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Carbon Dioxide Assimilation by Leaves, Isolated Chloroplasts, and Ribulose Bisphosphate Carboxylase from Spinach ¹

^a The University of Sheffield, Department of Botany, Sheffield S10 2TN, United Kingdom

The relationship between rate of photosynthesis and CO₂ concentration has been reinvestigated using isolated spinach (Spinacia oleracea) chloroplasts. The apparently low CO₂ concentration required for half-maximal photosynthesis is shown to result partly from a ceiling imposed by electron transport. In double reciprocal plots of rate against CO₂ concentration, this ceiling results in departures from linearity at high CO₂ concentrations. If these rate limitations are disregarded in extrapolation the "true" CO₂ concentration required for half maximal carboxylation by intact chloroplasts is approximately 46 µM (CO₂).

When assayed under comparable conditions, ribulose bisphosphate carboxylase from these chloroplasts also shows an apparent Km (CO₂) of approximately 46 µM, suggesting that its characteristics are not modified by extraction. An improved assay for ribulose bisphosphate carboxylase yielded rates of carboxylation considerably higher than those previously reported, the highest maximal velocities recorded approaching 1000 µmoles CO₂ fixed mg^–1 chlorophyll hr^–1 at 20 C. With such Km and V_max, values the carboxylase would be able to achieve, at concentrations of CO₂ less than atmospheric, rates of CO₂ fixation equal to those displayed by the parent tissue or by the average plant under favorable conditions in its natural environment.

¹ This work was supported by the Science Research Council and the Agricultural Research Council, U.K.