In thermotolerance tests of tropical tree leaves, the chlorophyll fluorescence parameter Fv/Fm measured soon after heat exposure is not a reliable predictor of tissue necrosis.

Tropical rainforests are hot and may be particularly sensitive to ongoing anthropogenic global warming. This has led to increased interest in the thermotolerance of tropical trees. Thermotolerance of leaves of two tropical tree species, Terminalia catappa and Coccoloba uvifera, was determined by exposing leaf samples to 15-min heat treatments, followed by measurements of potential photosystem II quantum yield (dark-adapted value of variable/maximum chlorophyll a fluorescence, F_v/F_m) after 24 h and 14 days, and visible damage (necrosis) after 14 days. T₅₀ (24 h), the temperature at which F_v/F_m declined by 50% 24 h after heat treatments, was associated with only ~10% leaf area damage in C. uvifera and no damage in T. catappa. In neither species was leaf necrosis observed at T₅ (24 h), the temperature at which F_v/F_m declined by 5%. In both species, temperatures significantly higher than T₅₀ (24 h) were required for 50% leaf area necrosis to occur. T₅₀ (14 days) was a better proxy of visible leaf damage than T₅₀ (24 h). The relationship between heat-induced F_v/F_m decline and tissue necrosis varies among species. In species surveys of leaf thermal tolerances, calibration of the F_v/F_m assay against the necrosis test is recommended for each species under investigation. F_v/F_m measurements soon after heat exposure do not reliably predict irreversible heat damage and may thus not be suitable to model and predict the thermostability of tropical forest trees.