Changes in the rates of gas exchange and the amount of ribulose 1,5-bisphosphate (RuBP) carboxylase protein were determined in the 12th leaf blades of rice during the reproductive stages. RuBP carboxylase exhibited a large change similar to that in the assimilation rate at 2% O2 throughout the leaf's life, but its decrease during senescence was barely faster than the decrease in the assimilation rate. Consequently, the overall relationship was slightly curvilinear. By contrast, leaf conductance decreased more slowly than the assimilation rate which resulted in the intercellular CO2 concentration increasing during senescence. In order to determine the maximum activity of RuBP carboxylase at the intercellular CO2 concentration, the kinetic parameters and their pH response were determined using purified, and completely activated, rice RuBP carboxylase. The maximum carboxylase activity at the intercellular CO2 concentration was linearly correlated with the assimilation rate at 2% O2 (r=0.989), and was very close to that needed to account for the assimilation rate. We conclude that changes in both the amount of RuBP carboxylase protein and leaf conductance reflect the change in photosynthesis during the life span of the leaf.
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Makino et al. (1984) studied this question.