Handbook for Irrigation of Wine Grapes in South Africa

Ideally, they should experience moderate water constraints to obtain high wine quality with ca. 5% reduction in yield. Since high to severe water constraints will reduce yield substantially in sandy soil, it must be avoided. Grapevines in heavier soil should experience high water constraints to obtain high wine quality with ca. 15% reduction in yield. If yield is the primary objective, grapevines should experience mild water constraints to obtain a balance between yield and wine quality. In this regard, -0.6 MPa is considered as a critical midday Ψ S threshold. Although high wine quality is expected where grapevines experience severe water constraints, i.e. midday Ψ S lower than -1.4 MPa, it will only be economically viable if high wine prices can compensate for the expected ca. 40% yield loss. Since water potential responses to soil and atmospheric conditions can vary between cultivars, all cultivars might not respond in exactly the same way to water constraints as Cabernet Sauvignon. For example, water potential in Shiraz is lower, compared to other cultivars under comparable soil and atmospheric conditions. Therefore, anisohydric behaviour of water potential, as well as related growth, yield and quality responses of the more popular cultivars, should be determined in more detail by further research.

176 CHAPTER 6 – GRAPEVINE WATER STATUS