Inorganic Carbon Diffusion between C(4) Mesophyll and Bundle Sheath Cells: Direct Bundle Sheath CO(2) Assimilation in Intact Leaves in the Presence of an Inhibitor of the C(4) Pathway.

Photosynthesis rates of detached Panicum miliaceum leaves were measured, by either CO(2) assimilation or oxygen evolution, over a wide range of CO(2) concentrations before and after supplying the phosphoenolpyruvate (PEP) carboxylase inhibitor, 3,3-dichloro-2-(dihydroxyphosphinoyl-methyl)-propenoate (DCDP). At a concentration of CO(2) near ambient, net photosynthesis was completely inhibited by DCDP, but could be largely restored by elevating the CO(2) concentration to about 0.8% (v/v) and above. Inhibition of isolated PEP carboxylase by DCDP was not competitive with respect to HCO(3) (-), indicating that the recovery was not due to reversal of enzyme inhibition. The kinetics of (14)C-incorporation from (14)CO(2) into early labeled products indicated that photosynthesis in DCDP-treated P. miliaceum leaves at 1% (v/v) CO(2) occurs predominantly by direct CO(2) fixation by ribulose 1,5-bisphosphate carboxylase. From the photosynthesis rates of DCDP-treated leaves at elevated CO(2) concentrations, permeability coefficients for CO(2) flux into bundle sheath cells were determined for a range of C(4) 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 C(3) plants. These results support the concept that a CO(2) permeability barrier exists to allow the development of high CO(2) concentrations in bundle sheath cells during C(4) photosynthesis.