Winegrape growers prepare vines for rising temperatures
“Sun-exposedberriesarewarmberries,” Keller said. Leaves, he said, are designed to max- imize photosynthesis. Water loss has an adverse consequence, but evaporative cooling provides a beneficial side effect. Berries, meanwhile, are designed to minimize transpiration. They must ac- cumulate water to grow. Sunburn has ad- verse consequences, but water loss facili- tates sugar imports and ripening. Kellersaidgrapeberrieshavenostomatal control after veraison. Ripening berries are vulnerabletoheat injury.Hesaidpost-verai-
son irrigationmayprevent, but not reverse, berry shrinkage fromdehydration. Yield and berry size are determined before veraison, and compensation after veraison is not possible. Before veraison, Keller said, less irriga- tion will help minimize berry size. After veraison, a farmer should irrigate enough to limitberryshrinkageandfavor ripening. Keller said heat waves are a recur- ring issue in grape production and “very hot days” are on the rise. That
ByDennis Pollock Temperatures on the rise worldwide have turned up the heat on vineyards. Different varieties of grapes respond differently—some more tolerant than others—but all face consequences, and growers are taking steps to mitigate the harmthat can come to them. Those steps were discussed in a semi- nar at Fresno State University presented by Markus Keller, professor of viticulture atWashingtonStateUniversity. The semi- narwas part of a series of classes stagedby theFresnoStateDepartment ofViticulture and Enology. Keller discussed the adverse effects of heat and drought and talked of ways to combat them, includingan innovativeuse of water-saving canopy misters that rely on sensors. He started his talk by pointing out Vitis viniferaevolved inwarm, arid regions and grapes are, in fact, heat- and drought-tol- erant. But he saiddifferences among some 5,000 varieties “are poorly understood; some may be better adapted to heat and drought than others.” Keller forecast “more heat ahead.” “That’skindof alarming,”hesaid, “and it getsworsewhenyou lookat the futureand the greater number of hot days in the late 21st century.” He said he is especially concerned about forecasts there will be more hot days—warmer than 95 degrees—during the grape-growing season and the ripen- ing period. He noted that a vine planted in 2020 would face a different climate by the end of its useful lifetime 30 years later. Keller said some varietieswill no longer besuitableand, over time,will bereplaced. He displayed a chart that showed how a wide variety of grapes are better able to copewith various temperature ranges. The cool end of the spectrum included pinot gris. Chardonnay was among those in an intermediate range. Cabernet sau- vignonwas among those able to handle a warm climate; at the hot end of the scale were raisin and table grapes. “Interactionbetweenvarietyandclimate determines fruit quality and wine style,” Kellersaid,addingthat it’s important tolook at theenvironment inwhichgivenvarieties alreadyarethriving, tohelpdeterminetheir suitability for cultivation inother regions. Keller said grape varieties cope with stress through such processes as transpi- ration or evaporative cooling, which pre- vents leaves fromoverheating. Stomata close under water stress. Heat stress bringsmorewater loss, dehydration with lowsoilmoistureand leafwiltingand canopy collapse. Drought stress compromises yield. As stomata close, photosynthesis declines, meaning less sugar is available and there is less carbon in the fruit. Drought stress before veraison results in smaller berries. Keller said, whereas drought stress after veraisonresults inberryshrinkageand less total soluble solids. “Heat stress can also have an effect on
yield, just likewater stress,”he said, noting that as heat rises, results can include poor bud fruitfulness and poor fertilization. Keller said the combination of heat stress and drought stress brings a worse response thanwhen dealingwith just one of the two. A water deficit results in small berries and low shoot vigor.
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March 24, 2021 Ag Alert 9
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