Factors affecting photosynthetic induction in spinach leaves

Induction of photosynthetic carbon assimilation in spinach leaves in saturating CO₂ has been studied in relation to such factors as the length of the preceding dark period, intensity of pre-illumination, irradiance and the phosphate status of the leaf, through measurements of O₂ evolution and of changes in leaf metabolites. (i) In low light, induction comprised two phases, an oxygen burst and a slow rise to the steady-state rate. Both the oxygen burst and induction (measured as induction gain or loss) correlated with the amount of glycerate-3-P present in the leaf prior to illumination. (ii) The induction period in dark-adapted leaves was dependent on the irradiance, being greater at higher irradiance. In the early stages of induction at different irradiances metabolite levels rather than the rate of electron transport appeared to limit the rate of photosynthesis. (iii) The induction period was extended in leaves from plants grown in low phosphate or where phosphate was sequestered by the use of 2-deoxyglucose while the induction period was shortened by feeding phosphate to phosphate-deficient leaves. Phosphate deficiency was characterized by low glycerate-3-P in the dark and a slow build-up to lower steady-state levels of metabolites in the light, with the absence of characteristic peaks in the contents of glycerate-3-P, Fru-1,6-P₂ and the ATP ADP ratio during induction. These observations serve to emphasise that the conversion of existing metabolite pools and metabolite build-up are important determinants of the induction period in leaves.