Effects of long-term UV-exposure and plant sex on the leaf phenoloxidase activities and phenolic concentrations of Salix myrsinifolia (Salisb.).

The accumulation of flavonoids on the leaf surface is a well-characterized protective mechanism against UV-B radiation. Other protective mechanisms, such as the induction of antioxidative enzymes and peroxidase-mediated lignification may also be important. The effects of UV-B radiation have mainly been considered in short-term studies, whereas ecologically more relevant long-term field studies are still rare. Here we examined the effects of long-term exposure to enhanced UV-B radiation on the activities of two antioxidative enzymes, polyphenol oxidase (PPO; EC 1.10.2.2 and EC 1.14.18.1) and guaiacol peroxidase (POD; EC 1.11.1.7), as well as the phenolic concentrations in two sexes of the dioecious species, Salix myrsinifolia. After three consecutive growth seasons with enhanced UV-B radiation, we found that PPO activity was decreased by UV radiation in male plants, which might explain their lower UV-B tolerance when compared to female plants. In addition, male plants had higher specific activity than did female plants under ambient conditions, supporting the idea that males of S. myrsinifolia are generally more growth-oriented than females. By contrast, neither UV treatment nor sex had significant effects on the POD activities of willows. Gender differences in the concentrations of phenolic compounds are in line with the general concept that males are less well defended than females. We suggest that the inability to increase PPO and POD activity, along with lower accumulation of UV-B absorbing compounds under UV-B exposure, might be one of the reasons why males had thinner leaves and were less tolerant of UV-B than were females.