In this study, the function of the NAD(P)H dehydrogenase (NDH)-dependent pathway in suppressing the accumulation of reactive oxygen species in chloroplasts was investigated. Hydrogen peroxide accumulated in the leaves of tobacco (Nicotiana tabacum) defective in ndhC-ndhK-ndhJ (DndhCKJ) at 42°C and 4°C, and in that of wild-type leaves at 4°C. The maximum quantum efficiency of PSII decreased to a similar extent in both strains at 42°C, while it decreased more evidently in DndhCKJ at 4°C. The parameters linked to CO 2 assimilation, such as the photochemical efficiency of PSII, the decrease of nonphotochemical quenching following the initial rise, and the photosynthetic O 2 evolution, were inhibited more significantly in DndhCKJ than in wild type at 42°C and were seriously inhibited in both strains at 4°C. While cyclic electron flow around PSI mediated by NDH was remarkably enhanced at 42°C and suppressed at 4°C. The proton gradient across the thylakoid membranes and light-dependent ATP synthesis were higher in wild type than in DndhCKJ at either 25°C or 42°C, but were barely formed at 4°C. Based on these results, we suggest that cyclic photophosphorylation via the NDH pathway might play an important role in regulation of CO 2 assimilation under heat-stressed condition but is less important under chilling-stressed condition, thus optimizing the photosynthetic electron transport and reducing the generation of reactive oxygen species.

The expression and activity of type-1 NAD(P)H dehydrogenase (NDH-1) was compared between cells of Synechocystis PCC6803 grown in high (H-cells) and low (L-cells) CO 2 conditions. Western analysis indicated that L-cells contain higher amounts of the NDH-1 subunits, NdhH, NdhI and NdhK. An NADPH-specific subcomplex of NDH-1 showed higher NADPH-nitroblue tetrazolium oxidoreductase activity in L-cells. The activities of both NADPHmenadione oxidoreductase and light-dependent NADPH oxidation driven by photosystem I were much higher in Lcells than in H-cells. The initial rate of re-reduction of P700+ following actinic light illumination in the presence of DCMU under background far-red light was enhanced in Lcells. In addition, rotenone, a specific inhibitor of NDH-1, suppressed the relative rate of post-illumination increase in Chl fluorescence of L-cells more than that of H-cells, suggesting that the involvement of NDH-1 in cyclic electron flow around photosystem I was enhanced by low CO 2 . Taken together, these results suggest that NDH-1 complex and NDH-1-mediated cyclic electron transport are stimulated by low CO 2 and function in the acclimation of cyanobacteria to low CO 2 .