Handbook for Irrigation of Wine Grapes in South Africa

Cultivars and treatments

Isohydric irrigated

Isohydric water stress

Anisohydric irrigated

Anisohydric water stress

-1.2 -1.4 -1.6 -1.8 -0.8 -1.0 -0.6 -0.4 -0.2 0

Ȍ L (MPa)

FIGURE 6.12. Mean midday Ψ L in isohydric and anisohydric grapevine species or cultivars that were either well-watered or experienced some degree of water stress (after Lovisolo et al ., 2010). Vertical bars indicate ± one standard deviation.

Measuring midday Ψ S in commercial vineyards growing in deep, red soils in the Devon valley near Stellenbosch revealed that cultivars differ slightly in their response to soil water depletion under the same atmospheric conditions. In this case, readily available water was taken as the difference between the highest neutron probe count ratio measured after heavy rainfall in winter, and the CR measured when the water content in all soil layers reached a plateau as the soil dried out. Since the slopes of the Ψ S vs readily available water depletion were comparable ( ca. 0.022), the rate at which water constraints developed was more or less the same in the four cultivars (Fig. 6.13). However, the y-axis interception, i.e. the Ψ S when the readily available water had been depleted, was lowest in Shiraz, followed by Merlot, Cabernet Sauvignon and Pinotage. This means that for a given level of soil water depletion, water constraints were the highest in the Shiraz and the lowest in the Pinotage grapevines.

156 CHAPTER 6 – GRAPEVINE WATER STATUS