Yun Zhang and Markus Keller* 
*Washington State University, 24106 N Bunn Rd, Prosser, WA 99350
(mkeller@wsu.edu)

Irrigation is a commonly used management tool to manipulate yield and quality of grapes, especially in arid and semiarid regions. However, long-standing myths still have strong influences on vineyard irrigation practice. A better understanding of water relations of grape berries is fundamental to applying evidence-based irrigation practices. Contrary to previous beliefs, by tracing water movement in the xylem with xylem-mobile dye, it was found that berries remain hydraulically connected to the mother vine. When the pressure in the xylem was manipulated, inward dye movement into ripening berries (xylem inflow), which normally ceases at veraison, was restored and outward dye movement from berries (xylem back-flow) was stopped. Using a fruit growth model, it was estimated that, to meet the demand of berry sugar accumulation, phloem inflow exceeded the water demand for berry growth and transpiration. Along with apoplastically unloaded sugar, this surplus phloem water may alter the pressure gradient inside the xylem and thus the direction of xylem flow during berry ripening. As a practical implication, the surplus phloem water buffers ripening berries from changes in xylem water supply; therefore, unlike unripe berries, ripening berries become insensitive to xylem water supply (e.g., water taken up by roots from the soil). Consequently, the most effective time to control berry size is before ripening, and postveraison drip irrigation does not enlarge berries by adding water to them. It is thus recommended to apply adequate irrigation after veraison to maintain a healthy canopy for sugar accumulation in the berries and to replenish storage reserves in the permanent structures. Avoiding excess water stress close to harvest may also alleviate berry weight loss (dehydration) during extended hang-time.

Funding Support: USDA Northwest Center for Small Fruits Research, Washington State University, Chateau Ste. Michelle Distinguished Professorship, and Rhone Rangers