Pr Vanhasselt

INHIBITION OF CHILLING-INDUCED PHOTOOXIDATIVE DAMAGE TO LEAVES OF CUCUMIS-SATIVUS L BY TREATMENT WITH AMINO-ALCOHOLS

The effect of pretreatment of cucumber (Cucumis sativus L.) roots with choline chloride or ethanolamine on leaf phospholipid composition and light-induced leaf damage during chilling was studied. Photooxidative chlorophyll degradation was similarly inhibited by both amino alcohols. The decrease of the chlorophyll a/chlorophyll b ratio and the increase of polyunsaturated-fatty-acid degradation during chilling in the light were equally inhibited by pretreatment with choline chloride or ethanolamine. Treatment with choline chloride and ethanolamine caused, respectively, 43% and 26% increases in the total phospholipid contents of the leaves. After treatment with choline chloride, the phosphatidylcholine content was higher than the content of phosphatidylethanolamine; the reverse was true after treatment with ethanolamine. The chlorophyll concentration increased less than the phospholipid concentration, resulting in a decreased chlorophyll/phospholipid ratio of treated leaves. During chilling in the light, degradation of phosphatidylcholine, ethanolamine and phosphatidyl glycerol occurred. Phosphatidyl glycerol was less sensitive than phosphatidylcholine and ethanolamine. The degradation was equally inhibited by pretreatment with either amino alcohol. Possible connections between the phospholipid content of leaf membranes and the inhibition of chilling-induced photooxidative leaf damage are discussed.

TEMPERATURE-DEPENDENCE OF CHLOROPHYLL FLUORESCENCE INDUCTION AND PHOTOSYNTHESIS IN TOMATO AS AFFECTED BY TEMPERATURE AND LIGHT CONDITIONS DURING GROWTH

The temperature dependence of chlorophyll fluorescence induction and photosynthesis of tomato plants grown at different temperatures and light intensities was studied. Chlorophyll fluorescence induction and photosynthetic activity of leaf discs was determined between 0-degrees and 30-degrees-C. Two breakpoints, around 16-degrees and 24-degrees-C, were observed when the maximum of chlorophyll fluorescence induction, F(p), was plotted against temperature. DCMU was used to study the role of photosynthetic electron transport on the breakpoints of F(p). Inhibition of photosynthetic electron transport by DCMU caused the disappearance of the breakpoint at 16-degrees-C. The temperature breakpoint at 24-degrees-C remained intact. This result is used to discuss the mechanisms underlying the occurrence of the breakpoints. The low temperature breakpoint at 16-degrees-C was ascribed to electron transport limitation between photosystem I and photosystem II at temperatures below 16-degrees-C. The high temperature breakpoint was attributed to alterations in light distribution between photosystem I and photosystem II. An increased State II adaptation at temperatures above 24-degrees-C is believed to cause a decrease in F(p). Temperatures of both breakpoints were affected by growth conditions of the tomato plants. Compared with optimal growth conditions, suboptimal growth conditions caused a significant decrease in the low temperature breakpoint. Concomitantly, photosynthetic activity and leaf chlorophyll content were decreased. The possibility of using breakpoint temperatures to indicate an adaptation of the thylakoid membrane organisation and functioning to suboptimal growth conditions is discussed.

The quantum efficiency of photosystem II and its relation to non-photochemical quenching of chlorophyll fluorescence; the effect of measuring-and growth temperature

The relation between the quantum yield of oxygen evolution of open photosystem II reactions centers (Φp), calculated according to Weis and Berry (1987), and non-photochemical quenching of chlorophyll fluorescence of plants grown at 19°C and 7°C was measured at 19°C and 7°C. The relation was linear when measured at 19°C, but when measured at 7°C a deviation from linearity was observed at high values of non-photochemical quenching. In plants grown at 7°C this deviation occurred at higher values of non-photochemical quenching than in plants grown at 19°C. The deviations at high light intensity and low temperature are ascribed to an increase in an inhibition-related, non-photochemical quenching component (qI).The relation between the quantum yield of excitation capture of open photosystem II reaction centers (Φexe), calculated according to Genty et al. (1989), and non-photochemical quenching of chlorophyll fluorescence was found to be non-linear and was neither influenced by growth temperature nor by measuring temperature.At high PFD the efficiency of overall steady state electron transport measured by oxygen-evolution, correlated well with the product of qN and the efficiency of excitation capture (Φexe) but it deviated at low PFD. The deviations at low light intensity are attributed to the different populations of chloroplasts measured by gas exchange and chlorophyll fluorescence and to the light gradient within the leaf.The relation between the quantum yield of oxygen evolution of open photosystem II reactions centers (Φp), calculated according to Weis and Berry (1987), and non-photochemical quenching of chlorophyll fluorescence of plants grown at 19°C and 7°C was measured at 19°C and 7°C. The relation was linear when measured at 19°C, but when measured at 7°C a deviation from linearity was observed at high values of non-photochemical quenching. In plants grown at 7°C this deviation occurred at higher values of non-photochemical quenching than in plants grown at 19°C. The deviations at high light intensity and low temperature are ascribed to an increase in an inhibition-related, non-photochemical quenching component (qI).The relation between the quantum yield of excitation capture of open photosystem II reaction centers (Φexe), calculated according to Genty et al. (1989), and non-photochemical quenching of chlorophyll fluorescence was found to be non-linear and was neither influenced by growth temperature nor by measuring temperature.At high PFD the efficiency of overall steady state electron transport measured by oxygen-evolution, correlated well with the product of q N and the efficiency of excitation capture (Φexe) but it deviated at low PFD. The deviations at low light intensity are attributed to the different populations of chloroplasts measured by gas exchange and chlorophyll fluorescence and to the light gradient within the leaf.

Genotypic Variation in Chilling-induced Leakage of Electrolytes from Leaf Tissue of Tomato (Lycopersicon esculentum Mill.) in Relation to Growth under Low-energy Conditions

Summary Differences between 10 genotypes of tomato ( Lycopersicon esculentum Mill.) with respect to electrolyte leakage from leaf tissue after a chilling treatment (EL) were demonstrated. Effects of growing temperature and light intensity on EL were also observed. The inheritance of EL was studied. The variation in this membrane character was controlled by additive gene action. Comparison between EL data and literature data on the growth of the different genotypes indicated an effect of membrane characters related to EL in plant performance.

Temperature-induced alterations of in vivo chlorophyll a fluorescence induction in cucumber as affected by DCMU

Induction of chlorophyll a fluorescence and photosynthesis as affected by temperature were measured in cucumber leaf discs. Abrupt changes of the maximal variable fluorescence, Fv(p), and photosynthesis were observed around 9° and 21°C when the temperature was decreased from 30° to 0°C. The temperature-dependent maximal fluorescence of DCMU-treated leaf discs showed a single change around 21°C. Temperature-induced chlorophyll a fluorescence alterations are discussed in relation to electron transport activity of the two photosystems and photosynthetic activity of the cucumber leaf discs.

Temperature Effects on Chlorophyll Fluorescence Induction in Tomato

Chlorophyll fluorescence induction of tomato leaf discs was measured at a low actinic light intensity of 10 mu mol.m(-2).s(-1) and at decreasing temperatures from 30 degrees to 0 degrees C. F-o remained constant within the temperature range assessed. In contrast, the peak of fluorescence induction, F-p, showed a strong temperature dependence. F-p increased by lowering the temperature within two temperature regions, from 30 degrees to 22 degrees C and from 14 degrees to 0 degrees C, respectively. F-p was hardly affected by temperatures between 14 degrees and 22 degrees C. Consequently, two temperature breakpoints of F-p were observed at 14 degrees and 22 degrees C. F-p reflected a maximum reduction state of the primary electron acceptor of photosystem II, Q(A). The relation between F-p and maximal Q(A) reduction was used to explain the observed temperature effects on F-p. The reduction state of Q(A) depended on both photosystem I oxidizing and photosystem II reducing activity. Consequently, breaks in the temperature response of F-p were attributed to dissimilar effects of temperature on photosystem I and photosystem II functioning. The steep increase of F-p below 14 degrees C was attributed to a low temperature induced impaired electron transport. This was sustained by the absence of a low temperature break when electron transport was inhibited by DCMU. A limitation of Q(A) oxidizing activity below 14 degrees C could be deduced from the temperature dependence of F-p of control discs. The second break in the temperature response of F-p at 22 degrees C was not caused by temperature effects on electron transport. The break of F-p at 22 degrees C was also found for F-m of electron transport inhibited leaf discs. While F-m decreased above 22 degrees C, the ratio of F-m determined at 687 and 730 nm (F(m)730/F(m)687) increased. This indicates a redistribution of excitation energy between the photosystems. Kinetic analysis of fluorescence induction traces showed a temperature effect on PSII heterogeneity. From 22 degrees to 30 degrees C, the PSII alpha fraction decreased from 70 to 50 % while the PSII beta fraction increased from 35 to 50 %. These results also suggest a temperature induced redistribution of excitation energy above 22 degrees C in favour of PSI. Implications of temperature effects on thylakoid membrane associated processes and plant functioning are discussed.