Optimization theory of stomatal behaviour: II. Stomatal responses of several tree species of north Australia to changes in light, soil and atmospheric water content and temperature

Abstract

In a companion paper several methods of calculating the marginal unit water cost of plant carbon gain (∂E/∂A) were tested to determine whether stomata were behaving optimally in relation to regulating leaf gas exchange. In this paper one method is applied to several tropical tree species when leaf-to-air vapour pressure difference (D), photosynthetic photon flux density, leaf temperature, and atmospheric soil water availability were manipulated. The response of leaves that had expanded during the dry season were also compared to that of leaves that had expanded in the wet season. Few differences in absolute value of ∂E/∂A, or the form of the relationship, were observed between species or between seasons. In the majority of species, ∂E/∂A increased significantly as either leaf-to-air vapour pressure difference increased, at a leaf temperature of either 33°C or 38°C, or as photosynthetic photon flux density increased. In contrast, as leaf temperature increased at constant D, ∂E/∂A was generally constant. As pre-dawn water potential declined, ∂E/∂A declined, indicating increased efficiency of stomatal behaviour. The relationship between ∂E/∂A and D did not differ whether internal or ambient carbon dioxide concentration were kept constant. It is concluded that stomata are only behaving optimally over a very small range of D. If a larger range of D is used, to incorporate values that more closely reflect those experienced by tropical trees in a savanna environment, optimization is incomplete.