Chloroplastic NAD(P)H dehydrogenase in tobacco leaves functions in alleviation of oxidative damage caused by temperature stress

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 ( Delta ndhCKJ) at 42 degrees C and 4 degrees C, and in that of wild-type leaves at 4 degrees C. The maximum quantum efficiency of PSII decreased to a similar extent in both strains at 42 degrees C, while it decreased more evidently in DndhCKJ at 4 degrees C. The parameters linked to CO2 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 degrees C and were seriously inhibited in both strains at 4 degrees C. While cyclic electron flow around PSI mediated by NDH was remarkably enhanced at 42 degrees C and suppressed at 4 degrees C. The proton gradient across the thylakoid membranes and light-dependent ATP synthesis were higher in wild type than in DndhCKJ at either 25 degrees C or 42 degrees C, but were barely formed at 4 degrees C. Based on these results, we suggest that cyclic photophosphorylation via the NDH pathway might play an important role in regulation of CO2 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.