SELECTIVE PHOTOINHIBITION OF PHOTOSYSTEM-I IN ISOLATED THYLAKOID MEMBRANES FROM CUCUMBER AND SPINACH

The site of photoinhibition at low temperatures in leaves of a chillin g-sensitive plant, cucumber, is photosystem I [Terashima et al. (1994) Planta 193: 300]. As described herein, selective photoinhibition of P SI can also be induced in isolated thylakoid membranes in vitro. Inhib ition was observed both at chilling temperatures and at 25 degrees C, and not only in the thylakoid membranes isolated from cucumber, but al so in those isolated from a chilling-tolerant plant, spinach. Comparis on of these observations in vitro to the earlier results in vivo indic ates that (1) photoinhibition of PSI is a universal phenomenon; (2) a mechanism exists to protect PSI in vivo; and (3) the protective mechan ism is chilling-sensitive in cucumber. The chilling-sensitive componen t seems to be lost during the isolation of thylakoid membranes. Very w eak light (10-20 mu umol m(-2) s(-1)) was sufficient to cause the inhi bition of PSI. About 80% of the oxygen-evolving activity by PSII was m aintained even after the activity of PSI had decreased by more than 70 %. This is the first report of the selective photoinhibition of PSI in vitro.