Handbook for Irrigation of Wine Grapes in South Africa

Chapter 2

losses. However, in the case of grapevines bearing high crop loads, crop reduction may indirectly increase ET (Table 2.2). It is hypothesised that more water is being absorbed and stored during the night in high crop loads, compared to grapevines bearing lower crop loads (Van Zyl, 1987). During daytime, water stored in the high crop load flows into the transpiration stream, thereby reducing the soil water content more rapidly, compared to grapevines bearing fewer bunches. TABLE 2.2. The effect of crop load on yield, the difference in SWC during a 27 day drying cycle and mean daily ET of Colombar near Robertson in the Breede River valley (adapted from Van Zyl, 1987). Crop load (%) Yield (t/ha) ΔSWC (mm) ET (mm/d) 100 39.4 67.2 5.5 75 27.8 64.1 2.4 50 19.9 52.9 2.0 – – LSD = 11.2 (p ≤ 0.05) – 2.4.2.1.2 Edaphic factors Atmospheric conditions: Stomatal opening is, amongst other things, induced by solar radiation. Therefore, transpiration occurs mainly during the day. Overcast conditions and fog during the day (Fig. 2.20) will also have a significant effect by reducing transpiration due to partial stomatal closure, irrespective of leaf area per grapevine (Fig. 2.15). 0 1 2 3 4 5 6 7 0 2 4 6 8 10 12 14 16 Sap flow (Ɛ grapevine d) Leaf area (m 2 /grapevine)

A

B

C

FIGURE 2.20. Optimal transpiration will occur under sunny skies (A) compared to clouds (B) and fog (C) that will induce partial stomatal closure, thereby reducing vineyard transpiration. Soil water status: The decline in grapevine transpiration as the soil dries out is well documented. In sandy soil, transpiration will decrease more rapidly, compared to soil with a higher water holding capacity (Fig. 2.21). In the case of field grown Colombar/99 R near Robertson in the Breede River Valley region, the effect of soil water deficits on stomatal resistance of sunlit leaves became more prominent in the afternoon, compared to earlier in the day (Table 2.3). Stomatal resistance of shaded leaves followed the same trend. Due to partial stomatal closure, shading increased the stomatal resistance, compared to sunlit leaves except around noon, irrespective of the level of plant available water depletion. Higher soil water content

IRRIGATION OF WINE GRAPES 45