Handbook for Irrigation of Wine Grapes in South Africa

Chapter 6

Figure 6.23 6.23). The VSP trained grapevines were planted in a duplex soil near Stellenbosch. Bunches were removed during berry ripening, and each crop load treatment was replicated four times in a randomized block design. It must be noted that the highest crop load was one bunch per shoot, which amounted to a relatively low yield of 8.2 t/ha. This suggested that water stored in bunches did not make any contribution to transpiration under the prevailing conditions. The different crop loads were probably too small to act as a significant sink for water. adaptations of the roots. In contrast, 36% bunch thinning had no effect on midday Ψ S where Tempranillo received 100% of the crop water requirement, although the yield was reduced from 23.6 to 13.8 t/ha (Esperanza Valdés et al. , 2009). Likewise, 29% less bunches had no effect on grapevine water status where only 25% of the water requirement was applied. In the latter case, yield was reduced from 13.6 to 9.0 t/ha. In a pilot study, six levels of crop load ranging between zero and 20 bunches per grapevine did not have any effect on midday Ψ S in Cabernet Sauvignon (Fig.

Bunches per grapevine

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-1 8 -1 6 -1 4 -1 2 -1 0 -0 8 -0 6 -0 4 -0 2 0 0

Ȍ S (MPa)

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a

a

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FIGURE 6.23. Effect of crop load on midday Ψ S during berry ripening in Cabernet Sauvignon near Stellenbosch (unpublished data). Bars designated by the same letter do not differ significantly (p ≤ 0.05).

In another scenario, one bunch per primary shoot and the full crop load did not affect midday Ψ S in high yielding Mourvèdre, compared to grapevines bearing no grapes, when the soil was wet at the beginning of a drying cycle (Fig. 6.24). The VSP trained grapevines were drip irrigated in a sandy soil near Rawsonville. Crop load treatments were applied at véraison, and were replicated seven times in a fully randomized block design. At harvest, the actual yields of one bunch per shoot and the full crop load amounted to 20 and 31 t/ha, respectively. At the end of a three week drying cycle, midday Ψ S in the Mourvèdre grapevines bearing the full crop load was lower than in the ones bearing no grapes (Fig. 6.24). A possible

IRRIGATION OF WINE GRAPES 165