Tolerance of Greek Indigenous Winegrape Cultivars to Heat Stress under Variable Light Conditions

This study investigates the effects of rising temperatures on photosynthetic efficiency and stress tolerance in major Greek grapevine cultivars (Savvatiano, Muscat, Assyrtiko, Mavrodafni, Moschofilero, and Agiorgitiko), using Sauvignon Blanc and Merlot as ref-erence varieties. The primary aim was to identify heat tolerant genotypes suitable for cultivation in warm regions with high solar radiation. Another objective was to indicate shade-tolerant cultivars that could benefit from canopy shading techniques, contributing to broader climate adaptation strategies in viticulture. Muscat and Assyrtiko displayed the most heat-tolerant photosynthetic apparatus among white cultivars, while Mavrodafni was the most heat-tolerant among the red ones, by effectively managing excess light energy. Sauvignon Blanc although exhibiting heat susceptibility, it maintained high PSII functionality under heat stress by activating photoprotective mechanisms. Temperatures above 35 °C increased grapevine’s susceptibility to strong light, thereby intensifying photooxidative stress. Savvatiano and Agiorgitiko were more vulnerable to photooxi-dative stress above 35°C, though Agiorgitiko maintained a functional photosynthetic apparatus even at 40°C by shifting to a more photoprotective strategy. In contrast, Merlot, despite its resistance to photooxidative stress, lacked photoprotective investment, re-sulting in suppressed PSII under heat stress. Moschofilero was the most susceptible cul-tivar to photooxidative stress. Leaf morphological traits contributed to heat and light stress tolerance, with smaller, thicker leaves facilitating thermoregulation. Taken together, Merlot, Mavrodafni, Sauvignon Blanc, and Assyrtiko are recommended for high-altitude cultivation due to their high tolerance to light. Conversely, Savvatiano, Moschofilero, and Agiorgitiko were better suited to lower light conditions, favoring higher latitudes and shading practices under heat stress. The present results provide important insights into cultivar-specific responses to heat and light stresses, presenting for the first time the respective tolerances of major Greek cultivars. This knowledge is essential for selecting heat-tolerant genotypes, optimizing vineyard site selection and management strategies, thereby supporting the sustainability and climate resilience of viticulture.