FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

Zinc (Zn) Plants absorb zinc as Zn 2+ , but it can also be assimilated as chelates by the roots, leaves and bark of vines. It is the metal component and activator of various enzyme systems. A variety of dehydrogenase, anhydrase, proteinase and peptidase systems have been found to be Zn dependent. It is essential in relation to carbohydrate, protein and fat metabolism and also plays a role in the formation of auxins. Zinc is found in various rocks and Zn containing minerals. Zn 2+ is released through weathering and is then adsorbed onto the soil colloids where it exists in equilibrium with soil solution ions. There is no correlation, however, between the total Zn content of soil and the accessibility thereof for plants. The accessibility of Zn decreases with an increase in soil pH, excessive phosphate applications and also on account of an absolute deficiency in sandy soils, as a result of leaching. Zn deficiencies have also occasionally been found on the highly weathered, acidic mountain slope soils on granite. Zinc, like Mn, is extracted from soil samples using EDTA, and for soils with a pH KCl of between 5.0 and 6.5, the Zn content should be at least 0.5 mg.kg –1 (see Table 5 in Chapter 3). For Zn supplementation in soils of existing vineyards, a 15 kg ha –1 Zn sulphate solution can be applied on the berms or via the irrigation system and for young vineyards it can be applied evenly on the future vine row zone. A Zn deficiency is manifested by abnormal growth and severe effects on bunch development. Deficiency symptoms first appear on apical leaves as general interveinal chlorosis, comparable to that of Mn. Leaves remain small (known as small leaf symptom), the petiole sinus angle enlarges, and internodes are short, which lead to a rosette appearance (known as witches’ broom) (Figure 20). Berries shatter and subsequently bunches are straggly, with berry size varying due to it containing few or no pips and also tending to remain hard and green (Figure 21). Certain cultivars, like, inter alia the muscat types, are very sensitive to Zn deficiencies and bunches can be affected before leaf symptoms become visible. The norms for the Zn contents of grapevine leaf blades and petioles are indicated in Table 24.

FIGURE 20: Zn deficiency manifests firstly in the apical leaves (left), while it also leads to smaller leaves, short internodes (witches’ broom) and enlarged petiole sinuses (right) (Photos: P. Raath).

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