Handbook for Irrigation of Wine Grapes in South Africa

FIGURE 6.21. Examples of (A) VSP, (B) sprawling and (C) mechanical or “box” pruning. A B C

FIGURE 6.22. Effect of VSP, as well as sprawling canopies and box pruning, on midday Ψ S during berry ripening in Merlot/99 R near Stellenbosch (unpublished data) and Shiraz near Robertson (Lategan & Howell, 2016).

6.3.7 CROP LOAD The Ψ S in daily drip irrigated Sauvignon blanc bearing two bunches per shoot was higher, compared to grapevines bearing one bunch per shoot (Naor et al. , 1997). Measuring stomatal conductance revealed that two bunches per shoot enhanced transpiration of these well-watered grapevines. Since the higher transpiration was not related to berry shrinkage during the day, the higher crop load did not provide additional water for transpiration. Therefore, in the case of the higher crop load, higher transpiration was probably due to an increase in root permeability, root length, or root diameter (Naor et al. , 1997). It must be noted that the bunch thinning was carried out before flowering, which allowed enough time for these physiological

164 CHAPTER 6 – GRAPEVINE WATER STATUS