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

The specificity factor of Rubisco (S-f) was estimated in intact leaves from the carboxylation of ribulose-1,5-bisphosphate (RuBP) at various CO2/O-2 ratios. As oxygenation is calculated by the difference of the (CO2)-C-14 uptake by RuBP in the absence and presence of oxygen, it is important to choose the optimum CO2/O-2 ratios. At high CO2 concentration (1,000 cm(3) 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 O-2-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 (CO2)-C-14 and O-2. The specificity factor of Rubisco was calculated from the amount of the tracer taken up under different (CO2)-C-14/O-2 ratios by the exhaustion of the RuBP pool. Application of (CO2)-C-14 allowed us to discriminate between the CO2 uptake and the concurrent respiratory CO2 release which proceeded at the expense of unlabelled intermediates.