The site of photoinhibition at low temperatures in leaves of a chilling-sensitive plant, cucumber, is photosystem I [Terashima et al. (1994)Planta193: 300]. As described herein, selective photoinhibition of PSI can also be induced in isolated thylakoid membranes in vitro. Inhibition was observed both at chilling temperatures and at 25°C, and not only in the thylakoid membranes isolated from cucumber, but also in those isolated from a chilling-tolerant plant, spinach. Comparison of these observations in vitro to the earlier results in vivo indicates that (1) photoinhibition of PSI is a universal phenomenon; (2) a mechanism exists to protect PSI in vivo; and (3) the protective mechanism is chilling-sensitive in cucumber. The chilling-sensitive component seems to be lost during the isolation of thylakoid membranes. Very weak light (10–20μmol m-2s-1) was sufficient to cause the inhibition of PSI. About 80%of the oxygen-evolving activity by PSII was maintained even after the activity of PSI had decreased by more than 70%. This is the first report of the selective photoinhibition of PSI in vi
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