Handbook for Irrigation of Wine Grapes in South Africa

Chapter 6

radiation (Fig. 6.16A) will cause partial stomatal closure, or a reduction in stomatal aperture, which will lower water constraints in grapevines (Fig. 6.16B). The vapour pressure deficit is a measure of the moisture content of the atmosphere. The higher the VPD, usually expressed in kPa, the drier the atmosphere. In the grape growing regions of South Africa, maximum daily VPD is around 3 kPa on normal sunshine days. Grapevine leaf water potential generally declines with an increase in VPD, i.e. water constraints tend to increase as the air becomes drier (Fig. 6.17). However, the effect of VPD recedes as grapevine water constraints increase, whereas the role of soil water becomes more prominent (Williams & Baeza, 2007). According to the data in Figure 6.17, Ψ L will decrease by ca. 0.08 MPa per 1 kPa VPD increase if the soil is wet, and only by ca. 0.02 MPa per 1 kPa VPD increase if grapevines experience water deficits. Figure 6.16

5

A

4

2 Radiation ( M J / m / s ) 3

Veld fire

Clouds

1

0

04 00

08:00 12:00 16:00 20:00 24:00

04:00

0 0

B

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

Ψ L (MPa)

FIGURE 6.16. The decline in net radiation caused by clouds and the smoke column of a veld fire (A) and the concomitant increase in Merlot Ψ L near Wellington (after Myburgh, 2011a).

IRRIGATION OF WINE GRAPES 159