Prevalence of Drought Over Heat Stress in Modulating Physiological and Metabolic Responses of Castanea sativa Mill. Var. Martaínha

The productivity of chestnut trees (Castanea sativa Mill.) is strongly affected by high temperatures and reduced water availability, phenomena exacerbated by climate change. However, the physiological effects of heat and drought exposure in this species remain poorly characterised, particularly, in local varieties of national relevance. In this study, we investigated the primary metabolism of young plants of the Martaínha variety subjected to drought (25% of maximum water-holding capacity) and/or high temperatures (42°C, 4 h day^-1). After 21 days of stress, exposure to heat alone induced distinct physiological adjustments, including increased stomatal conductance, transpiration and net CO₂ assimilation, as well as changes in mineral composition. Conversely, drought alone or in combination with heat, impaired plant growth, inducing clear signs of oxidative damage and redox imbalance. This was accompanied by negative impacts on photosynthesis, including reduced gas exchange and dysregulation of photochemical processes. In addition, parameters related to nitrogen and carbon metabolism were also affected under drought, as evidenced by reduced levels of nitrate, ammonium and amino acids, downregulation of glutamine synthetase activity and increased sugar accumulation. Notably, water scarcity also led to oxidative damage. Finally, both drought and heat influenced potassium and calcium accumulation in leaves. Overall, our findings support that drought is the primary driver of physiological impairment in chestnut, while heat employs significant but distinct effects and modulates plant responses under combined stress. In addition, the responses observed in Martaínha suggest genotype-specific strategies and highlight the need for further research into the mechanisms underlying stress tolerance in this species.