Sunburn dynamics: exploring occurrence and evolution on Vitis vinifera berries – a case study of ‘Sangiovese’ and ‘Pignoletto’ cultivars.

In recent years, sunburn and heat-related damage have increased in subtropical regions, posing a novel threat to vineyards in temperate climates. Elevated temperatures and prolonged drought have intensified berry necrosis and shriveling, resulting in reduced yields and berry quality. The SHEET project (sunburn and heat prediction in canopies for evolving a warning tech solution) aimed to address these specific objectives: i) study the effect of sunburn on berry composition; ii) study damage appearance and evolution in relation to berry temperature; iii) develop weather-based model to realize a warning-tech-solution for alerting growers on sunburn risk; iv) investigate the role of management practices (e.g., irrigation during ripening) in mitigating sunburn. Focusing on the points i and ii, in the first year (2021), sunburn damages were observed and characterized on both ‘Sangiovese’ (black) and ‘Pignoletto’ (white) cultivars. Additionally, the biochemical composition of healthy and damaged berries was analyzed. Sunburn necrosis appeared on the most exposed berries. ‘Sangiovese’ exhibited berry shriveling about three weeks after veraison, progressing until harvest, leading to dehydration of many berries. These resulted in lower anthocyanin and flavonol content compared to healthy berries. Conversely, sunburn browning was evident only on ‘Pignoletto’, in particular on those berries more exposed to sunlight, and the concentration of flavonols increased with the intensification of the brown color. During 2021 and 2022 vegetative seasons, berry temperature of ‘Sangiovese’ vines was continuously recorded and the evolution of the sunburn damages visually estimated every week. These measurements showed that intense sunburn damages occurred when the temperature of the most exposed berries raised over 40°C. Moreover, we observed that irrigation during ripening mitigated berry temperature reducing necrosis severity. Finally, collected data were used for developing a prototype of a warning IoT solution to inform growers on the risk of damage by means of a mobile-phone application.