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

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In limiting light the activation of ribulose-1,5-bisphosphate (RuP2) 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 RuP2 carboxylase, while RuP2 levels remained unchanged. The degree of activation varied from 20% to 60% of full activation at irradiances of 225-1650 μE/m2·s (photosynthetically active radiation; E = einstein, 1 mol of photons). Between 225 μE/m2·s and darkness, activation approached 50% while RuP2 levels dropped more than 90%. During steady-state photosynthesis, levels of the substrate RuP2 were 250-300 nmol/mg of chlorophyll in the leaves and were similar at all irradiances above 225 μE/m2·s (25% of light saturation). When velocities of the carboxylase in leaf extracts were corrected for CO2 levels estimated to exist within the leaf, they compared favorably with the photosynthetic rates of the intact seedlings. Comparison of CO2 exchange rate, RuP2 level, and activation of the carboxylase indicates that light limitation of photosynthesis can be due to two factors: the availability of RuP2 in dark to dim light and activation of the RuP2 carboxylase in dim light and higher irradiances.

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