Handbook for Irrigation of Wine Grapes in South Africa

Chapter 7

irrigation increased vegetative growth of the Merlot grapevines, it probably caused more shaded leaves and berries, compared to no irrigation. The tendency towards higher pH and lower TTA suggests that the effect of more shaded leaves dominated the effect of more shaded berries under the prevailing conditions (Fig. 7.10). In contrast to dryland conditions or three irrigations, five irrigations reduced sensorial wine colour and berry character (Table 7.3). Furthermore, five irrigations produced poorer overall wine quality than dryland conditions or three irrigations. It must be noted that three irrigations will reduce the risk of yield losses without any detrimental effects on wine quality (Table 7.3). It is also important that, although non-irrigated grapevines produced the smallest berries, it did not necessarily improve the wine quality. Figure 7.10

A

High water constraints

More exposed leaves

More exposed berries

More efficient photosynthesis

Higher berry temperature

Leaves export less K + to berries Malic acid respiration higher Lower pH & higher TTA Lower malic acid, higher pH & lower TTA

Higher pH & lower TTA if berry exposure dominates

Lower pH & higher TTA if leaf exposure dominates

Lower pH & lower TTA if leaf exposure dominates the pH equilibrium and berry exposure dominates the TTA measure

Low water constraints

B

More shaded leaves

More shaded berries

Less efficient photosynthesis

Lower berry temperature

Leaves export more K + to berries Malic acid respiration is slowed Higher pH & lower TTA Higher malic acid, lower pH & higher TTA

Lower pH & higher TTA if berry shading dominates

Higher pH & lower TTA if leaf shading dominates

Higher pH & higher TTA if leaf shading dominates the pH equilibrium and berry shading dominates the TTA measure

FIGURE 7.10. Schematic illustration of pH and total titratable acid (TTA) dynamics in the juice when grapevines experience (A) high and (B) low levels of water constraints (Iland, 1989).

Low frequency drip irrigation caused extremely dry conditions, i.e. close to field capacity, in a fine sandy loam soil near Robertson in the Breede River valley, compared to high frequency irrigation (Fig. 7.11). The dry soil induced substantially

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