The effects of decreased flux in the glycolate pathway on photoinhibition was investigated in transgenic tobacco (Nicotiana tabacum L. cv. SR1) plants. These plants harbored a pumpkin cDNA for glycolate oxidase (GO), an enzyme in the glycolate pathway, under the control of the cauliflower mosaic virus 35S promoter. Some transformants showed both reduced amounts and reduced activities of GO. The decrease of GO was enhanced at a later growth stage of these transformants, whereas no changes were observed in the amounts of other enzymes in the glycolate pathway, such as hydroxypyruvate reductase and serine glyoxylate aminotransferase. The phenotype grown under a low light condition (30 microE s(-1) m(-2)) resembled that of the wild type. Transformants with about 35% lower GO activity than wild type, had a lower Fv/Fm under 500 microE s(-1) m(-2) irradiation for 8 h. After 60 microE s(-1) m(-2) irradiation for 8 h, Fv/Fm was lowered in some transformants with less than 20% of the GO activity of the wild type. These results suggest that photosynthesis was susceptible to photoinhibition with reduction to below threshold levels of GO activities and that higher activities of GO are required under a higher irradiation. The increase in the electron transport rate (ETR) with increased irradiation was suppressed only in transformants that had GO activity one-third less than the wild type, suggesting that the regeneration of the substrate for the Calvin cycle was decreased only when there was an extreme reduction of GO. These results also suggest that the photosystem was disturbed when the concentration of the substrate for the Calvin cycle decreased until it became insufficient to receive the excess photon energy generated in each light environment.