The transition from passive to active metabolic control of stomata and plant water balance occurred about 360 million years ago.

Significance: A major determinant of plant species distribution on Earth is a specific tolerance to soil drying, yet there are currently no functional or anatomical characteristics that can predict species’ requirement for rainfall. This study examines the systems responsible for controlling water delivery and water loss in the leaves of conifers and finds functional evidence of how conifers have evolved in drying climates over the course of the last 150 million years. Two “strategies” for conserving water during water stress emerged. One group relied on the plant hormone abscisic acid to maintain stomata closed during sustained drought, and another, more derived group allowed leaves to dehydrate and resisted damage by producing a water transport system capable of functioning under the extreme tension that develops in water-stressed plants.

Stomatal responses to vapor pressure deficit (VPD) are a principal means by which vascular land plants regulate daytime transpiration. While much work has focused on characterizing and modeling this response, there remains no consensus as to the mechanism that drives it. Explanations range from passive regulation by leaf hydration to biochemical regulation by the phytohormone abscisic acid (ABA). We monitored ABA levels, leaf gas exchange, and water status in a diversity of vascular land plants exposed to a symmetrical, mild transition in VPD. The stomata in basal lineages of vascular plants, including gymnosperms, appeared to respond passively to changes in leaf water status induced by VPD perturbation, with minimal changes in foliar ABA levels and no hysteresis in stomatal action. In contrast, foliar ABA appeared to drive the stomatal response to VPD in our angiosperm samples. Increased foliar ABA level at high VPD in angiosperm species resulted in hysteresis in the recovery of stomatal conductance; this was most pronounced in herbaceous species. Increased levels of ABA in the leaf epidermis were found to originate from sites of synthesis in other parts of the leaf rather than from the guard cells themselves. The transition from a passive regulation to ABA regulation of the stomatal response to VPD in the earliest angiosperms is likely to have had critical implications for the ecological success of this lineage.

• The stomata of angiosperms respond to changes in ambient atmospheric concentrations of CO₂ (C_a) in ways that appear to optimize water-use efficiency. It is unknown where in the history of land plants this important stomatal control mechanism evolved. Here, we test the hypothesis that major clades of plants have distinct stomatal sensitivities to C_a reflecting a relatively recent evolution of water-use optimization in derived angiosperms. • Responses of stomatal conductance (g_s) to step changes between elevated, ambient and low C_a (600, 380 and 100 µmol mol⁻¹, respectively) were compared in a phylogenetically and ecologically diverse range of higher angiosperms, conifers, ferns and lycopods. • All species responded to low C_a by increasing g_s but only angiosperm stomata demonstrated a significant closing response when C_a was elevated to 600 µmol mol⁻¹. As a result, angiosperms showed significantly greater increases in water-use efficiency under elevated C_a than the other lineages. • The data suggest that the angiosperms have mechanisms for detecting and responding to increases in C_a that are absent from earlier diverging lineages, and these mechanisms impart a greater capacity to optimize water-use efficiency.