Stomatal Behavior and CO(2) Exchange Characteristics in Amphistomatous Leaves

The possibility that differences in stomatal conductance between upper and lower surfaces of amphistomatous leaves are adaptations to differences in CO(2) exchange characteristics for the two surfaces was investigated. The ratio of upper to lower stomatal conductance was found to change little in response to light and humidity for well-watered sunflower (Helianthus annuus L.) plants. Stressing the plants (psi = -17 bars) and rewatering 1 day before gas exchange measurements reduced upper conductance more severely than lower in both indoor- and outdoor-grown plants, and caused small changes in conductance ratio with light and humidity. A similar pattern was found using outdoor grown sunflower and cocklebur (Xanthium strumarium L.) plants. Calculated intercellular CO(2) concentrations for upper and lower surfaces were always close to identical for a particular set of environmental conditions for both sunflower and cocklebur, indicating that no differences in CO(2) exchange characteristics exist between the two surfaces. By artificially creating a CO(2) gradient across the leaf, the resistance to CO(2) diffusion through the mesophyll was estimated and found to be so low that despite possible nonhomogeneity of the mesophyll, differences in CO(2) exchange characteristics for the two surfaces are unlikely. It is concluded that differences in conductance between upper and lower stomates are not adaptations to differences in CO(2) exchange characteristics.