Characterization of sunburn damage on grapes

Project duration: May 2020 to October 2022

Coordinated by the Forschungskreis der Ernährungsindustrie (FEI) e.V. and funded by the Federal Ministry of Economics and Climate Protection (BMWK).
This project is being worked on as part of a doctoral thesis by Jonas Waber.

Contact    

• Jonas Waber; Room A1-106; jonas.waber☞ Please insert an @ at this point ☜dlr.rlp☞ Please insert a period at this point ☜de
• Prof. Dr. Jochen Bogs; Room A2-161; jochen.bogs☞ Please insert an @ at this point ☜dlr.rlp☞ Please insert a period at this point ☜de

Summary

Climate change has a significant impact on viticulture. Phenological earlier stages of development such as the time of flowering and ripening, the challenge of certain grape varieties to adapt to higher temperatures and drought stress, up to the occurrence of undesirable off-flavors in the later wine represent a wide range of impairments. The growing conditions of many autochthonous varieties are deteriorating, as they are adapted to a cooler climate due to the long-term selection process. In addition, since the 1990s there has been an increase in sunburn damage to grapes, which is associated with increased exposure to radiation, among other things. The phenological stage of development is decisive for the severity of the damage, with a higher susceptibility 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 strong sunlight, high temperatures and low humidity.

In Riesling, the most frequently planted grape variety in Germany, sensory deviations known as "petrol off-notes" have also occurred in this context in recent years. According to previous studies, the causes of this off-note are attributed to rising temperatures and greater overall exposure to radiation, which causes carotenoids in the grapes to be broken down into _C13 norisoprenoids, among other things. Further chemical reactions ultimately result in the compound 1,1,6-trimethyl-1,2-dihydronaphthalene (TDN), which gives the wines a dull, petrol-like odor and a bitter and astringent taste. Sunburn damage therefore not only reduces the quantity of yields, but also the quality of the wines to a considerable extent.

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

Another aim of the research project is to develop an avoidance strategy in which optimized foliage management in combination with the application of various preparations is intended to reduce the susceptibility of grapes to sunburn. These sunscreens are based on kaolin or lime and are therefore harmless for the future wine. Alternatively, the reduction of sunburn through different shading nets is to be evaluated. The underlying mechanisms range from adaptation and reflection to shading and are assessed in terms of effectiveness, costs and possible effects on the subsequent wine. The aim is to find a solution that is as sustainable as possible and can be achieved with little effort.