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

Abstract

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.