Stomatal traits and the control of gas exchange

Stomata are found on the aerial surfaces of land plants. These pores, which are composed typically of a pair of guard cells, are responsible for facilitating gaseous exchange between plants and their environment. Changes in guard cell turgor pressure causes pores to open and close to tightly regulate gas exchange. The stomata of a wide range of species are known to respond to light, but the speed and magnitude of stomatal responses has received less attention. The magnitude and speed of stomatal responses was measured, using infra-red gas exchange analysis, across species from broad evolutionary lineages with a range of stomatal sizes, densities, geometries and distributions across both leaf surfaces. This revealed that guard cell geometry is a primary determinant of stomatal response speed and magnitude with dumbbell-shaped guard cells conveying an advantage. Interestingly, no relationship between stomatal size and speed was found, indicating that this may not be an important determinant of speed when looking across distantly related species. It has been suggested that more rapid stomatal responses provide an advantage in a dynamic light environment by enabling plants to capture more carbon dioxide and minimise water loss. To investigate this further, plants were grown under constant daytime light or dynamic light conditions. Under well-watered conditions there was no difference in plant growth or water use between light conditions. Under drought conditions, plants grown under fluctuating light accumulated significantly less biomass and were less water-use efficient. Nonetheless, there was no significant advantage to having more rapid stomatal responses. Stomatal opening and closure is fundamentally a mechanical process. Therefore, guard cell composition was investigated for species with kidney shaped guard cells. This revealed a large diversity in guard cell wall components across species from different evolutionary lineages and showed that arabinans contribute to the magnitude of stomatal response in soybean.