Handbook for Irrigation of Wine Grapes in South Africa

Chapter 8

the baseline (°C/kPa). Previous field studies have shown that grapevines begin to experience water constraints when the CWSI exceeds ca. 0.3 (Anconelli & Battilani, 2000).

Figure 8.44 i r .

2

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B

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1 0

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-2 -1 -3 -4 -5 -6

-2 -1

0

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ΔT (°C) T (° ) -3

ΔT (°C) T (° )

-2

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-4 -5 -6

ΔT = 4.198 - 2.699VPD ΔT = 4.198 - 2.69 VPD

ΔT = 1.32 - 0.98VPD (R 2 = 0.68; MSE = 0.546) ΔT = 1.32 - 0.98VPD (R 2 = 0.68; SE = 0.546)

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0 0.5 0 0.5

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VPD (kPa) (k a)

VPD (kPa) (k a)

FIGURE 8.44. Examples of Δ T vs VPD baselines for well-watered (A) grapevines (redrawn from Anconelli & Battilani, 2000) and (B) peppermint (redrawn from Gallardo, 1993).

Based on the foregoing, it is clear that additional instrumentation is required to measure real time VPD in order to calculate the CWSI. Furthermore, it is not sure to what extent baselines may vary between grapevine cultivars. For example, low stomatal conductance in Grenache on a hot day might not indicate that irrigation is required, whereas a relatively high conductance in Shiraz does not necessarily imply that irrigation would not be beneficial (Loveys et al. , 2008). These aspects may limit the use of Δ T for irrigation scheduling in commercial vineyards. However, field studies have shown that VPD measurements can be replaced by measuring the temperature of dry and wet filter paper disks when canopy temperature is measured (Loveys et al. , 2008). These values can be used to calculate a canopy conductance index as follows: I g = (T dry - Tc) ÷ (Tc -T wet ) Eq. 8.9 where T dry and T wet are the temperatures in °C of the dry and moist filter paper reference disks, respectively. Since the I g is relatively easy to measure, and seems to be responsive to irrigation treatments (Fig. 8.45), it looks promising for practical application in vineyards.

IRRIGATION OF WINE GRAPES 275