What chlorophyll fluorescence reveals about PSII functioning during water deficiency in European beech (Fagus sylvatica L.) saplings?

Drought is a major abiotic stressor that constrains key physiological processes in forest trees and influences their resilience. Climate models predict increasing frequency and intensity of drought events, which will shorten the intervals available for recovery between successive stress episodes. This study investigated the effects of water deficit on photosystem II (PSII) functioning in European beech (Fagus sylvatica L.) saplings, and recovery of PSII-related functional parameters following rewatering.A total of 41 F. sylvatica saplings were grown under a transparent roof with controlled irrigation in a forest gap in central Poland. The experimental design included a well-watered control and a water deficit treatment consisting of a 30-day irrigation withdrawal followed by 40 days of rewatering to track recovery. Eight measurement series were conducted during the summer of 2024 at regular 7–10-day intervals. Chlorophyll a fluorescence was measured after 20 min of dark adaptation using a HandyPEA fluorimeter.The progressive decrease in soil water content led to a significant decline in PSII efficiency (FV/FM) in F. sylvatica saplings. The applied stress prolonged the time required to reach maximum fluorescence (TFM) and decreased the maximum fluorescence level (FM) indicating slower and incomplete reduction of QA molecules​ and a reduced pool of available reaction centres (Area). These changes increased excitation pressure per reaction centre, reflected by higher ABS/RC, thereby elevating the risk of photodamage. However, we also observed increased dissipation of excess energy as heat (DI₀/RC), providing evidence for the activation of PSII protective mechanisms. Following rewatering, F. sylvatica saplings exhibited partial recovery of PSII performance, suggesting that drought-induced impairments of photochemical efficiency were at least partly reversible under the applied experimental conditions. Taken together, our results suggest that while F. sylvatica can engage photoprotective responses under drought, incomplete post-drought recovery may increase vulnerability under scenarios of recurrent drought with short recovery intervals, with implications for the management of beech-dominated forests.