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 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 megapascals. This value matches the range (1.4 ± 0.6 megapascals) 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 species growing across variable environments. Editor's summary: Plants transport columns of water through the vasculature to leaves under tension. Looking across diverse species inhabiting varying environments, Carminati et al. established that there is a narrow range of leaf vascular tension thresholds at which transpiration starts to decline. Using a theoretical model, the authors found that this convergence in leaf vascular tension threshold can be explained by the physical constraints of soil properties on water extraction by roots. Large pockets of water are easily drained, but as these diminish in size, capillary and viscous forces increase, making water challenging to extract. The greater vascular tension required to extract water from drier soils makes transpiration physically inefficient. This concept explains universal constraints on plant water relations. —Madeleine Seale [ABSTRACT FROM AUTHOR]