Separating intrinsic from extrinsic stomatal control in different soils

Summary The regulation of stomata in response to environmental changes is one of the most dynamic and important behaviours in vascular plants, playing a central role in determining gas exchange and vulnerability to drought. Yet whether this regulation is determined by intrinsic species physiology or by soil hydraulic properties or a combination of both remains unsolved. Here, we use a long‐term dataset of needle water potential (Ψ needle ) measurements to compare stomatal regulation in a hardy conifer, Callitris rhomboidea , growing at two sites with contrasting soil textures: sand and clay‐loam. Inferred stomatal sensitivity to water potential differed between soil types when expressed against soil water potential but exhibited a universal relationship to Ψ needle . This observation aligned with the predictions from an optimal soil water extraction model, indicating that distinct optimal water extraction in the two soil types could be achieved through a common relationship between stomatal conductance and Ψ needle . Our results suggest that the primary driver of stomatal response to soil and atmospheric dryness lies in a common response to leaf water potential. This work reconciles competing ideas of stomatal control, indicating how the appearance of soil‐specific behaviour is in fact the expected consequence of a single, optimal physiological behaviour across soil types. Characterising the variation in species‐specific stomatal response to leaf water potential will enable comparisons of water extraction behaviour between species in different soils.