Issue link: http://read.dmtmag.com/i/114539
photo by melissa hansen photo by yun zhang 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. • 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. Yun Zhang, left, explains her research poster to Ste. Michelle Wine Estates's Mimi Nye during a poster session at the Washington Association of Wine Grape Growers convention. 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. • Trees on ® 9 7 97 OHxF 6 , 8 , & Rootstocks OHxF® 69, 87, & 97* – Match your specific variety and orchard to one of these rootstocks for maximum results. *OHxF® 69 Rootstock Shown Patent Information Available Call today for more detailed information (800) 675-6075 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: www.goodfruit.com b GOOD FRUIT GROWER March 15, 2013 37