PARTITIONING OF THE LEAF CO2 EXCHANGE INTO COMPONENTS USING CO2 EXCHANGE AND FLUORESCENCE MEASUREMENTS

Photorespiration was calculated from chlorophyll fluorescence and ribu lose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) kinetics and com pared with CO2 evolution rate in the light, measured by three gas-exch ange methods in mature sunflower (Helianthus annuus L.) leaves. The ga s-exchange methods were (a) postillumination CO2 burst at unchanged CO 2 concentration, (b) postillumination CO2 burst with simultaneous tran sfer into CO2-free air, and (c) extrapolation of the CO2 uptake to zer o CO2 concentration at Rubisco active sites. The steady-state CO2 comp ensation point was proportional to O-2 concentration, revealing the Ru bisco specificity coefficient (K-sp) of 86. Electron transport rate (E TR) was calculated from fluorescence, and photorespiration rate was ca lculated from ETR using CO2 and O-2 concentrations, K-sp, and diffusio n resistances. The values of the best-fit mesophyll diffusion resistan ce for CO2 ranged between 0.3 and 0.8 s cm(-1). Comparison of the gas- exchange and fluorescence data showed that only ribulose-1,5-bisphosph ate (RuBP) carboxylation and photorespiratory CO2 evolution were prese nt at limiting CO2 concentrations. Carboxylation of a substrate other than RuBP, in addition to RuBP carboxylation, was detected at high CO2 concentrations. A simultaneous decarboxylation process not related to RuBP oxygenation was also detected at high CO2 concentrations in the light. We propose that these processes reflect carboxylation of phosph oenolpyruvate, formed from phosphoglyceric acid and the subsequent dec arboxylation of malate.