Issue link: http://winesandvines.uberflip.com/i/946131
38 WINES&VINES March 2018 GRAPEGROWING PRACTICAL WINERY & VINEYARD But during véraison, very few changes in berry size were observed in response to water stress or rewatering. Soil-water availability affects xylem water supply to the berries directly. The reason that ripening berries become insensitive to soil moisture changes is because the surplus phloem water buffers the ripening berries from changes in the xylem water supply. To further prove this, the roots of well-watered, pot-grown vines were pressurized at 1 MPa (145 psi) for three hours. During pressurization, water was pushed up- ward through the xylem (as if the soil water supply was increased by root pressurization). From the start of pressurization, the size of pre- véraison berries increased. But the size of post- véraison berries remained the same. Clearly, post-véraison berries did not expand with in- creased soil water supply. But, berries do still grow during ripening, so where do they get water to support their growth? The answer is the phloem. At the start of ripening, the phloem brings both water and sugar into the berries. Since the sugar concen- tration (commonly measured as Brix) of berries increases during ripening, the phloem deposits more sugar than water inside the berries. Late-season irrigation benefits Consequently, it was found that the most ef- fective time to control or influence berry size is before véraison. Since berries are sensitive to soil moisture availability before véraison, smaller berry size can be achieved by impos- ing water stress. While berry growth still occurs during and post-véraison, increase in berry size is supported by phloem inflow and thus is accompanied with sugar import. Therefore, it is a myth that late-season drip irrigation just adds water to the berry and plumps it up. Irrigation close to harvest helps maintain a healthy canopy for leaf photosynthesis, sugar transport through the phloem and sugar accumulation in the berries to continue ripen- ing. A healthy canopy helps replenish storage reserves in the vine's permanent structures before dormancy. In addition, post-véraison irrigation can reduce or stop any previous decrease in berry size from dehydration. But there is a difference between drip and overhead sprinkler irrigation. For clari- fication, drip irrigation after véraison will not decrease berry sugar concentration or dilute berry quality. The reason behind this is that drip irrigation supplies water to the soil; as mentioned, berries become insensi- tive to soil moisture after véraison. Post- véraison berry enlargement is contributed by phloem inflow, which brings both sugar and water. Berry dehydration Generally, it has been found that berries reach IMPLICATIONS OF GRAPE BERRY WATER RELATIONS FOR VINEYARD MANAGEMENT Growth stage Physiological function Berry response Vineyard-management practice Before ripening Xylem supplies most of the water to berries. Berry size is sensitive to soil water availability. Best time to control berry size through deficit irrigation. During ripening Phloem supplies most of the water to berries. This is a time of rapid sugar accumulation. Berry size is insensitive to soil water availability. Need adequate irrigation for photosynthesis and sugar accumulation. After maturation No more phloem inflow and sugar accumulation. Weight loss in berries can occur due to dehydration. Need adequate irrigation to avoid excess dehydration or new canopy growth.