Light limitation of photosynthesis and activation of ribulose bisphosphate carboxylase in wheat seedlings

Abstract

In limiting light the activation of ribulose-1,5-bisphosphate (Ru P 2 ) carboxylase [3-phospho-D-glycerate carboxylyase (dimerizing), EC 4.1.1.39] in leaf extracts of 7- to 8-day-old wheat seedlings changed proportionally with the photosynthetic rate of the intact plants. Higher rates of photosynthesis, induced by increasing irradiances, were accompanied by an increase in activation of the leaf Ru P 2 carboxylase, while Ru P 2 levels remained unchanged. The degree of activation varied from 20% to 60% of full activation at irradiances of 225-1650 μE/m 2 ·s (photosynthetically active radiation; E = einstein, 1 mol of photons). Between 225 μE/m 2 ·s and darkness, activation approached 50% while Ru P 2 levels dropped more than 90%. During steady-state photosynthesis, levels of the substrate Ru P 2 were 250-300 nmol/mg of chlorophyll in the leaves and were similar at all irradiances above 225 μE/m 2 ·s (25% of light saturation). When velocities of the carboxylase in leaf extracts were corrected for CO 2 levels estimated to exist within the leaf, they compared favorably with the photosynthetic rates of the intact seedlings. Comparison of CO 2 exchange rate, Ru P 2 level, and activation of the carboxylase indicates that light limitation of photosynthesis can be due to two factors: the availability of Ru P 2 in dark to dim light and activation of the Ru P 2 carboxylase in dim light and higher irradiances.