Handbook for Irrigation of Wine Grapes in South Africa

Figure 7.9

250

Budbreak

Pea size

Véraison

Harvest

FC

200

150

No irrigation Three irrigations Five irrigations

100

PWP

50

Soil water content (mm/0.9 m)

0

Sep

Oct

Nov

Dec

Jan Feb

FIGURE 7.9. The effect of non-irrigated (dryland) conditions and low frequency drip irrigation on the variation in soil water content over the season near Wellington, where FC and PWP indicate field capacity and permanent wilting point, respectively (after Myburgh, 2011a).

TABLE 7.3. The effect of low frequency irrigation on midday Ψ S at harvest, cane mass, berry mass, yield and sensorial wine quality of Merlot near Wellington in the Coastal region (Myburgh 2011b). Data are means for three seasons. Parameter Number of irrigations 0 3 5 PAW depletion at harvest (%) 95 90 66 Ψ S (MPa) -1.4 b* -1.4 b -1.2 a Cane mass (t/ha) 1.2 a 1.3 ab 1.5 b Berry mass (g) 0.9 b 1.1 ab 1.2 a Yield (t/ha) 8.2 b 9.7 ab 11.7 a Juice pH 3.32 a 3.36 a 3.36 a Juice TTA (g/ ℓ ) 6.0 a 5.9 a 5.7 a Colour (%) 80.0 a 78.0 a 71.0 b Berry character (%) 50.7 b 57.3 a 51.3 b Wine quality (%) 57.0 a 57.7 a 51.3 b * Values followed by the same letter within a row do not differ significantly (p ≤ 0.05). Irrigation affects the juice pH and TTA indirectly via the effect of water constraints on the vegetative growth (Iland, 1989). The subsequent effects of shading or exposure of leaves and berries on the pH and TTA depend on which process is dominant under the prevailing conditions (Fig. 7.10). Since the low frequency

186 CHAPTER 7 – IRRIGATION STRATEGIES