Most stomatal closure in woody species under moderate drought can be explained by stomatal responses to leaf turgor

Reduced stomatal conductance (g(s)) during soil drought in angiosperms may result from effects of leaf turgor on stomata and/or factors that do not directly depend on leaf turgor, including root-derived abscisic acid (ABA) signals. To quantify the roles of leaf turgor-mediated and leaf turgor-independent mechanisms in g(s) decline during drought, we measured drought responses of g(s) and water relations in three woody species (almond, grapevine and olive) under a range of conditions designed to generate independent variation in leaf and root turgor, including diurnal variation in evaporative demand and changes in plant hydraulic conductance and leaf osmotic pressure. We then applied these data to a process-based g(s) model and used a novel method to partition observed declines in g(s) during drought into contributions from each parameter in the model. Soil drought reduced g(s) by 63-84% across species, and the model reproduced these changes well (r(2)=0.91, P<0.0001, n=44) despite having only a single fitted parameter. Our analysis concluded that responses mediated by leaf turgor could explain over 87% of the observed decline in g(s) across species, adding to a growing body of evidence that challenges the root ABA-centric model of stomatal responses to drought.