Sun Helps Grapes, Heat Does Not

For the past several years, savvy growers of red winegrapes have done all they could to open up their canopies in an effort to get more sun on their grapes. The theory was notably espoused by Australian winegrape guru Richard Smart, who’s 2001 seminal work, Sunlight Into Wine, detailed principles of canopy management to improve winegrape yield and quality. The book’s title says it all.

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But in recent years, growers have noticed that Smart’s theories can be taken too far. Growers observed that it seemed too much sunshine wasn’t improving quality. In fact, the grapes were losing color. Exacerbating the situation in Washington was that growers started using regulated deficit irrigation to further boost quality, says Julie Tarara, a USDA-Agricultural Research Service (ARS) horticulturist based in Prosser, WA.

“They wanted a smaller canopy for more radiation, but they were getting sunburn on the fruit, especially on the west side of the vine,” she says. “They came to us and said it must be because the sun is hotter in the afternoon. But no, that’s not the case.”

Award-Winning Study

Tarara, based at the Irrigated Agriculture Research and Extension Center in Prosser, and a team of scientists from ARS and Washington State University, Jungmin Lee, Sara Spayd, and Carolyn Scagel, embarked on a study of the problem. Their efforts resulted in a paper titled “Berry Temperature and Solar Radiation Alter Acylation, Proportion, and Concentration of Anthocyanin in Merlot Grapes,” which was recently named the Best Viticulture Paper for 2008 in the American Journal of Enology and Viticulture.

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The scientists punctured a long-held idea that seems to be common sense that it was the afternoon sun alone that was hurting the quality of the grapes by causing sunburn or sunscald. “It’s just that the world is warmer in the afternoon, so we think intuitively that the sun is more intense in the afternoon, but that’s not true,” says Tarara. “The same intensity of sunlight hits the grapes on the east side of the vine at 10 in the morning as hits the grapes on the west side at 2 in the afternoon.”

But if that’s true, the growers wanted to know, why weren’t their grapes on the east side of the vines getting burned as much as the grapes on the west side? (Most vineyards in the state are planted on a north-south orientation.) “What we found was that it was temperature extremes that were really driving the difference in berry quality,” Tarara says.

 

Cool Those Grapes

By using heaters and air conditioners, the team found that, in general, assuming the same amount of sunshine, the cooler the grape, the greater amount of anthocyanin it will contain. For example, grapes on the east side of the vines that were cooled had nearly double the anthocyanin concentration of grapes that were on the west side in full sun in the afternoon. Because anthocyanin levels correlate with deep, rich color in grapes, and because deep, rich color correlates with generally higher prices, growers would be advised to try and cool off those “hot grapes,” says Tarara.

That’s not easy, of course, as it certainly wouldn’t be financially feasible to air condition a commercial vineyard. But there are some steps growers could take, says Tarara. Some progressive growers are experimenting with misters for evaporative cooling. Short bursts of very fine droplets can effectively cool the fruit. Besides cost, there are some concerns about disease or excess irrigation, she notes.

Another alternative is managing the grapes on the west sides of the vines differently through the use of a modified sprawl system that creates dappled sunshine. But that too can be overdone, Tarara cautions. “We don’t want growers to go out and over-shade the fruit,” she says, noting that after all, there was a reason Sunlight Into Wine became so popular. “You have to have light on fruit.”

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