RESPONSES OF ADAXIAL AND ABAXIAL STOMATA TO LIGHT AND WATER DEFICITS IN SUNFLOWER AND SORGHUM

The responses of adaxial and abaxial stomata to light and water deficits were followed in normally‐oriented and inverted leaves of sunflower (Helianthus annuus L. cv. Hysun 30) and sorghum (Sorghum bicolor (L.) Moench cv. RS 610) In both species, but especially in sunflower, the adaxial and abaxial stomata closed at the same leaf water potential. However, as a water deficit was imposed, the stomata facing away from the light source, i.e. abaxial stomata in the normal leaf and the adaxial stomata in the inverted leaf, closed more rapidly than the stomata facing the light source. In both sorghum and sunflower the resistance of the adaxial epidermis was higher than that of that abaxial at the same photon flux densities; in sunflower, but not in sorghum this was correlated with the stomatal frequency. The sorghum stomata acclimated to High light, i.e. the adaxial stomata in a normal leaf and the abaxial stomata in an inverted leaf had high resistances on inversion, but the resistance decreased slowly over a per tad of days. Acclimation was not the sole factor involved in the difference in response to light of adaxial and abaxial sorghum stomata. Short‐term leaf inversion reduced the rate of net photosynthesis by 20 to 30% at all photon flux densities. We conclude that adaxial stomata in sorghum, hut not in sunflower, differ inherently from stomata in their response to light. However, in both species there was no inherent between adaxial and abaxial stomata to rapidly‐imposed water deficits.