Handbook for Irrigation of Wine Grapes in South Africa

Figure 6.8

Chapter 6

0.12

0.10

0.08

0.06

0.04

0.02 Daily contraction (mm)

0.00

0 10 20 30 40 50 60 70 80 90 100 PAW depletion (%)

FIGURE 6.8. Effect of PAW depletion on the daily shrinkage of grapevine trunks (Myburgh, 1996).

6.2

QUANTIFICATION OF GRAPEVINE WATER STATUS

6.2.1 GRAPEVINE WATER POTENTIAL Grapevine water status is usually quantified by measuring the predawn water potential in leaves when water constraints are the lowest, or around midday when the constraints are the highest on normal sunshine days (Fig. 6.9). The midday measurements can be carried out using mature leaves that are fully exposed to the sun to obtain leaf water potential. Alternatively, leaves can be covered in bags to assess the stem water potential. Since the stomata are partially closed due to less radiation, the water potential in shaded leaves is usually higher, compared to leaves that are fully exposed to the sun (Fig. 6.9). Given the fact that the amount of light or the degree of shading can be quite variable within grapevine canopies, measuring water potential in shaded leaves is not recommended. Details of the procedures for measuring grapevine water status are presented in Chapter 8.

IRRIGATION OF WINE GRAPES 151