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.