Soils drive convergence in the regulation of vascular tension in land plants

Terrestrial vascular plants operate under negative water potential, which results in hydraulic tension in the vascular system. Vascular tension varies with transpiration and soil drying, and it is regulated by stomata, pressure-activated valves on the leaf surface. We hypothesize that soil physical constraints drive convergence in the operational range of leaf vascular tension. Based on a meta-analysis of 19 diverse species, we find that stomatal regulation of transpiration is activated when leaf vascular tension reaches a narrow target of 1.3±0.6 MPa. This value matches the range (1.4±0.6 MPa) predicted from an optimal soil water extraction model. Optimality in plant vascular tension appears to have evolved by selection for a narrow range of osmotic pressure in the leaves of diverse AASAQAQSAspecies growing across enormously variable environments.