Assimilatory power as a driving force in photosynthesis

Abstract

Intact spinach chloroplasts were permitted to photoreduce added 3-phosphoglycerate until oxygen evolution was replaced by oxygen uptake. The chloroplast suspensions were then analyzed for dihydroxyacetone phosphate and residual 3-phosphoglycerate. Ratios of dihydroxyacetone phosphate to phosphoglycerate served to calculate assimilatory power ([ ATP] [ ADP][ P i ]) × ([ NADPH] [ NADP]) . Extrapolation yielded maximum values of assimilatory power P A of about 4000 (M −1) as long as the chloroplasts continued to oxidize dihydroxyacetone phosphate via ribulose bisphosphate oxygenase. When the oxygen concentration was reduced to 25 μM, maximum values of P A approached 25 000 (M −1) at light saturation of phosphoglycerate reduction. Maximum P A declined as chloroplasts aged. When the light intensity was reduced, P A decreased and intact chloroplasts oxidized dihydroxyacetone phosphate which had previously been exported into the medium. This observation explains the transient inhibition of photosynthesis of leaves after a sudden reduction in light intensity. Maximum P A calculated for broken chloroplasts by multiplication of NADPH NADP ratios and maximum phosphorylation potentials [ ATP] [ ADP][ P i] measured separately in thylakoid suspensions at light saturation was as high as 2.5·10 6 ( M −1). The discrepancy between P A in the stroma of intact chloroplasts and the values calculated for thylakoid suspensions is explained by inefficient cyclic electron flow which is incapable of raising phosphorylation potentials to high levels when NADP is reduced in chloroplasts. In leaves, maximum P A in chloroplasts was even lower than in isolated chloroplasts. Turnover of ATP and NADPH in situ prevents P A from reaching high levels even when net assimilation is zero. P A was higher in leaves at low light intensities when carbon reduction was slow than at high light intensities when it was fast. This can explain the Kok effect. The apparent paradox that photosynthetic flux is increased as the driving force P A is decreased is explained by regulation of enzymes of the Calvin cycle. Maximum rates of electron flow and phosphorylation and therefore also of photosynthesis are possible only when levels of P A are kept low. Rapid use of P A requires high activities of the enzymes of the Calvin cycle and may explain the necessity of enzyme activation.