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/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?C5% 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.