Silicon priming enhances leaf-level resilience to heat and drought in young chestnut plants

Climate change-driven increases in heat and drought events pose a serious threat to chestnut trees, demanding the development of mitigation strategies, especially for young plants growing in orchards. This study investigated whether silicon (Si)-based priming (1 mM) can enhance the resilience of chestnut seedlings (3 months-old) to combined heat and drought (42 °C, 4h d^-1; 25% maximum water holding capacity). After 21 days of stress exposure, data showed that combined stress significantly reduced leaf area and the production of leaves without affecting relative water content in both organs. In addition, soluble sugars increased in leaves and decreased in roots, suggesting a slowdown of metabolic activity rather than a hydraulic limitation, as well as a redistribution of carbon assimilates towards other plant tissues. In leaves, co-exposure to heat and drought decreased the effective quantum yield of PSII and electron transport rate; net CO₂ assimilation was also negatively impacted, although no changes in stomatal conductance were detected, pointing towards non-stomatal photosynthetic limitations along with a reduced intrinsic water use efficiency. Additionally, these physiological changes were accompanied by shifts in redox metabolism, particularly in roots, where a downregulation of the antioxidant (AOX) defences was observed, along with reduced levels of reactive oxygen species and oxidative stress indicators. In leaves, stressed plants exhibited enhanced levels of superoxide anion and lipid peroxidation, but AOX defences remained essentially unchanged, except for glutathione and guaiacol peroxidase activity. In response to exogenous Si treatment, stress-induced effects on photosynthesis and oxidative damage were efficiently mitigated, although AOX metabolism did not substantially change compared with the stress treatment alone. Overall, Si priming was able to partially limit the effects of combined heat and drought on leaf photochemistry, carbon assimilation, and oxidative damage, but did not prevent growth inhibition nor root metabolic slowdown. These results highlight functional organ specificity and indicate that Si alone cannot provide whole-plant chestnut resilience under combined heat and drought stresses.