Handbook for Irrigation of Wine Grapes in South Africa

-0.4

C.S. 1.12ET C.S. Deficit irrigated T.S. 1.0ET T.S. No water applied

-0.6

-0.8

-1.0

Ȍ L (MPa)

-1.2

-1.4

-1.6

0

2

4

6

8

10

VPD (kPa)

FIGURE 6.17. The effect of VPD on Ψ L in Cabernet Sauvignon (C.S.), either irrigated at 1.12 times the reference ET, or receiving only deficit irrigation, as well as Thompson Seedless (T.S.), either irrigated at 1.0 times the reference ET, or receiving no irrigation (redrawn from Williams & Baeza, 2007). 6.3.3 SOIL WATER STATUS The decrease in grapevine transpiration by means of partial stomatal closure as the soil dries out, is well documented. Likewise, there is a concomitant increase in grapevine water constraints as the plant available water in the soil becomes less (Williams & Araujo, 2002; Myburgh, 2011a). Grapevine water status is well related to soil water status in anisohydric grapevines, and can be used as an indication of when irrigation is required (Patakas et al ., 2005). However, the rate at which water constraints develop in grapevines depends on the soil texture. When the soil water matric potential is higher than -30 kPa, grapevines in some sandy soils tend to experience less water constraints, compared to ones in heavier soils (Fig. 6.18). This indicates that the water supply to grapevines is also a function of the hydraulic conductivity of soils. In fact, when the Ψ m in sandy soil is above -30 kPa, the hydraulic conductivity is higher than in heavier soil (Chapter 3, Fig. 3.17). This is due to the high flow rate through the coarse pores in the sandy soil when the water content is high. If the soil water matric potential is less than -30 kPa, grapevines in sandy soils will experience more water constraints at a given Ψ m , compared to grapevines in heavier soils (Fig. 6.18). In this case, a higher volume of water flowing through the many fine pores of heavier soils will result in lower grapevine water constraints, compared to lower volume of water flowing through the lower number of fine pores in sandy soils.

160 CHAPTER 6 – GRAPEVINE WATER STATUS