Regulation of photosynthetic carbon assimilation in spinach leaves after a decrease in irradiance

The aim of this work was to investigate the factors which regulate photosynthetic carbon assimilation in leaves following a large step decrease in irradiance. Metabolites were measured in intact leaves and in chloroplasts isolated by non-aqueous means from leaves. The following observations were made. (i) After a decrease in irradiance, the [ATP] [ADP] and [NADPH] [NADP⁺] ratios decreased immediately due to a rapid decrease in the rate of electron transport. The assimilatory power, ATP·NADPH ADP·P_i·NADP⁺, decreased almost to dark levels during the transition from high to low irradiance. It then increased slowly to the new steady state. (ii) The decline in the ratios [ATP] [ADP] and [NADPH] [NADP⁺] resulted in the accumulation of a large pool of glycerate-3-P. (iii) The amount of Rbu-1,5-P₂ declined as it was carboxylated to glycerate-3-P and its regeneration was limited by the supply of ATP and NADPH. (iv) The activation state of Rbu-1,5-P₂ carboxylase changed little during the transient, but its activity was strongly inhibited by stromal concentrations of glycerate-3-P. (v) The rate of carbon drainage into starch adjusted slowly to the decrease in irradiance as indicated by a displacement of the reaction between Fru-6-P and Glc-6-P. This delay in adjustment to a lower rate of carbon assimilation may contribute to the initial decrease in sugar phosphates observed after the irradiance was lowered. (vi) Eventually the pools of metabolites assumed their correct proportions and the concentration of Rbu-1,5-P₂ was high enough for the resumption of steady-state photosynthesis at the lower irradiance.