Handbook for Irrigation of Wine Grapes in South Africa

Chapter 6

TABLE 6.2. Thresholds for water constraint classes for predawn leaf water potential ( Ψ PD ) according to Ojeda et al . (2002) and Deloire et al . (2004), as well as midday leaf water potential ( Ψ L ) according to Greenspan (2005) and stem water potential ( Ψ S ) according to Van Leeuwen et al . (2009). Class Water constraints Water potential (MPa) Ψ PD Ψ L Ψ S I No Ψ PD ≥ -0.2 Ψ L ≥ -1.0 Ψ S ≥ -0.6 II Low -0.2 > Ψ PD ≥ -0.4 -1.1 > Ψ L ≥ -1.2 -0.6 > Ψ S ≥ -0.9 III Moderate -0.4 > Ψ PD ≥ -0.6 -1.2 > Ψ L ≥ -1.4 -0.9 > Ψ S ≥ -1.1 IV High -0.6 > Ψ PD ≥ -0.8 -1.4 > Ψ L ≥ -1.6 -1.1 > Ψ S ≥ -1.4 V Severe Ψ PD < -0.8 Ψ L < -1.6 Ψ S < -1.4 More recently, cultivar specific water potential thresholds were determined for Merlot, Shiraz and Cabernet Sauvignon in field studies under local conditions. In these studies, thresholds for midday leaf and stem water potentials were determined. This was achieved by measuring Ψ PD , as well as midday Ψ L and Ψ S on the same day, and repeating the measurements as the soil dried out. The water constraint thresholds were obtained by plotting midday Ψ L and Ψ S against Ψ PD . An example of the midday Ψ S vs Ψ PD plot in Merlot is presented in Figure 6.28. The proposed Ψ PD thresholds in Table 6.2 were then used to deduct thresholds of the different water constraint classes for midday Ψ S (Table 6.3). Thresholds for midday Ψ L in Merlot were determined in a similar way. A water constraint classification based on the Ψ PD classification was also determined under field conditions near Robertson for Shiraz (Lategan, 2011). The thresholds were also obtained by plotting midday Ψ L and Ψ S against Ψ PD (Fig. 6.29), and are presented in Table 6.4. Thresholds for the water constraint classification for Cabernet Sauvignon are presented in Table 6.5.

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