No evidence of general CO2 insensitivity in ferns: one stomatal control mechanism for all land plants?

Stomatal regulation of plant carbon uptake and water loss under changing environmental conditions was a crucial evolutionary step in the colonization of land by plants. There are currently two conflicting models describing the nature of stomatal regulation across terrestrial vascular plants: the first is characterized by a fundamental mechanistic similarity across all lineages, and the second is characterized by the evolution of major differences in angiosperms compared with more ancient lineages. Specifically, the second model posits that stomata of ferns lack a response to elevated atmospheric CO2 concentration (c(a)) and therefore cannot regulate leaf intercellular CO2 concentration (c(i)). We compared stomatal sensitivity to changes in c(a) in three distantly related fern species and a representative angiosperm species. Fern and angiosperm stomata responded strongly and similarly to changes in c(a). As a result, c(i)/c(a) was maintained within narrow limits during c(a) changes. Our results challenge the model in which stomata of ferns generally lack a response to elevated c(a) and that angiosperms evolved new dynamic mechanisms for regulating leaf gas exchange that differ fundamentally from ferns. Instead, the results are consistent with a universal stomatal control mechanism that is fundamentally conserved across ferns and angiosperms, and therefore likely all vascular plant divisions.