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We studied the photooxidative effects of methyl viologen (MV) on PSII in rice (Oryza sative L). Leaves were held at either room temperature (RT) or 4℃. In the presence of MV, the photochemical efficiency of PSII, or F_v/F_ml was more depressed at RT than at the low temperature (LT), but the loss of D1 protein that was detected at RT was not observed at LT. However, the decline in the content of functional PSII, 1/F_0 - 1/F_ml was similar for MV-treated at either temperature. There results suggest that, at LT, PSII is not protected from MV-induced pthotooxidation, although degradation of the D1 protein is delayed. The 1/F_0 - 1/F_ml was also possible, even for tissues treated with MV at LT. Therefore, we believe that MV-induced reversible photoinactivation may exist. This possibility is further discussed in terms of changes in the de-epoxidation state and the rate of PSII-driven electron transport.
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