Technical Yearbook 2024

RF

SLD

DLD

2017/18 season, except for the backslope double dripper line (DLD) and footslope single dripper line (SLD) plots, the irrigated treatments did not experience any water constraints according to thresholds reported previously (Figure 1). 7,8 However, grapevines at the rainfed (RF) plots experienced low water constraints at the shoulder and backslope and moderate constraints at the footslope site (Figure 1A - C). On 18 December 2017 (véraison), all of the grapevines at the shoulder site experienced moderate water constraints, with Ψ S varying between -0.9 MPa and -1.1 MPa (Figure 1A). In contrast, grapevines of the RF plot at the backslope site were already experiencing severe water constraints (Figure 1B). Before harvest, there was little difference between the treatments and all the grapevines experienced severe water constraints, except for the grapevines at the footslope DLD plot (Figure 1C). According to water constraint thresholds, the maximum Ψ S measured at the footslope DLD plot fell under Class IV, namely “high water constraints”, which is regarded as ideal for producing quality Cabernet Sauvignon wine on a clay soil. 8 The substantially higher Ψ S measured at the footslope DLD plot during véraison and harvest was most probably due to high volumes of TMW applied at this plot 4 and subsequent greater soil water content (Figure 1C). At the back- and footslope sites, Ψ S was consistently higher at the SLD and DLD plots when compared to the RF plots, albeit very slightly (Figure 1B - C). Similarly, lower Ψ S in non-irrigated grapevines was reported compared to those irrigated with SLD and DLD in the Swartland. This was attributed to greater soil water content in irrigated plots. 9 From the results of the current study, it is clear that irrigation with TMW was only beneficial in preventing water constraints up until véraison, whereafter irrigated grapevines experienced similar levels of water stress compared to non-irrigated grapevines. Similar results were

-2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 -2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 -2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0

No

Low Moderate

Ψ S (MPa)

High Severe

A

Pea size Véraison Harvest

RF

SLD

DLD

No

Low Moderate High Severe

Ψ S (MPa)

B

Pea size Véraison Harvest

RF

SLD

DLD

No

Low Moderate

Ψ S (MPa)

High Severe

C

Pea size Véraison Harvest

FIGURE 1. Effect of rainfed conditions (RF) and irrigation with treated municipal wastewater via single (SLD) and double line drip (DLD) on the midday stem water potential (Ψ S ) in (A) Sauvignon blanc on a shoulder and Cabernet Sauvignon on (B) a backslope and (C) a footslope at pea size, véraison and harvest during the 2017/18 season. Figure 1.

reported for Tempranillo grapevines under RF and irrigated conditions during seasons with limited rainfall. 10 Vegetative grapevine measurements Leaf chemical status All experimental grapevines had leaf blade N levels (Table 1) exceeding the recommended norms of 1.5% to 2.4%. 11 No substantial differences were observed between treatments, but leaf N content tended to increase slightly

with the amount of irrigation water applied. Given that the N content of the leaves was above the recommended norm of 2.4%, care should be taken to avoid over-fertilisation that could lead to excessive vegetative growth and reduced fruitfulness. 12 The grapevine leaf blade P content (Table 1) was within the recommended range of 0.12% to 0.45%, 11 except for slightly higher concentrations in the shoulder SLD, DLD and footslope DLD plots. Irrigation with TMW significantly

18

TECHNICAL YEARBOOK 2024