## 4 Discussion
### Traditional Monastrell Clones Show a High Variability in WUE
It is noticed that with the same irrigation water volume (around 80 mm year\({}^{-1}\), Table 2), and similar soil conditions, these traditional Monastrell clones (in the control treatments) reached WUEyield values up to 14-16 kg m\({}^{-3}\) (clones 188, 94, respectively) (Figure 9D), substantially higher than those obtained from a commercial Monastrell clone grafted on the same rootstock (110R) (10.47 kg m\({}^{-3}\)) and in the same experimental and soil conditions [26]. Probably, more demanding climatic conditions in the 2012-2016 period in the rootstock experiment (VPD 1.00 kPa, ETo 1240 mm year\({}^{-1}\) and a lower rainfall, with 302 mm year\({}^{-1}\)) compared to the 2018-2021 period in the clone experiment (VPD 1.16 kPa, ETo 1095 mm year\({}^{-1}\) and a higher rainfall, with 393 mm year\({}^{-1}\)) may also help explain these differences in WUE. Moreover, the very high WUEyield (>40 kg m\({}^{-3}\)) reached in high-vigor clones (94, 4, and 188) under WS (Figure 9D) indicates that these clones are very productive with very little irrigation water (23 mm year\({}^{-1}\), Table 2), suggesting a greater drought tolerance and water use efficiency than in low-vigor clones. Future research combining high-WUE drought tolerant rootstocks (e.g., 140 Ru) [26] and these high-WUE drought tolerant Monastrell clones (e.g., clones 4 or 188) under semi-arid-deficit irrigation (DI)/rainfed conditions would be necessary, because it could reduce reliance on supplemental irrigation and make vineyards more sustainable in semi-arid areas, even for dry farming. In spite of low water irrigation volumes (Table 2), physiological indicators (\(\Psi_{s}\) and \(g_{s}\)) were, in most of the clones and years, within the optimum water stress ranges and thresholds that were proposed (moderate water stress, \(-\)1.2 > \(\Psi_{s}\) > \(-\)1.4 MPa and 0.15 > \(g_{s}\) > 0.05 mol m\({}^{-2}\) s\({}^{-1}\)) in order to avoid severe damage in DI vineyards [44; 45; 46]. It is also noticed than high-vigor clones, such as 4, 94, and 188, were the most water use efficient vines from a productive point of view (WUEyield, Table 5), but were the least efficient at the leaf/organ level (the lowest \(A\)/\(g_{s}\), \(A\)/\(E\), \(\xi^{13}\)Cberries, Table 3), whereas the opposite effect was observed with low/medium vigor and less productive clones (360, 373, 372, and 276). This is also supported by the significant negative linear relationships found between \(A\)/\(g_{s}\), \(\delta^{13}\)Cberries, yield, and WUEyield (Figure 2). \(\delta^{13}\)Cberries, which integrates the entire grape ripening period, confirmed that the most negative genotypes (4, 94) had a lower WUEleaf(\(A/\!\!/g_{s}\)) and suffered lower WS. Meanwhile, 276 and 360 showed the least negative values (\(-24\%_{\infty}\)) and higher \(A/\!\!/g_{s}\), which is indicative of a greater WS [15; 47]. Significant relationships between long-term \(A/\!\!/g_{s}\) and \(\mathrm{WUE}_{\mathrm{yield}}\) have been also found in Monastrell and other grapevine varieties [13; 48; 49; 50]. In our study, improvements in \(\mathrm{WUE}_{\mathrm{yield}}\) were not related to an increase in \(A/\!\!/g_{s}\) (Figure 2A,C). A significant positive relationship was found between \(A/\!\!/g_{s}\) and \(\delta^{13}\mathrm{C}_{\mathrm{berries}}\), as reported in different grapevine genotypes [14; 15]. The agreement found between \(A/\!\!/g_{s}\) and \(\delta^{13}\mathrm{C}_{\mathrm{berries}}\) (Figure 2E), better than with \(\mathrm{WUE}_{\mathrm{yield}}\) (Figure 2C vs. 2G), as has also been found in other varieties (cv. Grenache) [13; 15], indicates that \(\mathrm{WUE}_{\mathrm{yield}}\), which refers to the fraction of carbon that is translocated into yield, does not only depend on purely physiological factors. Rather, carbon partitioning and sink/source adjustments due to different resources distribution and a different response to environmental conditions (i.e., VPD) could have been involved [15; 50].