Photosynthesis and Ribulose 1,5-Bisphosphate Carboxylase in Rice Leaves

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Abstract
Changes in the photosynthesis and the ribulose, 1,5-bisphosphate (RuBP) carboxylase level were examined in the 12th leaf blades of rice (O. sativa L.) grown under different N levels. Photosynthesis was determined using an open IR gas analysis system. The level of RuBP carboxylase was measured by rocket immunoelectrophoresis. These changes were followed with respect to changes in the activities of RuBP carboxylase, ribulose 5-phosphate kinase, NADP-glyceraldehyde 3-phosphate dehydrogenase and 3-phosphoglyceric acid kinase. RuBP carboxylase activity was highly correlated with the net rate of photosynthesis (r = 0.968). Although high correlations between the activities of other enzymes and photosynthesis were also found, the activity/leaf of RuBP carboxylase was much lower than those of other enzymes throughout the leaf life. The specific activity of RuBP carboxylase on 1 mg of the enzyme protein basis remained fairly constant (1.16 .+-. 0.07 .mu.mol of CO2/min per mg at 25.degree. C) throughout the experimental period. Kinetic parameters related to CO2 fixation were examined using the purified carboxylase. The Km (CO2) and Vmax values were 12 .mu.mol and 1.45 .mu.mol of CO2/min per mg, respectively, (pH 8.2 and 25.degree. C). The in vitro specific activity calculated at the atmospheric CO2 level from the parameters was comparable to the in situ true photosynthetic rate/mg of the carboxylase throughout the leaf life. The results indicated that the level of RuBP carboxylase protein can be a limiting factor in photosynthesis throughout the life span of the leaf.