Recovery of water potential and leaf gas exchange performance following drought stress in Quercus cerris populations

Drought and global warming are major environmental stressors that significantly affect forest ecosystems and the survival of tree species. This study investigates the physiological and biochemical responses of Turkey oak (Quercus cerris L.) seedlings from five populations across three bioclimatic zones in Turkey - semi-arid (Yozgat), semi-humid (Isparta, Balikesir), and humid (Çanakkale and Sinop) - to drought stress and subsequent re-watering. Seedlings were subjected to control (regular irrigation) and drought stress (no irrigation for 30 days), with ecophysiological (soil water content, midday water potential, gas exchange) and biochemical (total soluble sugars and chlorophyll content) traits measured on days 10, 20, 30, and after re-watering (days R3 and R10). Drought stress reduced midday water potential and gas exchange parameters across all populations. Total soluble sugar content increased under drought, except in the Sinop population, which showed a decline in photosynthetic rate and failed to accumulate soluble sugars. In contrast, the Yozgat population maintained higher midday water potential, suggesting effective osmotic regulation. Other populations (Isparta, Balikesir, and Çanakkale) coped with drought by increasing total soluble sugar content despite lower water potential. Following re-watering, water potential and sugar content recovered in all populations, with a gradual improvement in photosynthetic performance. These findings highlight significant intraspecific variation in drought response among Q. cerris populations and underscore the importance of considering population-level differences in forest management and afforestation strategies under changing climate conditions.