Carbon dioxide assimilation by leaves, isolated chloroplasts, and ribulose bisphosphate carboxylase from spinach

• Published in: Plant physiology (Bethesda) 1975-06, Vol.55 (6), p.1087-1092
• Main Authors: Lilley, R.M, Walker, D.A
• Format: Article
• Language: English
• Subjects: Bicarbonates; Carbon dioxide; Carboxylation; Chloroplasts; Diphosphates; Enzymes; Photorespiration; Photosynthesis; Plants; Spinach
• ISSN: 0032-0889; 1532-2548
• DOI: 10.1104/pp.55.6.1087

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 Vmax, 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.