Coordination of stomatal physiological behavior and morphology with carbon dioxide determines stomatal control.

Stomatal control is determined by the ability to alter stomatal aperture and/or the number of stomata on the surface of new leaves in response to growth conditions. The development of stomatal control mechanisms to the concentration of CO₂within the atmosphere ([CO₂]) is fundamental to our understanding of plant evolutionary history and the prediction of gas exchange responses to future [CO₂]. In a controlled environment, fern and angiosperm species were grown in atmospheres of ambient (400 ppm) and elevated (2000 ppm) [CO₂]. Physiological stomatal behavior was compared with the stomatal morphological response to [CO₂]. An increase in [CO₂] or darkness induced physiological stomatal responses ranging from reductions (active) to no change (passive) in stomatal conductance. Those species with passive stomatal behavior exhibited pronounced reductions of stomatal density in new foliage when grown in elevated [CO₂], whereas species with active stomata showed little morphological response to [CO₂]. Analysis of the physiological and morphological stomatal responses of a wider range of species suggests that patterns of stomatal control to [CO₂] do not follow a phylogenetic pattern associated with plant evolution. Selective pressures may have driven the development of divergent stomatal control strategies to increased [CO₂]. Those species that are able to actively regulate guard cell turgor are more likely to respond to [CO₂] through a change in stomatal aperture than stomatal number. We propose a model of stomatal control strategies in response to [CO₂] characterized by a trade-off between short-term physiological behavior and longer-term morphological response.