Recent research shows that Riesling and other white wine grape varieties may benefit from an irrigation strategy that uses partial root-zone drying. <b>Courtesy Jack Kelly Clark, University of California Statewide IPM Program)</b>

Recent research shows that Riesling and other white wine grape varieties may benefit from an irrigation strategy that uses partial root-zone drying. Courtesy Jack Kelly Clark, University of California Statewide IPM Program)

Growers have employed deficit irrigation in red wine grapes for years, knowing that smaller berries result in a higher proportion of berry skins favorable for tannins and other phenolic compounds that improve wine flavor and color.

What about white wine grapes?

White wine grapes are not fermented with the skins, and too many phenolic compounds generally make the wine more astringent and increase instability during aging anyway.

They also are more susceptible to sunburn, which can increase bitterness. For that reason, a deficit irrigation strategy that is beneficial to reds may not be suitable to whites.

Researchers at Washington State University are examining whether white wine grapes could receive less water through a technique called partial root-zone drying and still maintain the quality of the grapes and wine.

Results from the first two years of study are promising.

“The potential benefit is that it seems like partial root-zone drying is a technique that does not reduce berry size or yield as much as conventional deficit irrigation. We think that will be beneficial especially to white wine grapes,” Dr. Yun Zhang, a postdoctoral research associate at WSU’s Irrigated Agriculture Research and Extension Center in Prosser, Washington, told Good Fruit Grower.

The technique

Both irrigation strategies impose some water deficit to constrain canopy growth and control vigor and berry size, though they differ in their approach.

Regulated deficit irrigation imposes a soil water deficit over time and is applied at specific points in the growing season.

It results in a plant-water deficit and reduces berry size and yield.

Partial root-zone drying, a technique developed in Australia, imposes a soil water deficit spatially and, in this case, was applied from fruit set to harvest — essentially drying out one side of the root zone while the other side is being irrigated, supplying the vine enough water for growth.

With two drip irrigation lines running on both sides of the vine, the technique is more expensive. However, by alternating which side receives irrigation, the vines are tricked into thinking that there is soil water deficit, reducing their canopy growth, and berry size and yield reduction are less severe.

Yun Zhang, Washington State University

Yun Zhang

“It has enough water to support growth and photosynthesis,” Zhang said. “Meanwhile, the drying part of the root zone generates hydraulic and chemical signals to trick the vine.”

For the past two years, Zhang and WSU viticulturist Dr. Markus Keller have been evaluating a selection of deficit irrigation strategies, including partial root-zone drying, to try to determine when and where water is most needed, while improving productivity and grape quality.

They have focused on Chardonnay and Riesling grapes, which account for 75 percent of white wine production in Washington, applying several different irrigation strategies through the growing season for two years.

They included treatments where no water stress was intended, different deficit irrigation strategies and partial root-zone drying. In all cases, soil water content was replenished after harvest and again before bud break if necessary.

Overall, the partial root-zone drying showed some clear benefits relative to conventional deficit irrigation strategies, Zhang said. The berries were bigger and the yield was heavier, and when compared to strategies that employ no water stress, the technique reduced the amount of irrigation water applied.

Partial root-zone drying resulted in similar water use compared to the other deficit irrigation trials, she said.

“It’s definitely a lot less than having no water stress,” she said. “The idea is that by using this technique, you still reduce the amount of water usage, compared with not applying water stress at all. At the same time, it can give you less yield and berry size reduction compared with conventional deficit irrigation treatments.”

Ste. Michelle Wine Estates is collaborating on the research. Dr. Russell Smithyman, director of viticulture, said it’s nice to have researchers’ interest on irrigation in white varieties again.

“It goes back to trying to get the best quality and water savings that we can in the vineyard,” he said. “What we’re seeing is that we’re maybe applying more water than we need to for the quality purposes. And with the semi-deficit, the partial root-zone drying, the nice thing about that is they’re showing there are similar types of wines and maybe even better wines produced under partial root-zone drying.”

Ste. Michelle completed irrigation research on Sauvignon Blanc in the 1990s, Smithyman said. “There, our focus was to see if water deficit at that time could control canopy growth.

Since then, we’ve come to this philosophy of trying to stylize wine in the vineyard by manipulating canopy growth through irrigation or water additions,” he said.

However, in the years since, most industry research has focused on red varieties.

“Now we’re seeing the right way of watering white varieties. We’re working to be more efficient, maybe use less water and still have high quality,” he said. “We’re very excited about this work in whites, and we look forward to these final results.”

Another hypothesis the researchers want to study is whether partial root-zone drying will result in a slightly bigger canopy, better protecting white wine grapes from overexposure to sun. They will continue their study this year.

Sun exposure and skin phenolics

As part of this project, Dr. Letizia Rocchi, then a visiting doctoral student from the University of Milan, Italy, also studied the phenolics in Chardonnay and Riesling grapes in response to sunlight exposure under two irrigation regimes.

One featured full irrigation, where vines were irrigated to replace 100 percent crop evapotranspiration from fruit set to harvest, with no water stress imposed.

The other employed deficit irrigation where vines were irrigated to maintain a moderate water stress from fruit set to harvest. She also evaluated berries exposed to direct sunlight and others totally shaded from sunlight.

Rocchi said high light regimes potentially stimulated a protection mechanism in the skin, increasing flavonols over the season.

Flavanols also had their highest concentration in the sun; it could be speculated a possible involvement of these molecules in the photo-protection mechanisms of the berry, the study showed.

Hot temperatures did not affect biosynthesis of flavanols, but could have had a role in significantly reduced flavanol formation in Chardonnay, especially at harvest, Rocchi said.

Flavonols, found in grape skins, contribute to bitterness and protect fruit from sunburn. Flavanols, meanwhile, polymerize — attach to each other to get bigger — and get more astringent as they enlarge. They also contribute to bitterness and to tannins in wine.

Overall, the research showed that overexposure of clusters to sunlight led to higher concentration of compounds that potentially elicit bitterness and astringency in the final wine.

No effects on flavonol and flavanol accumulation were directly due to the irrigation regimes in either cultivar (Chardonnay and Riesling).

However, leaf area reduction as a consequence of water deficit, thus irrigation regime management, seems to indirectly affect the flavanols and flavonols by providing high exposure of fruit, also generating sunburn appearance.

The results suggest that when planting vineyards growers should consider orientation to avoid peak sun exposure; also, to avoid trellis systems that have excessive fruit exposure; to avoid severe leaf removal; and to manage the water deficit, thus irrigation regimes, to avoid an excessive canopy reduction, which could generate excessive cluster exposures.

Collaborating on the project were Keller, Smithyman, Dr. Jim Harbertson, WSU enologist, and Professor Osvaldo Failla of the University of Milan. The Washington Grape and Wine Research Program, the Washington Wine Commission and a WSDA Specialty Crop Block Grant Program funded the research. •

– by Shannon Dininny

Tip for growers

Dr. Letizia Rocchi’s research at the University of Milan also showed that the timing of leaf removal in Chardonnay and Riesling grapes could be key to reducing the appearance of brown color in berry skin.

In Chardonnay, growers are advised to avoid any leaf removal in the morning and to remove leaves in the afternoon. Because Riesling is more susceptible to sunburn in the afternoon, growers should do leaf removal during the early morning, if necessary.

This research was carried out under the COST Action and InnoVine project, a European collaborative project involving 27 partners from seven European countries.