Introduction

Most agricultural regions have been subjected to warming trends in recent decades, as noted in the 2019 Intergovernmental Panel on Climate Change (IPCC) (Shukla et al., 2020), making desert viticulture an increasingly relevant study model to understand grapevine acclimation and metabolic plasticity under high temperatures and solar irradiance. These conditions can cause berry sunburn and desiccation (Gambetta et al., 2021; Hulands et al., 2014; Krasnow et al., 2010) and an overall reduction in berry size and quality (Hulands et al., 2014), as well as higher sugar content. These factors may result in high alcohol percentages in wines (Laget et al., 2008; Rösti et al., 2018), high berry pH (Laget et al., 2008; Martínez-Pérez et al., 2020), and a reduction in malic acid, which leads to decreased berry titratable acidity (Laget et al., 2008; Spayd et al., 2002; Sweetman et al., 2014). In addition, a combination of high solar irradiance and temperature often exerts oxidative stress (Carvalho et al., 2015; Foyer et al., 1994; Mansour et al., 2022), a physiological condition resulting in an imbalance between the concentrations of antioxidants and reactive oxygen species (ROS) (Gashu et al., 2022). Excessive ROS accumulation may lead to cellular injuries and degradation of the metabolites that determine the wine quality and composition. Among the Vitis vinifera cultivars, white berries are more susceptible to radiation damage than red berries, as they lack antioxidant anthocyanins, photo-protective pigments against UV radiation, and high temperatures (Adams, 2006; Gashu et al., 2022). While red varieties are also sensitive to sunburn, the symptoms are not as apparent as in white varieties (Gambetta et al., 2021).

However, other antioxidant metabolite classes, namely carotenoids and polyphenols, have been proposed to protect white and red berries against sun and heat damage (Gashu et al., 2022). Abiotic stress triggers a complex system of responses, including secondary metabolites and antioxidants, which are not limited to ascorbic acid and occur regardless of berry skin colour (Yeshi et al., 2022). The prominent role carotenoids play in plants is the photoprotection of chlorophyll and the photosynthetic complex (Sun et al., 2022). They are considered to be a significant line of defence in plants against oxidative stress because of their capacity to quench ROS through a physical mechanism involving a transfer of excitation energy followed by thermal deactivation or by a chemical mechanism that involves their oxidation (Carvalho et al., 2015; Ramel et al., 2012; Sun et al., 2022). Carotenoid biosynthesis is induced by high light exposure and induced degradation by higher temperatures during the pre-veraison stages (Joubert et al., 2016; Mendes-Pinto, 2009; Young et al., 2016). Therefore, there appears to be a fine balance between the enhancing and the degrading effects of sunlight exposure and heat on carotenoids, which are also considered to be essential quality determinants and precursors of the low-threshold aroma compounds, C13-norisoprenoids (Mendes-Pinto, 2009).

Metabolic alterations, among other factors, in response to increasing temperature and increased radiation can significantly affect wine typicity (varietal characteristics). In a review by Drappier et al. (2019) on temperature effects on wine composition and typicity, climatic conditions substantially influence typicity more than other environmental factors. Additionally, the authors mention a shift in Bordeaux wine typicity under hot temperatures, illustrated by cooked fruit notes, higher tropical notes for white varieties, and changes in red wine colours (Drappier et al., 2019). In other regions, Gewürztraminer grapes and other white cultivars grown under high temperatures had a reduced aromatic expression compared to those grown in cooler conditions (Cataldo et al., 2021a; Ferretti, 2021).

One of the most essential aspects of grapevine cultivation is the training system applied in the vineyard. The training system can significantly impact the quality and productivity of the grapes produced (Gambetta et al., 2021). Training systems, including adjustments of the vine frame, are viticultural practices to alter the percentages of high-light-exposed leaves, microclimatic conditions in the cluster zone, and total leaf area (de Rességuier et al., 2020). Several training systems are commonly used in viticulture, each with advantages and disadvantages. The vertical Shoot Positioning (VSP) system (Gutiérrez-Gamboa et al., 2021) is one of the most popular training systems, while the Y-shaped open canopy system called SAYM (Musacchi et al., 2021) is the new training system.

Figure 1. (a) An illustration of the training systems in an in-row perspective: SAYM (left) and VSP (right). (b) Photos of the SAYM training systems: an open system (left) and the closing of the "Y" structure after harvest, to be opened again before the next season's bud burst (right).

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Viticultural practices can influence both the yield and wine quality of the grape. Shoot positioning can influence the metabolic change of grapevine as different training systems can affect the amount of light and nutrients the leaf and fruit receive. This can cause a change in the metabolic pathways that control the growth of the plant. Additionally, training systems can also lead to increased sugar content in the fruit, as the practice of pruning and training systems helps to balance the sink and source, and ultimately it can impact the productivity and grape quality (Cataldo et al., 2021b; He et al., 2020; Puelles et al., 2022; Smart et al., 1990; Verdenal et al., 2019). Manipulating leaf area exposure to maximise light interception can increase yield and grape quality (Reynolds and Vanden Heuvel, 2009). Traditional grapevine training (trellising) systems were designed to provide shading to grapes in old traditional wine regions, specifically in southern Europe and the Middle East. At the same time, those used in central Europe tended to enable higher fruit exposure to sunlight (Gambetta et al., 2021). VSP is a popular system that provides increased exposure to irradiation but can lead to sunburn damage (Vilanova et al., 2017). Consequently, alternative trellising systems such as single high-wire cordon (sprawl), head-training, tendon, pergola, Geneva Double Curtain, and closing Y-shaped open canopy (SAYM ) trellis system have been proposed as suitable alternatives due to their ability to maintain bunches under a diffuse light regime and reduce direct radiation (Yu et al., 2022). Minimum pruning, used in the warmer viticultural area, also provides adequate shade to protect berries from sunburn (Gambetta et al., 2021).

This research focused on the SAYM and the VSP trellis system, two popular training systems used in arid environments. The SAYM trellis is a free-standing system with two main wires running horizontally in opposite directions and a third wire running between them at a 45-degree angle. This trellis system provides good exposure to sunlight, adequate airflow, and protection from strong winds and sun irradiance (Palliotti, 2012). In most cases, the SAYM training system is better for increased grape yield and quality since it provides shade and light exposure to the cluster zone (Musacchi et al., 2021). The VSP trellis system is a more radiation-intensive system that involves training the vine vertically on an overhead trellis. This system is designed to provide increased exposure to sunlight and reduce the need for manual labour (Yu et al., 2022). When comparing the SAYM and VSP training systems in arid environments, the SAYM trellis system is generally seen as more effective for providing shelter from sunburn due to its ability to diffuse light, provide the grape clusters with more shade for a more extended period during the day than the VSP as it has a slightly open canopy nature. In addition, the SAYM presents itself as a practical training system capable of enhancing grape and wine quality. Its simplicity and cost-effectiveness make it an easily manageable choice (Palliotti, 2012). However, the VSP system is more suitable for mechanised harvesting and offers more support to the vine. It offers effective shading for fruit, reducing potential damage from high irradiance due to its upright shoot position. In general, both the SAYM and VSP trellis systems can be beneficial in arid environments, and the choice between the two systems will depend on the specific needs of the vineyard and the nature of the grapevine variety (Arias et al., 2022; Ferretti, 2021; Vilanova et al., 2017; Yu et al., 2022). We hypothesised that the innovative training system known as 'SAYM' characterised by an open canopy, can significantly enhance grape and wine quality while maintaining adequate yields compared to traditional vertically shoot-positioned (VSP) trellis systems. This research aimed to compare the effects of two different training methods, SAYM and VSP, on the fruit and wine quality of the aromatic Gewürztraminer cultivar in the arid environment.

Here, we report results from a two-year project integrating wine and grape metabolic analysis and quality assessments with microclimatic and physiological measurements of desert-grown Gewürztraminer vines trained on two distinct trellising systems, VSP and SAYM.

Materials and methods

Results

1. Vine physiology

In 2016/17 and 2017/18, the SAYM’s LAI was significantly higher throughout the measurement season (p < 0.01, Figure 2). In 2017/18, measurements started at day of year (DOY)135, when the SAYM’s LAI was 58 % higher than the VSP’s LAI. A necessary training adjustment was done on DOY 160 (marked by a green arrow). From this day onward, the SAYM’s LAI was higher by 88.5 %, 96 %, and 118 % at DOY 164, 178, and 206, respectively, while the VSP’s LAI remained constant throughout the measurement season. From DOY 164 to harvest, the SAYM's LAI increased by 20%, and from veraison (DOY 179) to harvest, it increased by 11%. The SAYM canopy generally boasted a notably higher LAI than the VSP, nearly doubling by the season's end. This larger canopy necessitated 60 % more irrigation to sustain stem water potential than the VSP (Figure 2).

Figure 2. Leaf Area Index (LAI) during the 2017/18 season by the VitiCanopy app. The green arrow marks a training adjustment event. Error bars are standard errors. An asterisk indicates significant differences between training systems (p < 0.01). n(DOY 135) = 40, n(DOY 164) = 30, n(DOY 178) = 30, n(DOY 206) = 31.

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Similarly, SWP was recorded in both training systems in the 2016/17 season, and no significant differences were found, ranging from –0.91  ±  0.11 MPa in both the SAYM and VSP (DOY 164) to lower values of –1.36  ±  0.11 MPa in the SAYM and –1.35  ±  0.1 MPa in the VSP (DOY 218). In contrast, in the 2017/18 season, the SWP pattern was characterised by shifting trends and statistically significant differences between treatments, pre and post-veraison. At DOY 178, the measured SWP in the SAYM was –1.54  ±  0.06 MPa and –1.41  ±  0.06 MPa in the VSP. Hence, supplementary tap drip lines (0.66 L/h) were used in the SAYM treatments (as well as in parallel SAYM rows) to align the SWP values of the two treatments. Two weeks later, at DOY 192, the mean of SAYM value increased to –1.34  ±  0.08 MPa, while the VSP mean plummeted further to –1.52  ±  0.08 MPa. The SWP means of the two training systems were successfully equalised only at the harvest point (–1.44  ±  0.05 MPa SAYM; –1.42  ±  0.07 MPa VSP).

2. Microclimatic conditions in the cluster zone

The diurnal pattern of incoming radiation in the cluster zone included three peaks; between 9:00 and 11:00, it reached 90.1 W/m² in the SAYM and 95.4 W/m² in the VSP; between 14:15 and 16:00, it reached 111.3 W/m² in the SAYM and 115.4 W/m² in the VSP; and between 18:00 and 18:45, it reached 44.4 W/m² in the SAYM and 55.7 W/m² in the VSP (Figure 3a). The ratio between cluster zone radiation and global incoming radiation indicates that the western side of the canopy, which is shaded until the afternoon, was not affected by treatment. In the afternoon, a clear difference between the treatments started to develop, with higher radiation intensity penetrating the cluster zone in the VSP trellising system, with the most significant difference occurring at 19:30, the time of maximum radiation penetration (Figure 3c). The diurnal temperature in the cluster zone was lower in the SAYM treatment during the daytime, reaching statistically significant differences from 9:15–11:00 (p ≤ 0.05). The accumulated degree hour (> 30 °C degrees/hour) of 15 consecutive days in the 2017/18 growth season was 184.4 °C in the SAYM cluster zone and 5.5 % higher in the VSP cluster zone (194.8 °C), (p > 0.05). The diurnal SAYM cluster zone temperature values ranged between a minimum of 14.9  ±  2.8 °C at dawn (~5:45) and a maximum of 32.5  ±  3.0 °C in the afternoon (15:45). In contrast, the cluster zone temperature in the VSP was ca. 1 °C greater, having almost the same maximum temperature of 32.6  ±  2.9 °C but a lower minimum temperature (13.9  ±  3.0 °C). While the minimum temperature occurred in both treatments simultaneously (at dawn), the maximum temperature for the VSP was reached at 13:10, almost three hours earlier.

The result we reported from the 2018 season was similar to the measurement we did in the 2016 and 2017 seasons (Figure 3b). The SAYM canopy, characterised by its more open structure, effectively reduced and postponed direct radiation exposure on grape clusters compared to the VSP system. This delay caused the maximum radiation to reach clusters around 16:00 instead of 13:00, as observed in the VSP, which is attributable to the SAYM's ability to block direct afternoon sunlight (Supplementary Figure 2). Although ambient air temperatures generally aligned between both training systems, minor differences emerged at specific times of the day. Notably, both treatments experienced cluster surface temperatures surpassing 40 °C (Supplementary Figure 4). The SAYM's canopy contributed to increased relative humidity around clusters during the afternoon compared to the VSP. This rise in humidity was likely a result of escalated transpiration from the expanded leaf area (Supplementary Figure 5). The more open structure of the SAYM canopy offered advantages in terms of delayed direct radiation and augmented humidity around clusters. However, the significantly amplified water demands may restrict its adoption in arid regions like the Negev Desert. The canopy design enhancement could potentially optimise radiation interception while mitigating irrigation needs.

Figure 3. Diurnal radiation. (a) Cluster zone radiation in the SAYM and VSP treatments. Measurements were taken during two weeks (VSP: 5–11/6/2018, SAYM: 13–19/6/2018). (b) The regression line indicates the two training’s radiation level was similar to the measurement done in 2017. (c) Diffused radiation in the SAYM and VSP treatments is the ratio between the cluster zone radiation and the global radiation measured during the separate weeks. Measurements were taken from the west side of the canopy.

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4. ROS and photosynthetic pigment content in berries

During the 2017 season, a notable disparity in ROS contents was observed in VSP berries during veraison, with higher concentrations of hydrogen peroxide and superoxide anion detected compared to SAYM berries, showcasing a statistically significant difference (p < 0.5). Interestingly, in the subsequent 2018 season, SAYM berries accumulated higher levels of ROS, particularly at veraison, but both training systems demonstrated lower and similar accumulations at harvest (Figure 4a,e). The dynamics of total carotenoid content seemed more influenced by the year rather than the specific training systems. This was evident through significantly higher accumulations detected during the 2018 season at the veraison stage, followed by a contrasting pattern of lower and similar responses observed at the harvest stage, showing a characteristic degradation typical of the harvest period (Figure 4b). Comparatively, the photosynthetic pigment content did not exhibit substantial differences between SAYM and VSP berries, except chlorophyll b, which notably increased during the 2018 season at veraison. Conversely, chlorophyll a showed a non-significant difference between the training systems (Figure 4c,d, Supplementary Table 1).

In a general sense, except for the superoxide anion, which was measured only in the 2017 season, a consistent trend emerged across ROS, total carotenoids, and chlorophyll pigments, demonstrating a characteristic decline from veraison to harvest in both treatment groups (Figure 4a–e). This temporal pattern highlights the common metabolic changes occurring from the ripening initiation (veraison) to the final harvesting stages, albeit with variations influenced by grapevine training systems and yearly fluctuations.

Figure 4. Mean  ±  standard deviation for ROS (Reactive Oxygen Species) levels (a), total carotenoids (b), and photosynthetic pigments (Chlorophyll a (c), Chlorophyll b (d), and Superoxide anion for the year 2017 (e)) content in SAYM and VSP berries assessed during veraison and harvest stages across the 2017 and 2018 seasons. Distinct letters denote significant variations among training systems and growth stages, determined via a three-way ANOVA with the subsequent TukeyHSD test, achieving a significance level of p < 0.05, n = 4–7).

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Discussion

This study demonstrates a relationship between reduced radiation and heat in the SAYM cluster zone and a higher quality of berries and wine, thus according to the hypothesis that an open canopy architecture can benefit viticulture in arid regions. In successive measurements of microclimate parameters such as radiation, temperature, and RH from 2016, we found a reasonably similar trend indicating that the Negev desert climatic conditions are less dynamic (Supplementary Figures 1–5). For example, the diurnal pattern of global radiation was pretty constant from the 6^th of July to the 10^th of August 2016/17 (Supplementary Figure 1) and also showed a similar trend in 2018 (Figure 3). The difference in light exposure between SAYM and VSP treatments is that in the VSP system, the cluster zone received significantly more radiation from the west than the east. During specific hours (13:00 to 17:30), the VSP received a maximum radiation flux of 773  ±  106 Wm^-2 predominantly from the west, while the east-facing pyranometer recorded lower maxima (123 ± 6 Wm-2 and 108 ± 4 Wm^-2) with a slight decrease around noon. Conversely, in SAYM, even less radiation reached the east-facing sensor (maximum 97 ± 2 Wm^-2), and most of the incoming solar radiation from the west reached the cluster zone between 16:00 and 18:00 (708 ± 60 Wm^-2 maximum). Overall, the VSP system exhibited higher daily radiation input than SAYM, mainly from east and west directions, showcasing notable differences in light exposure between the two treatments (Supplementary Figures 1–5 and 3).

The higher LAI and alleviated microclimatic conditions in the SAYM likely contributed to the improved yield obtained in the SAYM, as demonstrated by the higher ratio of clusters per vine and heavier clusters with a higher fresh berry weight. This supports the findings that excessive direct radiation and heat may reduce berry size (Hulands et al., 2014). Moreover, the higher ratio of clusters per vine indicates a higher production capacity of SAYM vines, which is determined by the total active leaf area (Winkler et al., 1974). The SAYM's LAI was higher than the VSP's, so it had more source tissues, allowing a better support of sink tissues such as clusters. In contrast, the SAYM's higher TA levels (2017/18) and lower pH levels (2016/17) contradict Palliotti's results from Sangiovese cv. grown on the same training systems (Palliotti, 2012), they do corroborate other studies that link high temperatures and lower TA (Laget et al., 2008; Spayd et al., 2002; Sweetman et al., 2014), as well as higher pH (Laget et al., 2008; Spayd et al., 2002). The higher pH in the VSP berries can indicate an oxidative reaction was higher than in SAYM, which can further deteriorate quality-related compounds. One of the most essential attributes of LAI can be used to predict the vine water status in semiarid vineyards where water is minimal for vine growth and development. Much research reported that LAI is strongly and significantly correlated with evapotranspiration and crop coefficients (K_c). Research on Superior Seedless (Netzer et al., 2009) and Cabernet Sauvignon table grape (Munitz et al., 2019) in semiarid vineyards demonstrates strong correlations between LAI and ETc/K_c, especially in the early season. Hence, in this research, our focus was drawn to LAI mainly. The LAI-ETc/Kc relationships allow LAI to estimate vine water needs and guide irrigation to control water status. A similar correlation was also reported in Apple orchards, which can be used to gauge plant water status using LAI (Jia and Wang, 2021). The K_c values of grapevines tend to fluctuate throughout the growing season due to differing agricultural practices (Williams and Ayars, 2005) such as irrigation (Allen et al., 2015) and grapevine trellising systems (Williams and Fidelibus, 2016). The canopy trellising systems had a substantial influence in this scenario, impacting the canopy structure and consequently affecting the dynamics of LAI (Netzer et al., 2009). These dynamics, in turn, impact the amount of solar radiation the canopy intercepts, all while being interconnected with the phenological stage of the crop (Williams et al., 2003).

Difficulties in equalising the SWP values between the training systems in the 2017/18 season may have caused the inter-seasonal differences in berry quality, as ROS and photosynthetic pigment contents could have been influenced by drought stress, as well as by solar irradiance with concomitant heat (Carvalho et al., 2015; Cramer, 2010). ROS and pigments are two crucial components of berry quality. When berries are exposed to stresses, such as drought, high temperatures, or nutrient deficiency, ROS production can increase, causing oxidative damage to the pigment molecules. This can cause a decrease in the levels of pigments, including anthocyanins and carotenoids, in the berries, resulting in a decrease in berry quality. Therefore, the relationship between ROS and pigments being affected by stresses can directly affect berry quality (Arias et al., 2022).

However, the 2016/17 season’s SWP levels were successfully equalised between training systems. Hence, in addition to the higher berry quality and quantity in the SAYM, the lower ROS levels in the 2016/17 season’s SAYM and 2018 season in VSP berries at veraison can also be attributed to the lower radiation and temperature measured in the cluster zone. Considering the oxidation damage exerted by ROS on various metabolic components, it is not surprising that wines made from SAYM berries that experienced lower levels of oxidative stress scored higher in all categories of the wine sensory assessment and retained the varietal fingerprint of Gewürztraminer wines better and SAYM training system can potentially mitigate the effects of oxidative stress in high-temperature environments.

One of the distinctions between the training systems' VOC profiles in this work was the higher content of esters in the SAYM wines. Esters are mainly produced by yeast metabolism during fermentation (Saerens et al., 2010; Sumby et al., 2010). Since the vinification process was identical for both the SAYM and VSP wines, we can assume that the ester precursor content was higher in SAYM berries, including branched and aromatic amino acids (Antalick et al., 2014) and lipids, such as fatty acids. A study by Boss et al. (2015) highlighted the effect of β-alanine on the formation of medium-chain fatty acids and their ethyl esters and acetate esters. We propose that the higher ROS content in the VSP berries likely had detrimental effects on the substrate availability of esters, as there appears to be a correlation between ROS formation and oxidative modifications of amino acids (Suto et al., 2006), as well as lipid peroxidation (Farmer and Mueller, 2013). Maoz et al. (2018) showed that grapes have a latent ability to produce esters limited by substrate availability, namely amino acids.

The absence of norisoprenoids in the wines is worth noting. While many representatives of this class were detected in Gewürztraminer wines by Furdíková et al. (2017), only two norisoprenoids were identified in the Ramon vineyard wines. One of them, (E) β-damascenone, an important varietal component of Gewürztraminer wines, was found in higher contents in the SAYM wines (p > 0.05). This disparity between Furdíková's results and ours is most likely terroir and yeast strain-related and could stem from the Negev Desert's high temperatures. Indeed, the results of Marais et al. (1999) showed lower norisoprenoid levels in Sauvignon Blanc berries and wines in two warmer seasons than in a cooler one. The lack of significant differences in the norisoprenoid content between the training systems in this study accords with the lack of significant differences in their carotenoid precursor content in the berries. In 2016/17 and 2017/18, the carotenoid content in the two training systems underwent a typical decrease between veraison and maturity (Joubert et al., 2016; Mendes-Pinto, 2009; Young et al., 2016). The norisoprenoid content can be predicted (Crupi et al., 2010) based on this difference, yet, as mentioned earlier, this was not illustrated in the VOC profile of the Ramon vineyard wines. It is possible that norisoprenoids were formed, but the high temperatures in the Negev Desert promoted their volatilisation during ripening. A similar idea was previously proposed as an interpretation of the lower free volatile terpene content in sun-exposed Gewürztraminer berries than in partially exposed berries (Reynolds and Wardle, 1989). To fully assess the carotenoid-norisoprenoid metabolic pathway in the Ramon Vineyard's Gewürztraminer, a UPLC analysis of carotenoids from photosynthetically active pre-veraison berries is needed at the stage of carotenoid production.

The significantly higher content of some monoterpenes (Z-β-Ocimene, E-β-Ocimene, and the putatively identified β-pinene), along with a slightly higher fraction of total monoterpenes in the VSP wines, is consistent with other studies that showed an increased monoterpene pool in exposed Gewürztraminer berries (Reynolds and Wardle, 1989), Sauvignon Blanc berries (Young et al., 2016), and V.labruscana Golden Muscat (Macaulay and Morris, 1993). Moreover, our data suggest a decreasing trend of VOCs inherent to Gewürztraminer berries, including linalool, geraniol, nerol, and 2-phenethyl acetate from SAYM to VSP vines (Slaghenaufi et al., 2022). This implies an accumulation of these compounds in SAYM wines while experiencing degradation in VSP wines. This may indicate a protective response to the higher radiation and heat experienced by the VSP berries, along with higher oxidative stress levels. Several studies have suggested monoterpenes' thermo- or photo-protective roles in plants (Loreto et al., 1998; Peñuelas and Munné-Bosch, 2005) and their roles in quenching or neutralising ROS. For example, β-pinene, a monoterpene putatively identified to have significantly higher content in the VSP than in the SAYM, has previously been shown to reduce ROS accumulation in maize and to regulate the activity of scavenging enzymes (Mahajan et al., 2019).

Acknowledgements

Acknowledgements: We thank Prof. Moshe Sagi and Dr Aigerim Soltabaya for their guidance in the ROS colorimetric assays, Mariela Leiderman of Agam's µMet lab for her help with the micro-climatic measurements, and all the Fait lab's members, notably Dr Kelem Gashu Alamrie, Khadija Ayarne, Dr Noam Reshef, Corrado Perrin, Mirjam Westram, and Noga Sikron.

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