[Different CO2-sensitivities of the gas exchange of the two leaf surfaces of Zea mays].

CO2 exchange and transpiration of the two leaf surfaces of Zea mays were measured at various external CO2 concentrations between 50 and 10000 ppm (V/V). The CO2 exchange of the upper surface followed an "optimum curve" with a maximum near 300 ppm, whereas the CO2 exchange of the lower epidermis approximated a "saturation curve" from 100 to 3000 ppm. The transpiration data of both leaf surfaces at three CO2 concentrations indicated that these results were due to stomatal interference with the saturation curve of photosynthesis. The results show, again that the guard cells of the two leaf surfaces can react differently. Data obtained with viscous flow porometers or with diffusion porometers employing an unidirectional diffusion of a gas, through an amphistomatic leaf should therefore be interpreted with great caution.