Acute exposure to UV-B sensitizes cucumber,tomato,and Arabidopsis plants to photooxidative stressby inhibiting thermal energy dissipation and antioxidant defense

To characterize a change in npq upon exposure to ultraviolet-b (uv-b),the xanthophyll cycle-dependent and -independent npqs were compared in cucumis sativus,lycopersicum esculentum,and arabidopsis thaliana leaves. the xanthophyll cycle-dependent npq was dramatically but reversibly suppressed by uv-b radiation. this suppression was correlated more strongly with a marked decrease in photosynthetic electron transport rather than changes in xanthophyll cycle enzymes such as violaxanthin de-epoxidase and zeaxanthin epoxidase. accordingly,the uv-b-induced suppression of npq cannot be attributed to changes in expressions of vde and zep. however,suppression of the xanthophyll cycle-dependent npq could only account for the 77 k fluorescence emission spectra of thylakoid membranes and the increased level of 1o2 production,but not for the decreased levels of ·o2 - production and h2o2scavenging. these results suggest that a gradual reduction of h2o2 scavenging activity as well as a transient and reversible suppression of thermal energy dissipation may contribute differentially to increased photooxidative damages in cucumber,tomato,and arabidopsis plants after acute exposure to uv-b radiation.