The Science Behind UV Light in Vineyards

Wine regions that receive higher levels of UV light are finding it comes with unexpected challenges—and benefits

At Adrianna Vineyard, 1,500 meters high in the Andes, there is less atmosphere between space and the vine. Photo courtesy of Catena Institute of Wine.
At Adrianna Vineyard, 1,500 meters high in the Andes, there is less atmosphere between space and the vine. Photo courtesy of Catena Institute of Wine.

Catena Institute’s Adrianna Vineyard lies at almost 1,500 meters, high up in the Andes, and the temperature is much cooler than in the lower-lying areas east of Mendoza. “It’s like Burgundy compared to Châteauneuf-du-Pape,” says Fernando Buscema, the executive director of the Catena Institute of Wine. But despite the cooler air, vineyard workers complained that the intensity of the sun made it too warm for them to work there. That, Buscema says, combined with the better quality of fruit coming from the higher elevation vineyard, prompted an investigation into the effects of ultraviolet light on grape growing.

What the Catena Institute and other researchers like Lincoln University in New Zealand discovered was that in the vineyard a certain band of ultraviolet light called UV-B punches above its weight in terms of its impact on the grapes and resulting wines.

In the vineyard, UV light can be just as important as temperature. It can have profound effects on the development of flavor, color, and fermentation activity. In certain places, managing this light in the vineyard can be a careful balancing act. SevenFifty Daily spoke with the researchers and producers investigating the topic to better understand how UV light affects grape growing and winemaking.

Understanding UV Light

Full spectrum sunlight stretches from long infrared waves through the spectrum visible to the human eye—the ROYGBIV of colors a prism reveals—to the shortest of the bunch, ultraviolet rays. Among the latter, scientists divide things up even further into three bands, UV-A, UV-B, and UV-C; the last is so short that it never penetrates the ozone layer, and our only exposure to it is from welding torches and similar artificial sources. The earth’s atmosphere barely filters UV-A light at all, while the middle band, UV-B, is partially filtered by the ozone layer, water vapor, oxygen, and carbon dioxide.

While infrared light is relatively harmless, UV-A and UV-B light is to blame for wrinkles, sunburn, and skin cancer, and can even damage human DNA. Research suggests that grapevines, in contrast, feel the impact of the shorter, UV-B wavelengths more than UV-A, even though the latter is more prevalent.

How UV Light Affects Vines

At higher latitudes, UV light enters the atmosphere at an angle, and therefore passes through more of the atmosphere before it strikes the earth. That means there’s more opportunity for the atmosphere to filter it out. But at a high-altitude site like Adrianna Vineyard there is less atmosphere between space and the vine, so it forgoes some portion of that protection, and the vine is forced to respond.

Fernando Buscema, the executive director of the Catena Institute of Wine. Photo courtesy of the Catena Institute of Wine.
Fernando Buscema, the executive director of the Catena Institute of Wine. Photo courtesy of the Catena Institute of Wine.

“The entire vine is sensing the higher UV light,” says Buscema, “and different tissues react differently. The skins are going to increase the concentration of certain compounds, mostly phenolic compounds, that work as a natural sunblock.” The vines reduce the size of the canopy and the leaves in an effort to control transpiration; water stress actually has similar effects on the vines. The result, according to the Catena Institute, is a one-two punch that concentrates flavors and aromatics, creates complexity, and deepens the color of red wines.

The primary phenolic compounds produced in the berries are called flavonoids. “How flavonoids influence the wine flavor is likely to be primarily through their antioxidant characteristics,” says Brian Jordan, an emeritus professor of plant biotechnology at Lincoln University in New Zealand, “but they may also play a role in mouthfeel.”

According to Jordan, UV-B’s effects on vines do not need to be stressful to be impactful, though the influence may be more subtle. They also vary depending on the grape variety. Pinot Noir responds less than Tempranillo, for example. In terms of vineyard practices, canopy management dictates the extent and nature of UV-B’s effects. Leaf removal in particular dictates to what extent the berries themselves will react to the light. 

“The exposure of the fruit directly to UV-B via leaf removal will have subtle effects on the metabolism and biochemistry right through to severe damage and skin browning,” says Jordan. The same holds true when deciding on row direction and vine density when planting a vineyard; both factors will affect how much of the plant’s leaves and its fruit bunches are exposed to UV-B. 

UV Light’s Global Variation

In general, UV light is strongest in the tropics, but New Zealand, and the Southern Hemisphere more generally, tends to have higher levels of UV-B light. In his chapter on the subject in a recent book, UV-B Radiation and Plant Life, Jordan notes that Christchurch, a region on the south island of New Zealand, sees 30 percent more UV-B light than Bordeaux, despite being at a similar latitude. This is in part due to ozone depletion as well as lower levels of atmospheric pollution.

Given the impact of climate change, the Northern Hemisphere may be looking to the work of Jordan and the Catena Institute in the future. Growers in both traditional winegrowing areas like Spain and lesser-known areas like Arizona are exploring higher elevation vineyards as a way to counterbalance heat and climate change. As they do, they’ll need to adapt their vineyard practices.

In the era of the so-called “flying winemakers,” Buscema has already seen the effects of consultants advising Argentine wineries to induce water stress in the vineyards; a common practice elsewhere, but one which yielded poor results in vineyards already feeling that intense UV-B light. Planting practices such as row direction will need to be reconsidered, and canopy management, in particular the practice of leaf thinning, will also need to be carefully evaluated. 

Cornell AgriTech's robotic arrays arch over the vines exposing them to UV-C light to prevent damaging mildew. Photo by David Gadoury courtesy of Cornell University.
Cornell AgriTech’s robotic arrays arch over the vines exposing them to UV-C light to prevent damaging mildew. Photo by David Gadoury courtesy of Cornell University.

UV’s New Role: Disease Prevention

Vines are not the only thing in the vineyard reactive to ultraviolet light; powdery mildew and downy mildew are reactive as well, but only when UV light is applied in isolation from other parts of the light spectrum. In isolation, UV light damages the DNA of the mildews, rendering them inactive. But what UV light takes away, the visible light spectrum gives back. 

“Blue light can repair the DNA damage that is caused by the UV,” says Kerik Cox, an associate professor of plant pathology at Cornell University. This means exposing the mildew to an artificial source of UV light when there’s no other light source around. “You have to do it at night to give them that chance,” says Cox.” This approach, initially used for sterilizing medical equipment in hospitals, was originally developed for agricultural use in greenhouses in the Netherlands, but has been adapted for strawberry fields, apple orchards, and vineyards.

“You build an array such that it has all these reflective surfaces,” says Cox. “It effectively makes a cloud of bouncing UV light particles. That way it gets under the leaves, and above them. Grapevines are a little less resistant than apples and strawberries, so we had to switch to UV-C, which is a little better and a little less penetrative.” 

These robotic arrays arch over the vines and slowly make their way down each row to make sure any potential mildew areas are adequately exposed to the UV-C light. They have been tested in New York and Oregon, with good results. Willamette Valley Vineyards is the first commercial operation to put them to use. It’s an entirely sustainable and organic process, leaving no residue or the copper and sulfur accumulations of traditional sprays, instead shedding new light on how to control two of the most troublesome diseases in the vineyard.

Vines, mildews, and human beings all have their own reactions to too much UV light. By better understanding these reactions, winegrowers are better able to adapt their vineyard management practices to balance the effects of UV light. Equally important, these insights are allowing winemakers to use UV light to their advantage, making wines that are deep, concentrated, and complex—and, potentially, disease free.


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Jim Clarke writes about wine, beer, and spirits for trade and consumer publications, including Beverage Media, Fortune, and World of Fine Wine. He is a sommelier and the U.S. marketing manager for Wines of South Africa.

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