Heat combined with salinity stimulate antioxidant defense but induce severe impairment in photosynthesis of rice plants

Abstract

Aims To test the hypothesis if high temperature could aggravate salt stress through stimulation of Na+ accumulation, impairment in photosynthesis and generation of oxidative stress in rice leaves.Methods Rice plants were previously exposed to 0 and 100 mM NaCl for eight days at 27°C and afterwards two groups were transferred to high temperature (42°C) for 10 hours (heat and heat + salt) whereas two others remained at 27°C (control and single salt).Results Heat combined with salinity greatly stimulated Na+ accumulation in leaves causing intense decrease in K+/Na+ ratios, inducing significant osmotic and ionic alterations. Stomata were closed intensely causing drastic impairment in CO2 assimilation and decrease in water use efficiency. In contrast, the PSII activity was much lesser affected, corroborated by low increase in closed reaction center fraction of PSII and slight decrease in electron transport rates. Unexpectedly, that stress combination partially favored oxidative protection as indicated by reduction in the levels of H2O2 and lipid peroxidation associated with reduction in the contents of reduced ascorbate and glutathione. These favorable antioxidant responses were accompanied by increases in the activities of ascorbate peroxidases, superoxide dismutases, glutathione peroxidases, and phenol peroxidases whereas catalases and glycolate oxidases decreased. These antioxidant responses were not enough to mitigate overall physiological damages caused by combined stress as indicated by drastic increase in membrane damage.Conclusions High temperature drastically aggravates negative effects caused by salt stress on the photosynthetic efficiency, especially CO2 assimilation, despite that combination has partially favored the antioxidant defense.