Plant Physiology 91:1356-1363 (1989)
© 1989 American Society of Plant Biologists
Metabolism and Enzymology
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
Colin L. D. Jenkins,
Robert T. Furbank1 and
Marshall D. Hatch
Division of Plant Industry, CSIRO, GPO Box 1600, Canberra ACT 2601, Australia
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.
1 Supported by a QEII Fellowship.
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