PHYSIOLOGICAL ECOLOGY OF AMPHISTOMATOUS LEAVES.

Abstract

Most plants produce leaves with stomata on either both surfaces (amphistomatous) or on the lower surface only (hypostomatous). The importance of stomata to plant survival suggests that these two stomatal distribution patterns may be adaptive, and this problem is explored. It is concluded that amphistomaty is an adaptation to produce a high conductance to CO₂ diffusion into the leaf. As such it is advantageous to plants with high photosynthetic capacity leaves in high light environments, experiencing rapidly fluctuating or continuously available soil water. Plants meeting these criteria are found to be almost exclusively amphistomatous; those not meeting the criteria are mostly hypostomatous. Also investigated is the adaptive significance of differences in stomatal conductances and conductance responses to environmental factors between the two surfaces of amphistomatous leaves. Although differences in stomatal conductance are found between the two surfaces in sunflower, differences in conductance response to light intensity and water vapor pressure difference across the stomatal pore were neglible. Water stress relieved one day prior to experiments caused upper stomatal conductance to be reduced more than lower, but responses to light and water vapor pressure difference remained essentially parallel for the two surfaces. For these differences in conductance to be adaptive differences in photosynthetic characteristics between the two surfaces. In addition, estimation of the resistance to diffusion of CO₂ across the mesophyll yields values low enough to preclude steep gradients in CO₂ partial pressure in the mesophyll. In the absence of CO₂ gradients within the leaf, differences in photosynthetic characteristics between the two surfaces cannot exist. It is concluded that differences in stomatal conductance between the two surfaces of amphistomatous leaves are not adaptations to differences in CO₂ uptake characteristics.