Inorganic Carbon Diffusion between C4 Mesophyll and Bundle Sheath Cells

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.