Inorganic Carbon Diffusion between C₄ Mesophyll and Bundle Sheath Cells: Direct Bundle Sheath CO₂ Assimilation in Intact Leaves in the Presence of an Inhibitor of the C₄ Pathway

Photosynthesis rates of detached Panicum miliaceum leaves were measured, by either CO2 assimilation or oxygen evolution, over a wide range of CO2 concentrations before and after supplying the phosphoenolpyruvate (PEP) carboxylase inhibitor, 3,3-dichloro-2-(dihydroxyphosphinoyl-methyl)-propenoate (DCDP). At a concentration of CO2 near ambient, net photosynthesis was completely inhibited by DCDP, but could be largely restored by elevating the CO2 concentration to about 0.8% (v/v) and above. Inhibition of isolated PEP carboxylase by DCDP was not competitive with respect to HCO3 -, indicating that the recovery was not due to reversal of enzyme inhibition. The kinetics of 14C-incorporation from 14CO2 into early labeled products indicated that photosynthesis in DCDP-treated P. miliaceum leaves at 1% (v/v) CO2 occurs predominantly by direct CO2 fixation by ribulose 1,5-bisphosphate carboxylase. From the photosynthesis rates of DCDP-treated leaves at elevated CO2 concentrations, permeability coefficients for CO2 flux into bundle sheath cells were determined for a range of C4 species. These values (6-21 micromoles per minute per milligram chlorophyll per millimolar, or 0.0016-0.0056 centimeter per second) were found to be about 100-fold lower than published values for mesophyll cells of C3 plants. These results support the concept that a CO2 permeability barrier exists to allow the development of high CO2 concentrations in bundle sheath cells during C4 photosynthesis.