Correlation of Stomatal Conductance with Photosynthetic Capacity of Cotton Only in a CO2-Enriched Atmosphere: Mediation by Abscisic Acid?

Some evidence indicates that photosynthetic rate (A) and stomatal conductance (g) of leaves are correlated across diverse environments. The correlation between A and g has led to the postulation of a "messenger" from the mesophyll that directs stomatal behavior. Because A is a function of intercellular CO(2) concentration (c(i)), which is in turn a function of g, such a correlation may be partially mediated by c(i) if g is to some degree an independent variable. Among individual sunlit leaves in a cotton (Gossypium hirsutum L.) canopy in the field, A was significantly correlated with g (r(2) = 0.41, n = 63). The relative photosynthetic capacity of each leaf was calculated as a measure of mesophyll properties independent of c(i). This approach revealed that, in the absence of c(i) effects, mesophyll photosynthetic capacity was unrelated to g (r(2) = 0.06). When plants were grown in an atmosphere enriched to about 650 microliters per liter of CO(2), however, photosynthetic capacity remained strongly correlated with g even though the procedure discounted any effect of variable c(i). This "residual" correlation implies the existence of a messenger in CO(2)-enriched plants. Enriched CO(2) also greatly increased stomatal response to abscisic acid (ABA) injected into intact leaves. The data provide no evidence for a messenger to coordinate g with A at ambient levels of CO(2). In a CO(2)-enriched atmosphere, though, ABA may function as such a messenger because the sensitivity of the system to ABA is enhanced.