Handbook for Irrigation of Wine Grapes in South Africa

Chapter 6

6.5.1

CLASS I – NO WATER CONSTRAINTS

Based on midday Ψ S , grapevines in only one plot fell in Class I. This was insufficient to identify consistent grapevine response trends. However, vegetative growth is expected to be more vigorous, compared to Class II, i.e. mild water constraints. Usually, shoot growth tips are active throughout the season and internodes are long. Light penetration into the bunch zone is poor and older leaves in the bunch zone may turn yellow. Berry mass could exceed 1.3 g. Due to excessive shading of buds early in the season and competition with vegetative vigour, yield may be lower than Class II. If leaves are shaded, juice K is usually high, TTA low and pH high (Iland, 1989). If berry temperature is reduced by shading, juice TTA could be high and pH low. Sugar accumulation might be delayed and juice dilution may occur. Wine colour and overall quality is expected to be less than 45%. In fact, where mean Ψ S of Cabernet Sauvignon in a sandy soil was higher than -0.6 MPa over three seasons, juice pH was 3.6 and wine quality 38% (Howell et al. , 2016). As a result of no water constraints, mean berry mass was 1.4 g and yield 14.6 t/ha in the sandy soil (Howell et al. , 2016). CLASS II – LOW WATER CONSTRAINTS Active growth tips occurred late in the growing season. Light penetration into the bunch zone was poor and leaf yellowing occurred inside the canopy. Strongest growth, biggest berries and highest yield were obtained in all soils, compared to the other classes (Fig. 6.30A, B & C). Berry mass was between 1.2 and 1.4 g. Juice K ranged from 1 600 to 1 800 mg/  , TTA from 7.3 to 8.0 g/  and pH from 3.4 to 3.6 (Fig. 6.30D, E & F). Since vigorous growth caused more shaded leaves, juice TTA was lower and pH higher in heavier than in sandy soils. Sugar accumulation may be retarded and juice dilution may occur. In sandy soils, wine colour was 65 to 70% and overall quality 45 to 50% (Fig. 6.30G & I). In heavier soils, wine colour was 50 to 55% and overall quality 40 to 45%. Berry character was more prominent in sandy soil than in heavier soil (Fig. 6.30H). 6.5.3 CLASS III – MODERATE WATER CONSTRAINTS In the pre-véraison period, drooping of the second tendril from the shoot apex may occur. Limited active shoot tips occurred in the post-véraison period. Leaves could show para-helionastic movement. Leaves did not show yellowing. Vegetative growth was reduced by ca. 25 and 30% in sandy and heavier soils, respectively, compared to Class II (Fig. 6.30A). Berry mass was between 1.1 and 1.3 g. In heavier soils, yield was comparable to Class II, but was reduced by ca. 5% in sandy soils. Juice K ranged from 1 200 to 1 400 mg/  , TTA from 7 to 7.5 g/  and pH from 3.4 to 3.5 (Fig. 6.30D, E & F). Since the stronger growth reduced berry temperature, juice TTA was higher and pH lower in heavier than in sandy soils. In sandy soils, wine colour was 75 to 80% and overall quality 55 to 65%. Wine colour was 60 to 70% and overall quality 45 to 55% in heavier soils (Fig. 6.30G & I). Berry character was stronger in sandy soil, compared to heavier soil (Fig. 6.30H). 6.5.2

IRRIGATION OF WINE GRAPES 173