Strigolactones as possible elicitors in sunburn defense mechanisms in grapes: preliminary results

Abstract

Due to altered climatic conditions, grape berry sunburn has become one of the main challenges in contemporary viticulture. This disorder results from a combination of excessive direct exposure to sunlight and high temperatures, leading to brown and necrotic patches on the fruit skin, potentially reducing vineyard yield and grape commercial value. Despite its prevalence, winegrowers lack effective and economically viable strategies, in line with sustainable agriculture principles to mitigate sunburn beyond the use of shade nets, temporary protection with kaolin and zeolite, or the avoidance of defoliation practices.

We explored the use of phytohormones, particularly strigolactones, as potential agronomic tools to enhance plant stress responses. Specifically, we investigated the potential benefits of multiple in-field strigolactone (racGR24, a synthetic analog of natural strigolactone) applications on “Muscat blanc” clusters during the green developmental phase to enhance stress response mechanisms. At veraison, berries were then subjected to artificial sunburn in a laboratory setting, utilizing a “UV Test Box”.

Clusters were treated with a 5 μM rac-GR24 solution twice during the green growth phase. The solution was sprayed to facilitate absorption into fully shaded berry tissues. This approach was aimed at mitigating the risk of sudden sun exposure, as severe sunburn damage typically occurs when berries are abruptly subjected to high light and temperature conditions without prior acclimation during early development. Both control and rac-GR24-treated berries underwent artificial sunburn testing under identical conditions to assess the protective effects of strigolactones.

Analyses included berry color assessment using a colorimeter (CIEL*a*b system), spectrophotometric measurements of total polyphenols, polyphenol oxidase activity, and the skin’s antioxidant capacity, both pre- and post-artificial sunburn treatments. racGR24 application revealed significant differences in all the parameters investigated, indicating a protective effect against oxidative stress. Further research is needed to decipher the molecular role of strigolactone applications in grape berry metabolism, in order to characterize the strigolactone’s induced mechanism.