Issue link: http://winesandvines.uberflip.com/i/137110
GRAPEGROWING balance of grape berries BY Yun Zhang and Markus Keller, Washington State University G rowers are constantly struggling to balance yield and quality in wine grapes. They are often asked to shut down irrigation sometime during ripening to achieve better quality. Moreover, a fear of dilution has led to the custom of not irrigating immediately pre-harvest. Surprisingly, there is little scientific evidence to support this practice. For every economy-minded vineyard manager, the goal should be achieving the highest potential yield without compromising quality. At ripeness, 70% to 80% of grape berry fresh weight is water.1 Also, water as a solvent determines the concentrations of all important compounds (sugars, acids, phenolics, etc.) that are essential to fruit quality. Therefore, it would be difficult to overestimate the importance of berry water balance to the commercial yield and quality of grape berries. Four components in water balance of grape berries Grape berries are storage organs whose growth and ripening are almost entirely supported by the water and sugar flows from the mother vine. Imagine grape berries as a checking account—there are two streams of "direct deposit" in terms of water: xylem and phloem. Xylem is the main pipeline transporting water and minerals into berries. Water flow in the xylem follows the hydrostatic pressure gradient, the same mechanism as in a garden hose; water always flows "downhill." On the other hand, phloem transports sugar solution, and it is virtually the only source of sugar supply to the berries. The grape "checking account" does not just receive deposits; water is also withdrawn as a normal physiological process. Grapes are not the only organs on a vine that require water supply. The berries are always competing with other vine components (mainly leaves) for water. As leaves are transpiring rapidly during a warm or hot day, the pressure gradient 64 p r acti c al w i ne ry & v i ne yard J U LY 20 13 points toward the leaves. Thus xylem flow can be reversed, going from fruit to the leaves. This is called "xylem backflow." Meanwhile, water also evaporates (transpires) from the berry surface. Although the rate of berry transpiration is much lower than that of the leaves (100 times or so), it is still not negligible, especially late in the season when the "deposits" from the xylem and phloem stop. The four components of berry water balance are summarized in Figure 1. Keeping a balance in the water "account" is important In order to mature normally, grape berries need to balance their water checking account. Unlike a bank checking account, grape berries have a "ceiling" (the skin), which limits the amount of water they can take in. The expandability of the skin is not boundless, and it declines during ripening.2 If there is too much water coming in but not enough water going out, the pressure exerted on the skin could crack or split the berries (Figure 2a). If water "deposits" do not equal "withdrawals" in grape berries, shrinkage will occur (Figure 2b). For example, pre- véraison berries may shrink during the Figure 1. Image of a peeled grape berry with pedicel showing the four components of berry water. Red (phloem) and blue (xylem) arrows indicate water inflows. Orange (xylem backflow) and pink (berry transpiration) arrows indicate water loss. Note that transpiration occurs at any surface while phloem and xylem are bundles embedded within the berry and pedicel. daytime, when competition from transpiring leaves is high, especially during water stress that limits water supplies from both the xylem and phloem. Shrinkage may also occur late in the season with extended hang-time (later harvest). Please note that this kind of shrinkage is dehydration, but not a physiological disorder (sour shrivel); the berries shrink because they lose water due to xylem backflow and transpiration. Late-season shrinkage (dehydration) may cause substantial yield loss, because when the shrinkage becomes visible, berries have already lost about 10% weight. OUTS INS Phloem inflow Transpiration Xylem backflow Xylem inflow (a) (b) INS Phloem inflow Xylem inflow OUTS Transpiration Xylem backflow Figure 2. When the berry water "account" is unbalanced, berries may (a) crack when water inflows exceed water outflows, or (b) shrink when water inflows are inadequate to balance water loss.