The Syrah cluster on the  right was treated with the antitranspirant Vapor Gard; nontreated cluster is on the left. The treated cluster showed slower coloration,  a sign of delayed ripening.

The Syrah cluster on the right was treated with the antitranspirant Vapor Gard; nontreated cluster is on the left. The treated cluster showed slower coloration, a sign of delayed ripening.

PHOTO BY YUN ZHANG

A Washington State University graduate student recently returned from the world’s most prestigious wine university where she collaborated in research on how grape berry water flow influences ripening. Her research highlights the detrimental effects overhead irrigation could have in delaying sugar accumulation and increasing cracking, especially with Concord juice grapes and, to a lesser degree, with Syrah wine grapes.

Doctoral student Yun Zhang spent three months in France analyzing grape berry samples she brought from Washington State. The University of Bordeaux has a specialized molecular biology laboratory designed for genetic work. She received grants and financial support from both WSU and Bordeaux’s Institut des Sciences de Vigne et du Vin (Institute of Vine and Wine Sciences).

Zhang grew up in China and received her bachelor’s degree in agricultural engineering there, studying dams, reservoirs, and canals. She first became interested in wine grapes while conducting irrigation experiments on grapes for her master’s research in China. According to Zhang, grapes are primarily grown in northwest China—raisin and table grapes in the Xinjiang province and wine grapes in the Gansu province, near the ancient silk trade route.

“What’s interesting to me is that Washington State and China’s wine grape regions share similar latitudes and varieties,” she told Good Fruit Grower, adding that Merlot, Cabernet Sauvignon, and other vinifera varieties are grown in China as they are in Washington.

Her grape irrigation research led her to WSU’s Dr. Markus Keller, a horticulturist who specializes in grape physiology. Zhang finished her master’s degree at WSU and will complete her doctorate research of grape berry water relations and their role in berry development later this year.

Water flow

“Water going into the berry is how we get berry growth and yield, but we tend to forget about how water flows out of the berry and what that means for ripening,” she said.

“Just like in our human bodies, the grape berry has two circulatory systems, the xylem and phloem,” she said.

The xylem is the main pipeline that brings water and nutrients into the grape berry, while the phloem mostly imports water and sugars to the berry. But while the xylem can bring excess water out, the phloem only transports water and sugars into the berry, not out.

The water status of grape berries has a direct relationship to quality and yield, she said. “The net amount of grape berry growth is determined by the balance of the water ins [phloem and xylem inflows] and the outs [berry transpiration and xylem backflow].”

Too much water retained in the berry causes cracking, while not enough causes shrinkage, she explained.

Zhang likened the vascular pathway to a conveyor belt that unloads the sugar solution (soluble solids) into the berry. But as the berry fills up with soluble solids, a surplus of water from the phloem exists in the ripening berries. Disposal of the excess water requires both xylem backflow and berry transpiration.

Previous research has shown that the xylem declines in its function of being the main water supplier to the berry after veraison. Zhang said that at one time, scientists believed that rapid expansion of the berry caused the xylem to collapse. Keller and others have since learned that the xylem pipeline is still intact, although a hydraulic gradient causes the xylem to stop or decline its import function.

Zhang’s research has focused on the “outs” of water flow, trying to identify the gradient responsible for the declined xylem inflow, while examining what happens when the water flow out from the berry is restricted. Additionally, she has studied the hypothesis that sugars may leak out from the berries along with the xylem backflow.

“Since xylem backflow exists, and there is no membrane barrier that stops unloaded sugar moving into the xylem, how is it that the berry doesn’t lose sugar from the backflow?” she asked.

To test the sugar-recycling hypothesis, she spent three months in the molecular laboratory at the Bordeaux university, looking for gene expression in the pedicel that would indicate sugar recycling.

“If sugar recycling is going on, there would have to be sugar transporters,” she said. “The good news is that I found gene expression of all ten of the different sugar transporters, so the machinery is there. That’s the first step in the recycling hypothesis.”

Further research will focus on where the transporters are located and understanding protein activity.

Restricting xylem backflow

Zhang’s research focused on three cultivars—Merlot, Syrah, and Concord. She used three treatment methods, applied to preveraison berries, to see the effects of restricting the xylem backflow and inhibiting berry transpiration:

  • Applying an antitranspirant to fruit to inhibit berry transpiration
  • Drilling through the cluster peduncle to restrict xylem flow
  • Combining both the antitranspirant and drilling ­treatments

For all cultivars, the rates of soluble solids (sugar) accumulation in berries with restricted xylem flow or berry transpiration were lower than in the controls. In an additional experiment, she was also able to slow the ­progression of ripening (sugar accumulation) in potted Syrah vines by restricting xylem backflow using root ­pressurization.

By doubling up the berry’s waxy cuticle layer through the application of Vapor Gard, an antitranspirant used in fruits like cherries to prevent cracking from rain, Zhang was able to restrict berry transpiration, which increased cracking incidence in all three varieties. Syrah and ­Concord showed more cracking than Merlot.

She used a cluster chamber to monitor berry transpiration throughout the day and found that the rate of berry transpiration was highly correlated with vapor pressure deficit. Vapor pressure deficit is a measure of how dry the air is. The drier the air, the higher the vapor pressure deficit, and the higher the berry transpiration rate will be.

Berry transpiration is mostly cuticular transpiration, without any stomatal control, she explained. Zhang also found that berry transpiration increases initially during berry development and then declines, which means that berry transpiration may not be a reliable pathway to ensure water disposal from berries.

Practical implications

Zhang’s physiology research has implications for growers who use overhead sprinkler irrigation or have a high water status in their vineyard. Overhead sprinklers can increase the humidity around the clusters, which reduces the rate of berry transpiration. Conversely, growers who could manipulate berry transpiration by reducing humidity within the canopy and cluster zone may be able to avoid delayed ripening and increased cracking.

“Compared to Merlot, Concord and Syrah berries are very susceptible to cracking,” she said. Concord berries showed susceptibility to cracking throughout fruit development, but Syrah berries showed susceptibility after the onset of ripening and became less susceptible when Brix reached 20 degrees.

“Cautions are needed when using overhead sprinkler irrigation in a Concord vineyard, especially,” Zhang ­concluded.