The Sequence of Drought‐Driven Stomatal Closure, Stem Xylem Embolism, Dehydration, and Aquaporin Gene Expression Differs Among Species

Abstract

Stem dehydration is critical in predicting drought-driven tree mortality. Yet, how it coordinates with stomatal closure, xylem embolism, and aquaporin-mediated water transport regulation remains unknown. Two angiosperms (beech and olive) and two conifers (pine and juniper) of contrasted embolism resistance were selected to assess the sequence of drought-driven responses. Leaf stomatal conductance, stem hydraulic conductivity, and dehydration of elastic tissues (via stem radial variations) were monitored across a gradient of stem water potential to fit the corresponding percentage loss curves. Leaf pressure–volume curves and aquaporin gene (PIP) expression in lateral branches were also measured. Stomatal closure was the first response to drought across species. Loss of hydraulic conductivity preceded, co-occurred, and succeeded elastic dehydration in angiosperms, pine, and juniper, respectively. PIP expression consistently decreased in response to drought stress. This downregulation aligned more closely with stem shrinkage than with embolism formation for all species except beech. The use of high-resolution dendrometers revealed (1) the absence of a consistent, generalizable sequence of drought-induced physiological responses across tree species, (2) a prioritization of stem elastic tissue hydration over vascular integrity in (resprouting) angiosperms, potentially enabled by enhanced overnight rehydration capacity, and (3) PIP gene repression, likely limiting cell-to-cell water movement and elastic dehydration.