Effects Of Low Temperature And Weak Light On Photoinhibition And Thylakoid Membrane Lipids In Cucumber Leaves

The relationship among thylakoid membrane lipids, photoinhibition and cold resistance was investigated under low temperature stress in chilling-sensitive cucumber (Cucumis sativus L. cv. Jin-yan 4) leaves. In addition, physiological changes of cucumber and rumex that with different sensitivity to low temperature under low temperature and weak light were also studied. The main results were as follows:1. Relationship between photoinhibition and membrane lipid fatty acids during low temperature and weak light treatment and recovery.Both Fv/Fm and ФPSII of cucumber leaves decreased obviously after 5 day low temperature and weak light treatment, and recovered in proper condition in one day. It was interesting to observe that polyunstatured fatty acids was closely related to recovery of photoinhibition. 2. Physiological changes in plants with different sensitivity to low temperature under low temperature and weak light. Difference in changes of chlorophyll fluorescence and fatty acid composition of PG in chilling-sensitive cucumber and chilling-resistant rumex under low temperature and weak light was conducted. Results showed that damage of PSII in cucumber was more seriously than that in rumex. 16:1(3t)of PG in rumex decreased less than that in cucumber under stress, which indicated that degradation of LHCII oligmer was slighter in rumex. Besides, the content of saturated fatty acids of PG in rumex decreased significantly under stress, which is benefit to decrease the temperature of membrane lipid phase transitions.3. Effects of low temperature and weak light on thylakoid membrane lipids and chlorophyll fluorescence in high temperature pre- treatment and control cucumber leavesChanges of membrane lipids in cucumber leaves were analyzed after cold-hardening and high temperature pre-treatment. Results indicated cold-hardening stimulated conversion of 18:2 to 18:3, while high temperature pre-treatment enhanced in conversion of 18:1 to 18:2. The pre-treatment (3 days at 32℃) and the control cucumbers were moved to low temperature and weak light. Changes of chlorophyll fluorescence and membrane lipids suggested that high temperature pre-treatment improved the chilling resistence of cucumber. Photoinhibition of pre-treatment cucumber leaves was slighter compared with control during low temperature and weak light. The increase of unsaturated fatty acids of membrane lipids resulted in increasing thylakoid membrane fluidity. However, degradation of chlorophyll in high temperature pre-treatment cucumber leaves was more seriously than that in control, and the increase of unstaturated fatty acids of PG in pre-treatment cucumber was not benefit to decrease in temperature of membrane lipid phase transitions.4. Effect of light intensity on fatty acid composition of thylakoid membrane lipids under short time treatment of low temperature During low temperature(10℃) treatment, the PSII maximumal photo-chemical efficiency (Fv/Fm) was higher in cucumber leaves under strong light compared to that under weak light, implying that srong light aggravated photoinhibition. However, the increase of polyunstasurated fatty acids and the slight decrease of unstasurated fatty acids of PG in cucumber leaves under strong light were beneficial to alleviating photoinhibition after 8h stress of low temperature. According to the different relation between membrane lipids and photoinhibition during long (5 days) and short (8 hours) time treatment of low temperature, it was deduced that alleviation of photoinhibition by changes in fatty acid composition of thylakoid membrane lipids in cucumber leaves need certain time. In addition, 18:3 of MGDG and DGDG under strong and weak light changed to the contrary, indicating that synthesized and unsaturated process of 18:3 in MGDG and DGDG were sensitive to light intensity.