T50 as a proxy of PS II heat tolerance: time dependence, differences in calculation methods and implications for its suitability to assess the heat tolerance of tree species

O_LIThe heat tolerance of tree species has recently received increased attention, as it is critical for the response of forest ecosystems to climate change. Most studies rely on laboratory experiments with detached leaves, where the thermostability of photosystem II (PS II) is analyzed using chlorophyll fluorescence analysis. The temperature at which the maximum quantum yield (Fv/Fm) of PS II is reduced by 50% (T50) is often used as key measure of heat tolerance despite of weak mechanistic corroboration and inconsistent definitions. We propose a new metric consisting of a critical temperature (TIP) and a corresponding fluorescence value (FIP). C_LIO_LIWe analyzed the effect of incubation time and temperature on different versions of T50. C_LIO_LIT50 was strongly dependent on the duration of heat exposure, decreasing exponentially with incubation time. T50 and TIP values derived from short-term treatments of different incubation times <1 h are not comparable, but stabilize after longer intervals of heat exposure. C_LIO_LIT50 of Fv/Fm specified without considering incubation time is a meaningless metric. Specifications derived from short-term heat treatments should be avoided, because the result is highly influenced by the experimental setup. Different calculation methods for T50 influence the result and elucidate different aspects of the heat response. C_LI