Reason for photosynthetic declination in rice from water stress induced by polyethylene glycol (PEG)

After rice plants (japonica rice Azucena) were treated with various concentrations of polyethylene glycol (PEG) for 16 days, characteristics of gas exchange, chlorophyll fluorescence, MDA content and activities of antioxidant enzymes (SOD, POD and CAT) in the leaves were measured. The reasons for photosynthetic declination caused by different PEG concentrations were remarkably different, varying in three phases. In the first phase, under 10% PEG treatment, net photosynthetic rate (P_(n)) significantly decreased, but stomatal conductance (G_(s)) and intercellular CO_(2) concentration (C_(i)) increased slightly, while chlorophyll fluorescence parameters and photosynthetic pigment content showed little change, suggesting that the decrease in P_(n) was not due to the stomatal limitation. In the second phase, under 15% PEG treatment, the pigment content and fluorescence parameters began to change but P_(n) decreased with a significant decrease in G_(s) and C_(i), indicating G_(s) became a main factor limiting P_(n). In the last phase, under 20% PEG treatment, P_(n), G_(s), fluorescence parameters and pigment content fell signifiantly further, whereas C_(i) exhibited no further decrease, demonstrating that the decline in P_(n) was largely attributed to the damage of photosynthetic membrane. Although severe water stress caused the reduction of the abilities for harvesting and transferring light energy, the excess excitation energy still occurred and might lead to the production of reactive oxygen species triggering the antioxidant system. However, it could still damage photosynthetic apparatus and result in the reduction of photochemical activity of PS II.