Seedlings of winter wheat (Triticum aestivum L. cv. Jubilejnaja 50) were grown under normal and dry conditions. Frost resistance (LT50) of 10‐day‐old control seedlings was −6°C. LT50 of the subsequently drought‐stressed leaves shifted to −16°C. In plants of the same physiological age (28 days) but grown without stress, LT50 was −12°C. Phosphatidylcholine accumulated and phosphatidylethanolamine decreased in drought‐stressed leaves. Fatty acid unsaturation of these phospholipids increased with leaf age, independently of water supply. Both ageing and drought stress produced a decrease in the apparent phase separation temperature of isolated total phospholipids as determined by electron spin resonance. The possible role of dehydration‐induced structural changes in the bilayer matrix in triggering adaptive alterations in membrane composition, similar to those observed during cold hardening, is discussed.
Spinach plunts (Spinacia oleracea L. cv. Monosa) were exposed to air with and without 0.25 μl l‐1 H2S. Effects of H2S exposure for up to 18 days on photosynthesis, dark respiration and on chlorophyll a fluorescence were studied. Dark respiration was not affected by H2S fumigation. Photosynthetic CO2 fixation decreased linearly with time in both control and fumigated plants. The rate of decrease in CO2 fixation was faster in the fumigated plants; after 14 days of exposure the fumigated plants showed a decrease in CO2 fixation of 23%äs compared with the control plants. The H2S‐induced decrease in CO2 fixation was accompanied by a decrease in quenching of the chlorophyll fluorescence. The most characteristic change in chlorophyll fluorescence was a decreased difference between maximum and steady‐state fluorescence [(P‐T)/P), suggesting a reduced efficiency in the use of photochemical energy in photosynthesis. Differences in CO2 fixation were more pronounced whcn measured at high light intensity; the maximum rate of CO2 fixation at light saturation decreased significantly with time in the H2S‐exposed plants; after 14 days of H2S exposure a decrease of more than 70% was noted. The decrease in CO2 fixation could not be attributed to a decreased chlorophyll content; on the contrary, chlorophyll content even slightly increased during fumigation. The initial increase in CO2 fixation rate with increasing light intensity was also reduced by prolonged H2S fumigation, indicating an effect of H2S fumigation on photosynthetic electron transport. Finally, the phytotoxicity of H2S is discusscd in relation to the H2S‐induced changes in photosynthetic CO2 fixation and chlorophyll a fluorescence, and the effect of H2S on leaf development observed in earlier studies.
Chlorophyll degradation in Cucumis leaf discs was measured at different temperatures between 1 and 25°C in the light and in darkness, and in the presence or absence of sucrose. Two different processes of chlorophyll degradation could be distinguished, a lightrequiring process operating at 1 and 5°C and another, light and sucrose enhanced degradation process which was evident at 25°C. Degradation of leaf pigments at low temperatures was of a photooxidative nature since there was no degradation in the dark. The chlorophyll a/b ratio was decreased, carotene was degraded at a faster rate than chlorophyll, and 3-(3,4-dichlorophenyl)-l,ldimethylurea (DCMU) and triphenyltetrazolium chloride (TTC) which prevent photo-oxidation, protected against chlorophyll degradation. The light and sucrose enhanced chlorophyll degradation at 25°C was of an enzymatic nature since it occurred in the dark as well as in the light. The chlorophyll a/b ratio was not affected, and carotene and chlorophyll degradation occurred at the same rate. Since DCMU completely inhibited the light enhancement at 25°C and experimentation in a low oxygen atmosphere also protected chlorophyll against the effect of light and sugar application, it is suggested that the enhancement of chlorophyll degradation by light and sucrose at 25°C may be due to increased sugar uptake of the chloroplasts and consequently excessive starch formation in the organelles.
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