Plants originating from more extreme biomes have improved leaf thermoregulation

Many plants have some capacity for leaf thermoregulation via stomatal conductance (gsw), such that leaf temperature (Tleaf) is rarely coupled with air temperature (Tair). The difference between leaf and air temperature (thermal offset, ΔT) and the slope (thermal coupling strength, β) is mediated by interactions between the immediate environment of the plant and its leaf traits. The aim of this study was to determine whether species originating from biomes with contrasting environmental conditions (alpine, desert and coastal temperate) would differ in their tendency to thermoregulate in a common environment.

Using benign-temperature (25 °C) and high-temperature (38 °C) glasshouse treatments, we measured paired canopy Tair and Tleaf for 15 diverse species, 5 from each biome, in a common garden experiment. Instantaneous stomatal conductance and a suite of leaf traits were measured and calculated to test for associations with leaf thermoregulation.

We found clear evidence for greater leaf cooling occurring during high-temperature exposure, especially in alpine and desert species. The leaves of temperate species were largely warmer than air in both treatments. Thicker leaves with higher water content and high stomatal conductance clearly were more effective at cooling. Species originating from different biomes displayed divergent responses of thermal offset and thermal coupling with leaf traits.

Our findings suggest that plants originating from more extreme biomes have innately greater scope for thermoregulation, especially desert plants, which could better counter the risk of reaching excess temperatures at the cost of higher water loss. Leaf thermoregulation is a complex plant-environment interaction, and our work contributes to the development of more accurate predictions of leaf temperature during heat exposure across diverse species and biomes.