Photosynthesis and Activation of Ribulose Bisphosphate Carboxylase in Wheat Seedlings

Photosynthetic carbon assimilation in plants is regulated by activity of the ribulose 1,5-bisphosphate (RuBP) carboxylase/oxygenase. Although the carboxylase requires CO(2) to activate the enzyme, changes in CO(2) between 100 and 1,400 microliters per liter did not cause changes in activation of the leaf carboxylase in light. With these CO(2) levels and 21% O(2) or 1% or less O(2), the levels of ribulose bisphosphate were high and not limiting for CO(2) fixation. With high leaf ribulose bisphosphate, the K(act)(CO(2)) of the carboxylase must be lower than in dark, where RuBP is quite low in leaves. When leaves were illuminated in the absence of CO(2) and O(2), activation of the carboxylase dropped to zero while RuBP levels approached the binding site concentration of the carboxylase, probably by forming the inactive enzyme-RuBP complex.The mechanism for changing activation of the RuBP carboxylase in the light involves not only Mg(2+) and pH changes in the chloroplast stroma, but also the effects of binding RuBP to the enzyme. In light when RuBP is greater than the binding site concentration of the carboxylase, Mg(2+) and pH most likely determine the ratio of inactive enzyme-RuBP to active enzyme-CO(2)-Mg(2+)-RuBP forms. Higher irradiances favor more optimal Mg(2+) and pH, with greater activation of the carboxylase and increased photosynthesis.