Handbook for Irrigation of Wine Grapes in South Africa

Figure 6.27

Unscathed

Damaged

-1 6 -1 4 -1 2 -1 0 -0 8 -0 6 -0 4 -0 2 0 0

Ȍ L (MPa)

FIGURE 6.27. Effect of black vine weevil damage on midday Ψ L

during berry ripening in

Mourvèdre leaves near Rawsonville.

6.4 GRAPEVINE WATER STATUS CLASSIFICATION Water status in anisohydric grapevines can be classified according to Ψ PD thresholds (Table 6.2). It was proposed that Ψ PD values of -0.2, -0.4, and -0.6 MPa, respectively, could serve as upper thresholds for “weak”, “medium” and “strong” water deficits in Shiraz (Ojeda et al. , 2002). A similar, but more general classification was proposed by Deloire et al. (2004). It was also suggested that grapevines experience no water constraints if Ψ PD is higher than -0.2 MPa, whereas -0.3 to -0.5 MPa indicate moderate water constraints (Carbonneau et al. , 2004). Based on the close relationship between soil water content and Ψ PD , the latter is the preferred indicator of grapevine water status for some researchers. For practical irrigation scheduling, the predawn time span is usually too short if the water potential in a number of vineyards needs to be assessed. Therefore, it is more practical to measure midday Ψ L or Ψ S . Similar to Ψ PD , midday Ψ L could also be used to indicate the level of water constraints in grapevines (Winkel & Rambal, 1993; Girona et al. , 2006; Sousa et al. , 2006). In this regard, Greenspan (2005) proposed Ψ L values of approximately -1, -1.2, -1.4 and -1.6 MPa, respectively, as lower thresholds for “no”, “mild”, “moderate” and “high” water constraints (Table 6.2). Likewise, -0.8, -1.2 and -1.5 MPa were considered to be Ψ L thresholds for “low”, “moderate” and “severe” water constraints, respectively (Girona et al. , 2006 and references therein). Midday Ψ S may also be used as a measure of grapevine water status (Choné et al. , 2001; Patakas et al. , 2005; Olivo et al. , 2009). It should be noted that midday Ψ S is considered to be a better indicator of grapevine water status in relation to soil water status than Ψ L (Naor et al. , 1997; Van Leeuwen et al. , 2009). In this regard, it was also reported that midday Ψ S in Cabernet Sauvignon in the Lower Olifants River region was more closely related to soil water status than Ψ L (Bruwer, 2010).

168 CHAPTER 6 – GRAPEVINE WATER STATUS