Stomatal Distribution, Density and Conductance of Three Perennial Grasses Native to the Southern True Prairie of Texas

Abstract

Stomatal distribution within three perennial grasses native to the southern true prairie of Texas, Schizachyrium scoparium (C4), Bothriochloa saccharoides (C4) and Stipa leucotricha (C3), was highly variable among species and between leaf surfaces. The greater proportion of stomata occurred on the abaxial (lower) surface of the two C4 species (99 and 95%), but on the adaxial (upper) surface of the C3 species (83%). A 1800 twist at the base of Stipa leaf blades effectively oriented the leaf surface with the greatest proportion of stomata in a position comparable to the abaxial surfaces of the C4 species. Leaf involution in response to water stress protected leaf surfaces with the greater proportion of stomata in only the C3 species. The leaf surface with greater stomatal density in each species approached or exceeded maximum densities reported for entire leaves (both surfaces) of cereals and forage grasses, while mean stomatal sizes approached the lower end of the reported range. A significant positive relationship existed between stomatal density and conductance, confirming the functional significance of stomatal distribution to gas exchange in these species. The tendency toward functional hypostomaty (gas exchange from only the abaxial surface) in these perennial grasses suggests that efficient water use functions as a stronger selective agent than maximum CO2 conductance in this environment.