In photoinhibited photosystem II particles pheophytin photoreduction remains unimpaired.

Oxygen-evolving photosystem II particles (DT 20) isolated from pea chloroplasts by digitonin-Triton X-100 fractionation were photoinhibited with 150 W. m(-2) white light, at 20°C under three conditions: aerobic, anaerobic and strongly reducing (E(h) poised to approx. -250 mV with dithionite). Hill reaction rate (H (20) + BQ)and variable fluorescence (Fv) declined in parallel in all three cases with shortening half times: 30, 10 and 2.5 min, respectively. Light-induced absorbance changes at 685 nm characteristic of reversible photo accumulation of reduced pheophytin (& z -2.50 mV) remained essentially unchanged. We conclude that the three types of photoinhibitory treatment do not impair the separation of charges between chlorophyll P-680 and pheophytin in the photosystem II reaction center.