Introduction

Environmental factors, such as topography, precipitation and soil type, affect the growth of grapevines and grape composition (Downey et al., 2004), whereas viticultural practices, such as row orientation and canopy management, primarily affect the grapevine microclimate (Hunter et al., 2016), which in turn has an effect on the grape chemical composition.

Grape growers attempt to minimise the heterogeneity of grape material within a single vineyard parcel/block to improve wine quality (Downey et al., 2004; Hunter et al., 2010). The development of phenolics (secondary plant metabolites) is affected by environmental factors (Gómez-Alonso et al., 2007), viticultural practices (Pérez-Magariño and González-SanJosé, 2002) and plant genetics (Czemmel et al., 2009).

The most common phenolic compounds present in red wine grapes are non-flavonoids (i.e., derivatives of hydroxybenzoic and hydroxycinnamic acids (Ribéreau-Gayon et al., 2006)), and flavonoids (i.e., anthocyanins, flavones, flavanonols, flavan-3-ols and flavonols (Margalit, 2004; Monagas et al., 2005)). Anthocyanins, flavan-3-ols and flavonols are most abundant in grape skin, whereas phenolic acids are mainly present in the grape pulp.

Phenolic concentrations increase throughout berry development (Pérez-Magariño and González-SanJosé, 2006), with differing evolution depending on phenolic group (Mateus et al., 2002).

The relationship between phenolic composition and grape ripeness levels has been reported by numerous authors. Phenolic evolution as a consequence of a delay in sugar accumulation during the grape development of artificially shaded Cabernet-Sauvignon has been reported by Rojas-Lara and Morrison (1989), which was likely the result of decreased photosynthetic capacity of the vine. Fournand et al. (2006) found that free anthocyanins change to derivatised anthocyanins during the grape ripening period, and Guidoni and Hunter (2012) reported differences between the anthocyanin profiles of grapes at different ripeness levels. Guidoni and Hunter (2012) showed that malvidin 3-O-glucoside and malvidin 3-O-(6-p-coumaroyl)-glucoside were lower when at 22 °Brix compared to 24 °Brix and 26 °Brix ripeness levels, whereas acetylated anthocyanins were higher at a lower ripeness level. The high proportion of malvidin 3-O-glucoside found at 22 °Brix appeared to occur at the expense of the acetyl derivative, which is expected to be more affected by degradation reactions in grapes. Non-acetylated and acetylated glucosides are sensitive to degradation processes induced by high temperature, whereas coumaroyl-glucoside seems to be more stable under these conditions. Treatment did not have an effect on coumaroyl-glucosides. The “insensitivity” of coumaroyl-glucoside to treatment can be ascribed to their participation in intramolecular co-pigmentation (Brouilard and Dangles, 1994).

Giacosa et al. (2015) found that the modification of the chemical profile of Shiraz grapes was more due to changes during grape ripening than to viticultural practices. During the over ripeness stage of grapes (> 26 °Brix), individual anthocyanins can undergo some transformation independently of light and temperature conditions.

Roubelakis-Angelakis and Kliewer (1986) suggested that temperature and light play an independent regulating role in the flavonoid biosynthetic pathway to the point of the formation of dihydroflavonols, taking into consideration that anthocyanins and flavonols share the same pathway. Hunter et al. (2016, 2021) reported that radiation profiles in the canopy of Shiraz grapevines have implications for photosynthetic rates and grape berry temperature.

Hydroxylation levels of anthocyanins can be affected by not only UV radiation, but also temperature variation (Spayd et al., 2002). However, Alonso et al. (2016) and Mori et al. (2005) reported that anthocyanins may not always respond to radiation. Increased temperature in the fruit zone can result in grape chemical composition variability (Romboli et al., 2017), since the response of anthocyanins to UV radiation depends on the interaction of grape cultivar, grape ripeness and temperature factors (Jordan, 2017).

Gallic acid and most flavan-3-ols in grape skins do not respond to UV radiation, but p-coumaric, caffeic and ferulic acid, as well as gallocatechin increase in UV blocked grape bunches (Spayd et al., 2002). Yu et al. (2016) reported that increased temperature in the fruit zone was associated with an increase in epigallocatechin and a decrease in epicatechin in Merlot and Shiraz grapes respectively. Allegro et al. (2019) found that increased light interception in the canopy and increased berry temperature (after leaf removal) did not affect the total flavan-3-ols in the skins of Grechetto Gentile grapes; however, high levels of epicatechin were counterbalanced by low levels of epigallocatechin.

Sternad-Lemut et al. (2011) showed that hydroxycinnamic acids in Pinot noir grapes respond only slightly to leaf removal (exposure to light), indicating that solar radiation has a limited role in regulating phenolic acid biosynthesis. Light exclusion (by brown paper bagging) of Cabernet-Sauvignon grape bunches have been found to affect phenolic acids less than flavonoids; however, differences in hydroxycinnamic acid contents in grape berries during ripening have been found between light-excluded grape bunches and bunches exposed to light (Sun et al., 2019).

Rescic et al. (2016) reported increased concentrations of gallic acid in Istrian Malvasia grapes harvested at 24 °Brix, with increased light in the fruit zone brought about by leaf removal/shoot thinning of grapevines planted in N-S (north-south) rows. Work by Tessarin et al. (2014) on grapevines planted in east-west (E-W) directions showed that caffeic acid concentrations in Uva Longanesi grapes were higher after 50 % defoliation compared to no defoliation treatment.

Martínez-Lüscher et al. (2014) found lower concentrations of kaempferol and quercetin in Tempranillo grapes from N-S orientated vines receiving reduced light due to artificial shading compared to low vigour vines. Friedel et al. (2015) reported that boxed-in Riesling grape bunches from vines planted in an E-W direction, resulted in an increase in rutin, quercitrin and isoquercetin. In a study by Downey et al. (2004), artificially shaded Shiraz grapes at harvest were lower in flavan-3-ol content than exposed bunches; however, the decrease in total flavan-3-ols during berry ripening was greater in exposed bunches, to the extent that the levels were virtually the same in shaded and exposed berries at harvest. Scrafidi et al. (2013) reported higher concentrations of total flavan-3-ols in exposed Grillo grapes at 21 °Brix and planted in N-S rows than in boxed-in grapes, whereas Cortell et al. (2005) found that (-)-epigallocatechin concentrations in Pinot noir grapes at harvest, planted in an E-W direction, were positively correlated to increased light in the canopy and fruit zone.

Light intensity and light quality (UV-B and blue light) have been associated with the regulation of anthocyanin biosynthesis (Jaakola, 2013). Shiraz and Pinot noir grape bunches that develop in low vigour vines, as opposed to those that develop in high vigour vines, have increased concentrations of berry skin anthocyanins (Downey et al., 2004; Cortell et al., 2007). According to Joscelyne et al. (2007) and Ristic et al. (2007), artificially shaded Syrah grapes of vines planted in an E-W direction had lower concentrations of total anthocyanins than grape bunches from low vigour vines. Feng et al. (2015) reported that increased light brought about by 50 % leaf removal of Pinot noir N-S planted vines, resulted in an increase in total anthocyanins, compared to no leaf removal treatment. Martínez-Lüscher et al. (2014) found that reduced light (artificial shading) in the canopy of Tempranillo grapevines planted in a N-S direction, resulted in an increase in acylated malvidin-glucosides in grapes at 23 °Brix. Fernandes-De Oliveira and Nieddu (2016) reported higher concentrations of delphinidin- and petunidin 3-O-(6-O-p-coumaroyl) glucosides in artificially shaded (reduced UV-radiation) Bovale Grande and Cannonau grapes from N-S orientated vines than naturally exposed grapes.

In the cited literature, flavonols, flavan-3-ols and phenolic acids generally increased in grapes of vines exposed to light due to defoliation, whereas artificial shading or high vigour vines resulted in a decrease in these phenolics. Increased light due to defoliation also resulted in an increase in total anthocyanins, but a decrease in certain monomeric anthocyanin glucosides and anthocyanin glucoside coumaroylates.

Internationally, work on the effect of grapevine row orientation under realistic field conditions has not been published, and in South Africa, only limited investigations have been undertaken on the effect of row orienation per se on phenolics. It was demonstrated that, despite total anthocyanins and phenolics in Syrah grapes from rows with different orientations being relatively similar (with highest values found for the NW-SE (northwest-southeast) rows), wooded and unwooded wines produced from E-W orientated vines contained the lowest concentrations (Hunter and Volschenk, 2017).

The aim of this study was to investigate the effect of microclimate induced by different grapevine row orientations under realistic field conditions on the concentrations of selected individual anthocyanins, flavan-3-ols, flavonols, and phenolic acids in Syrah grapes, harvested at three grape ripeness levels over four consecutive seasons.

Materials and methods

1. Vineyard layout and management

An experimental vineyard was established in 2003 (Figure 1). The first full harvest was carried out five years after the establishment of the vineyard. The vineyard was already in full production when this experiment was undertaken. Syrah, clone SH 9C was grafted onto 101-14 Mgt rootstock. The vineyard was established on a flat terroir with clayey loam soil on the research farm of the Agricultural Research Council (ARC) in the Breede River Valley, Robertson (33.8021° S, 19.8875° E, 187 metres above sea level), South Africa (Hunter et al., 2016). Row and vine spacing were 2.7 m and 1.8 m respectively. The vines were trained to a vertical shoot positioning (VSP) trellis system. A cover crop of rye grass was sown after harvest (April) and treated with herbicide prior to bud burst (October). The grapevine canopies consisted of three to four leaf layers, side by side. The leaf layers were measured using the point quadrat method (Hunter et al., 2016). Grapevines were subject to supplementary irrigation every seven days.

Figure 1. Aerial view of different grapevine row orientations indicating experimental replicate blocks (1-5).

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1 = East-west (low light penetration in the morning and afternoon in the canopy); 2 = North-south (high light penetration in the morning and afternoon in the canopy); 3 = Northwest-southeast (low light penetration in the afternoon and high light penetration in the morning in the canopy); 4 = Northeast-southwest (low light penetration in the morning and high light penetration in afternoon in the canopy).

The experimental design was a randomised complete block (Figure 1) with four different row orientations (i.e., N-S, E-W, NE-SW and NW-SE) replicated at random in each of five experimental blocks with a total surface area of 1860 m². An experimental unit consisted of all the grapevines within a vineyard block of a specific row orientation within the experimental vineyard.

Results

6. Principal component analysis

The PCA biplot (Figure 2) of the first two principal components (PC1 and PC2) illustrates the association of phenolic compounds at 22 °Brix in 2008, 2009, 2010 and 2011 with the N-S, E-W, NE-SW and NW-SE rows, explaining 59.42 % of the variation in the data. The major source of variation is row orientation with PC1 separating the NE-SW row from the E-W, N-S and NW-SE rows, while PC2 separates the NW-SE row from the E-W and N-S rows. Variables with the highest squared cosine values (data not shown) on PC1 are (+)-catechin, caffeic acid, ferulic acid, total flavonols, isoquercetin, quercetin, kaempferol, total anthocyanins, petunidin 3-O-glucoside, malvidin 3-O-glucoside, delphinidin 3-O-(6-O-p-coumaroyl) glucoside and petunidin 3-O-(6-O-p-coumaroyl) glucoside. These variables were positively associated with the NE-SW rows.

Flavan-3-ols, epigallocatechin 3-O-gallate, total phenolic acids, p-coumaric acid, peonidin 3-O-(6-O-acetyl) glucoside and malvidin 3-O-(6-O-acetyl) glucoside had the highest squared cosine values on PC2. The NW-SE rows were positively associated with flavan-3-ols, but negatively associated with total phenolic acids.

Figure 2. Principal component analysis bi-plot illustrating the association of 23 individual phenolic compounds measured in grapes harvested in 2008, 2009, 2010 and 2011 at 22 °Brix from N-S, E-W, NE-SW and NW-SE rows. Gluc = glucoside; Coum = coumaroyl; EGCG = Epigallocatechin 3-O-gallate.

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Principal component 3 explains only an additional 13.85 % of variation (data not shown) and does not improve interpretability; it was therefore not included in the biplot. Only quercitrin, (-)-epicatechin and peonidin 3-O-glucoside had the highest squared cosine value on PC3. Close groupings of the same rows from different vintages may be an indication that the largest variation in phenolic content are between rows, while variation between vintages are less pronounced.

At 24 °Brix, the PCA biplot (Figure 3) of the first two principal components (PC and PC2) illustrates the association of phenolic compounds in 2008, 2009, 2010 and 2011 with the N-S, E-W, NE-SW and NW-SE rows, explaining 57.61 % of the variation in the data. In general, the different vintages and rows separated and grouped reasonably well.

Figure 3. Principal component analysis bi-plot illustrating the association of 23 phenolic compounds measured in grapes harvested in 2008, 2009, 2010 and 2011 at 24 °Brix from N-S, E-W, NE-SW and NW-SE row orientations. Gluc = glucoside; Coum = coumaroyl; EGCG = Epigallocatechin-3-O-gallate.

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The main source of variation is row orientation with PC1 separating NW-SE from E-W, NE-SW and N-S rows, while PC2 separates individual vintages within rows. Variables with the highest squared cosine values (≥ 0.5) on PC1 were EGCG, phenolic acids, caffeic acid, p-coumaric acid, ferulic acid, total favonols, quercetin, quecitrin and petunidin 3-O-glucoside. Phenolic acids and flavonols were mainly associated with the NW-SE rows.

Catechin (+), total anthocyanins, malvidin 3-O-glucoside, delphinidin 3-O-glucoside, peonidin 3-O-(6-O-acetyl) glucoside and malvidin 3-O-(6-O-acetyl) glucoside had the highest squared cosine values on PC2. Gallic acid, rutin and cyanidin 3-O-glucoside had the highest squared cosine values on PC3, but it explained only an additional 15.17 % of variation in the biplot. It does not improve interpretability and was therefore not included in the biplot.

At 26 °Brix, the PCA biplot (Figure 4) of the first two principal components (PC1 and PC2) illustrates the association of phenolic compounds in 2008, 2009, 2010 and 2011 with the N-S, E-W, NE-SW and NW-SE rows, explaining 54.75 % of the variation in the data. The different vintages and rows showed excellent separation and grouping. PC1 mainly separates NW-SE from the other rows, while PC2 separates NE-SW from N-S and E-W. The phenolic compounds with the highest squared cosine values (≥ 0.5) on PC1 were total flavan-3-ols, (+)-catechin, total phenolic acids, gallic acid, caffeic acid, ferulic acid, total flavonols, kaempferol, quercitrin and malvidin 3-O-(6-O-acetyl) glucoside. NW-SE rows were mainly associated with flavonols and phenolic acids. Epigallocatechin 3-O-gallate, p-coumaric acid, isoquercetin, quercetin and petunidin 3-O-(6-O-acetyl) glucoside have the highest squared cosine values on PC2 (data not shown).

Most anthocyanins were associated with E-W rows and flavan-3-ols with NE-SW. Peonidin 3-O-glucoside, malvidin 3-O-glucoside and malvidin 3-O-(6-O-coumaryol) glucoside have the highest squared cosine values on PC3, but they contributed only an additional 13.76 % to the variability in the PCA biplot; PC3 does not improve interpretability and was therefore not included in the biplot.

Figure 4. Principal component analysis bi-plot illustrating the association of 23 phenolic compounds measured in grapes harvested in 2008, 2009, 2010 and 2011 at 26 °Brix from the N-S, E-W, NE-SW and NW-SE rows. Gluc = glucoside; Coum = coumaroyl; EGCG = Epigallocatechin-3-O-gallate.

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Discussion

The results show that row orientation and grape ripeness effects were detectable for individual phenolic compounds and phenolic classes. Generally, vintage effect was not evident among the phenolics quantified, except for gallic acid and malvidin 3-O-glucosides in 2011 (22 °Brix), epicatechin and total flavan-3-ols in 2008 and 2010 (24 °Brix) respectively, and rutin in 2011 (26 °Brix). Hunter et al. (2016) reported that climatic conditions among vintages were similar, reaching comparable maturity indices in all four seasons; therefore, the vintage factor was not taken into account and the grapes per ripeness level were considered as replications.

Ambient (air) temperature of the experimental vineyard during grape development never exceeded 35 °C (Hunter et al., 2016). Ambient temperature between 30 °C and 35 °C is the critical temperature range for flavonoid and non-flavonoid biosynthesis (Tarara et al., 2008; Bonada et al., 2015). Temperatures above 35 °C can be detrimental to, for example, anthocyanin and flavonol accumulation, since they share the same flavonoid biosynthetic pathway (Bonada et al., 2015).

Considering the relatively uniform vegetative characteristics of the canopy architecture (Figure 1), canopy interior temperature primarily reflected ambient (air) temperature at meso-level (Hunter et al., 2017) and therefore temperature profiles inside the canopy did not show marked differences among the different rows. Canopy temperature per se consequently played a minor role in the final berry temperature of VSP-trellised vines of the different rows. Despite this, berry temperature and light penetration were slightly affected by the canopy orientation. Berry temperature of grapes from the E-W and NE-SW rows were slightly lower than grapes from the N-S and NW-SE rows in mid-morning (Table 2). However, at around midday, significant differences in temperature among the rows were not found. In late afternoon, grapes from the NE-SW rows had the highest temperature. This may be a consequence of the high levels measured for total flavonols, isoquercetin and total malvidin 3-O-glucosides at 22 °Brix and isoquercetin and quercetin measured at 26 °Brix. The respective berry temperatures of the N-S and E-W rows in the late afternoon were significantly different, but those of the N-S and NW-SE rows were not different. Nevertheless, on average over the four consecutive seasons, only grapes from the E-W rows were significantly lower in temperature (Table 2).

Beggs and Wellman (1994) reported that increased light exposure of grape berries, and not temperature, enhances anthocyanin biosynthesis. Downey et al. (2006) reported that increased grape bunch temperature can result in a decrease in total anthocyanin content, and Cagnasso et al. (2011) reported that cooler climates seem to favour anthocyanin accumulation in Carema grapes. Anthocyanins can increase at higher grape ripeness levels as reported by Guidoni and Hunter (2012), who showed that increased anthocyanin levels in Shiraz grapes is related to cooler temperatures and moderate light penetration under the control of photoreceptors during the final stages of ripening, which may prolong anthocyanin synthesis. Despite this, these flavonoids can also undergo polymerisation during the last ripening phase before harvest, leading to a decrease in anthocyanin levels. Therefore, separating the effects of temperature and light on grape composition is difficult because many of the biochemical pathways are light and temperature (solar radiation) sensitive (Spayd et al., 2002).

Anthocyanins reached their highest levels at 22 °Brix from the N-S rows, whereas flavonols and flavan-3-ols seemed to be the highest at 24 °Brix from the same row orientation (Minnaar, 2018). Anthocyanin concentrations tended to decrease as grape ripeness levels increased. Hunter et al. (2021) reported that anthocyanins in Shiraz grapes from N-S rows decreased as the grapes ripend. Phenolic acids were not substantially affected by grape ripeness levels but were highest at 24 °Brix from N-S rows.

For E-W rows, flavan-3-ols and phenolic acids reached their highest levels at 22 °Brix, after which a decrease occurred as the grapes ripened further (Minnaar, 2018). Jordão et al. (2001) reported flavan-3-ols of Castelão Francês and Touriga Francesa grapes to decrease during the grape maturation process. Flavonols, on the other hand, appeared to be highest at a ripeness level of 24 °Brix. This is similar for grapes from N-S rows of this study. Anthocyanins tended to already be highest at 22 °Brix, after which a decrease occurred as the grapes ripened. This is similar to the findings for the grapes from N-S rows. The grapes from both the N-S and E-W rows tended to reach their highest levels of anthocyanins and flavonol at 22 °Brix and 26 °Brix respectively.

The NE-SW rows brought about only a slight variation in flavan-3-ol levels from 22 °Brix to 26 °Brix ripeness level. Flavonols and phenolic acids ostensibly reached their highest levels at 22 °Brix, after which a decrease occurred as the grapes ripened (Minnaar, 2018). Anthocyanins seemed to reach their highest levels at 22 °Brix, as was the case for the grapes from N-S and E-W rows.

Northwest-southeast rows were the cause of flavan-3-ols already reaching their highest levels at 22 °Brix, after which a decrease occurred as the grapes ripened. Flavonols, phenolic acids and anthocyanins tended to be highest at 24 °Brix. Grapes from NE-SW rows seemed to reach their highest anthocyanin, phenolic acid and flavonol levels before those from NW-SE rows, but they tended to reach their highest flavan-3-ol levels before those from the NE-SW rows (Minnaar, 2018). Certain treatments (rows) had no effect on phenolics (no main effects, p-values not significant) which means that row direction prevented sufficient UV-B radiation from entering the canopy, thereby delaying accumulation, but did not completely inhibit synthesis.

Monomeric anthocyanins not affected by treatment (not significant main effects) maybe a result of the shift in anthocyanin composition during grape maturation due to the hydroxylation of the B-ring (Ryan and Revilla, 2003); this is an indication of post maturity and the process in which cells irreversibly stop dividing and enter a state of permanent growth arrest without undergoing cell death, also known as senescence. In contrast to findings by Guidoni and Hunter (2012), Ryan and Revilla (2003), and Pérez-Magarino and González-SanJosé (2006) reported that maximum accumulation of anthocyanins occurred when grape sugar levels reached between 20 and 25 °Brix in Tempranillo and Cabernet-Sauvignon grapes planted in an E-W direction. This is in agreement with the results of this study, in which maximum anthocyanin accumulation already occurred at 22 °Brix, but in grapes from NE-SW rows. Furthermore, De la Hera Orts et al. (2005) reported that maximum anthocyanin levels were reached at between 20 and 24 °Brix in Monastrell grapes, but Fourmand et al. (2006) found that anthocyanins remained unchanged from 20 °Brix to 26 °Brix.

Chorti et al. (2010) reported increased anthocyanins in artificially (partially) shaded Nebbiolo grapes at harvest and planted in N-S rows (northern-hemisphere), compared to grape bunches without shading. Partially shaded grapes from N-S rows with limited light penetration in the canopy can be considered to be similar to the NW-SW rows of this study in terms of light penetration (low light penetration in the afternoon). Contrary to Chorti et al. (2010), Bubola et al. (2017) and Feng et al. (2017) have reported increased total anthocyanins in Teran grapes at 23 °Brix and Pinot noir grapes at 22 °Brix from N-S-orientated vines. Vines underwent 50 % defoliation thereby increasing the light penetration and temperature in the fruit zone, compared to no leaf removal.

Ultraviolet-B radiation can upregulate chalcone synthase; i.e., flavonoid biosynthesis and phenylalanine ammonia-lyase in the phenylpropanoid genes pathway (Jansen et al., 1998). Flavonols can therefore increase when grapes are exposed to increased light penetration in the canopy (Alonso et al., 2016; Pastore et al., 2017a) and expression of gene encoding flavonol synthase in grape skins (Pastore et al., 2013). Flavonols are sensitive to changes in environmental conditions and can respond as a protectant against UV-containing light. Temperatures above 35 °C can, however, have a detrimental effect on flavonol biosynthesis (Pastore et al., 2017b). In contrast to the above, flavonol levels in this study were lowest in grapes from N-S rows (high light penetration all day) at 22 °Brix, 24 °Brix and 26 °Brix; this is similar to work by Cohen et al. (2012), who found lower concentrations of flavonols in Merlot grapes at 22 °Brix of vines planted in N-S rows subjected to heating in the fruit zone (via heat blowers), compared to grapes at ambient temperature. Increased temperature in the fruit zone may have inhibit flavonol biosynthesis.

In this study, flavonols that decreased during grape ripening (Table 5) may be attributed to oxidation through coupled reactions and may act as co-pigments with anthocyanins in co-pigmentation processes (Bimpilas et al., 2015). The increase in flavonols (i.e., grapes from NW-SE rows at 24 °Brix and 26 °Brix) could be due to the high light penetration in the canopy in the morning, resulting in increased grape bunch temperature (Table 2), which stimulates flavonol biosynthesis; some UV radiation is required to stimulate gene transcription to produce flavonol synthase (Holton and Cornish, 1995). Flavonol synthase catalysis the conversion of dihydrokaemferol to kaempferol and dihydroquercetin to quercetin, among other flavonols. Friedel et al. (2015) have also reported that flavonols increased in Riesling grape bunches at harvest with increased light penetration in the canopy (E-W direction) due to leaf removal in the fruit zone. It seems that flavonol biosynthesis favours increased light penetration in the fruit zone in the morning; i.e., in NW-SE rows.

In the present study, the decrease in total phenolic acids in grapes from the E-W, NE-SW and NW-SE rows as grapes ripened (24 °Brix to 26 °Brix) may be a result of down-regulated flavonoid biosynthesis due to limited light penetration in these canopies. Grapes from N-S rows showed a slight increase in phenolic acids with ripeness. Additionally, the differences in grape phenolic acid levels among rows may be a consequence of these reactive compounds changing to galloylated derivatives when exposed to increased PAR, thereby decreasing free acids (Jordão et al., 2001). Trihydroxybenzoic acids in grapes do not usually respond to UV radiation; hydroxycinnamic acids, however, respond to variation in light penetration in the canopy (Spayd et al., 2002), which could have contributed to the observed differences in their concentrations in grapes depending on the row orientation in this study (Table 5). Rescic et al. (2016) reported that total phenolic acids were higher in Uva Longanesi and Istrian Malvasia grapes from E-W and N-S rows respectively at 22 °Brix after 50 % defoliation, compared to vines with no defoliation treatment. In this study, the phenolic acid levels were highest in grapes from NW-SE rows at 24 °Brix and 26 °Brix.

Flavan-3-ol levels in grapes from the NE-SW and NW-SE rows at both 22 °Brix and 24 °Brix were higher than those in grapes from to N-S and E-W rows. Similar results have been reported by Romboli et al. (2017), who found increased flavan-3-ol levels in low vigour Sangiovese grape planted in NE-SW rows at harvest, compared to high vigour vines. In the present study, total flavan-3-ols generally increased until grape ripeness reached 24 °Brix, after which it was not affected by the treatment as grapes ripened to 26 °Brix.

In a study by Downey et al. (2004), flavan-3-ol levels in artificially shaded Shiraz grapes during berry ripening were found to be lower than in exposed bunches; however, the decrease in total flavan-3-ols during berry ripening was greater in exposed bunches, to the extent that the levels were virtually the same in shaded and exposed berries at harvest. Pastore et al. (2017a) reported flavan-3-ols to have minimal response to environmental factors; however, PAR rather than UV radiation had a positive effect on flavan-3-ol synthesis. In addition, Scrafidi et al. (2013) reported higher concentrations of total flavan-3-ols in exposed Grillo grapes at 21 °Brix and planted to N-S rows than in boxed-in grapes. In contrast to work by Pastore et al. (2017a) and Scrafidi et al. (2013), the results of this study show that grapes from N-S rows (high light penetration all day) had the lowest levels of total flavan-3-ols. However, in agreement with the flavan-3-ol levels found in this study, Allegro et al. (2019) reported that increased light penetration in the canopy/fruit zone of Grechetto Gentile grapes did not increase the accumulation of flavan-3-ols. Higher levels of total flavan-3-ols, phenolic acids and flavonols are to be expected in N-S rows (high light penetration in the canopy), compared to E-W, NE-SW and NW-SE rows; however, this was not the case in this study. The low levels of total flavan-3-ols and total flavonols at 22 °Brix and total flavonols at 24 °Brix may be due to over-exposure of grape bunches to light (i.e., high initial PAR at 22 to 24 °Brix grape ripeness), which could have inhibited some metabolic processes (Downey et al., 2006).

The variation in phenolic concentrations reported in the cited literature was likely caused by defoliation and artificial shading, resulting in grape bunches being over- or under-exposed to light. Artificial shading of grape bunches (light manipulation/boxed-in) caused a decrease in certain phenolic compounds and over-exposure/artificial heating an increase. The results reported by international authors were therefore primarily due to the manipulation of the canopy and fruit zone in addition to “natural” row orientation creating a microclimate. The cited results may, however, be linked to the results and deductions of this study regarding the effect of different row orientations on the light penetration in the fruit zone. Hunter et al. (2016) have reported that the microclimate profiles (light) of canopies are affected by changes in row orientation.

This is the first investigation in South Africa to demonstrate the effect of grapevine row orientation on individual phenolic compounds in Syrah grapes, as well as the combined effect of vintage and grape ripeness level, although vintage did not play a major role. The results showed that phenolic compound biosynthesis involves the interaction of light and grape ripeness (and temperature). The effect of different light regimes brought about by the different row orientations and grape ripeness on phenolic compound development was shown.

Under realistic field conditions (i.e., different row orientations), planting grapevines in N-S and NE-SW rows can result in higher temperatures in the fruit zone (Hunter et al., 2016) than planting in E-W and NW-SE rows. An increase in temperature in the fruit zone resulting from grapevine row direction has been found to cause a decrease in certain phenolics, which is probably due to a combination of both degradation and biosynthesis inhibition (Mori et al., 2005; Tarara et al., 2008; Feng et al., 2017). In this study, NE-SW and NW-SE row orientation generally had a positive effect on phenolic development, which may be linked to shorter periods of light penetration/exposure in the fruit zone; i.e., low light exposure in the morning for NE-SW and low light exposure in the afternoon for NW-SE rows. However, the effect of light on the biosynthesis of phenolics can vary depending on the grape cultivar (Rustioni et al., 2011; Bonada and Sadras, 2014). Ambient temperatures above 35 °C may inhibit phenolic biosynthesis even under different light regimes. Nevertheless, light intensity differences in the fruit zone are an important factor for phenolic compound development/accumulation, irrespective of row orientation per se (Koyama et al., 2012; Zoratti et al., 2014). Accumulation is also dependent on a combination of moderate exposure of grape bunches to light and moderate temperature in the canopy.

Within the context of global warming, many grape growers are facing premature véraison due to early-season temperature increases, which could lead to sunburn of grape-berry skin tissue, as well as the inhibition of the biosynthesis of, for example, anthocyanins and flavan-3-ols (Hunter et al., 2010). Achieving phenolic maturity when grape sugar content and acidity are at optimum levels can sometimes be challenging in warm, semi-arid wine-growing regions, such as in Robertson in South Africa. As a result, growers have to either wait for phenolic maturity, at the cost of producing wines with high alcohol content and sometimes too low acidity, or harvest at desired sugar-acid levels and risk producing wines with poor colour and unripe tannins.

The desirable conditions for grapevines growing in a warm semi-arid environment are moderate to low light penetration of the canopy. However, the orientation of vineyard rows has proven to be one of the viticultural practices enabling successful canopy microclimate manipulation, leading to grape phenolic improvement. Since in practice, it may not be possible to achieve suitable grapevine row orientation in all environments, the management of the fruit zone by means of pruning, shoot positioning, removal of infertile shoots and leaf removal remains an option for increasing/decreasing grape exposure to light, irrespective of row orientation.

Conclusion

The results indicate that row orientation and ripeness level can be applied to control phenolic development. The effect of vintage, however, cannot be controlled. In the southern hemisphere, light conditions induced in the grape bunch zone primarily in the NE-SW (at 22 °Brix) and NW-SE (at 24 °Brix and 26 °Brix) rows positively affected most phenolics. Grapes from these row orientations could result in Syrah wines with an improved phenolic profile and have the potential to extend the range of Syrah wine styles. However, it is unlikely that a single factor can be applied to all growing conditions and grape cultivars. It is advised that viticulture-related factors, such as fertility, berry set, berry size and berry yield, be taken into account before making a decision on a specific row orientation. It is important to consider the characteristics of specific growing regions when determining the effect of row orientation on grape phenolic content. In the light of this, the results of this study can be applied to grapevines planted in specific row orientations in clayey loam soil and trained to a VSP trellis system. Commercial vineyard parcels/blocks, different terroirs and the hombre influencia can also have an effect on grape phenolic profiles. Therefore, further research is needed to understand the associations that exist among vine phenology, microclimate/light penetration and grape phenolic content.

Acknowledgements

The authors would like to thank the Agricultural Research Council and Winetech for providing funding and infrastructure. This research forms part of a PhD thesis obtained by P Minnaar at Stellenbosch University, South Africa.

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