Handbook for Irrigation of Wine Grapes in South Africa

FIGURE 8.34. Grapevine leaf covered with a bag for the measurement of Ψ S .

Some controversy surrounds the duration of the period that leaves must be covered before Ψ S is being measured. Periods of between one and six hours did not impact on Ψ S in Cabernet Sauvignon leaves (Choné et al. , 2001). During a trial carried out at Nietvoorbij near Stellenbosch, bags were placed around one leaf on each of five Sauvignon blanc grapevines at various times before measuring Ψ S at 14:00. The measurements were completed within 15 minutes by five people using five pressure chambers. All the pressure chambers were calibrated against a reference pressure gauge. The stage at which the leaves were covered before the measurements were carried out hardly had any effect on Ψ S (Fig. 8.35). The same trend was observed with apple leaves in Elgin (T. Volschenk, unpublished data). Growers who prefer to measure Ψ S can therefore decide when it suits them best from a practical point of view to affix the bags, provided it is between 30 minutes and 24 hours.

268 CHAPTER 8 – PRACTICAL IRRIGATION SCHEDULING