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Rubisco activase (RCA) plays a regulatory role in photosynthetic function, by facilitating removal of inhibitors from the Rubisco active site. The RCA variants in most plant species thus far surveyed lose function between 35°C and 40°C, which is significantly lower than the putative thermostability of Rubisco itself. As such, RCA may be a ‘weak link’ in photosynthesis at moderate temperatures, and so may prove a fruitful target for crop fortification to improve crop yields. This study surveyed five tropical species chosen for their potential to possess highly thermostable RCA variants. We found one unusually thermostable RCA variant in the South American native Caesalpinia pulcherrima, and found two distinct RCA isoforms which exhibited differing thermostabilities. We also found that Rubisco was much less thermostable than previous literature suggests, becoming decreasingly functional in three of the five species studied between 25°C and 50°C. We suggest that these findings 1) provide support for RCA as a potential crop improvement target in a warming world, 2) suggest thermostable RCA variants may occur in a wide variety of locations and taxonomic groups, and 3) may have important implications for our understanding of temperature limitations to photosynthesis.