Handbook for Irrigation of Wine Grapes in South Africa

Chapter 6

6.5.4 CLASS IV – HIGH WATER CONSTRAINTS Shoot tips were inactive in the pre-véraison period. Limited drying of tendrils occurred in some plots. Leaf folding, drooping and para-helionastic movement may occur. Basal leaves showed limited yellowing. Bunch exposure was increased. Vegetative growth was reduced by ca. 30 and 40%, respectively, in the sandy and heavier soils, compared to Class II (Fig. 6.30A). Limited berry shrinkage occurred. Berry mass was between 1 and 1.2 g. Yield was reduced by ca. 35 and 15% in the sandy and heavier soils, respectively, compared to Class II (Fig. 6.30C). Juice K ranged from ca. 1 200 to 1 300 mg/  , TTA from 6.5 to 7.5 g/  and pH from 3.35 to 3.45 (Fig. 6.30D, E & F). Juice TTA was higher and pH lower in heavier soils than in sandy soils, since stronger growth reduced berry temperature (Iland, 1989). Wine colour was 65 to 75%, irrespective of soil texture (Fig. 6.30G). In sandy soil, overall quality was 50 to 55%, compared to 55 to 60% in heavier soil (Fig. 6.30I). Berry character was lower in the sandy soil, compared to heavier soil (Fig. 6.30H). 6.5.5 CLASS V – SEVERE WATER CONSTRAINTS Shoot elongation stopped when berries reached pea size. Desiccated shoot tips occurred in the pre-véraison period, particularly in sandy soils. Basal and bunch zone leaves showed yellowing. General tendril drying and leaf shedding may occur. Bunches were excessively exposed, particularly in sandy soils. Vegetative growth was reduced by ca. 50%, compared to Class II in all soils (Fig. 6.30A). Berries showed shrinkage or shrivelling. Berry mass was between 0.9 and 1.1 g. Yield was reduced by ca. 40%, compared to Class II in all soils (Fig. 6.30C). Negative carry-over effects to the following season’s yield may occur, particularly in sandy soil. Juice K ranged from ca. 900 to 1 000 mg/  and TTA from 6.5 to 7.5 g/  (Fig. 6.30D & E). Juice pH was ca. 3.5 and 3.35 in sandy and heavier soils, respectively (Fig. 6.30F). Since the sparse growth allowed berry temperatures to increase, juice TTA was lower and pH higher in sandy than in heavier soils (Iland, 1989). Sugar accumulation is likely to be limited. In sandy soil, sensorial wine colour was 70 to 75% and overall quality 50 to 55% (Fig. 6.30G & I). In heavier soil, wine colour was 75 to 85% and overall quality 55 to 65%. Berry character was stronger in heavier soil, compared to sandy soil (Fig. 6.30H). CONCLUSIVE REMARKS It is important to note that water potential, like many other plant based measurements, does not indicate how much irrigation is required. In addition to soil water content, prevailing atmospheric conditions also influence grapevine water potential. Viticultural practices may also play an indirect role in determining grapevine water status. Since midday Ψ S is better related to soil water content than Ψ L , and more convenient to measure than Ψ PD , it is recommended for commercial application. According to midday Ψ S , when grapevines in sandy soil experience no water constraints, yield might be slightly higher, but wine quality will be low. 6.6

IRRIGATION OF WINE GRAPES 175