Issue link: https://read.dmtmag.com/i/705606
w w w. v w m m e d i a . c o m J u l y - A u g 2 016 | V I N E YA R D & W I N E RY M A N A G E M E N T 6 3 ment from the berry to the leaves and shoots. "Previously, the textbook belief was that the xylem became dys- functional at veraison and only brought water and minerals into the berry up to that time, while the phloem brought the sugars in solu- tion," Keller says. "We saw that the xylem can easily move water out of the berry. But we're still fig- uring out the how and why." In explaining the why, Keller says that, after sugars in the berry are unloaded from the phloem's sugar water solution, there's a need to remove the excess water. The berry can't transpire enough to do the job, so the xylem pathway helps the berry in disposing excess water. YIELD LOSS Keller is often asked how much yield loss occurs when winemak- ers want to hang fruit for extended ripening to maximize flavor profiles and reach high sugars of 26 °Brix and above. At 23 to 25 °Brix, ber- ries reach their sugar maximum. "Any further increase in sugar is a result of sugar concentration from dehydrating and shrinking berries, not from sugar import," he says. In general, berries lose 5% to 10% in weight for each increase above 23 to 25 °Brix, but the weight loss varies by cultivar. With- out quantifiable data regarding wine grape weight loss, it's diffi- cult for wine industry personnel to Joel Perez, one of Keller's most recent grad students, took weekly berry weight and Brix measure- ments once fruit reached 20 °Brix on nearly 25 wine grape variet- ies for two years to quantify berry weight loss due to dehydration or shrinkage. His study found that, while some didn't lose weight, some lost up to 45% during the extended hang time. The prelimi- nary research showed that berry weight loss always began before visual symptoms of shrivel were observed. INDUSTRY ADOPTION As a result of the WSU research, many Washington winemakers have changed their irrigation mindset. Jim Holmes, owner of Wash- ington's acclaimed Ciel du Cheval Vineyards on Red Mountain near Benton City, shared that when Keller's research was first pub- lished, he sent copies to all of his winemaker customers. "At one time, I had several winemakers concerned about late season irriga- tion, but Markus Keller's research clearly showed there's no problem with drip irrigation and dilution," says Holmes. His customers are told up-front that he irrigates up to the end of the season. "In my world, there haven't been any winemaker issues with our irrigation practices." Holmes, who takes rigorous grape chemistry measurements throughout fruit ripening, notes that he hasn't seen any change in their measurements to show dilution or decreased fruit quality effects from irrigation. "The vine can be shortchanged if irrigations are withheld during fruit ripening, especially during warm vintages," he says. "If you have a long hang time and no irrigation, the plants are ready to call it a day before the fruit is picked." Yakima Valley appellation grape grower Dick Boushey of Boushey Vineyards agrees that most wine- makers have changed their think- ing to now allow drip irrigation after veraison. He admits to discuss- ing research results often with his estimate yield and economic losses from extended hang time. "Grow- ers know they're losing tonnage from extended ripening because they can see their fruit shrivel," says Keller. "But it's hard to ask for compensation if you don't know how much you're losing." An example of Cabernet Sauvignon grapes shriveling from dehydration before harvest. + Ripening Grape Berries Remain Hydraulically Connected to the Shoot, Journal of Experimental Botany (Vol. 57:2577-2587) + Sugar Demand of Ripening Grape Berries Leads to Recycling of Surplus Phloem Water via the Xylem Plant, Cell & Environment (Vol. 38:1048-1059) + Water Economy in Grape Berries http://wine.wsu.edu/research-extension/files/2013/04/2013-VEEN-Spring- FINALsmaller.pdf + Pre-Harvest Irrigation Dilutes Grape Quality! Or does it? http://winegrapes.wsu.edu/Newsletters/vol18-1-2008.pdf ADDITIONAL READING