Effects of Photoinhibition on Photosynthetic Carbon Metabolism in Intact Isolated Spinach Chloroplasts.

Abstract

Pools of intermediates of the Calvin cycle were measured during photosynthetic ¹⁴CO₂ fixation by intact isolated spinach chloroplasts. Photoinhibitibn (illumination for 8 min with 4000 µmol m^-2 s^-1 light in the absence of bicarbonate) decreased the subsequently measured rate of CO₂ fixation. Individual compounds were differently affected: the ribulose-1,5-bisphosphate (RuBP) pool was drastically lowered, while that of fructose-1,6-bisphosphate (FBP) was increased, suggesting that photoinhibition causes a limitation in RuBP regeneration. An increase in FBP and decrease in RuBP were not observed during photosynthesis in low light at rates of CO₂ fixation comparable to those in photoinhibited chloroplasts. This indicates that changes of the metabolite pools induced by photoinhibition were not due solely to decreased rates of electron transport. Activities of RuBP carboxylase and fructose-1,6-bisphosphatase (FBPase) were decreased by the photoinhibitory treatment. However, the activity of both enzymes in photoinhibited chloroplasts was still well in excess of that required to sustain the measured rates of carbon flux. Photoinhibition largely abolished the light-induced proton gradient across the chloroplast envelope. The concomitant acidification of the chloroplast stroma could inhibit FBPase activity. It is concluded that photoinhibition does not result in irreversible modification of the FBPase protein but that its activity may be decreased by changes in pH and possibly other factors in the chloroplast stroma.