Differing substomatal and chloroplastic CO2 concentrations in water-stressed wheat.

Gas exchanges of wheat (Triticum aestivum L. cv. Courtot) shoots were measured before and during a water stress. While photosynthesis, transpiration and dark respiration decreased because of the stress, photorespiration increased initially, up to a maximum of 50% above its initial value. The CO2 concentration in the intercellular space was calculated from gas-diffusion resistances, and remained approximately constant before and during the stress. On the other hand, the CO2 concentration in the chloroplast, in the vicinity of Ribulose-1,5-biphosphate carboxylase/oxygenase (Rubisco), was evaluated from the ratio of CO2 to O2 uptake, using the known kinetic constants of the oxygenation and carboxylation reactions which compete for Rubisco. In the well-watered plants, the calculated chloroplastic concentration was slightly smaller than the substomatal concentration. During water stress, this concentration decreased while the substomatal CO2 concentration remained constant. Hypotheses to explain this difference between substomatal and chloroplastic CO2 concentrations are discussed.