Growth under UV-B radiation increases tolerance to high-light stress in pea and bean plants

Pea (Pisum sativum L.) and bean (Phaseolus vulgaris L.) plants were exposed to enhanced levels of UV-B radiation in a growth chamber. Leaf discs of UV -B treated and control plants were exposed to high-light (HL) stress (PAR: 1200 mu mol m(-2) s(-1)) to study whether pre-treatment with UV-B affected the photoprotective mechanisms of the plants against photoinhibition. At re gular time intervals leaf discs were taken to perform chlorophyll a fluores cence and oxygen evolution measurements to assess damage to the photosystem s. Also, after 1 h of HL treatment the concentration of xanthophyll cycle p igments was determined. A significantly slower decline of maximum quantum e fficiency of PSII (F-v/F-m), together with a slower decline of oxygen evolu tion during HL stress was observed in leaf discs of UV-B treated plants com pared to controls in both plant species. This indicated an increased tolera nce to HL stress in UV-B treated plants. The total pool of xanthophyll cycl e pigments was increased in UV-B treated pea plants compared to controls, b ut in bean no significant differences were found between treatments. Howeve r, in bean plants thiol concentrations were significantly enhanced by UV-B treatment, and UV-absorbing compounds increased in both species, indicating a higher antioxidant capacity. An increased leaf thickness, together with increases in antioxidant capacity could have contributed to the higher prot ection against photoinhibition in UV-B treated plants.