Photosynthetic pathway and ecological adaptation explain stomatal trait diversity amongst grasses

The evolution of C-4 photosynthesis in plants has allowed the maintenance of high CO2 assimilation rates despite lower stomatal conductances. This underpins the greater water-use efficiency in C-4 species and their tendency to occupy drier, more seasonal environments than their C-3 relatives. The basis of interspecific variation in maximum stomatal conductance to water (g(max)), as defined by stomatal density and size, was investigated in a common-environment screening experiment. Stomatal traits were measured in 28 species from seven grass lineages, and comparative methods were used to test for predicted effects of C-3 and C-4 photosynthesis, annual precipitation and habitat wetness on (max). Novel results were as follows: significant phylogenetic patterns exist in g(max) and its determinants, stomatal size and stomatal density; C-4 species consistently have lower gmax than their C-3 relatives, associated with a shift towards smaller stomata at a given density. A direct relationship between g(max) and precipitation was not supported. However, we confirmed associations between C-4 photosynthesis and lower precipitation, and showed steeper stomatal sizedensity relationships and higher g(max) in wetter habitats. The observed relationships between stomatal patterning, photosynthetic pathway and habitat provide a clear example of the interplay between anatomical traits, physiological innovation and ecological adaptation in plants.