The ndhBand psaEmutants of the cyanobacterium Synechocystis sp. PCC 6803 are partly deficient in PSIdriven cyclic electron transport. We compared photoinhibition in these mutants to the wild type to test the hypothesis that PSI cyclic electron transport protects against photoinhibition. Photoinhibitory treatment greatly accelerated PSI cyclic electron transport in the wild type and also in both the mutants. The psaEmutant showed rates of PSI cyclic electron transport similar to the wild type

more »

... er all conditions tested. The ndhBmutant showed much lower rates of PSI cyclic electron transport than the wild type following brief dark adaptation but exceeded wild type rates after exposure to photoinhibitory light. The wild type and both mutants showed similar rates of photoinhibition damage and photoinhibition repair at PSII. Photoinhibition at PSI was much slower than at PSII and was also similar between the wild type and both mutants, despite the known instability of PSI in the psaEmutant. We conclude that photoinhibitory light induces sufficient PSI-driven cyclic electron transport in both the ndhBand psaEmutants to fulfill any role that cyclic electron transport plays in protection against photoinhibition. Abbreviations: DS 820 , the change in reflectance signal at 820 nm that results from photooxidation of P700; DMBQ, 2,6-dimethylbenzoquinone; FQR, the putative ferredoxin-quinone oxidoreductase; KFeCN, potassium ferricyanide; MV, methyl viologen (paraquat); NDH-1, the thylakoid NAD(P)H dehydrogenase; PAR, photosynthetically active radiation; PET, photosynthetic electron transport.