Handbook for Irrigation of Wine Grapes in South Africa

TABLE 7.13. Effect of low frequency irrigation until véraison, followed by deficit irrigation (DI) during berry ripening, compared to high frequency irrigation throughout the season on midday Ψ S , cane mass, berry mass, yield and sensorial wine quality of Shiraz in a fine sandy loam soil in the Breede River valley (Lategan, 2011). Data are means for three seasons. Parameter RAW depletion pre-véraison→post-véraison 35%→35% 75%→DI 90%→DI Pre-véraison Ψ S (MPa) -0.9 a* -1.2 a -1.4 a At harvest Ψ S (MPa) -0.8 a -1.4 a -1.3 a Cane mass (t/ha) 3.6 a 2.7 a 2.2 b Berry mass (g) 1.4 a 1.2 a 1.0 b Yield (t/ha) 25.6 a 21.4 a 16.0 b Juice pH 3.84 a 3.74 b 3.66 c Juice TTA (g/  ) 5.2 a 5.2 a 5.3 a Wine colour (%) 38.6 b 45.3 b 61.6 a Berry character (%) 42.0 b 45.3 b 51.4 a Spicy character (%) 36.7 b 38.0 ab 42.2 a Overall wine quality (%) 40.2 b 42.4 b 49.5 a * Values followed by the same letter within a row do not differ significantly (p ≤ 0.05). PARTIAL ROOT ZONE DRYING Previous studies suggested that irrigation of vineyards according to the PRD strategy required substantially less water without reducing yield and grape quality, compared to conventional drip irrigation. With the PRD strategy, one half of a grapevine’s root system is irrigated using the “A” lines while the other half is allowed to dry out (Fig. 7.19). After approximately 10 to 14 days, depending on the soil and atmospheric conditions, the irrigation is switched to the dry half of the root system, i.e. the irrigation is applied via the “B” lines. The switching between the “A” and “B” lines is continued through the season (Fig. 7.20). The key principle of the PRD strategy is that hormones, e.g. abscisic acid, which are produced by the roots when the one half of the soil dries out, only restrict vegetative growth, but not yield. Therefore, it can be assumed that the more open canopies resulting from the limited shoot growth could be beneficial to wine quality. 7.2.8

206 CHAPTER 7 – IRRIGATION STRATEGIES