Grafting onto thermotolerant hybrid rootstocks enhances heat stress tolerance in sweet pepper

High temperatures significantly limit sweet pepper ( Capsicum annuum L.) production, particularly in mediterranean greenhouse systems. This study evaluated the physiological, productive, and fruit quality responses of a commercial sweet pepper variety grafted onto three hybrid rootstocks (H211, NIBER® and TERMABER®) under thermal stress (38/22 °C day/night) and control conditions (28/22 °C). The results showed that grafting onto thermotolerant rootstock, TERMABER®, enhanced plant performance under high temperatures by increasing marketable yield through reducing blossom-end rot (BER) incidence compared to non-grafted plants. Grafted plants exhibited higher phenolic compound accumulation, and lower electrolyte leakage, contributing to thermal stress tolerance. Respect to fruit quality parameters, °Brix and titratable acidity were not negatively affected by thermal stress, and nutraceutical antioxidant compounds (ascorbic acid and lycopene) were significantly enriched in the fruits of grafted plants. These findings support grafting onto thermotolerant rootstocks as a viable strategy to improve sweet pepper heat tolerance and maintain yield and fruit quality under rising temperatures.