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South African Journal of Enology and Viticulture

On-line version ISSN 2224-7904
Print version ISSN 0253-939X

S. Afr. J. Enol. Vitic. vol.43 n.2 Stellenbosch  2022 



Determining a Midday Stem Water Potential Threshold for Irrigation of Table Grapes



P.A. MyburghI, II; C.L. HowellI

IARC Infruitec-Nietvooibij, Private Bag X5026, 7599, Stellenbosch, South Africa
IIRetired in November 2021




Sustainable table grape production depends on sufficient water supply. Water potential is a useful indicator of water constraints in grapevines. In this regard, midday stem water potential (Ψ8) is considered to be a better indicator of grapevine water status than leaf water potential (*FL). The objective of the study was to determine a water potential threshold to set soil water refill lines for table grape irrigation. However, in previous studies carried out locally, only *FL was measured. The relationship between Ψ8 and *FL was determined for ten selected table grape cultivars. Since there were no differences between cultivars, a single equation could be used to convert midday *FL measured in previous studies with table grapes to Ψ8. Vegetative growth, berry mass and colour, as well as juice total soluble solids (TSS) data were pooled, and related to midday Ψ8. This showed that -0.8 MPa seems to be a Ψ8 threshold for water constraints in the pre-harvest period that will allow sustainable growth and berry size for anisohydric table grape cultivars. The optimum Ψ8 for berry colour is between -0.8 MPa and -1.0 MPa. Consequently, a midday Ψ8 threshold of -0.8 MPa can be used to set refill points for irrigation where soil water content is measured on a regular basis in table grape vineyards.

Key words: Berry mass, colour, grapevine, vegetative growth, water potential.




Water plays an important role in grapevine physiology. Consequently, management of grapevine water status to avoid water constraints in table grapes is essential to ensure optimum yield and grape quality. Predawn (Ψ^), as well as midday *FL and Ψ5 are proven measures to assess the water status in table grapes (Myburgh, 1996; Myburgh, 2003; Selles et al., 2004; Williams & Ayars, 2005; Myburgh & Howell, 2006a; El-Ansary & Okamoto, 2007; Reynolds et al., 2009; Williams et al., 2010a; Williams et al., 2010b; Myburgh, 2012; Myburgh & Howell, 2012; Silva-Contreras et al., 2012; Williams, 2012; Williams et al., 2012; Howell et al., 2013; Gálvez et al., 2014; Mabrouk, 2014; Conesa et al., 2015; Zúñiga-Espinoza et al., 2015; Pinillos et al., 2016; Conesa et al., 2018; Al-Fadheel et al., 2018; Weiler et al., 2019). These studies have shown that water potential relates to important grapevine responses such as physiological processes, vegetative growth, berry size, yield and grape quality. This implies that water potential can be used to establish guidelines for irrigation scheduling of table grapes. However, there is a need to determine a water potential threshold that will prevent unnecessary irrigation, but still allows optimum yield and grape quality.

Midday Ψ5 is considered to be a more sensitive indicator of grapevine water status than *FL (Van Leeuwen et al., 2009; Tuccio et al., 2019). Hence, a classification was proposed according to midday for wine grapes where water constraints were defined as none (> -0.6 MPa), weak (-0.6 to -0.9 MPa), weak to moderate (-0.9 to -1.1 MPa), moderate to severe (-1.1 to -1.4 MPa) and severe (< -1.4 MPa). Since

measurements are time consuming and require skilled persons, it might not be suitable for irrigation scheduling at the commercial level. A more practical approach would be to monitor soil water content (SWC) and apply irrigation when grapevines experience a critical level of water constraints, or reach a threshold. The refill points can be set by measuring SWC and Ψ5 simultaneously as the soil dries out until reaches the threshold. Once the SWC refill point is set, no further measurements would be necessary. Using water potential thresholds was previously proposed for irrigation scheduling of wine grapes (Baeza et al., 2007; Acevedo-Opazo et al., 2010; Centeno et al., 2010 and references therein, Charrier et al., 2018). Likewise, pre- and post véraison midday *FL thresholds of -0.8 MPa and -1.1 MPa, respectively, were proposed for table grapes in Tunisia (Mabrouk, 2014).

ARC Infruitec-Nietvoorbij carried out several irrigation studies with table grapes where only midday ΨL was measured. The preferred trellis systems for table grape production in South Africa are Slanting, Gable and Factory trellises (Ferreira, 2020). Since it is difficult to access sun-exposed leaves on the upper side of these horizontal canopies, measuring midday in bagged leaves on the underside of canopies provides a more practical option than It is also easier to standardize by picking mature leaves opposite bunches or close to bunches when leaves are removed as the season progresses. This is an important consideration where measurement of grapevine is required to set SWC refill points for irrigation scheduling in commercial vineyards. However, in order to relate the previously reported grapevine responses to midday to determine an optimum threshold, the midday *FL values need to be converted to Grapevines were subjected to different levels of plant available water depletion in the previous studies. In addition to midday vegetative growth, berry size and grape colour responses were measured.

It is well established that grapevine water potential is affected by VPD (Williams & Baeza, 2007; Gálvez et al.,, 2014; Conesa et al., 2018; Suter et al., 2019). Similar to ΨL (Williams & Baeza, 2007), becomes less susceptible to the effect of VPD as water constraints develop when the soil dries out (Gálvez et al., 2014). In fact, the latter study showed that there was no relationship between and VPD where table grapes were irrigated at 50% plant available water depletion. Furthermore, the effect of VPD was not considered where water potential thresholds for grapevines were determined in previous studies (Baeza et al., 2007; Acevedo-Opazo et al., 2010; Centeno et al., 2010 and references therein; Mabrouk, 2014). Air temperature can also affect (Williams & Baeza, 2007; Suter et al., 2019). In this regard it was shown that modelling can be used to standardize Ψ,, when climatic conditions differ (Suter et al., 2019). However, growers might find it difficult to implement such models, particularly with respect to obtaining real time weather data.

The objectives of the study were (i) to determine the relationship between and ΨL, (ii) convert existing ΨL data to and (iii) find a stem water potential threshold for table grape irrigation.



The study to establish the relationship between and ΨL was carried out during the 2015/16 season in full bearing commercial vineyards in the Noorder-Paarl region of the Western Cape. Five white and five red cultivars were included in the study (Table 1). The cultivars were seedless, except for Tropical Delight, Victoria and Waltham Cross. All vineyards were irrigated by means of micro-sprinklers and trained onto Gable trellises (Ferreira, 2020). In each vineyard, a plot comprising an experiment row and two buffer rows were selected. The experiment rows consisted of at least eight grapevines. From the beginning of berry ripening, the water supply to the experiment plots was cut off for approximately four weeks. As the soil dried out, midday and ΨL were measured weekly between 12:00 and 14:00 mean solar time according to the protocol described by Myburgh (2010) using a pressure chamber (Scholander et al., 1965). A custom-made pressure chamber mounted on a motor cycle was used. In the case of Ψ,,, leaves were covered using aluminium bags with black linings one hour before measurements were made. The bags were not removed during the measurements. Since the vineyards were approximately 5 km apart, water potentials were only measured in three grapevines per plot to stay within the midday time limit.

The irrigation studies carried by ARC Infruitec-Niet-voorbij included Barlinka (Myburgh, 1996), Thompson Seedless (Myburgh, 2003; Myburgh, 2012), Sunred Seedless (Myburgh & Howell, 2006a; Myburgh & Howell, 2006b; Myburgh & Howell, 2007) and Dan-ben-Hannah (Myburgh & Howell, 2012; Howell et al., 2013). In these studies, ΨL was generally measured before irrigations, thereby indicating the maximum water constraints the grapevines would have been subjected to by various treatments. Vegetative growth was quantified by weighing cane mass at pruning and berry mass at harvest. Juice TSS and sensorial berry colour were also determined at harvest. Berry colour was evaluated using the colour chart for each cultivar as prescribed by the table grape industry. To allow more data for relating grapevine responses to water status, data of the different experiments were pooled. Due to differences in locality and cultivar, relative values for cane mass, berry mass and grape colour were calculated for each experiment.

Regression analyses were carried out using STATGRAPHICS® version XV (StatPoint Technologies, Warrenton, Virginia, USA). To allow comparison between the regression lines of the different cultivars, upper and lower 95% confidence limits for the slope of each regression line were calculated as ±1.96 times the standard error of the slope (Ott, 1998).



For each cultivar, Ψ,, and ΨL correlated linearly (Fig. 1). However, in the case of Sugranineteen two distinct outlier values occurred which suggested that the water potential in this cultivar might be more susceptible to variability in atmospheric conditions as was previously shown (Williams & Baeza, 2007; Suter et al., 2019). The linearity between Ψ5 and ΨL agrees with previous reports for grapevines (Williams & Araujo, 2002; Montoro et al., 2012; Williams, 2012). The linear relationship also applies to predawn Ψ& and ΨL in table grapes (Mabrouk, 2014). When the soil was wet, the difference between ΨL and Ψ5 (ΔΨ) was notably bigger compared to drier conditions (Fig. 1). For well-watered wine grapes, ΔΨ is c. 0.6 MPa compared to c. 0.1 MPa when severe water constraints occur due to low soil water contents (Choné et al., 2001). Grapevine transpiration is high when water is readily available, and decreases as the soil dries out (Winkel & Rambal, 1993; Centeno et al., 2010; Rogiers et al., 2010). Since transpiration declines linearly as ΔΨ decreases, ΔΨ provides an indirect assessment of grapevine transpiration as it varies with soil water content and atmospheric VPD (Choné et al., 2001).

The linear correlations between *ΨL and were highly significant for all cultivars (Table 2). Furthermore, comparison of the regression lines for the ten cultivars showed that there were no statistical differences (Fig. 2). The latter indicated that the development of water constraints as the soil dried out did not differ between cultivars. Furthermore, it appeared that row direction did not affect grapevine water status. Consequently, the data for all cultivars were pooled to obtain the following equation:

Equation 1 was used to convert the midday ΨL to for the previous table grape irrigation studies mentioned earlier.

The foregoing results implied that the selected cultivars showed anisohydric behavior under the prevailing conditions. This means that ΨL follows a distinct diurnal pattern, and decreases in response to soil water deficits (Schultz, 2003 and references therein). In contrast, ΨL remains more or less constant during the day in isohydric or near-isohydric grapevines and does not respond to changes in soil water status (Schultz, 2003). This suggested that Equation 1 is most likely not applicable to isohydric table grape cultivars.

However, it must be noted that there is some controversy about the consistent hydric behavior, and subsequent classification, of grapevine cultivars (Hugalde & Vila, 2014; Charrier et al., 2018; Levin et al., 2020).

Vegetative growth vigour, i.e. as quantified in terms of cane mass at pruning, began to decline when midday Ψ5 fell below c. 0.8 MPa (Fig. 3). This value corresponded more or less with the transition from weak to moderate water constraints (Van Leeuwen et al., 2009). Below this threshold, relative cane mass declined at a rate of c. 11% per 0.1 MPa decrease in Ψ,,. Although grapevine shoot growth and cane mass declined linearly as ΨL decreased, no distinct threshold was observed (Baeza et al., 2007; Williams et al., 2010a). This was probably due to the highest ΨL being c. -0.7 MPa in both studies. These results indicate that irrigation applied before midday Ψ5 reaches -0.8 MPa is likely to induce ex cessive vegetative growth. The latter could cause unfavourable micro-climatic conditions in the bunch zone. Furthermore, excessive growth will require more canopy management inputs that could increase production costs. Similar to vegetative growth, berry mass also remained unaffected up to a threshold of c. -0.8 MPa (Fig. 4). The decline in berry mass with decreasing grapevine water potential agrees with earlier findings (Baeza et al., 2007; Williams et al., 2010b). The rate of berry mass decline below the threshold was c. 8% per 0.1 MPa decrease in Ψ,,. This suggested that berry size appeared to be less sensitive to water constraints than vegetative growth.

In contrast to vegetative growth and berry size, grape colour did not have a prominent threshold with respect to Ψ,. In fact, berry colour responded curvilinear to Ψ, and seemed to reach an optimum between -0.8 MPa and -1.0 MPa (Fig. 5). The poor colour score of Thompson Seedless was due to the presence of yellow coloured berries that are not suitable for the fresh market. It was previously shown that Thompson Seedless produced more yellow berries where water constraints reduced vegetative growth and solar radiation interception (Zúñiga-Espinoza et al., 2015). Although the colour of Crimson Seedless grapes improved where Ψ3 was lower than -0.8 MPa throughout most of the pre-harvest period, berry mass was reduced (Pinillos et al., 2016). Furthermore, this response was not consistent over seasons. Excessively high berry temperatures reduced the total monomeric anthocyanin concentrations in berry skins, but cooling of sun-exposed grapes had the opposite effect (Spayd et al., 2002). The foregoing suggested that the effect of over-irrigation, as well as excessive water constraints on berry exposure could have a negative effect on berry colour development. Juice sugar content at harvest did not correlate well with midday Ψ3 (data not shown). The insensitivity of juice TSS where table grapes were subjected to different irrigation regimes was in agreement with previous findings (Serman et al., 2004; Blanco et al., 2010; Mabrouk, 2014; Zúñiga-Espinoza et al., 2015; Pinillos et al., 2016; Al-Fadheel et al., 2018). This is probably due to table grapes being harvested at relatively low TSS for export. Yet, this does not rule out the possibility that irrigation induced water constraints have no effect on TSS in table grapes (Selles et al., 2004; El-Ansary & Okamoto, 2007; Tangolar et al., 2007; Reynolds et al., 2009). Inconsistent juice TSS responses to water deficits were also reported for a number of table grape cultivars (Permanhani et al., 2016 and references therein). If the midday thresholds proposed by Mabrouk (2014) are converted to by means of Equation 1, the pre-véraison threshold of -0.4 MPa appears to be too high for table grapes in South Africa. However, the post-véraison threshold of -0.8 MPa for table grapes in Tunisia will be applicable for local conditions. Based on the foregoing, the following water constraint classification according to midday Ψ3 is proposed for table grape production: none (Ψ8 > -0.6 MPa), weak (-0.6 < Ψ3 > -0.8 MPa), moderate (-0.8 < Ψ3 > -1.0 MPa), strong (-1.0 < Ψ3 > -1.2 MPa) and severe (Ψ3 < -1.2 MPa).



Within the constraints of the methodology, -0.8 MPa seems to be a water status threshold that will allow sustainable growth and berry size for anisohydric table grape cultivars. If midday is consistently above -0.8 MPa or below -1.0 MPa, it could restrict berry colour development. It is recommended that irrigation advisors and managers set soil water refill lines for table grape vineyards when midday reaches -0.8 MPa in the pre-harvest period. Adjusting thresholds for the post-harvest period is part of an ongoing study.



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Submitted for publication: June 2021
Accepted for publication: August 2022



Corresponding author: E-mail address:
Acknowledgements: The ARC for infrastructure and other resources, Mrs. M Van Der Rijst for statistical analyses and JDK (Pty) Ltd for permission to work in their vineyards

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