Handbook for Irrigation of Wine Grapes in South Africa

Chapter 6

FIGURE 6.5. Diurnal Ψ L in Shiraz irrigated at two different levels of PAW depletion, as well as continuous deficit irrigation near Robertson (Lategan, 2011). Vertical bars indicate LSD (p ≤ 0.05). -2 2 -2 0 -1 8 -1 6 -1 4 -1 2 -1 0 -0 8 -0 6 -0 4 -0 2 0,0 02:00 04:00 06:00 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 00:00 02:00 04:00 Ȍ L (MPa) Time T1 - 35% PAW depletion T4 - 75% PAW depletion T8 - Continuous deficit irrigation early morning, minimum evaporation occurs from leaf surfaces. Consequently, a positive sap pressure in the grapevine can cause water droplets to form on the tips of the serrations on the edge of young leaves (Fig. 6.6). This physiological condition is known as guttation (Latin: gutta = drop). When the water droplets evaporate, they may leave slight salt deposits on the edge of the leaves. However, the latter is unlikely to be harmful to plants (Kramer, 1983). In fact, guttation indicates that the grapevines experience no water constraints. Under normal atmospheric conditions, the lowest water potential in grapevines usually occurs between 12:00 and 14:00, i.e. irrespective of the soil water content (Fig. 6.5). Figure 6.5

Figure 6.6

FIGURE 6.6. Grapevine leaves showing early morning guttation.

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