Shriveling grape berries on severely water-stressed vines mysteriously begin expanding again at veraison, even while the leaves are wilting.
For years, wine grape growers have been discouraged from applying irrigation near harvest for fear of diminishing fruit quality. Recent discoveries by Washington State University scientists have debunked that theory as well as another long-held belief about grape physiology.
Since the mid-1980s, grape physiologists believed that the xylem inside grape berries broke down at veraison, said Dr. Markus Keller, Washington State University viticulturist. This was based on a "not very focused" photograph showing what was thought to be damage to the xylem at the base of the berry. Previous experiments had shown that dye infused at the shoot base was taken up by the xylem and moved throughout the berry before veraison. But the same dye infusion made after veraison was always confined to the brush area of the berry, he explained.
"The xylem rupture theory became accepted truth."
A discussion Keller had with a winemaker brought to a head the xylem theory and the widely accepted belief that berries increased in size from late irrigations. "They cannot both be true," he recalls saying to the winemaker. "Either the textbooks are right, and the berries don’t mind irrigation at all because the berries don’t get bigger. Or the textbooks are wrong," he said, adding that after the Âdiscussion, he lost a few nights of sleep.
By working backwards with the dye and infusing it into the stylar end of attached berries, Keller, visiting Australian scientist Jason Smith, and WSU viticulturist Dr. Bhaskar Bondada showed that the grape berries remain hydraulically connected to the shoot. They found the infused dye moved throughout the berry and recycled back to the leaves when it was applied to the top of the sliced berry.
There’s a widespread belief that rain or irrigation close to harvest may increase berry size and cause a dilution of solutes, like sugars, acids, tannins, and such, or even result in cracking or splitting of berries, Keller said. "In Europe, this belief is often written into law, and irrigation is prohibited or strictly restricted."
There’s also the notion that rain close to harvest is bad for fruit quality. "That notion is true," he said. "But it’s also been believed that irrigation is the same as rain."
By water-stressing Merlot and Concord vines before and after veraison, he found that berries shrank in size when stressed before veraison, only to "pop back up like balloons" when they were rewatered. However, vines stressed after veraison did not plump back up–the shrinking berries merely stopped shrinking when watered again. "There was not an increase in berry size, except for one Merlot berry that began changing color and gaining volume before rewatering," he said.
"It made no sense," Keller said, puzzled. "Here, the leaves were wilting, and one berry was getting bigger."
That one berry led to Keller’s research paper published in the Journal of Experimental Botany, a paper that has changed grape physiology textbooks around the world. Put simply, as sugar production increases inside the berry, the phloem and xylem act as conveyor belts, moving sugar in and excess water out of the berry. The hydraulic pressure inhibits the dye infused at the shoot from moving into the berry, which is why earlier experiments didn’t detect dye throughout the berry.
"The problem was that the textbooks and the winemakers have been wrong," Keller said, explaining that late-season irrigation did not result in any increase in berry size. Irrigation did, however, keep the berries from shrinking further, although the difference would not be noticed with bare eyes.
Grape growers withholding late-Âseason irrigation were losing tonnage, even if the shrinkage was hard to discern, he said.
The growers were losing crop and hurting their plants because the vines need to start putting reserves down for the coming year to become cold hardy. Irrigation late in the season allows growers to maintain quality–especially during hang time while they are trying to get more flavors in their grapes–and have bigger crops. For the grower, that’s huge."
Wine-growing companies like Gallo and Ste. Michelle Wine Estates already have made changes to their late-season irrigation practices based on the new findings, Keller said. Gallo is now applying water when grapes reach 20° Brix to maintain quality and tonnage, although Keller said that he would probably start Âirrigation earlier around 15 ° Brix.
Keller is quick to point out that he is not advocating "overwatering" wine grapes. Growers must monitor shoot growth and not apply so much water that shoots begin growing vigorously.
For late-season irrigation, he recommends that growers wait until all the berries have turned color after veraison and then apply water–but not so much that shoot growth is stimulated.
Keller and WSU graduate student Marco Biondi did find a difference between rainfall, overhead sprinkler irrigation, and furrow or drip irrigation. When attached berries were dipped into small cups of water late in the season they expanded, absorbing the water.
"You can have a terrible cracking problem, especially in Concord grapes," Keller said, adding that they rarely got wine grapes to crack in their study, although they only experimented with a few varieties (Merlot, Cabernet Sauvignon, and Chardonnay). "But with Concords, they just popped."
Most wine grapes are already on drip irrigation, but many juice grapes are still under sprinkler, he said. "I encourage juice grape growers to switch from sprinkler to drip if cracking is an issue."
Once Concord berries begin to crack, their volume increases, and sugars leach out of the berries. In his studies, Concords at 16Â° Brix were reduced to 2 ° Brix after berries soaked in water for five days.
"I’m not so concerned about our dry climate and cracking from rainfall, but for juice grape growers in New York, Pennsylvania, and Michigan, it’s a different story."
A WSU postdoctoral scientist in Keller’s lab, Dr. Pradeep Shrestha, is further studying sugar accumulation inside the berry, looking at cells and cell walls. Keller also hopes to have another graduate student continue these studies. Thus far, Âlaboratory studies have been funded through the Northwest Center for Small Fruits Research, but he doesn’t have Âfunding to conduct field studies.
He said he has no idea if the hydraulic connection concept applies to other fruits, but noted that Dr. Matthew Whiting, stone fruit horticulturist at WSU’s Prosser research center, plans to look at the concept as it relates to cherries.â€‚