Study of sugar phloem unloading in ripening grape berries under water stress conditions

Sugar phloem unloading in ripening grape berries (Vitis vinifera L. cv. Syrah) was studied under water stress conditions using the «beny-cup» technique. After veraison, berry growth, the potential Exposed Leaf Area (pELA) and photosynthetic activity are clearly reduced in water-stressed vines (- 0.5 > Ψb > - 0.6 MPa) as compared to normal 1 Ψ-watcred vines (Mb = - 0.2 MPa). The ratio pELA/yield is also reduced, which is particular to this experiment. The beiries' ripening period (between veraison and maturity) can be divided into three growth phases, Illa, Illb and IIlc. During phase Ma, the berries grow rapidly; at this point, water stress severely inhibits cell expan¬ sion of the berries but does not impact on daily sugar accumulation. During phase Mb, the berries grow slowly in both water-stressed and control vines. Water stress can shorten this phase and reduce sugar accumulation in the berries by decreasing daily sugar unloading. During phase II le, the Iresh weight and volume of the berries decreases as does the daily sugar unloading. During the day, sugar unloading in ripening berries occurs mainly in the morning (7 am to 10.30 am) and at noon (1 to 1.30 pm); little sugar is unloaded in the afternoon (4 pm to 4.30 pin). Moderate water stress from veraison to maturity affects végétative growth (i.e. the growth of primary and secoridary shoots), and reduces the exposed leaf area, photosynthetic activity, berry growth, and the accumulation of sugar at the end of ripening (phases Mb and IIlc).