ARTICLES

 

Response of Vitis vinifera L. cv. Pinotage to Irrigation Strategy and Trellis System in the Breede River Valley Region: Vegetative Growth, Yield and Juice Characteristics

 

 

C.L. Howell; P.A. Myburgh

ARC Infruitec-Nietvoorbij¹, Private Bag X5026, 7599, Stellenbosch, South Africa

 

 

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ABSTRACT

Pinotage is a South African bred red wine cultivar and is second only to Shiraz in terms of the country's wine exports. Since rainfall in the Breede River region is low, vineyards in this region depend on irrigation. The sensitivity of Pinotage/99R to water deficits and the most suitable irrigation strategy during water restrictions were studied in a field trial from 1998/99 until 2000/01. The possibility to produce more grapes with the same volume of irrigation water was also investigated. Irrigation strategies entailed combinations of 50% readily available water (RAW) depletion, 75% RAW depletion and no irrigation between various phenological stages, viz. budbreak, flowering, pea size berries, véraison, 17°B and harvest. Irrigation applied at 50% RAW depletion from budbreak in September until harvest in February was regarded as the control. Each experiment plot was split into a six-strand vertical hedge and a two-tier vertical trellis. The experiment layout was a split plot, randomised block design. Cane mass of grapevines on the two-tier trellis was lower compared to those on the six-strand hedge. Irrigation at 75% RAW depletion level until harvest tended to reduce cane mass compared to more frequent irrigation. Drier soil conditions reduced berry mass, regardless of the trellis system. Sustained water deficits reduced yield, but had little effect on juice acidity and pH. More Pinotage grapes were produced on the two-tier vertical trellis compared to the six-strand hedge with the same amount of irrigation, thereby reducing the blue water footprint and increasing the irrigation water use efficiency substantially.

Key words: bunch mass, grapevines, irrigation strategies

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INTRODUCTION

Irrigation resources are generally limited in the grape-producing regions of South Africa (Myburgh, 2018). The Breede River Valley production region, which has a Mediterranean climate, i.e. long, dry summers and rainfall in winter, receives on average of 116 mm of rainfall in spring and summer, followed by 164 mm in autumn and winter. Therefore, vineyards in this particular region depend totally on irrigation. Considering the possible effects of climate change on viticulture, the worst-case scenario would be that lower rainfall reduces natural water resources and higher air temperatures increase vineyard water requirements. Even if climate change does not affect vineyard evapotranspiration (ET), grape growers still need to use irrigation water more efficiently, but without compromising yield and wine quality.

Pinotage is a South African red wine grape cultivar that was bred locally in 1925 (Goussard, 2008). It yields approximately 10 to 15 t/ha of fruit. It is the most widely planted, locally bred cultivar in South Africa. The total area of vineyards planted to Pinotage comprises ca. 7.5% of the total area of wine grape vineyards in South Africa (SAWIS, 2023). Thirty two percent of the Pinotage vineyards are located in the Breedekloof, Robertson and Worcester areas of the Breede River Valley. In terms of wine exported from South Africa, Pinotage is only second to Shiraz. Despite the popularity of the cultivar, there is no knowledge on the sensitivity of Pinotage growth, yield and wine quality characteristics to water constraints if irrigation is, or becomes limited. Generally, vegetative growth of may impact directly and/or indirectly on grape yield and wine quality. Less irrigation reduces shoot growth tempo (Van Zyl, 1984a) as well as total shoot mass compared to more frequent irrigation (Myburgh et al., 1996; Conradie & Myburgh, 2000; Myburgh, 2003b; Myburgh, 2007; Bruwer, 2010; Lategan, 2011; Myburgh, 2011; Lategan & Howell, 2016;). Although vegetative growth can be manipulated by means of irrigation, yield also decreases with less irrigation (Myburgh et al., 1996; Conradie & Myburgh, 2000; Myburgh, 2003b; Myburgh, 2007; Bruwer, 2010; Lategan, 2011; Mybuigh, 2011; Lategan & Howell, 2016; Munitz et al, 2017; Vilanova et al., 2019). In drought situations, water restrictions could be imposed, thereby forcing growers to manage the limited available water to obtain maximum benefits. In order for growers to make informed decisions regarding the irrigation of their vineyards if water becomes limited, information is needed on the effect of water constraints during different phenological stages.

The water footprint (WF) of an agricultural product is the volume of water required to produce one unit of the product (Mekonnen & Hoekstra, 2010). The purpose of the WF is to create awareness of the impact that the water used for agricultural production has on the environment. The total water footprint of a crop is subdivided into three components, i.e. the green (WF ), blue (WF_bl ) and grey (WF ) water footprints, respectively (Mekonnen & Hoekstra, 2010). In short, the WF_green is the volume of rainwater involved in thecrop production process. The WF_blue is the volume of surface and groundwater used to produce a crop. The WF_grey is the volume of fresh water that is required to assimilate the load of pollutants which originate from the production cycle. Globally, the total WF for grapes is estimated at 608 m³/t of which the WFgreen, WF grey and WFblue are 425 m³/t, 97 m³/tand 87 m³/t, respectively (Mekonnen & Hoekstra, 2010).

Given the drive towards eco-friendly grape production, as well as limited water resources, grape growers need to follow measures to ensure the lowest water footprint that is practically possible. In addition, ever increasing water tariffs and energy costs are continuously challenging growers to adopt irrigation practices that will ensure sustainable grape production. In this regard, the published water footprint values are useful as benchmarks for growers to evaluate the situation on their farms, and to reduce the water footprint of their vineyards where practically possible. In practice, however, grape growers can only manipulate the WF_blue. The irrigation water use efficiency (WUE_i), which is defined as the unit crop produced per unit of irrigation water, is also a measure for the efficacy of irrigation management. At the farm level, the WFblue can be reduced or the WUE_i. can be iincreased based on two approaches,i.e. either use less water to produce the same yield, or produce higher yields with the same volume of water.

There are various practical ways in which the WF_blue or WUE_i vineyards can be improved. Based purely on the application efficiency of different irrigation systems (Ley, 1994; Myburgh, 2018), converting from full surface overhead irrigation to drip irrigation could improve the WF_blue or WUE_i substantially. A previous study showed that the WUE for full surface flood irrigation could be increased from 1.9 kg/m³ to 3.3 kg/m³ if the grapevines are irrigated by means of narrow furrows (Myburgh, 2003a). Less frequent irrigation will reduce evaporation losses from the soil surface (Myburgh, 1998; Lategan & Howell, 2016), and thereby reduce vineyard ET (Myburgh, 2003b; Myburgh, 2007; Myburgh & Howell, 2007; Lategan & Howell, 2016). Hence, if less frequent irrigation does not decrease yield and/or wine quality substantially, it could also result in the more efficient use of irrigation water. Furthermore, it is possible to produce more grapes on bigger trellis systems (Wolf et al, 2003) with the same volume of irrigation water (Van Zyl & Van Huyssteen, 1980; Swanepoel et al., 1990). However, higher yields are mostly obtained on horizontal trellis systems. Since adequate water and deep, fertile soils are crucial for sustaining grapevines on large trellis systems, the latter might not be a viable option where water resources are limited. Another option is to increase the bearing capacity of a vineyard vertically. This can be achieved by training every second grapevine onto a higher cordon wire. Although this so-called two-tier vertical trellis system is already used by some growers, no scientific evidence on the advantages and disadvantages of the two-tier trellis could be found in literature. However, this system holds a promise to increase the effective use of irrigation water.

Considering the above-mentioned, the primary objectives of the study were to determine (i) during what stage(s) Pinotage per se is sensitive to water deficits, (ii) the most suitable irrigation strategy when water restrictions are imposed during periodic droughts and (iii) if more grapes can be produced with the same volume of irrigation water when the bearing capacity of grapevines is increased.

 

MATERIALS AND METHODS

Experiment vineyard

The field trial was carried out over three seasons, i.e. from 1998/99 until 2000/01, in a three-year-old Pinotage/99Richter vineyard on the Agricultural Research Council (ARC) Research Farm near Robertson in the Breede River Valley of South Africa. The locality has a BSk (semi-arid, cold) climate according to the Köppen-Geiger climate classification (Peel et al., 2007). Based on the growing degree days (GDD) of 1497°C from 1 September to 31 March (Amerine & Winkler, 1944), the specific locality is in a class II climatic region that has the potential for the production of good quality red and white table wine (Le Roux, 1974). Given the mean February temperature of 23.1°C, low acid, high pH wines can be produced (De Villiers et al., 1996). The sandy loam soil was representative of the Hutton and Sterkspruit forms (Soil Classification Working Group, 1991) and was deep delved to ca. 90 cm using a crawler tractor before planting. Grapevines were planted at a spacing of 2.75 m χ 1.50 m. Irrigation was applied over the total area using 32 L/hour Eintal® micro sprinklers. Standard viticultural management practices were applied in the vineyard. The vineyard was mechanically cultivated only to establish Avena sativa L. cv. Pallinup (oats) as a winter cover crop as proposed by Fourie (2021). Full surface chemical control was applied before budbreak.

Experiment layout

To determine the effect of water deficits at various stages, different irrigation strategies were applied. These irrigation strategies consisted of eight different combinations of 50% readily available water (RAW) depletion, 75% RAW depletion and no irrigation between various phenological stages, viz. budbreak, flowering, pea size berries, véraison, 17°B and harvest as indicated in Table 1. Irrigation applied at 50% RAW depletion from budbreak in September until harvest in February was regarded as the control. For the purpose of this study, RAW was defined as the water available between -5 kPa and -100 kPa soil matric potential. It must be noted that plant available water (PAW) is the water available between -5 kPa and -1500 kPa soil matric potential. The experiment layout was a split plot, randomised block design. Each experiment plot was split into a six-strand vertical trellis and a two-tier vertical trellis to give a total of sixteen irrigation strategy/trellis system combinations, or treatments. Each of the sixteen irrigation strategy/trellis system combinations were replicated three times. Grapevines were developed onto the respective trellis systems from establishment onwards. The cordon arms of grapevines on the two-tier trellis were developed to a length of 3 m (Fig. 1). The total height of both trellis systems was 1.8 m. All grapevines were spur pruned in August, leaving two buds per spur. Plots consisted of eight experiment grapevines with two border grapevines at each end, as well as two border rows on either side of the experiment row to limit possible overlapping of treatment effects. Each experiment plot covered 247.5 m².

Application of irrigation strategies

Soil water matric potential was measured weekly, as well as before and after irrigations, using tensiometers at 30 cm, 60 cm and 90 cm depths. A soil water retention curve was determined for each 30 cm depth increment using undisturbed soil cores and the ceramic pressure plate technique (Klute, 1986). The water retention curves were used to determine RAW between -5 kPa and -100 kPa. The soil water retention curves were comparable for the three depth increments and total RAW amounted to 94 mm/m. Based on the soil water retention curves, 50% and 75% RAW depletion amounted to soil matric potentials of ca. -35 kPa and -65 kPa, respectively. Irrigation volumes were measured on S1, S3, S6 and S7 plots by means of water meters. Grapevines were not irrigated during winter months.

Vegetative growth

To quantify growth vigour, cane mass was measured at pruning in early August. The cane mass of all the experiment grapevines in each plot was determined using a hanging balance. Cane mass per plot (kg) was converted to tons per hectare. Shoots were slightly topped at the beginning of December. Topping of principal shoots was only required where irrigation was applied at 50% RAW depletion before véraison. On the two-tier trellis, shoots were topped on the lower, as well as upper cordons. In July 2001, the number of primary shoots per grapevine were counted and their length was measured using a measuring tape.

Yield components

Véraison was defined as the stage when visual observation showed that c. 95% of grape berries had changed colour. This was equivalent to stage 36 of the modified Eichhorn and Lorenz grapevine growth identification system (Coombe, 1995). At harvest, all bunches in each experiment plot were picked and counted using mechanical counters. The grapes were weighed to obtain the total mass per plot. Mean yield per grapevine was calculated and converted to tons per hectare. Bunch mass was determined by dividing the total grape mass per plot by the number of bunches per plot. The number of bunches per grapevine was calculated by dividing the total number of bunches per plot by the number of experiment grapevines per plot. Fresh berry mass was determined at harvest in all the plots. Berry samples were obtained by picking 20 berries along the longitudinal axis from each of ten bunches per experiment plot. Berries were removed from bunches by cutting through the pedicle as close as possible to the berry using a small pair of scissors (Van Schalkwyk, 2004). Berry mass was determined in the laboratory by weighing the samples using an electronic balance.

Juice characteristics

The objective was to harvest grapes when the total soluble solids (TSS) in the juice reached 24°B. The TSS, total titratable acidity (TTA) and pH in the juice were determined according to the standard procedures of Infruitec-Nietvoorbij which is the Fruit, Vine and Wine Research Institute of the Agricultural Research Council (ARC) near Stellenbosch. Total soluble solids were determined using a digital refractometer (Pocket PAL-1, Atago U.S.A. inc., Bellevue, WA, U.S.A.). Total titratable acidity and juice pH was measured using an automatic titrator (Metrohm 785 DMP Tritino, Metrohm AG, Herisau, Switzerland), against sodium hydroxide (NaOH) at a concentration of 0.33 M.

Statistical analyses

Raw data was captured and sorted in Microsoft^® Excel. The data were subjected to an analysis of variance (ANOVA) by using Statgraphics®. Least significant difference (LSD) values were calculated to facilitate comparison between treatment means. Means which differed at p < 0.05 were considered significantly different.

 

RESULTS AND DISCUSSION

It must be noted that some of the results of S1 and S7 on the two trellis systems have been summarised previously (Myburgh, 2018 and references therein). The results obtained with all the irrigation strategy/ trellis system combinations will be discussed in detail below.

Soil water status and irrigation volumes

Grapevines were generally irrigated once a week to maintain 50% RAW depletion during summer. In order to allow 75% RAW depletion, irrigation was applied every 10 to 14 days depending on the weather (Fig. 2). On average, 619 mm of water as applied to the control strategy (S1) over the three seasons. Irrigation of grapevines at 75% RAW depletion from budbreak in September until harvest (S7), required 462 mm of water. Where water deficits were imposed from flowering to véraison (S3), an average of 572 mm of water was applied over the three years of the study. For grapevines irrigated at 75% RAW depletion with 50% RAW from flowering to véraison (S6), 509 mm of water was applied.

It must be noted that grapevines on the two trellis systems received the same volume of irrigation water.

Vegetative growth

Since some cordon arm development was still required in 1998/99, mean cane mass was lower than in 1999/00 and 2000/01 when the cordon arms were fully developed (Table 1). Although grapevines on the two-tier trellis had double the cordon length, cane mass of grapevines on the two-tier trellis generally tended to be lower compared to the six-strand hedge. Cane measurements carried out in the 2000/01 season showed that grapevines on the six-strand hedge had less primary shoots per grapevine (Fig. 3A), but longer shoots compared to the two-tier trellis (Fig. 3B). The distribution of growth potential over the longer cordons reduced the length of single primary shoots on the two-tier trellis. Based on the number of shoots per grapevine, spurs were ca. 14 cm apart on the six-strand hedge, whereas the spacing was ca. 17 cm on the two-tier trellis. However, the total cane length of grapevines on the two-tier trellis was comparable to the six-strand hedge (Fig. 4A). Visual observations revealed that the canes of grapevines on the two-tier trellis were thinner compared to the those on the six-strand hedge. In fact, cane mass per unit cane length was considerably lower for grapevines on the two-tier trellis compared to the six-strand hedge (Fig. 4B). This suggested that the tendency towards lower cane mass on the two-tier trellis was due to thinner canes compared to the six-strand hedge in the first two seasons (Table 1). In the 2000/01 season, thinner canes also contributed to the lower cane mass on the two-tier trellis compared to the six-strand hedge, except where grapevines received the least irrigation (S7 & S8).

Irrigation at 75% RAW depletion from budbreak until harvest (S7) tended to reduce cane mass of grapevines on the six-strand hedge compared to irrigation applied at 50% RAW depletion over the same period (S1) in all three seasons (Table 1). In the case of the two-tier trellis, there were no consistent trends. On average, S7 tended to reduce cane mass compared to S1, irrespective of trellis system (Fig. 5). The growth reduction where irrigation was applied at higher soil water depletion levels from budbreak to harvest agrees with previous results (Van Zyl, 1984a; Myburgh, 1996; Lategan 2011). Likewise, cane mass of suckered VSP-trained Shiraz grapevines decreased with an increase in the level of PAW depletion (Stolk, 2014; Lategan & Howell, 2016). In contrast, deficit irrigation did not reduce cane mass of Castelão grapevines, whereas no irrigation caused a substantial reduction compared to well-watered grapevines (Santos et al., 2005). Colombar grapevines irrigated every seven days throughout the season produced higher pruning mass in comparison to those that were irrigated every two, three or four weeks (Myburgh, 2007). Merlot grapevines subjected to continuous deficit irrigation also produced lower cane mass than those that were irrigated more frequently (Munitz et al., 2017).

There was generally no reduction in cane mass of grapevines on the six-strand hedge when water deficits were applied from flowering up to pea size berries (S2) compared to the control (S1) in the three seasons (Table 1). In the case of the two-tier trellis, S2 tended to reduce cane mass compared to S1 in the 1998/99 and 1999/00 seasons. Where grapevines on the six-strand hedge were subjected to water deficits over a longer period, i.e. from flowering to véraison (S3), cane mass tended to be lower than that of S1 grapevines in the 1998/99 and 1999/00 seasons. In the case of the two-tier trellis, S3 also tended to induce a reduction in the cane mass of grapevines compared to S1. Mean cane mass over three years followed similar trends (Fig. 5). Previous studies also showed that vegetative growth was most sensitive to soil water constraints after flowering (Van Zyl, 1984a; McCarthy, 1997).

Where irrigation at 50% RAW depletion was terminated at 17°B (S4) or véraison (S5) grapevines tended to have lower cane mass compared to S1, irrespective of trellis system for most of the seasons (Table 1). Where irrigation at 75% RAW depletion was terminated at véraison (S8), cane mass tended to be lower compared to that of grapevines that irrigated at 75% RAW depletion from budbreak until harvest for all of the seasons (S7) (Table 1). Mean cane mass over three years showed similar trends (Fig. 5). This agrees with reduced grapevine cane mass caused by post-véraison deficits compared to a fully irrigated control (McCarthy, 1997). In general, excessive shading due to vigorous vegetative growth can be detrimental to wine colour (Smart, 1982). Visual observations revealed that water deficits tended to terminate shoot elongation. Termination of shoot growth could impact positively on red grape cultivars (Williams et al., 1994).

Yield components

Bunch number: In the 1998/99 season, grapevines on the two-tier trellis only tended to bear more bunches compared to those on the six-strand hedge (Table 2). In contrast, grapevines on the two-tier trellis bore significantly more bunches in the 1999/00 and 2000/01 seasons when the cordon arms were fully developed. Considering the means over the three seasons, grapevines on the two-tier trellis bore significantly more bunches than the six-strand hedge (Fig. 6). Based on the shoot counts carried out in 2000/01, grapevines bore 1.8 bunches per shoot, irrespective of trellis system or irrigation strategy. This was slightly less than the target of two bunches per shoot. However, grapevines on the two-tier trellis bore only ca. 45% more bunches per grapevine than those on the six-strand hedge (Fig. 6). This was due to the wider spur spacing on the two-tier trellis which on average resulted in only 60% more shoots per grapevine than on the six-strand hedge (Fig. 3). Although the different irrigation strategies caused differences in bunch numbers, it was not consistent over the three seasons (Table 2). However, considering the mean values, grapevines which received the least irrigation (S7 & S8) on the six-strand hedge had less bunches per grapevine compared to S1 and S2 which received the most irrigation (Fig. 6). Likewise, severe water constraints tended to reduce number of bunches compared to well-watered grapevines (Hardie & Considine, 1976; Santos et al., 2005). In the case of the two-tier trellis, irrigation at 75% RAW depletion which was stopped at véraison (S8) caused lower bunch numbers than some of the wetter treatments.

Berry mass: In general, trellis system did not seem to have any effect on berry mass in any of the seasons (Table 3). This trend also reflected in the mean values over the three seasons (Fig. 7). Water deficits from flowering to pea size berries (S2) only tended to reduce berry size compared to continued irrigation at 50% RAW depletion (S1), regardless of the trellis system (Table 3). In contrast, water deficits from flowering until véraison (S3) caused a more pronounced reduction in berry size in the 1999/00 and 2000/01 seasons. Similarly, irrigation applied at 75% RAW depletion and stopped at véraison (S8) also reduced berry size compared to S1 in the 1999/00 and 2000/01 seasons. Previous studies also showed that continued water deficits from budbreak until harvest reduced berry size (Santos et al., 2005; Myburgh, 2011; P'erez-Alvarez et al., 2021). On average, water deficits from flowering to véraison (S3) significantly reduced berry mass compared to S1 on both trellis systems (Fig. 7). The reduced berry size caused by pre-véraison water deficits agrees with earlier findings (Hardie & Considine, 1976; Van Zyl, 1984b; McCarthy, 1997). Irrigation applied at 75% RAW depletion until harvest (S7) also reduced berry mass compared to S1 (Fig. 7). Similarly, irrigation applied at 75% PAW depletion from budbreak until harvest reduced berry mass compared to irrigation at 10%, 30% and 50% depletion (Van Zyl, 1984b). Irrigation applied at 50% RAW depletion and stopped at véraison (S5), as well as 75% depletion stopped at véraison (S8) also reduced berry mass (Fig. 7). Post-véraison water deficits caused a similar reduction in berry size of Cabernet Franc (Hardie & Considine, 1976).

Bunch mass: The trellis system had no effect on bunch mass in the 1998/99 and 1999/00 seasons (Table 4). In the 2000/01 season, bunches were smaller on the two-tier trellis than on the six-strand hedge for some irrigation strategies. Considering the mean bunch mass over the three seasons, the trellis system did not have any effect on bunch mass (Fig. 8). Although there were differences in bunch mass between the irrigation strategies in the 1998/99 and 2000/01 seasons, there were no consistent trends (Table 4). On average, grapevines on the six-strand trellis only produced smaller bunches where irrigation at 75% depletion was stopped at véraison (S8) compared to S1 (Fig. 8). In the case of the two-tier trellis, pre-véraison deficits (S2 & S3) as well as deficits during berry ripening (S5) reduced bunch mass compared to S1. The sensitivity of bunch mass to pre-véraison water constraints was also found in a previous study (McCarthy, 1997). The driest irrigation strategy, i.e. where irrigation at 75% depletion was stopped at véraison (S8), also produced smaller bunches on the two-tier trellis compared to S1 (Fig. 8). This trend agrees with previous findings (Hardie & Considine; McCarthy, 1997; Santos et al., 2005). Surprisingly, irrigation at 75% RAW depletion from budbreak until harvest (S7) only tended to produce smaller bunches compared to S1 (Fig. 8). In contrast, grapevines that were subjected to water deficits from budbreak until harvest produced smaller bunches compared to more frequent irrigation (Santos et al., 2005; Lategan, 2011; Myburgh, 2011; Lategan & Howell, 2016).

Yield: In the 1998/99 and 1999/00 seasons, yields were generally lower compared to 2000/01 (Table 5). The lower yield in 1998/99 was due to the fact that some cordon arm development was still required. Although cordon arms were fully developed in 1999/00, mean yield was appreciably lower than in 2000/01. During 1999 and 2000, a period of drought occurred over the entire Western Cape (Araujo et al., 2014). This probably contributed to the low grapevine yield in the 1999/00 season. Since grapevines received adequate water via irrigation in summer, mild winter conditions such as higher air temperatures and lower humidity could have affected grapevine yield in the 1999/00 season. In fact, it was previously shown that low relative humidity and dry soil conditions in winter can reduce grapevine yields substantially (Myburgh, 2003a; Myburgh, 2003c; Myburgh & Van der Walt, 2005; Myburgh, 2008).

In the 1989/99 season, yield on the two-tier trellis was higher compared to the six-strand hedge where irrigation was applied at 50% PAW depletion from budbreak to véraison (S1) (Table 5). Where less irrigation was applied, i.e. S2 to S5, yield only tended to be higher on the two-tier trellis. In contrast, yield was significantly higher on the two-tier trellis where irrigation was applied at 75% RAW depletion, i.e. S6, S7 and S8. In the other seasons, yield was consistently higher on the two-tier trellis than on the six-strand hedge, irrespective of irrigation strategy. Although irrigation strategy had an effect on yield, it was not consistent over the three seasons (Table 5). However, on average, Pinotage grapevines irrigated at 75% RAW depletion throughout the season (S7) produced less grapes than those irrigated at 50% depletion throughout the season (S1), irrespective of trellis system (Fig. 9). Likewise, higher levels of soil water depletion maintained from budbreak to harvest reduced yield of Manto Negro and Tempranillo (Medrano et al., 2003), Castelão (Santos et al., 2005), Merlot (Myburgh, 2011), Shiraz (Lategan, 2011; Lategan & Howell, 2016), Cabernet Sauvignon (Williams & Heymann, 2017), Verdejo (Vilanova et al., 2019) and Bobal (P'erez-Alvarez et al., 2021). In contrast, Merlot grapevines only tended to produce lower yields when subjected to a higher level of soil depletion from budbreak to harvest (Munitz et al., 2017). On average, irrigation at 50% RAW depletion from budbreak to harvest (S1), as well as where irrigation at 75% depletion was changed to 50% depletion from flowering to véraison (S6), grapevines produced the highest yields on the two- tier trellis (Fig. 9). The lowest yields on the two-tier trellis were produced where irrigation at 50% RAW depletion was stopped from flowering to véraison (S3) and where irrigation at 75% depletion was stopped at véraison (S8). The sensitivity of grape yield towards pre-véraison, as well as post-véraison water deficits agrees with previous findings (Hardie & Considine, 1976; McCarthy, 1997). In the case of the six-strand hedge, the highest yields were produced by irrigation at 50% from budbreak to harvest (S1), as well as where irrigation at 50% was stopped between flowering and pea size berries (S2) (Fig. 9). The lowest yields on the six-strand hedge were obtained where irrigation at 50% RAW depletion was stopped from flowering to véraison (S3), as well as where irrigation was applied at 75% RAW depletion until harvest (S7) or terminated at véraison (S8). It is noteworthy that the average yield obtained on the six-strand hedge with the least irrigation, i.e. S8 (Fig. 9), was within the norms for Pinotage as proposed by Goussard (2008).

Juice characteristics

TSS: Given that grapes from the respective irrigation strategies were harvested as close as possible to 24°B, there were no meaningful differences in TSS of the different treatments. The sugar content at harvest varied between 23.5°B and 24.2°B (data not shown). However, it should be noted that the increase in sugar content of the grapes on the two-tier trellis was slower than that on the six-strand hedge. Depending on the season, grapes on the two-tier trellis were harvested one to two weeks after those on the six-strand hedge. This agrees with slower sugar accumulation as yield increases (Williams & Heymann, 2017; Myburgh & Howell, 2023).

TTA: In the 1998/99 season, trellis system had no effect on juice TTA, except for the higher level in grapes produced on the six-strand hedge where 75% RAW depletion was changed to 50% depletion between flowering and pea size berries (S6) (Table 6). In the 1999/00 season, S6 grapes on the six-strand hedge again had higher levels of TTA than those on the two-tier trellis. However, five more irrigation strategies, i.e. S1, S3, S5, S7 and S8 produced more TTA on the six-strand hedge than on the two-tier trellis. In the 2000/01 season, i.e. the first full-bearing season, grapevines on the six-strand hedge produced higher levels of juice TTA than those on the two-tier trellis, irrespective of irrigation strategy (Table 6). This trend was probably caused by slightly cooler grapes due to less exposure to solar radiation on the six-strand hedge than those on the two-tier trellis (Kliewer 1971; Iland, 1989). However, more exposed leaves on the six-strand trellis could also have contributed towards higher juice TTA (Iland, 1989). Irrigation at 50% RAW depletion from budbreak until harvest (S1) had no effect on juice TTA compared to irrigation at 75% depletion (S7), irrespective of trellis system (Table 6). This insensitivity of titratable acidity to water constraints agrees with previous findings (Medrano et al., 2003; Acevedo-Opazo et al., 2010; P'erez-Alvarez et al., 2021). Although there were some differences between irrigation strategies within a trellis system, they were not consistent in any of the three seasons. On average, irrigation at 50% PAW depletion before véraison followed by water deficits during berry ripening (S5 & S6) produced higher levels of juice TTA in grapes on the six-strand hedge compared to most of the other strategies over the three seasons (Fig. 10). A previous study also showed that post-véraison water constraints can increase juice TTA (Hardie & Considine, 1976). In the case of the two-tier trellis, irrigation strategy had no effect on juice TTA, except where irrigation at 50% PAW depletion which was terminated at véraison (S5) increased TTA compared to some of the other strategies (Fig. 10). On average, the juice TTA produced by grapevines of all irrigation strategy/trellis system combinations were within the typical range of 5 to 8 g/L for grapes (Rajković et al., 2007).

pH: In the 1998/99 season, trellis system had no effect on juice pH, except for the low pH in grapes produced on the six-strand hedge where 75% RAW depletion was changed to 50% depletion between flowering and pea size berries (S6) (Table 7). In the 1999/00 season, the trellis system had no effect on juice pH. In the 2000/01 season, the pH in juice produced on the two-tier trellis was higher compared to the six-strand hedge, except where irrigation at 50% RAW depletion was stopped from flowering to véraison (S3) and during berry ripening (S5). On average, irrigation strategy had no effect on juice pH, except that grapes produced on the six-strand hedge where irrigation at 50% RAW depletion was stopped from flowering to véraison (S3) had higher pH compared to some of the other strategies (Fig. 11). In contrast, juice pH was the lowest where 75% RAW depletion was changed to 50% depletion between flowering and véraison (S6). The foregoing agrees with previous studies which showed that irrigation strategies do not have major effects on juice pH (Hardie & Considine, 1976; Van Zyl, 1984b; Medrano et al., 2003; Santos et al., 2005; Myburgh, 2006; Acevedo-Opazo et al., 2010; Myburgh, 2011; Williams & Heymann, 2017; P'erez-Alvarez et al., 2021). It is also noteworthy that the juice pH for all the trellis system/irrigation strategy combinations was in the optimum range of 3.0 to 3.5 for winemaking (Kodur, 2011; Walker & Blackmore, 2012).

Blue water footprint and water use efficiency

The average WF_blue of the grapes (Table 8) produced with micro sprinkler irrigation was higher than the global value of 97 m³/t proposed by Mekonnen and Hoekstra (2010). Since most vineyards are drip irrigated (Way, 2014) which generally require less water than micro sprinklers (Van Zyl & Van Huyssteen, 1988; Myburgh, 2012), the higher than global WF_blue was to be expected. Under the prevailing conditions, the WF_blue of grapes produced on the two-tier trellis was substantially lower compared to the six-strand hedge, irrespective of the level of RAW depletion (Table 8). This confirmed that is possible to reduce the WF_blue if the bearing capacity of grapevines is extended vertically. In doing so, more grapes can be produced with the same irrigation volume. Although yields were marginally lower, irrigation at 75% RAW depletion tended to reduce the WF_blue of grapes produced on both trellis systems compared to irrigation at 50% RAW depletion. Furthermore, the WF_blue of grapes produced on the two-tier trellis where irrigation was applied at 75% RAW depletion was 44% lower compared to the six-strand hedge where irrigation was applied at 50% depletion (Table 8). It should be noted that the 139 m³/t WF_blue of grapes produced on the two-tier trellis was similar to 138 m³/t reported for drip irrigated Cabernet Sauvignon grapes on a Scott Henry trellis with a vertically split canopy (Williams & Heymann, 2017). In the latter study, the WF_blue was only reduced when less irrigation caused a concomitant yield reduction. However, such an approach is certainly not an economically viable option to reduce the WF_blue of wine grapes.

Surprisingly, the WUE of the micro sprinkler irrigated grapevines on the six-strand hedge (Table 8) was comparable to 5.01 kg/m³ (Mirás-Avalos et al., 2016), and even higher than 2.69 kg/m³ (P'erez-Alvarez et al., 2021) reported for drip irrigated grapevines on vertical trellises. In contrast, the WUE_i was appreciably lower than 10.4 kg/m³ obtained with drip irrigated Merlot on a vertical trellis (Myburgh, 2011). In the case of the two-tier trellis, the WUE_i of grapevines was only slightly less than the 7.9 kg/m³ of drip irrigated Thompson seedless grapevines on a horizontally orientated trellis system in the Breede River Valley (Myburgh & Howell, 2023). The WUE of grapes produced on the two-tier trellis where irrigation was applied at 75% RAW depletion was 78% higher compared to the six-strand hedge with irrigation applied at 50% RAW depletion (Table 8). Results confirmed that it is possible to increase the WUE, or reduce the WF_blue, substantially by producing more grapes with less irrigation water.

 

CONCLUSIONS

This was the first study where Pinotage responses to water deficits were determined. Vegetative growth of Pinotage was insensitive to water deficits during various stages, except where drier soil conditions prevailed from budbreak to harvest. The distribution of growth vigour over longer cordons on the two-tier trellis tended to reduce the primary shoot thickness. Consequently, the cane mass of grapevines on the two-tier trellis was unexpectedly lower compared to the six-strand hedge with shorter cordon arms. Pinotage berry mass was most sensitive to water deficits during the pre-véraison period, as well as continued water deficits from budbreak to harvest. This eventually reflected in the yield. In the case of the six-strand hedge, the average yield of grapevines that received the least irrigation was within the industry norms. Furthermore, water deficits did not have any detrimental effects on juice TTA and pH. This suggested that Pinotage can withstand a fair amount of water constraints. It also explains why Pinotage is one of the preferred cultivars for rain fed viticulture in the coastal region of the Western Cape. The best irrigation strategy for Pinotage is to avoid water constraints during the pre-véraison period. If irrigation water is limited, or when water restrictions are imposed during droughts, irrigation can be reduced, or even terminated during the post-véraison period. This study showed that it is possible to produce more grapes with the same volume of irrigation water by extending the bearing capacity of grapevines vertically. Furthermore, it was shown that it is also possible to produce more grapes with less water, and in doing so reduce the WF_blue, or increase the WUE_i, of wine grapes in a profitable way. If the two-tier trellis is to be considered as a means to increase grapevine yield, or to use irrigation water more effectively, it would be advisable to restrict the in-row plant spacing to 1.2 m, or probably even narrower in less fertile soil.

 

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Accepted for publication: October 2024

 

 

1 The Fruit, Vine and Wine Institute of the Agricultural Research Council
 * Corresponding author: howellc@arc.agric.za
 Acknowledgements: The Agricultural Research Council (ARC) and Winetech for funding the project. The ARC for infrastructure and other resources. Soil and Water Science Division personnel at ARC Infruitec-Nietvoorbij for technical support, in particular Mr. Trevor Harris. Staff at ARC Robertson Research Farm.