Photorespiration Coupled With CO2Assimilation Protects Photosystem I From Photoinhibition Under Moderate Poly(Ethylene Glycol)-Induced Osmotic Stress in Rice

Photorespiration coupled with CO(2)assimilation is thought to act as a defense system against photoinhibition caused by osmotic stress. In the present study, we examined whether such a mechanism is operative for the protection of photosystem I (PSI) in rice (Oryza sativaL.) including transgenic plants with decreased and increased Rubisco content (RBCS-antisense andRBCS-sense plants, respectively). All plants were hydroponically grown and moderate osmotic stress was imposed using hydroponic culture solutions containing poly(ethylene glycol) (PEG) at 16% or 20% (w/v) for 2 d. In wild-type plants, the rates of CO(2)assimilation (A) were significantly decreased by the PEG treatment, whereas the photorespiration activity estimated from the rates of electron transport in photosystem II (PSII) andAwere not affected. The maximal quantum efficiency of PSII (F-v/F-m) and the maximal activity of PSI (P-m) were also not affected. InRBCS-antisense plants,Aand the estimated photorespiration activity were considerably lower than those in wild-type plants in the presence or absence of the PEG treatment.P(m)and bothF(v)/F(m)andP(m)decreased in the 16% PEG-treated and 20% PEG-treatedRBCS-antisense plants, respectively. Thus, the decrease in Rubisco content led to the photoinhibition of PSI and PSII, indicating the importance of photorespiration coupled with CO(2)assimilation for the protection of PSI from moderate PEG-induced osmotic stress. It was also shown that PSI was more sensitive to osmotic stress than PSII. In the PEG-treated wild-type andRBCS-antisense plants, osmotic-stress responses of the photosynthetic electron transport reactions upstream of PSI led to the oxidation of P700, which is thought to prevent PSI from over-reduction. Although such a defense system operated, it was not sufficient for the protection of PSI inRBCS-antisense plants. In addition, there were no large differences in the parameters measured between wild-type andRBCS-sense plants, as overproduction of Rubisco did not increase photorespiration activity.