Impact of natural defoliation and heat shock on the recovery of photosystem ii parameters in european beech (Fagus sylvatica)

Marginal populations of European beech ( Fagus sylvatica ) are vulnerable to heatwaves and prolonged droughts, which affect photosynthetic function and tree resilience. The aim of this study was to investigate the leaf-level responses of European beech trees, depending on defoliation class (I-IV), to a heat shock and to compare the recovery processes of PSII parameters. For each defoliation class, five trees were selected. Detached leaves were exposed to a heat shock of 50°C for 10 minutes, and during ten days of recovery, chlorophyll content, the effective quantum yield of PSII (ΦPSII), the electron transport rate (ETR), and the maximum quantum yield (F _v /F _m ) were measured. Trees from classes I and II exhibited moderate initial values of photosynthetic parameters and showed faster recovery, although final values did not fully reach control levels. In contrast, trees from classes III and IV had lower initial values and exhibited slow and incomplete recovery, indicating reduced PSII resilience. On the first day of recovery, ΦPSII, ETR, and F _v /F _m values decreased significantly, but recovery dynamics moderately accelerated thereafter, without reaching control levels. Variation component analysis revealed that the source of variation represented by individual trees largely influenced chlorophyll content, whereas treatment and recovery day primarily explained variations in ΦPSII, ETR, and F _v /F _m . ΦPSII values reflected photochemical efficiency and persistent PSII photoinhibition, while ETR provided complementary information on electron flux dynamics, suggesting that PSII recovery involves a coordinated process among parameters. Integration of the contributions from different sources of variation indicates that leaf recovery in European beech depends on the complex interaction between inter-individual variability and heat shock, highlighting the role of initial vitality and physiological state of the trees in PSII resilience.