DETERMINING PHOTOSYNTHETIC PARAMETERS FROM LEAF CO2 EXCHANGE AND CHLOROPHYLL FLUORESCENCE - RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASESPECIFICITY FACTOR, DARK RESPIRATION IN THE LIGHT, EXCITATION DISTRIBUTION BETWEEN PHOTOSYSTEMS, ALTERNATIVE ELECTRON-TRANSPORT RATE, AND MESOPHYLL DIFFUSION RESISTANCE

Using simultaneous measurements of leaf gas exchange and chlorophyll f luorescence, we determined the excitation partitioning to photosystem II (PSII), the CO2/O-2, specificity of ribulose-1,5-bisphosphate carbo xylase/oxygenase, the dark respiration in the light, and the alternati ve electron transport rate to accepters other than bisphosphoglycerate , and the transport resistance for CO2 in the mesophyll cells for indi vidual leaves of herbaceous and tree species. The specificity of ribul ose-1,5-bisphosphate carboxylase/oxygenase for CO2 was determined from the slope of the O-2 dependence of the CO2 compensation point between 1.5 and 21% O-2. Its value, on the basis of dissolved CO2 and O-2 con centrations at 25.5 degrees C, varied between 86 and 89. Dark respirat ion in the light, estimated from the difference between the CO2 compen sation point and the CO2 photocompensation point, was about 20 to 50% of the respiration rate in the dark. The excitation distribution to PS II was estimated from the extrapolation of the dependence of the PSII quantum yield on F/F-m to F = 0, where F is steady-state and F-m is pu lse-saturated fluorescence, and varied between 0.45 and 0.6. The alter native electron transport rate was found as the difference between the electron transport rates calculated from fluorescence and from gas ex change, and at low CO2 concentrations and 10 to 21% O-2, it was 25 to 30% of the maximum electron transport. The calculated mesophyll diffus ion resistance accounted for about 20 to 30% of the total mesophyll re sistance, which also includes carboxylation resistance. Whole-leaf pho tosynthesis is limited by gas phase, mesophyll diffusion, and carboxyl ation resistances in nearly the same proportion in both herbaceous spe cies and trees.