RESPONSES OF THE CHLOROPLASTIC ANTIOXIDANT SYSTEM OF Hedysarum Mongolicum AND Caragana Korshinskii TO INCREASING DROUGHT STRESS AND THEIR EFFECTS ON THE PHOTOSYNTHESIS

To investigate the effects of increasing drought on the photosynthesis of xerophytic shrubs, a water controlled pot culture of Hedysarum mongolicum and Caragana korshinskii, two commonly planted shrubs in the arid regions in northwest China, was conducted. The shrubs were planted in light, moderate and serious drought treated soils (LD, MD and SD, the soil moistures are 65%, 50% and 30% of the field capacity, while the common soil moisture is 75% of that). The responses of the chloroplastic antioxidant system, photosynthetic response parameters and the relationships among them were detected. The results indicated that (1) the chloroplastic ascorbate peroxidase (APX) and dehydroascorbic acid reductase (DHAR) activities of H. mongolicum significantly increased in LD conditions, while the glutathione reductase (GR) activity significantly decreased. Chloroplastic superoxide dismutase (SOD), DHAR and GR activities of C. korshinskii and antioxidants contents (ascorbic acid, glutathione and carotenoids) significantly increased in LD conditions. Along with the increasing drought stress, chloroplastic antioxidase activities and antioxidants contents in both shrubs exhibited increasing tendencies, except for the APX and GR activities of C korshinskii and the DHAR activities of both the shrubs, which exhibited significant decreases after temporary increasing. Significant chloroplastic membrane lipid peroxidationstarted to be observed in SD and LD conditions for the 2 species, respectively. (2) the maximum net photosynthetic rate (P-nmax), light saturation point (LSP), the maximum carboxylation rate was (V-cmax), the maximum electron transfer rate (J(max)) and the utilization rate of triose phosphate (TPU) of H. mongolicum and the P-nmax, apparent quantum efficiency (AQY), J(max) and TPU of C. korshinskii significantly decreased in LD conditions. Along with the increasing drought stress, all the photosynthetic response parameters except dark respiration rate (Rd) and light compensation point (LCP) exhibited decreasing tendencies. (3) there were significant negative relationships among parts of the chloroplastic antioxidant indices and photosynthetic response parameters, however, the oxidative damage of chloroplasts is not the only dominant limiting factor of the photosynthesis under drought stress. (4) H. mongolicum was more suitable for the revegetation of arid regions in LD and MD conditions than C. korshinskii considering their abilities of chloroplastic antioxidation and maintaining photosynthesis.