Handbook for Irrigation of Wine Grapes in South Africa

Chapter 6

Figure 6.13

RAW (%)

RAW (%)

0

10

20

30

40

50

0

10

20

30

40

50

-2 0 -1 8 -1 6 -1 4 -1 2 -1 0 -0 8 -0 6 -0 4 -0 2 0 0

-1 8 -1 6 -1 4 -1 2 -1 0 -0 8 -0 6 -0 4 -0 2 0 0 -2 0 -1 8 -1 6 -1 4 -1 2 -1 0 -0 8 -0 6 -0 4 -0 2 -2 0 0 0

A

B

Ȍ S (MPa) Ȍ S (MPa) -0 -0 -0

y = 1.3 + -0.0223x R 2 = 0.786

y = 1.6 + -0.0218x R 2 = 0.869

0

10

20

30

40

50

0

10

20

30

40

50

0 0

C

D

-0 4

-2 0 -1 8 -1 6 - 4 -1 2 -1 0

y = 1.4 + -0.0234x R 2 = 0.902

y = 1.5 + -0.022x R 2 = 0.861

FIGURE 6.13. The relationship between midday Ψ S and RAW in commercial (A) Shiraz, (B) Pinotage, (C) Cabernet Sauvignon and (D) Merlot vineyards in the Devon valley near Stellenbosch (unpublished data).

Grapevine cultivars inherently have different stomatal densities as discussed in Chapter 1. The effect of different stomatal densities is in part reflected by the variation in level of water constraints experienced by different cultivars under the same set of soil and atmospheric conditions (Fig. 6.14). In fact, the Ψ S is closely related to stomatal density, i.e. the lower the stomatal density, the lower the Ψ S , irrespective of soil water content (Fig. 6.15). This trend implies that under a given set of conditions, partial stomatal closure of Merlot and Sauvignon blanc will occur to a higher degree and/or begin earlier, compared to Shiraz. More pronounced stomatal control is likely to reduce water loss via transpiration, thereby avoiding excessive water constraints in cultivars such as Merlot and Sauvignon blanc.

IRRIGATION OF WINE GRAPES 157