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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 degrees C per degrees C increase in growth temperature). The glasshouse study showed that T-crit was inherently higher in species from warmer habitats (increasing 0.16 degrees C per degrees C increase in origin annual mean maximum temperature) and acclimated to increasing growth temperature (0.24 degrees C degrees 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.