Handbook for Irrigation of Wine Grapes in South Africa

TABLE 6.5. Thresholds for water constraint classes for midday Ψ L

and Ψ S

water potential in

Cabernet Sauvignon (Myburgh et al ., 2016).

Class Water

Water potential (MPa)

constraints

Ψ L

Ψ S

Ψ L

Ψ S

≥ -1.15

≥ -0.6

I

No

-0.6 > Ψ S

-1.15 > Ψ L

≥ -1.3

≥ -0.85

II

Low

-0.85 > Ψ S -1.15 > Ψ S

-1.3 > Ψ L

≥ -1.45

≥ -1.15

III

Moderate

-1.45 > Ψ L

≥ -1.6

≥ -1.4

IV

High

Ψ S

Ψ L

< -1.6

< -1.4

V

Severe

Similar to water potential, grapevine water constraints can be classified according to the carbon isotope discrimination in the juice of ripe grape berries (Table 6.6). According to this classification, the non-irrigated, dryland Merlot grapevines as discussed in Section 6.2.2 (Fig. 6.10) experienced moderate to severe water constraints. On the other hand, grapevines irrigated twice a week, or according to the partial root zone drying concept, only experienced weak water constraints. TABLE 6.6. Thresholds for water constraint classes for carbon isotope discrimination ( δ 13 C) determined in ripe grape berries as proposed by Van Leeuwen et al. (2009). Class I II III IV V Water constraints No Weak Moderate to weak Moderate to severe Severe δ 13 C < -26 -24.5 to -26 -23 to -24.5 -21.5 to -23 > -21.5 When water potential is being used to improve vineyard irrigation, it is essential to know how grapevines will respond to water constraints. Grapevine growth and yield, as well as juice and sensorial wine quality characteristics differ in their responses to increasing levels of water constraints. In this regard, Cabernet Sauvignon responses were related to midday Ψ S in 54 experiment plots. These plots were located around Stellenbosch and in the Lower Orange River region (Myburgh, 2011l), as well as in the Swartland (Mehmel, 2010) and Lower Olifants River region (Bruwer, 2010). In order to increase variation, grapevines were either grown dryland, or irrigated by means of single or double dripper lines. In some plots, grapevines were also subjected to water deficits by reducing irrigation frequencies to expand the range of responses. For further details of the study, refer to Myburgh (2011l). The responses to the different midday Ψ S water constraint classes and soil texture were previously summarized as shown below (Myburgh et al. , 2016). 6.5 GRAPEVINE RESPONSES IN RELATION TO MIDDAY STEM WATER POTENTIAL

172 CHAPTER 6 – GRAPEVINE WATER STATUS