Drought, thermal response, and climate-patterning in cuticular conductance of the widespread C4 grass Themeda...

Grasslands underpin global biodiversity, carbon storage, and ecosystem resilience. However, grasslands are increasingly threatened by rising temperatures and water scarcity, making it essential to understand physiological responses to drought and heat to sustain their function under future climates. We investigated intraspecific variation in minimum cuticular conductance after stomatal closure (gmin) and its thermal response (phase transition temperature; Tp) in six Australian accessions of Themeda triandra spanning a broad climatic gradient. We also tested whether gmin and its response to drought reflected climate-at-origin. Plants were grown under well-watered glasshouse conditions and then exposed to a short-term drought. We measured gmin of control and droughted plants and assessed gmin responses to increasing temperature (30-55°C) using fresh, fully hydrated leaves (well-watered). Climatic data from accession origin were used to examine trait-environment relationships. Under well-watered conditions, gmin was unrelated to climate-at-origin. However, under drought, gmin was lower in accessions from cooler and wetter regions, contrary to expectations, with drought effects varying strongly among accessions. gmin generally decreased with temperature, and no accession showed a distinct Tp, indicating thermally stable leaf cuticles in this species. These findings reveal intraspecific gmin diversity and highlight water availability as a key filter for selecting stress-tolerant accessions.