Changes in the activities of enzymes involved in scavenging active oxygen species were followed after exposing bean seedling leaves (Phaseolus vulgaris L.) to various cross stresses of irradiance and temperature. The activities of superoxide dismutase (SOD, EC 1.15.1.1) and ascorbate peroxidase (AsAPOD, 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. CAT activity was more sensitive to HT than to HI. Ascorbate (AsA) content slightly decreased and then increased during the treatment of HI, but decreased under the cross stress of HI 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 cycle, which was difficult to be started by HT. The antioxidant systems tended 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 dissipation was one major energy dissipation pathway during the cross stress of HI and HT.

By analysis of gas exchange and chlorophyll fluorescence, the effects of NaCl treatment and supplemental CaCl2 on photosynthesis, photosystem II (PSII) photochemistry and photoinhibition were investigated in Rumex leaves. Photosynthesis in Rumex leaves was strongly inhibited by 200mM NaCl treatment. Such inhibition of photosynthesis was ameliorated by CaCl2 supplement. Neither NaCl treatment nor CaCl2 supplement had any significant effects on the PSII primary photochemical reaction in dark-adapted leaves. In light-adapted leaves, however, 200mM NaCl2 treatment significantly decreased photochemical quenching (qp), efficiency of excitation energy capture by open PSII reaction centers (FV’/FM’) and quantum yield of PSII electron transport (φPSII). These decreases in qp, FV’/FM’ and VPSII were mitigated by CaCl2 supplement with the maximum of its effect appearing at a concentration of 8 mM CaCl2. A similar mitigating effect was shown in 200 mM NaCl-treated Rumex leaves when susceptibility of φPSII to photo inhibition was determined under high irradiance. It is suggested that the mitigation of photo inhibition in NaCl-treated leaves is because of the amelioration of inhibition of photosynthesis.

Gas exchange, chlorophyll a fluorescence kinetics, chloroplast pigments and antioxidant enzymes were investigated to explore the development of photo protective mechanisms in soybean leaves from emergence to full expansion under field conditions. During leaf development, photosynthesis (Pn) gradually increased. Although the maximum quantum yield of PSII photochemistry (Fv/Fm) was quite high at the initial stages of leaf development, it was appreciably lower than that in fully expanded leaves. During daily courses, reversible decrease in Fv/Fm was noted at various stages, implying that no photodamage occurred. When exposed to irradiance, a substantial elevation in the actual PSII efficiency (φPSII) together with a remarkable decrease in non-photochemical quenching (NPQ) was observed with the process of leaf development. In this study, we observed that newly initiating leaves possessed relative larger xanthophyll pool size (V + A + Z)/Chl, but it began to decrease with leaf expanding until fully expanded. Further experiments revealed that the decline of relative xanthophyll cycle pool size during leaf expanding was owing to the fact that the chlorophyll contents per area increased faster than that of the xanthophyll cycle pigments. In addition, activities of the antioxidative enzymes, such as SOD, APX, CAT, POD and GR, were also enhanced at the early stages of leaf expansion. We suggested that photoprotective mechanisms, including xanthophyll cycle and antioxidant enzymic system, have developed firstly as soon as leaf emerged. It was the timely development of the mechanisms in young leaves that dissipate excess excitation energy protecting leaves against photodamage at the early stages of leaf development. The relative leaf water content increased with the process of leaf development and exhibited signicant negative linear correlation with NPQ, xanthophyll cycle pigments and antioxidant enzyme activities in soybean leaves. We proposed that the changes in leaf turgor might be involved in triggering the activation of photoprotective mechanisms together with high irradiance at the initiating stages of leaf expanding.