Characterization of sunburn damage on grapes

Climate change is having a significant impact on viticulture. A wide range of challenges has emerged, from earlier phenological stages—such as flowering and ripening—to the difficulty certain grape varieties face in adapting to higher temperatures and drought stress, all the way to the development of undesirable off-flavors in the final wine. Growing conditions for many indigenous varieties are deteriorating, as these varieties have adapted to a cooler climate through a long-term selection process. In addition, since the 1990s, there has been an increase in sunburn damage to grapes, which is associated, among other factors, with increased exposure to solar radiation. The phenological stage of development is a key factor in the severity of this damage, with grapes being more susceptible before ripening begins. In general, sunburn occurs when a prolonged period of cool, cloudy weather with high humidity is followed by a sudden change in which the grapes are exposed to intense sunlight, high temperatures, and low humidity.

In Riesling, the most widely planted grape variety in Germany, sensory abnormalities known as a “petrol off-flavor” have also occurred in recent years in this context. According to previous studies, the causes of this off-note are attributed to rising temperatures and overall greater exposure to solar radiation, which causes carotenoids in the grapes to be broken down into, among other compounds, C₁₃-norisoprenoids. Through further chemical reactions, this ultimately results in the compound 1,1,6-trimethyl-1,2-dihydronaphthalene (TDN), which imparts a dull, petrol-like aroma and a bitter, astringent taste to the wines. Sunburn damage therefore significantly reduces not only the quantity of the harvest but also the quality of the wines.

For this reason, a deeper understanding of the causes of sunburn and the associated changes in the metabolism of the grapes is essential. To this end, the activity of radiation-dependent genes and the resulting metabolites in the grape skins of the susceptible Riesling variety and the genetically related but sunburn-tolerant Calardis Blanc variety are being analyzed comparatively. The berry samples are obtained both under controlled conditions in a climate chamber and from field-grown vines. Particular focus is placed on phenylpropanoid and isoprenoid metabolism, as polyphenols and carotenoids are synthesized to protect against radiation stress and are stored in the grape skin.

Another goal of the research project is to develop a prevention strategy in which optimized canopy management, combined with the application of various preparations, is intended to reduce the grapes’ susceptibility to sunburn. These sunscreens are based on kaolin or lime and are therefore safe for the resulting wine. Alternatively, the reduction of sunburn through the use of various shading nets will be evaluated. The underlying mechanisms range from adaptation to reflection to shading and will be assessed in terms of effectiveness, costs, and potential effects on the resulting wine. The goal is to find a solution that is as sustainable as possible and can be implemented with minimal effort.

Coordinated by the Research Association of the Food Industry (FEI) e.V.