Heat treatment duration affects in vitro-induced photosynthetic impairment and development of necrotic leaf tissue in three Mediterranean oak species

Short heat treatments are commonly used to estimate the leaf-level thermal tolerance of tree species. The effect of varying treatment duration has recently been challenged to have significant effects on the estimates of thermal tolerance thresholds and how it reflects the development of leaf necrosis. Here, we aim to elucidate the effect of treatment duration on the thermal tolerance estimates of three Mediterranean oak species. We applied heat treatments with varying treatment durations (15, 30, 60, 120 and 240 min) to excised leaves. The temperature response of the leaf maximum quantum yield of photosystem II (Fv/Fm) was measured immediately, 24 h, and 2 weeks after the treatment. Necrotic leaf damage was visually assessed 2 weeks after the treatment. Both the intensity of decline in Fv/Fm and the development of necrotic tissue due to longer heat treatments were described by a logistic function. The longer the heat treatment lasted, the lower was the required temperature to induce irreversible damage to the photosystems and to reach a certain degree of visible damage to the leaf tissue. Fv/Fm measurements carried out 24 h after the treatment predicted well the development of necrotic tissue. Leaf-level thermal tolerance of Mediterranean oak species depends largely on the exposure duration to heat. The heat duration response follows a predictable logarithmic relationship, which allows for modelling the potential heat damage during heatwaves and forecasting climate change responses of Mediterranean forests.