Figure 2.

Illustrating two current models for the evolution of stomatal control: the general and passive origin (or gradualistic) models. Both models describe the key elements of the stomatal control system and the evolution of these elements as new plant lineages emerged. The major plant divisions, all with stomata, are arranged phylogenetically with respect to their common ancestor (distances not to scale). The main difference between the two models is that the general model incorporates, in some form, all of the core elements of passive and active stomatal control in each plant division, whereas the passive origin (or gradualistic) model assumes that certain active stomatal control elements, including sensitivity to elevated CO₂ and ABA, evolved first in angiosperms (possibly gymnosperms in the case of ABA sensitivity) and are absent in all other divisions. Colors represent genes, gene families, or gene regulatory networks involved in discrete elements of the stomatal control system and their respective feedback control loops in the regulation of stomatal conductance; different color hues represent homologs or other variants resulting from evolutionary processes: black = active response to light; blue = passive response to water potential; green = active response to water potential (including changes imposed by vapor pressure difference); yellow = active ABA response; and red = active response to elevated CO₂.