Unique responsiveness of angiosperm stomata to elevated CO2 explained by calcium signalling

Angiosperm and conifer tree species respond differently when exposed to elevated CO₂, with angiosperms found to dynamically reduce water loss while conifers appear insensitive. Such distinct responses are likely to affect competition between these tree groups as atmospheric CO₂ concentration rises. Seeking the mechanism behind this globally important phenomenon we targeted the Ca²⁺-dependent signalling pathway, a mediator of stomatal closure in response to elevated CO₂, as a possible explanation for the differentiation of stomatal behaviours. Sampling across the diversity of vascular plants including lycophytes, ferns, gymnosperms and angiosperms we show that only angiosperms possess the stomatal behaviour and prerequisite genetic coding, linked to Ca²⁺-dependent stomatal signalling. We conclude that the evolution of Ca²⁺-dependent stomatal signalling gives angiosperms adaptive benefits in terms of highly efficient water use, but that stomatal sensitivity to high CO₂ may penalise angiosperm productivity relative to other plant groups in the current era of soaring atmospheric CO₂.