Specificity factor of Rubisco: estimation in intact leaves by carboxylation at different CO2/O2 ratios

摘要

The specificity factor of Rubisco (S f) was estimated in intact leaves from the carboxylation of ribulose-1,5-bisphosphate (RuBP) at various CO2/O2 ratios. As oxygenation is calculated by the difference of the 14CO2 uptake by RuBP in the absence and presence of oxygen, it is important to choose the optimum CO2/O2 ratios. At high CO2 concentration (1,000 cm3 m−3 and higher) oxygenation consumes less than 50% RuBP but the difference of concentrations of CO2 at cell walls (C w) and at the carboxylation centers (C c) is 2–5% and the influence of mesophyll resistance (r md) is of minor importance. To accumulate large endogenous pool of RuBP, the leaves were preilluminated in the CO2- and O2-free gas environments for 8 to 10 s. Thereafter the light was switched off and the leaves were flushed with the gas containing different concentrations of 14CO2 and O2. The specificity factor of Rubisco was calculated from the amount of the tracer taken up under different 14CO2/O2 ratios by the exhaustion of the RuBP pool. Application of 14CO2 allowed us to discriminate between the CO2 uptake and the concurrent respiratory CO2 release which proceeded at the expense of unlabelled intermediates.