Plasticity of photosynthetic heat tolerance in plants adapted to thermally contrasting biomes

In many biomes, plants are subject to heatwaves, potentially causing irreversible damage to the photosynthetic apparatus. Field surveys have documented global, temperature-dependent patterns in photosynthetic heat tolerance (P_HT); however, it remains unclear if these patterns reflect acclimation in P_HT or inherent differences among species adapted to contrasting habitats. To address these unknowns, we quantified seasonal variations in T_crit (high temperature where minimal chlorophyll-a fluorescence rises rapidly, reflecting disruption to photosystem II) in 62 species native to 6 sites from 5 thermally contrasting biomes across Australia. T_crit and leaf fatty acid (FA) composition (important for membrane stability) were quantified in three temperature-controlled glasshouses in 20 of those species. T_crit was greatest at hot field sites and acclimated seasonally (summer > winter, increasing on average 0.34 °C per °C increase in growth temperature). The glasshouse study showed that T_crit was inherently higher in species from warmer habitats (increasing 0.16 °C per °C increase in origin annual mean maximum temperature) and acclimated to increasing growth temperature (0.24 °C °C⁻¹). Variations in T_crit were positively correlated with the relative abundance of saturated FAs, with FAs accounting for 40% of T_crit variation. These results highlight the importance of both plastic adjustments and inherent differences determining contemporary continent-wide patterns in P_HT.