Linking drought strategy to climate-of-origin in a widespread C4 grass: hydraulic traits, xylem anatomy and stomatal behaviour in Themeda triandra.

Abstract

Hydraulic, stomatal, and anatomical traits strongly influence plant performance under drought, but the extent to which these trait-types are coordinated and relate to climate-of-origin remains vastly underexplored in herbaceous species relative to woody plants. We quantified variation in these traits in eight populations of Themeda triandra selected from contrasting summer temperature and precipitation regimes across Australia, grown under common conditions and exposed to drought. Trait-climate-of-origin relationships were quantified using linear mixed models. Plants from drier climates exhibited lower stomatal conductance, earlier drought-induced stomatal closure, and higher stomatal safety margins. Plants from warmer climates exhibited greater embolism resistance and hydraulic conductance, with wider vessels and lower vein density, suggesting adaptation to high evaporative demand; embolism resistance was unrelated to precipitation. These findings reveal, for the first time in any grass species, the intraspecific coordination of xylem embolism resistance, stomatal regulation and xylem anatomy across a climate gradient, identifying distinct hydraulic strategies: conservative water use and dehydration avoidance in drier climates, and embolism resistance and hydraulic efficiency in warmer climates. These findings provide critical new insights into the adaptive strategies that facilitate success of this widespread C₄ grass in contrasting climates, with direct implications for understanding grassland responses to climate change.