ROLE OF CHLOROPLAST RESPIRATION IN THE INACTIVATION OF PHOTOSYSTEM-IIIN CHLORELLA

Nitrogen deficiency, dark incubation on glucose, and dark incubation a t an elevated temperature (41 degrees C) were previously shown to inac tivate photosystem II (PS LI) in Chlorella pyrenoidosa Chick, strain C ALU-175. These treatments also increased chloroplast respiration by 7- 11 times, At the same time, any attempt to inhibit the accumulation of substrates for chloroplast respiration (CO2 deprivation during nitrog en starvation, inhibition of glucose metabolism by a nonmetabolizable analog of glucose, 2-deoxy-D-glucose, or inhibition of protein synthes is by cycloheximide during dark incubation on glucose or by heat shock ) prevented the stimulation of chloroplast respiration and PS II inact ivation, Inhibition of the oxygen-dependent oxidation of the plastoqui none pool under anaerobic conditions or in the presence of salicylhydr oxamate, an inhibitor of chloroplast oxidases, markedly increased the extent and rate of PS II inactivation in cells subjected to heat shock . The dependencies of chloroplast respiration and the PS II inactivati on rate on the hear-shock temperature exactly matched one another. Diu ron, an inhibitor of photosynthetic electron transport between the pri mary and secondary quinone electron accepters, did not affect the rate of chloroplast respiration, but prevented PS II inactivation. We prop ose that the inactivation of PS II caused by these treatments is due t o the loss of the primary quinone electron acceptor as a consequence o f its two-electron reduction from the plastoquinone reduced by the ele ctron flow from the substrates of chloroplast respiration.