3.2.2. Stilbene Composition

Figure 1 shows the stilbene concentration of the vine shoot extracts of the investigated varieties while the Rsv, Vf, and total stilbenes concentrations (mean ± standard deviation) are reported in Table S1 (Supplementary Material). The mean total concentration of stilbenes, approximately 4500 mg kg−<sup>1</sup> DW, varied greatly depending on the variety, with values ranging between 2700 and 6400 mg kg−<sup>1</sup> DW for Verdeca and Palieri, respectively,

with 2.4-fold higher results for the latter. Nevertheless, the Palieri, Montepulciano, and Italia varieties presented the highest total stilbene concentration. In contrast, the Verdeca, Bianco d'Alessano, and Trebbiano varieties presented the lowest total stilbene concentration. In previous studies, a wide variability (from 2.5 to 4-fold) of total polyphenol amounts was already observed among different vine shoot varieties [18,30,37].

**Table 5.** The Pearson's correlation coefficients between the TPC, ABTS, and DPPH in 23 vine shoot extracts.


**Figure 1.** The stilbene contents in vine shoots from 23 different Italian varieties. Red dotted lines— the mean contents of *trans*-resveratrol (Rsv) and ε-viniferin (Vf); blue solid line—the mean content of *trans*-resveratrol + ε-viniferin. Black labels indicate black grape varieties; red labels indicate white grape varieties. For sample codes, see Table 1.

The major stilbene compounds found in all the collected samples were Rsv (mean of 3422.2 mg kg−<sup>1</sup> DW), followed by Vf (mean of 1040.0 mg kg−<sup>1</sup> DW). An example of HPLC-DAD chromatograms of the Palieri vine shoot extract is provided in the Supplementary Material Figure S1. These results agree with those observed in the studies by Vergara et al. [29] and Gorena et al. [33], in which the major stilbene compound found in most grape cane extracts considered were Rsv. In contrast, according to Guerrero et al. [18,32] and Lambert et al. [30], Vf was the most abundant compound in vine shoots of different *Vitis* varieties.

The highest mean concentration of Rsv was determined for the Nero di Troia (5298.1 ± 45.2 mg kg−<sup>1</sup> DW) and Negroamaro vine shoots (5249.4 ± 129.8 mg kg−<sup>1</sup> DW), followed by the Montepulciano and Palieri varieties. On the other hand, the Primitivo vine shoots (1861.3 ± 9.8 mg kg−<sup>1</sup> DW) showed the lowest concentration of Rsv, about 64.9% less than Nero di Troia. There are many studies showing the differences between the stilbene contents in vine shoots from different varieties and species of vines [26,32], but there are no studies concerning the variation in vine shoots of these Italian varieties. Nevertheless, comparable concentrations of Rsv were found in vine shoots of different Chilean varieties, in which Gewurztraminer (mean 4628 ± 568 mg kg−<sup>1</sup> DW) and Pinot noir varieties (mean 3676 ± 353 mg kg−<sup>1</sup> DW) were determined to contain high levels of this compound [29]. Recently, Zwingelstein et al. [31] showed that vine shoots of the Mondeuse variety contained higher levels of Rsv (3759–4636 mg kg−<sup>1</sup> DW) than those of the Jacquère variety (2259–2994 mg kg−<sup>1</sup> DW). Lower concentrations were found by Zhang et al. [28], in which the *Vitis Vinifera* vine shoots grown in China exhibited an Rsv content ranging from 664 to 1751 mg kg−1.

In regard to Vf, Figure 1 clearly reveals that most vine shoot extracts of red berry varieties had a concentration of Vf above the average. Nevertheless, the highest concentration was found in the vine shoot extracts of the Italia variety (2038.4 ± 15.8 mg kg−<sup>1</sup> DW), when compared to other varieties. The Bombino Bianco variety (175.9 ± 19.6 mg kg−<sup>1</sup> DW) showed a concentration 91.37% lower than that of the Italia variety. These results agreed with those observed in the studies by Guerrero et al. [18] in which the highest concentration of Vf, found in Gewurztraminer (2810.4 mg kg−<sup>1</sup> DW), was similar to that found in this study. Similarly, according to Lambert al. [30], the most abundant stilbenoid in grape canes of sixteen *Vitis Vinifera* varieties was Vf (mean of 2171 mg kg−<sup>1</sup> DW).

To evaluate the correlation between the TPC and the concentration of Rsv and Vf, the Pearson's correlation coefficient test was applied (Table 6). A significant correlation between TPC and Rsv and between TPC and Vf was observed, whilst no correlation was found between the two considered stilbenes (*p* = 0.697). A clear explanation for this absence of correlation is difficult to determine, considering that several sources of variability could affect the stilbene synthesis and outcome. From the genetic point of view, stilbene synthase (STS) is the key enzyme in the stilbene biosynthetic pathway, and grapevines contain a large number of STS genes [53,54]. Moreover, as reported in a recent review [11], the expression of these genes also varies according to environmental stress. At the same time, the specific varieties affect the accumulation of stilbenes, even under the same environmental conditions [32,34]. Vf is an oligomer of Rsv that accumulates in plants by oxidative coupling, affected by different biotic and abiotic stresses [11,55]. Thus, it could be supposed that Vf accumulation is independent of the original Rsv content, ye<sup>t</sup> much more correlated to environmental the stresses on the plant material.

**Table 6.** The Pearson's correlation coefficients between the TPC, *trans*-resveratrol, and ε-viniferin in 23 vine shoot extracts.

