Physiological responses of Marsol and CA-90 chestnut plants to high temperature and water scarcity co-exposure: a comparative study

The cultivation of chestnut trees (Castanea sativa Miller), one of the main forest crops in the Mediterranean, has been relying on grafting with selected genotypes resistant to diseases, such as the ink disease. However, the current climatic instability urges the selection of rootstocks also capable of enduring prolonged periods of drought and high temperatures. Thus, this study aimed to compare the response of chestnut plants (3 months-old) from two disease-resistant varieties - Marsol and CA-90 - to single and combined drought (25 % WHC) and heat (4h/day at 42 °C). After 3 weeks of exposure, all stress conditions led to a reduction in the leaf area in Marsol plants. Still, relative water content in this variety was only decreased upon drought stress, whether isolated or combined with high temperatures, while the production of new leaves was not changed. Contrastingly, CA-90 plants were affected to a higher extent, showing greater reductions in biometric attributes under all stress conditions. Data concerning the photosynthetic performance followed the same pattern, with Marsol exhibiting milder inhibitory effects on both chlorophyll fluorescence and gas-exchange parameters, when compared to CA-90. Notably, carbon assimilation in stressed Marsol plants remained identical to that of non-stressed plants, whereas it significantly declined in CA-90 (66 %). Oxidative stress markers were differently modulated between the two varieties and across stress treatments, with CA-90 plants showing clear signs of oxidative stress, mainly under drought. In addition, Marsol was able to ensure its redox homeostasis by coordinately up-regulating proline (3.6-fold), and glutathione (38 %), while the antioxidant system of CA-90 is inactivated, leading to ROS accumulation (H₂O₂; 54 %) mainly at the leaf level. Altogether, results suggest that heat and drought co-exposure severely impacts the physiology of chestnut plants, with CA-90 showing a higher susceptibility. Thus, chestnuts grafted onto Marsol rootstocks can represent an advantage in terms of climatic resilience to cope with global warming.