Wines & Vines

March 2018 Vineyard Equipment & Technology Issue

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36 WINES&VINES March 2018 E ssential in arid and semiarid grape- growing regions, irrigation is a management tool used to manipu- late yield and enhance the quality of wine grapes. 4 However, longstanding myths of the wine industry still have strong influence on irrigation practices—to the detriment of vine health and the wine grape grower's pocketbook. The irrigation myth, known as the grape berry or wine-dilution theory, is the belief that late-season irrigation increases berry size and dilutes berry sugars and other quality-related components of the grapes. It is an Old-World irrigation dogma that often results in withhold- ing irrigation from grapevines during fruit ripening for fear water will plump up berry size, and wine quality will suffer. This theory is not supported by science. A better understanding of water relations of grape berries is fundamental to applying research-based irrigation practices. The Wash- ington State Wine Commission, a state agency that represents all wine grape growers and wineries, has helped support research at Wash- ington State University to develop optimum irrigation management for white grape variet- ies and regulated deficit-irrigation strategies for red cultivars. Research funded by the Northwest Center for Small Fruits Research has focused on grape berry water relations and the role of water in berry development. Both basic and applied research have brought better understanding of berry water movement and have practical implications for vineyard irriga- tion management by laying the foundation for development of more effective irrigation strat- egies for the wine industry. Water ins and outs The grape berry has two circulatory systems: the xylem and phloem. The xylem is the main pipeline that delivers water and mineral nutri- ents to the berry. Water can move in and out of the berry through the xylem, depending on the direction of pressure gradient. Pressure gradient in the xylem can be thought of as water coming out of a garden hose. When the faucet is turned on, water moves through the hose from the point of higher pressure to the point of lower pressure. The gradient is the difference in pressure between these two points, and the direction of flow is the same as the direction of the gradient. The phloem imports mostly water and sugar into the berry, but unlike the xylem, the phloem can transport only one direction: into the berry. Think of the vascular pathway as a conveyor belt that brings the sugary solution made from photosynthesis by the leaf (the source), into the berry (the sink). As the berry fills up with the sugary solution, a surplus of water can build up in the ripening fruit. The berry disposes of excess water by berry tran- spiration, which is evaporation across the berry skin into the atmosphere, and recycling it back into the vine through the xylem. Past research showed that the xylem lost function after véraison, and its role of being the berry's main water supplier declined. 2 Scientists thought the berry's rapid expansion caused the xylem to collapse, and therefore the berry was hydraulically isolated from the mother vine. By tracing water movement in the xylem with a xylem-mobile dye, it was found that ripening berries remain hydraulically con- nected to the mother vine. 1 The dye was put in the berry from the stylar end (the end op- posite the pedicel), and then it was traced in the pedicels, peduncles and even shoots of the vine. This showed that the water can flow out of ripening berries through the xylem, and therefore the xylem can still conduct water. In addition, the conductivity that determines how easily water can flow through the xylem remained high at the onset of ripening. 5 There- fore, it was still a mystery as to why water stopped entering ripening berries through the xylem. In order to understand this, dye move- ment was observed when pressure in the xylem was manipulated. When detached clusters with Grapes and Irrigation Myths Debunked Research shows post-véraison irrigation does not increase berry size but may reduce dehydration By Yun Zhang and Melissa Hansen DR. MARKUS KELLER, WASHINGTON STATE UNIVERSITY KEY POINTS The long-standing irrigation myth known as the grape berry or wine-dilution theory has had strong influence on irrigation practices and often results in withholding water from grape- vines during ripening in fear that water will plump up berry size and impact wine quality. Recent research has brought better under- standing of grapevine water movement and disproved this theory. Late-season drip irrigation (not overhead sprinklers that can induce berry cracking) does not increase berry size but can help prevent berry dehydration and potential loss of yield and help prepare vines for winter dormancy. Drip irrigation after véraison does not increase berry size (above) but can help prevent or stop berry dehy- dration. The red arrows identify dehydrated berries.

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