Handbook for Irrigation of Wine Grapes in South Africa

Figure 6.19

0.00

-0.25

-0.50

Ȍ S (MPa)

Control Water-Deficit Salinity

-0.75

-1.00

0

4

8

12

16

Day

FIGURE 6.19. Effect of water deficits and irrigation with saline water on Ψ S in potted Cabernet Sauvignon grapevines where the salinity in the water was gradually increased until day 16 (redrawn from Cramer et al ., 2007).

6.3.5 TRELLIS SYSTEM It can be expected that grapevines trained onto trellis systems that allow more leaf exposure, e.g. gable or T-trellises, will transpire more, which could cause more water constraints, compared to vertically trained grapevines. In fact, it was shown that Shiraz trained onto a horizontally divided canopy trellis system experienced more water constraints when the trellis was opened, compared to the closed position (Galat Giorgi et al. , 2011). However, the difference in leaf exposure had no effect on Ψ PD . It was concluded that the higher leaf exposure increased grapevine water use, i.e. if other aspects related with plant water status were unchanged. When trained onto vertical trellis systems, Pinotage grapevines on a 2-Tier system did not appear to induce more water constraints, compared to grapevines on a single cordon VSP trellis, irrespective of soil water status (Fig. 6.20). This trend occurred in spite of the fact that the cordon length per grapevine on the 2-Tier system was almost double the cordon length of the single cordon VSP grapevines. Due to the shorter cane lengths on the 2-Tier system, leaf area per grapevine was probably comparable to the VSP grapevines. It should be noted that the effect of soil water depletion dominated Ψ L in the grapevines over the course of the day. Although Scott Henry trellises may allow more leaf exposure, compared to VSP trellises, it did not have any effect on midday Ψ L in Cabernet Sauvignon (Williams & Heymann, 2017). In this case, the level of irrigation, rather than the trellis system, also seemed to dominate grapevine water status.

162 CHAPTER 6 – GRAPEVINE WATER STATUS