Heat-stress response of photosystem II in five ecologically important tree species of European temperate forests

Heat negatively affects photosynthesis, thus tree growth and eventually survivability. The light-dependent photosynthesis reactions are considered highly heat sensitive; therefore, we studied the impact of supra-optimal temperatures on photosystem II (PSII) in five important tree species (Abies alba, Fagus sylvatica, Picea abies, Pinus sylvestris and Quercus petraea agg.). We simulated short-term heat stress at different levels and assessed PSII thermotolerance via fluorescence parameters derived from the OKJIP transient. To capture the seasonal changes in PSII thermostability and the stress tolerance index, the measurements were performed twice: at the beginning of the vegetation season before the natural heat events occurred and during the peak of the vegetation season. Moreover, the content of photosynthesis-related pigments was analysed. We confirmed the relationship between the appearance of the K-step at the OKJIP curve and the concentrations of photosynthesis-related pigments. We found that broadleaved tree species showed overall higher PSII thermostability and had a higher content of pigments than coniferous species. The most heat-tolerant species was Q. petraea agg., which presented enhanced PSII thermotolerance after heat events. F. sylvatica responded by a decrease in PSII thermostability during the vegetation season, though its thermostability was still higher than that of the coniferous species and was comparable to that of Q. petraea agg. The PSII thermotolerance of P. abies was relatively high compared to that of the other coniferous species on both dates, but its response was inconsistent across the assessed parameters. Contrary, the PSII thermostability of A. alba and P. sylvestris was generally low on both measurement dates.