Abstract
Spinach leaves were illuminated at various temperatures or CO 2 concentrations until steady-state photosynthesis could be measured. Subsequently, they were frozen rapidly in liquid nitrogen and freeze-dried. From the dry material, chloroplasts were isolated in a mixture of organic solvents in which polar metabolites are insoluble. Metabolite levels were determined in the chloroplast fraction. From measured levels of dihydroxyacetone phosphate, fructose 6-phosphate (Fru-6-P), ribulose 1,5-bisphosphate (Rbu-1,5-P 2), ATP and ADP, mass-action ratios of the reaction dihydroxyacetone phosphate + 2 glyceraldehyde 3-phosphate + 3 ATP + Fru-6-P → 3 Rbu-1,5-P 2 + 3 ADP + P i were computed. They increased at constant light intensity with increasing CO 2 concentration or increasing temperature as photosynthetic flux increased. Surprisingly, however, mass action ratios decreased as flux increased with increasing light intensities. Moreover, mass-action ratios were linearly correlated to light-limitation coefficients which were obtained by computing the light limitation of photosynthesis from the slopes of light and CO 2 response curves and multiplying obtained values with that increment of photosynthesis which was measured on increasing the light intensity to saturation. The results are interpreted to indicate tight enzymic control of the formation of ribulose bisphosphate by light. As light intensities are increased, light-regulated enzymes are activated to an extent which permits a decrease in the mass action ratios instead of the increase expected to drive increased carbon flux. Since the reactions catalyzed by phosphoglycerate kinase, glyceraldehyde-3-phosphate dehydrogenase and triosephosphate isomerase are close to thermodynamic equilibrium even when photosynthetic fluxes are large, ratios of dihydroxyacetone phosphate to 3-phosphoglycerate indicated the state of chloroplast phosphorylation potentials and the redox state of NADP which together form the assimilatory power [ATP] · [ADP] −1 · [P i] −1 · [NADPH] · [NADP +] −1. Assimilatory power decreased as carbon flux increased with increasing light intensity and increasing CO 2 concentration, but increased as carbon flux increased with increasing temperature. Again this indicates a decrease in the flow resistance of the carbon cycle as light or CO 2 is increased. The decrease in the flow resistance is attributed to enzyme activation when light is increased, or to increased carboxylation when CO 2 is increased.
Published Version
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call