Handbook for Irrigation of Wine Grapes in South Africa

Chapter 8

With respect to irrigation scheduling, a useful application of this concept is to program the logger to open irrigation valves automatically when the trunk shrinkage reaches a pre-selected value. This value should correspond with the level of soil water depletion at which irrigation is required as indicated in Figure 8.39. This approach implies that the grapevine acts as indicator of its own water requirements. Similar to Ψ S measurements, daily shrinkage only indicates when irrigation is required, but not how much irrigation water should be applied. Therefore, measuring daily trunk shrinkage could be useful in combination with soil based measurements to set refill lines for irrigation scheduling. 8.4.2.3 INFRARED THERMOMETRY Due to the cooling effect of transpiration, leaf or canopy temperatures of well- watered grapevines are lower than the ambient temperature. As soil water is being depleted, partial stomatal closure of grapevine leaves begins, causing transpiration to decrease (Fig. 8.41). When this happens, the cooling effect of transpiration becomes less. Consequently, the temperature difference between canopy temperature and that of the surrounding air temperature becomes less as the soil continues to dry out (Fig. 8.42). The close, linear correlation between Δ T and soil water content is well documented (Van Zyl, 1986; Gallardo, 1993; Anconelli & Battilani, 2000; Loveys et al. , 2008). Therefore, using infrared thermometry to measure Δ T holds promise as a rapid, non-destructive way to assess grapevine water constraints (Van Zyl, 1986). Hand held infrared thermometers provide a rapid, non-destructive method for measuring leaf or canopy temperatures, as well as ambient temperature (Fig. 8.43). Measuring canopy temperature, which integrates the variability between leaves, seems to be a better option than measuring the temperature of single leaves. Figure 8.41

250

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50 g s (mmol/m 2 /s) 150 100

b

b

0

Wet

Drying

Dry

FIGURE 8.41. Midday stomatal conductance (g s ) of Shiraz grapevines measured on 24 January 2004 near Nuriootpa in Australia, where “Wet” = daily irrigation, “Drying” = irrigated 7 days before measurements and “Dry” = not irrigated since 1 December (redrawn from Loveys et al. , 2008).

IRRIGATION OF WINE GRAPES 273

Figure 8.42