Using a laboratory-constructed system that can measure the gas exchange rates of two leaf surfaces separately, the light responses of the adaxial and abaxial stomata in intact leaves of sunflower (Helianthus annuus L.) were investigated, keeping the intercellular CO(2) concentration (C(i)) at 300 microL L(-1). When evenly illuminating both sides of the leaf, the stomatal conductance (g(s)) of the abaxial surface was higher than that of the adaxial surface at any light intensity. When each surface of the leaf was illuminated separately, both the adaxial and abaxial stomata were more sensitive to the light transmitted through the leaf (self-transmitted light) than to direct illumination. Relationships between the whole leaf photosynthetic rate (A(n)) and the g(s) for each side highlighted a strong dependence of stomatal opening on mesophyll photosynthesis. Light transmitted through another leaf was more effective than the direct white light for the abaxial stomata, but not for the adaxial stomata. Moreover, green monochromatic light induced an opening of the abaxial stomata, but not of the adaxial stomata. As the proportion of blue light in the transmitted light is less than that in the white light, there may be some uncharacterized light responses, which are responsible for the opening of the abaxial stomata by the transmitted, green light.