Handbook for Irrigation of Wine Grapes in South Africa

Figure 7.21

A

B

Figure 7.22

FIGURE 7.21. Effect of low frequency irrigation during winter (A) and severe water deficits during post-harvest and dormancy periods (B) on budbreak of Sultanina in a sandy soil in the Lower Orange River region. Photographs were taken on 11 September 2007.

Figure 7.22

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40 41 42 43 44 45 46 47 48

Cane water content (%)

Cane water content (%)

y = 4.106lnx + 38.49 (R 2 = 0.996) y = 4.106lnx + 38.49 (R 2 = 0.996)

0

2

4

6

8

10

Soil water content (mass%)

0

2

4

6

8

10

Soil water content (mass%)

FIGURE 7.22. Relationship between cane and soil water content before budbreak where Sultanina grapevines experienced severe water deficits during the post-harvest and dormancy periods in the Lower Orange River region (after Myburgh & Van der Walt, 2005).

The negative effect of low soil water content appears to be aggravated by low relative humidity, or high vapour pressure deficit, of the atmosphere during the post-harvest and dormancy periods. Relatively dry atmospheric conditions frequently occur in the Lower Orange River region, and are considered to contribute to the yield variation between seasons (Fig. 7.23). In this regard, it was shown that low frequency irrigation in combination with overhead pulse irrigation during winter slightly increased the cane water content of Sultanina grapevines in a

210 CHAPTER 7 – IRRIGATION STRATEGIES