severity of chlorosis and necrosis are discussed in relation to photooxidation of thylakoid constituents by activated O2 species. Elevated levels of these toxic O 2

species are to be expected as a result of impairment in the photosynthetic carbon turnover and electron transfer in leaves of plants deficient in Zn, K or Mg.

Key words: Phaseolus vulgaris, carbohydrate accumulation, light intensity, magnesium deficiency, oxygen activation, photooxidation, potassium deficiency, sucrose transport, zinc deficiency.

In higher plants typical visual symptoms of Zn deficiency in leaves are inhibition of leaf expansion ( «little leaf»), inter­veinal chlorosis or chlorotic bands with reddish-brown patches (Rahimi and Bussler, 1978; Cakmak, 1988). Accord­ing to field observations (Thorne, 1957; Bergmann, 1983) de­velopment of these symptoms is affected by light intensity. Visual Zn deficiency symptoms in perennials are generally more severe during the summer month and, on a given tree,

on the branches exposed directly to sunlight. This light effect was discussed in relation to enhanced auxin oxidation under Zn deficiency by Skoog (1940).

Enhancement by high light intensities of chlorosis and ne­crosis in leaves are documented for a range of environmental stress factors, such as low temperature (Wise and Naylor, 1987 a). Light is also a major factor causally involved in for­est decline (Wild, 1987; 1988). These effects of light are con­sidered to be a consequence of photooxidation of thylakoid constituents by activated O 2 species, such as superoxide

© 1989 by Gustav Fischer Verlag, Stuttgan

radical (Oi -), hydroxyl radical (OR) and singlet oxygen (102) (Elstner, 1982; Powles, 1984; Osswald and Elstner, 1986; Wise and Naylor, 1987 b). The formation of activated O 2 species in chloroplasts predominantly occurs at the ex­pense of excessive levels of excitation energy and photore­ductants: These excesses are brought about by stress-depend­ent limitations on the photochemical turnover of absorbed light energy and/or decrease in the demand for ATP and NADPH2 (Asada et ai., 1977; Elstner and Osswald, 1984; Ba­renyi and Krause, 1985). In general, photo oxidation involves peroxidation by toxic O 2 species of membrane lipids to lipid radicals and corresponding co-oxidation (bleaching) of chlo­rophyll (Osswald and Elstner, 1986; Elstner, 1987).

Zinc, as a protective and stabilizing component of bio­membranes against activated O 2 species in both leaf (Cakmak and Marschner, 1987; Cakmak, 1988) and root (Cakmak and Marschner, 1988 a, b, c) cells, may playa cru­cial role in preventing photooxidation. Under Zn deficiency photosynthetic carbon metabolism is also impaired, and con­centrations of sugars, especially sucrose, are increased (Shrotri et ai., 1981; Sharma et ai., 1982). Accordingly, exces­sive levels of excitation energy and photo reduct ants may occur and create an additional factor towards enhanced photooxidation and development of visual Zn deficiency symptoms. In this paper, the interactions between the Zn nutritional status of bean plants, light intensity, carbohy­drate concentrations and visual deficiency symptoms are studied.

Moreover, deficiencies of potassium (K) (Haeder and Men­gel, 1972; Huber, 1984) and magnesium (Mg) (Terry and Ul­rich, 1974; Kirkby and Mengel, 1976; Kiippers et ai., 1985), showed decreased rates of photosynthetic CO2 assimilation or export of photosynthates from leaves, and chlorosis and necrosis are well-known visual deficiency symptoms (Berg­mann, 1983). Therefore, also the effect of light intensity on the development of visual symptoms in leaves was studied in K or Mg deficient bean plants.

Materials and Methods

French bean (Phaseolus vulgaris L., cv. Prelude) plants were grown under controlled climatic conditions (light/dark regimes of 16/Sh, temperature 24 °C/20°C, and relative humidity 70-75%) with varied quantum flux densities from SO /LE m - 2 S - 1 to 600/LE m- 2 s- l , provided by Osram HQIIT/2000/D bulbs.

Seeds were germinated in quartz sand moistened with nutrient solution without Zn supply. After 4 days the seedlings were trans­ferred to nutrient solutions either without ( - Zn) or with ( + Zn) Zn supply, and grown for 4 days at 490 JLE m - 2 S - I. Then, plants were shaded with white plastic screens so as to receive light intensities of SO, 230 or 490/LE m - 2 S - 1 at canopy height for 16 days. In an addi­tional experiment, plants were exposed to 600/L m - 2 S - 1 for 13 days. For the experiments with K deficiency the seedlings were supplied with 2 x 10 - 2 M K in quartz sand for S days (3 days in dark,S days at 230/LE m - 2S -I light intensity) and than transferred to low K nutri­ent solutions containing 5 x 10 - 5 M K as K2S04 and grown for 14 days at SO /LE m - 2 S - 1 or 490/LE m - 2 S - I. For the experiments with Mg deficiency the seedlings were supplied with nutrient solu­tion in quartz sand without Mg. After S days the seedlings were transferred to nutrient solutions containing either 2 x 10 - 5 M Mg (insufficient supply) or 0.65 x 10- 3 M Mg (sufficient supply,

Light-enhanced Zn, K, and Mg deficiency symptoms 309

control) as MgS04 and grown for 14 days at SO /LE m - 2 S - 1 or 490/LE m -2S-I.

The standard nutrient solutions had the following composition (M): O.SS x 1O-3K2S04; 2.0 x 10- 3 Ca(N03)2; 0.25 x 10- 3KH2P04; 0.6Sx10- 3 MgS04; 0.lx10- 3 KCI; 1x10- 5 H3B03; Sx10- 5

FeEDTA; 1x10- 6 MnS04; 1x10- 7 CuS04 and 1 x 10 - 8 (NH4hMo024. In the K deficiency (5 x 10 - 5 M K) experi­ments P and CI were supplied as NaH2P04 and NaC!, respectively. The Zn concentration in the nutrient solutions for the Zn sufficient plants (+Zn) was 1 x 1O- 6 M supplied as ZnS04. For Zn deficient plants ( - Zn) Zn was supplied at a concentration of 2 x 10 - 8 M for one week, the plants were then grown without Zn supply until harvest. In some treatments Zn was resupplied as ZnS04 to defi­cient plants at a concentration of 3 x 10- 6 M from 24h to 96h be­fore harvest.

After harvest, plant tissues were frozen in liquid nitrogen, freeze­dried and ground in the presence of dry-ice. Using SO or 100 mg dry ground plant material, sugars were extracted with hot 70 % (v/v) aqueous ethanol. After centrifugation the pellets were extracted twice with 3 ml portions of 70 % ethanol. The combined superna­tants were used for sugar analysis and the residue for starch analysis. Reducing sugars were measured using the p-hydroxybenzoic acid hydro azide reaction (Blakeney and Mutton, 19S0), and sucrose from the difference of reducing sugar concentrations before and after in­vertase treatment. Starch extraction from residue was performed ac­cording to Perez et al. (1971). Following extraction with dimethyl­sulfoxide and sodium acetate buffer (0.1 M, pH 4.S), starch in the extract was hydrolyzed by incubating 24 h at 37 °C with amyloglu­cosidase (Merck). The glucose liberated was determined by the glucose oxidase method (Blakeney and Matheson, 19S4).

Chlorophyll concentrations were measured spectrophotometri­cally after extraction of freeze-dried and ground leaf tissues with ac­etone (Arnon, 1949). For the Zn, K and Mg determination samples were ashed at 550°C. After the ash was dissolved in 3.3 % HN03, Zn and Mg were determined by atomic absorption spectrometry and K by flame photometry.

For the statistical treatments see legends of the tables and figures.

Results

In Zn sufficient plants ( + Zn) shoot length and dry weight of roots, and particularly of shoots, increased with light in­tensity (Table 1). In contrast, in Zn deficient plants (- Zn) high light intensities depressed shoot length and were with­out and with only small effects on shoot and root dry weight, respectively.

Concentrations of Zn in leaves and roots were much higher in Zn sufficient than in Zn deficient plants (Table 2). Increasing light intensities were without effect on the Zn concentrations in deficient plants, but significantly decreased

Table 1: Shoot length, shoot and root dry weight of 20-day-old bean plants grown at different light intensities with (+ Zn) or without (-Zn) Zn supply. All values followed by the same letter are not sig­nificantly different (Duncan's multiple-range test, P = 0.05).

Light intensity

490

Shoot length +Zn -Zn

(cm shoot -I) 19.7" 14.0b 20.7" 9.7c

22.7" 9.3c

Dry weight Shoot Roots

+Zn -Zn +Zn -Zn

(g dry wt plant -I) 1.24C 1.13C 0.27C 0.31C 2.3Sb 1.11 c 0.S9b 0.29c

3.80" 1.16c l.10b 0.41 be

310 HORST MARSCHNER and ISMAIL CAKMAK

Table 2: Concentrations of Zn in leaves and roots of 20·day·old bean plants affected by light intensity and Zn supply. Values for both lea· ves and roots followed by the same letter are not significantly differ· ent (Duncan's multiple.range test, P = 0.05).

Light intensity (~ m- 2 s- l)

80 230 490

80 230 490

SO 230 490

+Zn -Zn (JLg Zn g-I dry wt)

trifoliate leaves 50.7' 14.3c

47.2,b 14.2c 43.0b 13.0c

primary leaves 43.1' 13.9c

32.0b 10.9c

29.9b 9Ac

roots 176.1' 19.6c

53.6b 15.2c 67.0b 16.0c

Table 3: Chlorophyll concentrations in leaves of 20·day·old bean plants grown at different light intensities with ( + Zn) and without (-Zn) Zn supply. Values for trifoliate or primary leaves followed by the same letter are not significantly different. (Duncan's multi· ple·range test, P = 0.05).

Light intensity (JLE m- 2 s- l )

80 230 490

80 230 490

+Zn -Zn (mg g-I dry wt)

trifoliate leaves 14.0' l1.1b

10Ab 6.scd

9.5bc 6.1d

primary leaves 19.2' 17.3,b 16.6b 7.Sd 11.2c 4.5"

the concentrations in Zn sufficient plants. This latter effect is most likely the expression of a «dilution effect» due to the in· crease in dry matter production.

Chlorophyll concentrations in the leaves decreased with increasing light intensities in both Zn sufficient and Zn defi· cient plants (Table3). However, compared to Zn sufficient plants chlorophyll concentrations were much lower at the higher light intensities in the deficient plants, especially in the primary leaves. In Zn sufficient leaves no visual symptoms appeared at high light intensity (Fig. 1 A), where·

as in Zn deficient leaves severe symptoms of chlorosis and necrosis occurred (Fig. 1 B). Partial shading of the leaves merely prevented the enhancement of chlorosis and necrosis (Figs. 1 Band 1 C), although Zn concentrations were not sig. nificantly affected by shading; for example, Zn concentra· tions in the leaf dry weight were 8.7 ± 2.1 p,g and 9.3 ± 2.5 p,g/ g in the high light·exposed and shaded areas, respectively. This indicates that photooxidation of chloroplast pigments plays a major role in the interaction between Zn deficiency and high light intensity·induced chlorosis.

With increasing light intensity the concentrations of re· ducing sugars and sucrose increased in the leaves and roots of Zn sufficient plants; for starch this increase by light intensity was also evident in the leaves, but was less pronounced in the roots (Table 4). In Zn deficient plants the concentrations of sugars and starch in the trifoliate leaves were similar to those of Zn sufficient plants but less affected by light intensity (Table 4). On the other hand, in the primary leaves of the de· ficient plants, concentrations of starch and particularly sucrose increased steeply with light intensity, whereas in the roots the concentrations of sugars and starch remained at a low level. This distribution of carbohydrates between leaves and roots indicates impairment of sucrose export from the primary leaves in Zn deficient plants at high light intensities.

Resupply of Zn to deficient plants not only prevented or at least delayed development of chlorosis in the primary leaves, but also acted against excessive accumulation of sucrose (Table 5). Most probably, resupply of Zn led to par· tial restoration of phloem loading of sucrose in the formerly deficient leaves. This restoration depended on the duration of resupply and particularly on the severity of Zn deficiency at the onset of resupply.

Similar to Zn deficient plants, in K deficient plants severe visual deficiency symptoms (chlorosis and necrosis) in the leaves occurred at high (480 p,E), but not at low (80 p,E) light intensity (Fig. 2 A). In K sufficient plants such a light effect did not occur (results not shown). Shading part of the K defi· cient leaf blade with filter paper (Fig. 2 B) merely prevented the development of chlorosis and necrosis at high light in· tensity. The enhancement effect of high light intensities on chlorosis and necrosis in the K deficient plants was not caused by lower K concentrations in those leaves exposed to high light intensity (Table 6). The light intensity was with· out significant effect on shoot and root dry weight of the K deficient plants (Table 6).

Fig. 1: Primary leaves from Zn sufficient (1 A) and Zn deficient plants (1 Band 1 C) grown for 18 days at 480 JLE m -2 S-I (1 A and 1 B) or for 13 days at 600 JLE m - 2 S - 1 (1 C). Partial shading of primary leaves of Zn deficient plants was started before visual symptoms of chlorosis oc· curred and continued for 9 days (1 B) or 5 days (1 C).

Fig.2: Effect of different light intensities on chlorosis and necrosis in primary leaves of 14·day·old K deficient bean plants (K supply 5 x 10- 5 M). 2A: Plants grown at SO JLE m -2 S-1 (left) and 490 JLE m -2 S-1 (right). 2B: Partial shading of K deficient leaf blades for 5 days to 120~ m -2 S-1 in plants grown at 490 JLE m -2 S-I.

Fig. 3: Effect of different light intensities on chlorosis and necrosis of primary leaves from 14·day·old bean plants with sufficient (0.65 x 10- 3 M Mg) and deficient (2 x 10- 5 M Mg) Mg supply. 3 A: Leaves from Mg sufficient plants grown at 80 JLE m -2 S-I (left) and 480JLE m -2S -I (right). 3 B: Leaves from Mg deficient plants grown at 80!ili m -2S-1 (left) and 480JLE m -2 S-1 (right). 3 C: Primary leaves from a Mg deficient plant either completly shaded with filter paper for 7 days (120 ~ m - 2 S - I, left) or exposed to high light intensity (480 JLE m -2 s-1, right). 3 D: Partial shading for 7 days of a Mg deficient primary leaf (120 JLE m -2 S-I).

() c N

E ::J .-en en o +-'

~

E ::J en Q) c: tJ)

o ~

Light-enhanced Zn, K, and Mg deficiency symptoms 311

312 HORST MARSCHNER and ISMAIL CAKMAK

Table 4: Concentrations of carbohydrates in leaves and roots of Zn sufficient ( + Zn) and Zn deficient ( - Zn) 20-day-old bean plants af-fected by light intensity. Values represent the means of two inde-pendent replications.

Zn Light Reducing Sucrose Starch supply intensity sugars

(JIoE m- 2s- l) (mg Glucose equivalent g dry wt -I)

trifoliate leaves 80 37.6±2.2 14.8 ±2A 5.2± 1.0

+Zn 230 40.8± 1.4 19.6±0.2 12.6± 1.8 490 54.6± 1.4 26.2±2.6 19.8 ± 2.8

SO 26A±5.2 21.4±2.S 12.5± 3.0 -Zn 230 30.0±0.2 33.2±0.S 21.4± 0.8

490 27A±0.2 34.2±7.S 23.0± 0.5

primary leaves SO 27.2±0.2 10.2±2.S 2.9± O.S

+Zn 230 25.S±OA 10.6±2.2 4.6± 1.1 490 2S.2±OA lS.6±4.6 32.0± 16.3

SO 16.2±0.6 11.0±0.2 15.3 ± 0.0 -Zn 230 15.0±0.2 54A±0.2 55A± 9.1

490 16.6± 1.0 82.2±4A 3S.S± 10.5

roots SO 9.5±0.3 14.5± 1.6 1.0± 0.3

+Zn 230 37.1±0.0 33.6±0.0 1.7± 0.1 490 22A± 1.1 30A±3.0 2.6± 0.3

SO 10.9±0.2 14A± 1.1 1.4± 0.2 -Zn 230 11.1 ±0.6 14.6±2.5 1.2± 0.1

490 11.6±0.5 14.5± 1.5 lA± 0.1

Table 5: Concentrations of Zn, reducing sugars and sucrose in pri­mary leaves of 20-day-old bean plants grown at different Zn supply and 490 JIoE m -2 S-I light intensity.

Zn supply Zn Reducing sugars Sucrose (JIog g-I dry wt) (mg glucose equiv g-I dry wt)

+Zn (lx 10- 6 M) 22bc 27.9' 21.3d

-Zn 6c 21.9bc 73.4' -Zn + 24h Zn l 46b 24.6'b 75.2' -Zn + 4Sh Zn 84' 22.8bc 58.3b

-Zn + 72h Zn lOS' 19.7c 35.8c

- Zn + 96 h Zn 106' 22.2bc 38.4c

I Resupply of Zn to -Zn plants at a concentration of 3xl0- 6 M. All values within a row followed by the same letter are not signifi­cantly different as judged by Duncan's multiple-range test at 0.05 probability.

Also in Mg deficient but not in Mg sufficient plants, in­creasing light intensity led to chlorosis and necrosis with reddish coloration (Figs. 3 A and 3 B). Complete (Fig. 3 C) or partial (Fig. 3 D) shading of Mg deficient leaf blades pre­vented or markedly delayed occurrence of these symptoms. As shown in Table 7, this effect of high light intensity (490/LE m- 2s- l) on the visual Mg deficiency symptoms oc­curred at Mg concentrations in the leaf blades similar or only slightly lower than in the leaf blades exposed to lower light intensity (80 or 120/LE m -2 s-I).

Discussion

The enhancement effect of high light intensities on the de­velopment of visual Zn deficiency symptoms in leaves

Table 6: Concentrations of K in different organs, and shoot and root dry weight of 14-day-old bean plants grown at deficient K supply (5x 10- 5 M) at two light intensities. Each value of K concentration represents the mean of duplicated measurements in a sample from 6 plants. The values for dry weight were from two independent repli­cations with 3 plants each.

Plant organ

Shoot tips 2nd trifoliate leaf 1 st trifoliate leaf Primary leaves Stem Roots

Shoot Roots

Light intensity (JIoE m - 2 S - I) SO 490

K (% in dry wt) 0.53 0.78 0.39 0.39 0.27 O.lS 0.15 0.17 0.59 0.59 0.66 0045

Dry weight (g 3 plants-I) 2.2±0.1 2.3±0.3 1.0±0.2 1.1 ±0.1

Table 7: Concentrations of Mg in primary leaves of 14-day-old bean plants grown at different Mg supply (Mgl: 2x 10- 5 M, deficient; Mgz: 0.65x 10- 3 M, sufficient) and exposed to varied light intensity.

Experiment No.

II*

III*"

Light intensity (JIoE m- 2s- l)

SO 490

120 490

120 490

Mg concentration (JIog Mg g-I leaf dry wt)

Mgl Mgz

62S± 56 9574± 57 476± 76 9264±320

521± 64 447± 15

644±177 407± lS

* On the same plants one primary leaf was shaded for 7 days (120JloE m- 2s- l) and other exposed to high light intensity (490 JIoE); see Fig. 3C.

*" Partial shading of a primary leaf for 7 days (120 JIoE m - 2 S - I) as compared to exposure of the remaining leaf blade to high light intensity (490 JIoE m -2s-I); see Fig. 3D.

(Table 3; Figs. 1 Band 1 C) is in agreement with correspond­ing field observations in Zn deficient plants (Thorne, 1957). Slight decreases in chlorophyll concentrations at increasing light intensities are, however, also to be observed in Zn suffi­cient plants (Table3). Decrease in chlorophyll concentra­tions both per unit dry weight or surface area of leaves ex­posed to high light intensity is a well-known phenomena (Nii and Kuroiwa, 1988).

Besides the Zn, K, and Mg deficiencies reported in this paper, other stress conditions which lead to chlorosis and necrosis are also accentuated at high light intensity. Such stress conditions are air pollution, herbicide treatment, infec­tion by pathogens, chilling, heating or water deficiency (Elstner, 1982; Powles, 1984; Osswald and Elstner, 1986; Wise and Naylor, 1987 a). According to these reports the oc­currence of these foliar symptoms reflects the photooxida­tion of thylakoid constituents by activated O 2 species, such as O 2 - OR and 102. The generation of these O 2 species is often associated with disturbances in the photosynthetic car­bon turnover, such as restrictions in availability of CO2 or in

THYLAKOID STROMA

Fig. 4: Schematic representation of the activation of molecular oxy­gen in chloroplasts.

its fixation, or in the export of photosynthates (As ada et aI., 1977; Elstner and Osswald, 1984; Barenyi and Krause, 1985). As a consequence of these disturbances the demand for NADPH2 and ATP decreases, and thus the electron flow is intensified from photoreductants (e.g. ferredoxin) to molecu­lar O 2, forming Oi - and other toxic O 2 species (Elstner, 1987; Wise and Naylor, 1987b; Robinson, 1988). This reduc­tive O 2 activation is schematically shown in Fig. 4. Besides reductive activation, molecular O 2 can be also activated photodynamically within the thylakoid membranes to give rise to 102 by transfer of excitation energy to molecular O 2

(Fig. 4). Accumulation of carbohydrates in leaves, e.g. due to insuf­

ficient export of sucrose, should lead, especially at high light intensity, to rates of production of reducing equivalents which exceed their rates of utilization for carbon turnover and other processes (e.g. nitrite reduction). Most probably, such a situation exists in leaves of Zn deficient plants where photosynthetic electron transport and CO2 reduction are impaired (Ohki, 1976; Shrotri et aI., 1981) and carbohydrates simultaneously accumulate in source leaves, sucrose in par­ticular (Sharma et aI., 1982; Table 4). The latter effect is at least in part due to impaired sucrose export from these leaves (Tables 4 and 5).

The reasons for the decrease in sucrose export from Zn de­ficient leaves are not well understood. Similar decreases in export from Zn deficient leaves have been found for P and Fe (Marschner and Cakmak, 1986; Cakmak, 1988). Rapid in­crease in the export of sucrose (Table 5; Sharma et aI., 1982) and P after resupply of Zn to deficient plants support the idea that Zn affects phloem loading, probably via its role in the structural and functional integrity of biomembranes (Bettger and O'Dell, 1981; Cakmak and Marschner, 1988 a, b).

High light intensities also induce chlorosis and necrosis in K (Figs. 2 A, 2 B) and Mg (Figs. 3 B, 3 C and 3 D) deficient leaves. Potassium affects photosynthesis at various levels, such as photosynthetic electron transport (Pfliiger and Men­gel, 1972), CO2 reduction (Peoples and Koch, 1979) and phloem loading and export of sucrose (Mengel and Haeder, 1977; Huber, 1984). Magnesium deficiency also disturbs photosynthetic carbon metabolism, such as CO2 assimila­tion and export of photosynthates (Terry and Ulrich, 1974; Kirkby and Mengel, 1976). Thus, in K and Mg deficient leaves exposed to high light intensities, levels of absorbed light energy and production of reducing equivalents may ex­ceed their consumption for carbon assimilation in a manner similar to Zn deficiency. As a result, extensive photo­dynamic and reductive O 2 activation is to be expected in Zn, K, and Mg deficient leaves, leading to powerful oxidants

Light-enhanced Zn, K, and Mg deficiency symptoms 313

with corresponding peroxidation of membrane lipids and, thus, cooxidation of chlorophyll. This suggestion of photo­oxidative destruction of thylakoid membranes and of chloro­phyll in Zn, K, and Mg deficient leaves is supported by the results with different light intensities and partial shading of leaf blades (Figs. 1, 2 and 3).

Moreover, membrane stability is impaired in Zn deficient leaf cells and superoxide dismutase activity is depressed (Cak­mak and Marschner, 1987; Cakmak, 1988). It is therefore likely that under Zn deficiency, O 2 activation in chloroplasts and susceptibility of thylakoid membranes to photooxida­tion are additionally accentuated.

Thylakoid membranes may dissipate excess light energy, and thus minimize over-energization of the photosynthetic apparatus (Krause and Laasch, 1987; Krause et aI., 1988). This energy dissipation, which reflects pH-dependent quenching of chlorophyll fluorescence might be caused by structural changes in the membrane due to H + - Mg2+ ex­change at the internal thylakoid surface (Krause et aI., 1983; Krause and Bahrend, 1986). These studies suggest that Mg might also be involved in the dissipating-mechanism of ex­cess light energy in the chloroplasts and thus in the protec­tion of thylakoid constituents against photooxidative damage.

In conclusion, under conditions which impair photo­synthetic electron flow for CO2 reduction, or the export of photosynthates, photooxidative destruction of chloroplasts is to be expected, especially in source leaves at high light in­tensities. Zinc, K, and Mg deficiencies are examples for such conditions. Particularly Mg, but also Zn, deficiency is wide­spread in forest ecosystems (Mies and Zottl, 1985; Lange et aI., 1987; Rothe et aI., 1988; Fiedler, 1988) and photo­synthetic carbon metabolism is impaired under these condi­tions (Bosch et aI., 1983; Kiippers et aI., 1985). The inten­sified chlorosis of needles from light-exposed branches of forest trees (Bosch et aI., 1983; Zech and Popp, 1983; Wild, 1987) most probably also reflects a mineral nutrient defi­ciency induced, light-enhanced photooxidation, as described in this paper for bean leaves. In Norway spruce, for example, strong experimental evidence has been presented that in pre­vious years needles close relationships exist between bleach­ing and critical deficiency concentrations of Mg and perhaps also Zn (Lange et aI., 1987).

Acknowledgements

The authors thank Evelyn Herkner and Anke Hlilster for excel­lent assistance and Dr. Michael Treeby for correction of the English text. This study was supported by the Deutsche Forschungsgemein­schaft (DFG).

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