Thermotolerance thresholds of the photosystem II increase with elevation in conifer species in the Spanish Pyrenees

In vitro-measured photosystem II (PSII) thermotolerance, indicated by the temperature-dependent decline of the quantum use efficiency of PSII, has been broadly applied to evaluate plants' ability to deal with future climate warming. Nonetheless, deriving PSII thermotolerance thresholds, such as T5, T15 and T50, representing 5%, 15% and 50% temperature-dependent decline in quantum use efficiency, has rarely been tested against distributional ranges and associated environmental variables. Therefore, we evaluate the predictive power of these three PSII thermotolerance thresholds, obtained from seven temperate conifer species in the Spanish Pyrenees, in association with their respective distributional pattern in regard to altitude and accompanying environmental variables. Both T5 and T15 correlated with the mean altitudinal distribution, with species growing in higher elevations exhibiting higher PSII thermotolerance thresholds compared to species in the lowlands. The thresholds were negatively correlated with the mean annual temperature while displaying a positive correlation with the mean annual precipitation. No correlation was found between T50 and elevational as well as environmental gradients. Both T5 and T15 indicate an initial decline of the quantum use efficiency. We speculate that plants of greater altitudes, by being exposed to larger temperature fluctuations, have a greater need for higher PSII thermotolerance. As climate change-induced heat stress intensifies, high-altitude conifer species might be more resistant to increasing temperatures than previously thought.