Bisulfite at low concentrations(L-NaHSO3) increases cyclic electron transport around photosystem I(PSI) and photosynthesis.However,little is known regarding the detailed contribution of cyclic electron transport to the promoted photosynthesis by L-NaHSO3.In the present work,we used tobacco mutant defective in ndhC-ndhK-ndhJ(ndhCKJ) to investigate the role of NAD(P)H dehydrogenase(NDH)-dependent cyclic electron transport around PSI in an increase in photosynthesis by L-NaHSO3.After the treatment of tobacco leaves with L-NaHSO3(10 μmol L-1),the NDH-dependent cyclic electron transport,monitored by a transient post-illumination increase in Chl fluorescence and the amount of NDH,was notably up-regulated in wild type(WT).The NDH-dependent cyclic electron transport was severely impaired in ndhCKJ and was not significantly affected by treatment with L-NaHSO3.Accordingly,the NDH-dependent transthylakoid membrane proton gradient(pH),as reflected by the slow phase of millisecond-delayed light emission(ms-DLE),was increased by L-NaHSO3 in WT,but not in ndhCKJ;the enhancement of cyclic photophosphorylation(PSP) activity by L-NaHSO3 was more obvious in WT than ndhCKJ.The accumulation of both superoxide and hydrogen peroxide was reduced in WT when subjected to L-NaHSO3 treatment,but not in ndhCKJ.Furthermore,the increase of photosynthetic O 2 evolution rate by L-NaHSO3 was more significant in WT than in ndhCKJ.We therefore conclude that L-NaHSO3 alleviates the photo-oxidative damage by the enhancement of NDH-dependent cyclic PSP,thereby improving photosynthesis. Bisulfite at low concentrations (L-NaHSO3) increases cyclic electron transport around photosystem I (PSI) and photosynthesis. However, little is known about the detailed contribution of cyclic electron transport to the promoted photosynthesis by L- Na HSO3.In the present work, we used tobacco mutant defective in ndhC-ndhK-ndhJ (ndhCKJ) to investigate the role of NAD (P) H dehydrogenase (NDH) -dependent cyclic electron transport around PSI in an increase in photosynthesis by L-NaHSO3.After the treatment of tobacco leaves the NDH-dependent cyclic electron transport, monitored by a transient post-illumination increase in Chl fluorescence and the amount of NDH, was not up-regulated in wild type (WT). NDH-dependent cyclic electron transport was severely impaired in ndhCKJ and was not significantly affected by treatment with L-NaHSO3.Accordingly, the NDH-dependent transthylakoid membrane proton gradient (pH), as reflected by the slow phase of millisecond-delayed light em The enhancement of cyclic photophosphorylation (PSP) activity by L-NaHSO3 was more obvious in WT than ndhCKJ. The accumulation of both superoxide and hydrogen peroxide (ms-DLE) was increased by L-NaHSO3 in WT, but not in ndhCKJ; was reduced in WT when subjected to L-NaHSO3 treatment, but not in ndhCKJ. Focus again, the increase of photosynthetic O 2 evolution rate by L-NaHSO3 was more significant in WT than in ndhCKJ. We therefore conclude that L-NaHSO3 alleviates the photo -oxidative damage by the enhancement of NDH-dependent cyclic PSP, thereby improving photosynthesis.