The Mendoza Connection

When Daniela Romero heard Washington State University’s Markus Keller talking about irrigation during grape ripening, her curiosity was piqued. After all, applying water close to harvest time was simply not done.

Keller was teaching a grape physiology course at the Universidad Nacional de Cuyo in Mendoza, deep in the heart of Argentina’s wine country. Romero is a graduate student at the university, and she asked if she could join Keller’s research team in Prosser to learn more about his tradition-defying research.

“Keller’s research is important to the wine industry, because it will influence the way growers add water to their vineyards. In most of the world’s wine regions, irrigation during grape ripening is thought to dilute the sugars in grapes–but this belief does not have any scientific foundation,” Romero said.

As Keller pointed out, “The European wine industries and their many regulators have it all figured out: irrigation during grapes’ critical ripening period is generally a bad thing and must be strictly regulated.”

A quote from the International Organization for Biological and Integrated Control in their 1999 Guidelines for Integrated Production of Grapes illustrates Keller’s point: “Irrigation of vines for wine production will not be applied after véraison or highly restricted by the regional guidelines in order to guarantee the good quality of the wine.”

“The tacit assumption is that irrigation boosts berry size and dilutes the quality-impact components of the grapes,” Keller said. “So pervasive is this argument that, even in the New World, many wineries encourage growers to withhold irrigation water during fruit ripening to avoid any perceived adverse effects.”

Keller and former graduate student Marco Biondi put the assumptions to the test–with startling results that fly in the face of viticultural tradition.“We proved that berries are not hydraulically isolated during ripening,” Biondi said in a 2008 interview. Indeed, Biondi’s experiments show that berries absorb water in a variety of ways, including through the skin and not just through the root system, as commonly believed.

“Late season irrigation doesn’t decrease Brix,” Biondi said, “but irrigation does increase photosynthesis in the leaves, and photosynthesis in turn increases Brix.”

“In other words,” said Keller, “irrigation accelerated ripening!”

Now, Romero is putting Keller and Biondi’s results to the test in a series of field-based experiments in Argentina.

“This is important, for a number of reasons,” Keller said. Better knowledge of the potential contribution of late-season irrigation and water stress to variations in berry size will lay the foundation for better vineyard irrigation management. It also will end the long-standing debates whether irrigation close to harvest will dilute grape sugar and flavors, or whether heavy irrigation may increase berry volume in juice grapes.

Such knowledge is needed to avoid excessive water application or deficit close to fruit maturity, which is potentially detrimental to fruit quality, canopy health, cold acclimation, and vine longevity. It will also improve efforts to estimate yield and make yield prediction more accurate and reliable. This will improve harvest planning, grape and wine quality management, and marketing to ensure a consistent, high-quality supply of fruit for both domestic and export markets.

Large wine companies have already begun to modify their irrigation practices based on results from this study, Keller said, because conservative estimates suggest that the strategy of increasing water supply close to harvest may prevent a greater than five percent yield loss. For Washington alone this would amount to increased returns to growers in the order of $6 million per year and almost $2 million per year to juice grape growers.

In the mean time, Romero’s experiments should yield results soon. When they do, you’ll read about them here, in Higher Learning.