An evolutionary perspective on the interaction of the parallel pathways responsible for the CO₂ sensitivity of angiosperm stomata.

<p>Stomatal conductances of all species were responsive to CO₂ in the light, but the nature of these responses differed markedly in the angiosperms (n=11, ±SE) compared with the non-angiosperms (gymnosperms, ferns and lycophytes; n=22, ±SE). Values for stomatal conductance in the light at ambient CO₂ concentration for each species are presented in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0082057#pone-0082057-g003" target="_blank">Figure 3C</a>. All species appear to possess the photosynthesis-dependent pathway (A and B, green icons), responsible for stomatal responses to changes in CO₂ below current ambient atmospheric concentrations. In angiosperms only, stomatal responses to CO₂ are driven by dual signals, coming from both photosynthetic signalling and the photosynthesis-independent Ca²⁺-signalling pathway (C and D, blue icons). The Ca²⁺-dependent pathway appears to be active across a range of CO₂ concentrations above and below current atmospheric levels, leading to stomatal responses at CO₂ concentrations above ambient levels (<b>C</b> and <b>D</b>), as well as conferring enhanced rates of stomatal response to CO₂ (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0082057#pone-0082057-g006" target="_blank">Figure 6C</a>).