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

Background and Aims : Grasslands underpin global biodiversity, carbon storage, and ecosystem resilience, yet are increasingly threatened by rising temperatures and water scarcity. Understanding how key physiological traits respond to drought and heat is essential for ensuring grassland function under future conditions. Here, we investigated intraspecific variation in leaf cuticular traits—specifically minimum cuticular conductance after stomatal closure (g _min ) and its response to temperature—in six Australian accessions of Themeda triandra spanning a wide climatic gradient. We asked whether g _min and its response to drought reflect climate-of-origin and whether the cuticle shows a thermal threshold (Tp) beyond which conductance rises sharply.

Methods : Plants from six accessions were grown under well-watered (control) glasshouse conditions and then exposed to drought. We measured g _min in both control and drought treatments and assessed the response of gmin to increasing temperature (30−55 °C) using fresh fully hydrated leaves (control). Climatic data for each site of origin were used to explore trait−environment relationships.

Key results: Under well-watered conditions, g _min showed no link to climate-of-origin. Under drought, however, g _min displayed clearer climate-linked patterns: accessions from cooler, wetter regions had lower values, contrary to expectations. Drought responses varied strongly among accessions, ranging from marked reductions to significant increases in g _min. With increasing temperature, g _min declined gradually and no accession exhibited a distinct phase transition, indicating a thermally stable cuticle.

Conclusions : Themeda triandra shows considerable intraspecific diversity in g _min and its response to drought. The clearer alignment between climate of origin and g _min measured under drought conditions suggests drought stress is an important filter for g _min expression. These findings provide a physiological basis for identifying genotypes with enhanced resilience for use in grassland conservation and restoration under a warming, drying climate.