Responses of the antioxidant systems and xanthophyll cycle in Phaseolus vulgaris to the combined stress of high irradiance and high temperature

Changes in the activities of enzymes involved in scavenging active oxygen s pecies were followed after exposing bean seedling leaves (Phaseolus vulgari s L.) to various cross stresses of irradiance and temperature. The activiti es of superoxide dismutase (SOD, EC 1.15.1.1) and ascorbate peroxidase (AsA POD, EC 1.11.1.11) increased to different extent with prolonged irradiation of the leaves, and were stimulated by high temperature (HT). The activity of catalase (CAT, 1.11.1.6) decreased when exposed to strong irradiance (HI ), and the decrease was further exacerbated when HI was combined with HT, C AT activity was more sensitive to HT than to HI. Ascorbate (AsA) content sl ightly decreased and then increased during the treatment of HI, but decreas ed under the cross stress of KI and HT. On the contrary, glutathione (GSH) content increased all the time during various treatments of irradiance and temperature. The increase in the combined stress was even more pronounced. Irradiance is the major reason in triggering the operation of xanthophyll c ycle, which was difficult to be started by HT. The antioxidant systems tend ed to be inactivated with prolonged exposure to the cross stress of HI and HT. The de-expoxidated state of xanthophyll cycle, however, was increasing all the time, which indicated that the zeaxanthin-dependent thermal dissipa tion was one major energy dissipation pathway during the cross stress of HI and HT.