Vine architecture and crop-regulation techniques, such as cluster thinning, leaves removal and branch grouping, influenced the grapes content of sugars, anthocyanins, and polyphenols as well as the berries and clusters weight (Table 3; Figs. 1 and 2). The DP and luminosity of the musts obtained were also influenced by the studied treatments (Fig. 1). From the results obtained, it was observed that the crop-regulation techniques applied caused a higher impact on the variables studied than that of the vine architecture. We also observed that the influence of the crop season was reduced in the entire study, showing that the results were consistent mainly in the first three crop seasons considered (2012-2013, 2013-2014, and 2014-2015). In the fourth season (2015- 2016), a reduction was found in the values of all the treatments studied, possibly related to the climatic conditions (Table 2).

In the four crop seasons studied, the berries and clusters weight increased in all the crop-regulation techniques applied (Fig. 1), with the only exception of T3 (basal leaf removal) (Table 3). In both vine architectures, the berries weight was higher in T6 (grouping of branches and removal of 33% of clusters) with 2.18 g and 2.31 g (trellis and vase, respectively). Regarding the clusters weight, treatment T2 (removal of 33% of the clusters) reported the highest weight in trellis (483 g). In the case of vase architecture, treatment T4 (removal of basal leaves and 33% of the clusters) reported the highest cluster weight (456 g). These results are in line with those obtained by other authors (Gatti et al., 2012; Gil et al., 2013; Bogicevic et al., 2015). These authors reported higher berries and clusters weights with cluster thinning, and a reduction with vine defoliation. This is a compensation response from the vines. With clusters thinning, the vines can redistribute the available resources among a small number of clusters and grapes, improving their nutrition and consequent development.

As in the case of berries and clusters weight, the DP (Fig. 1C) and luminosity (Fig. 1D) of the obtained musts also increased with the different crop-regulation techniques studied, regardless of the vine architecture. The DP of the musts was higher with treatment T4 (Fig. 1C) in both vine architectures. Regarding musts luminosity, it was higher in treatments T3 to T6 in both architectures, while T1 and T2 (removal of 33% of the clusters at 75 and 85 BBCH, respectively) did not considerably influence this variable (Fig. 1D). The increase of DP is mainly related to the composition of grapes, namely the phenolic compounds. As we will report ahead, the different crop-regulation treatments also caused an increase in phenolic compounds. Since the DP is the polymerization and association of phenolic molecules, including tannins and anthocyanins, its values also increased with the higher levels of phenols (Table 3). The increase in the polymerization degree and luminosity improves the stability and intensity of wines color but they can also contribute to a higher astringency and bitterness (González-Manzano et al., 2006).

Regarding sugars, anthocyanins, and polyphenols, treatment T6 (grouping of green branches, and removal of 33% of the clusters) followed by T4 (removal of the leaves at the base of the branches and removal 33% of the clusters) caused a higher increase in the contents. On average, in the four crop seasons studied, sugars contents increased from 22.0 mg/L in T0 to 24.3 mg/L in T6 (trellis), and from 22.1 mg/L in T0 to 24.4 mg/L in T6 (vase). Regarding anthocyanins, the same pattern was observed for vines grown under trellis architecture. Anthocyanins contents increased from 617 mg/L in T0 to 787 mg/L in T6 (trellis). In vase architecture, anthocyanins increased from 536 mg/L to 802 mg/L for T0 and T4, respectively (Table 3). With respect to polyphenols the same pattern was observed in both vine architectures. Polyphenols increased from 48.0 mg/L (T0) to 66.8 mg/L (T6) in trellis, and from 46.7 mg/L (T0) to 68.9 mg/L (T6) in vase vines (Table 3).

In Fig. 2, the data distribution allows perceiving the impact of treatments T6 and T4 in the contents of sugars (Fig. 2A), anthocyanins (Fig. 2B), and polyphenols (Fig. 2C) of ‘Shiraz’ grapes. Such results are in line with those obtained in several grape varieties and crop-regulation techniques, mostly cluster thinning. Gatti et al. (2012) also found an increase in soluble solids (ºBrix), phenols and anthocyanins in ‘Sangiovese grapes’, while Guidoni et al. (2002) improved anthocyanins content in the skin of grapes from the variety ‘Nebbiolo’.

The considerable increase in polyphenols contents in treatments T4 and T6 could be attributed to the balance between the leaf surface area and fruit. The removal of leaves or the grouping of branches result in the increase of substrates necessary for the synthesis of phenolic compounds (Prajitna et al., 2007). Also, the higher exposure of the cluster-thinned grapevines to sunlight enhances the production of substrates that in turn enhance the activity of the phenylalanine ammonia lyase, an enzyme involved in the biosynthesis of phenolic compounds in grapes (Chen et al., 2006).

To verify the impact of the crop-regulation techniques studied on the changes inflicted on the grapes, the data were analyzed by PCA. Figure 3 shows the analysis of sugars, anthocyanins, phenolic compounds and berries and clusters weights per crop season, and the differentiation between the treatments applied can clearly be seen. In the first three crop seasons studied (2012-2013, 2013-2014, and 2014-2015), it is visible that those samples from T4 and T6 were represented in the positive region of PC1. In the same region, and for the same three crop seasons, all the variables studied were represented. This means that in general, the higher values were obtained for these two treatments (T4 and T6; Fig. 3). However, in the last crop season studied (2015-2016), there was a noticeable differentiation of those samples from T4 in vase architecture, which reported higher values concerning anthocyanins, berries and clusters weight. In the four crop seasons studied, the control samples (treatment T0) were always represented in the extreme opposite region of the variables studied (Fig. 3). This means that grapes from the control treatment reported lower values, thus highlighting that the crop-regulation techniques improved the grapes composition, berries, and clusters weight, and consequently the musts DP and luminosity.

Such crop-regulation treatments influence grapes composition, which will change the final composition and quality of the wine. For instance, Gil et al. (2013) were able to obtain wines from Syrah variety with greater polyphenols, flavonols, proanthocyanidins and polysaccharide concentrations, and lower titratable acidity. These wines originated from grapes whose vines were submitted to cluster thinning. Using similar cluster thinning techniques, Prajitna et al. (2007) also improved the amounts of polyphenols, anthocyanins, and resveratrols in wines from the ‘Chambourcin’ variety, improving the antioxidant properties as well. Condurso et al. (2016) were able to improve grapes and wine polyphenol contents as well as volatile fraction through cluster thinning, thus improving the overall quality of the wine obtained.

In summary, the data obtained allowed concluding that treatment T6, i.e., grouping of green branches and removal of 33% of the clusters, produce ‘Shiraz’ grapes with higher levels of anthocyanins, polyphenols, and sugars, which will possibly result in better-quality wines. The removal of 33% of the clusters alone also increased these components levels, but the effect in higher when applied in combination with grouping of branches. We also concluded that cluster thinning is more effective if applied at the 75 BBCH stage (T1) rather than at the 85BBCH stage (T2), mainly regarding sugars content. It was also concluded that all the treatments applied improved the luminosity and degree of polymerization when compared to control. Therefore, the stability and intensity of wines color may be increased as well. Another conclusion drawn from the study was that grapes characteristics and musts composition were not considerably influenced by the crop season or by the vine architecture. Nevertheless, based on the overall results, we would recommend the application of treatments T4 and T6 in the vase architecture, to improve the quality of grapes and wines from cv. ‘Shiraz’.

Table 2. Climatic data of the evaluated crop seasons

[1] Reserve period: between October and February. [2] Vegetative period: between March and September.

Table 3. Influence of treatment, crop season, and vine architecture in the chemical composition of grapes (sugars, anthocyanins, and polyphenols) and in the berries and clusters weight.

Table 3 Continued. 

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Figure 1.  Boxplot of physico-chemical characteristics of grapes and musts from the ‘Shiraz’ variety grown under different thinning treatments and vine architecture. A, berries weight; B, clusters weight; C, degree of polymerization; D, luminosity

Figure 2.  Boxplot of chemical composition of grapes from the ‘Shiraz’ variety grown under different thinning treatments and vine architecture. A, sugars; B, anthocyanins; C, polyphenols

Figure 3.  Principal component analysis of the data obtained according to the treatment applied and the vine architecture studied per crop season. The principal component analyses explain 70.6% (2012-2013), 78.7% (2013-2014), 78.7% (2014-2015), and 65.1% (2015-2016) of the total variance. A, sugars; B, anthocyanins; C, phenolic compounds; D, berries weight; E, clusters weight.