## 4 Discussion
### Clones Show Different Hydraulic Behavior, Stomatal Regulation, and Vine Water Use  
Vitis species possess the ability to show different strategic behaviours in response to drought [51]. In our study, differences in the \(g_{s}\)-\(\Psi_{s}\) and \(A\)-\(\Psi_{s}\), relationships (Figure 5B,D) indicate a different stomatal regulation among Monastrell clones, as observed in other varieties [15]; for instance, for the same \(\Psi_{s}\), high-vigor clones (4 and 188) maintained higher \(g_{s}\) than low-vigor clones (276, 360, 373). Moreover, some clones displayed a tighter stomatal control and tended to close their stomata earlier than others when water stress developed (e.g., clones 188, 94, 373 or 276 had higher slopes, between 0.19 and 0.28, data not shown, vs. clone 4 with a lower slope of 0.062, data not shown) (Figure 5B). Slopes higher than 0.25 in \(g_{s}\)-\(\Psi_{s}\) relationship have been related with a tight stomatal regulation in winegrapes [15; 52]. A tighter and earlier stomatal closure indicates a short-term cost-effective strategy (stress avoidance strategy) that slows the dry-down by decreasing water loss by transpiration [53; 54; 55]. These genotypes are considered \"plastic\", due to their ability to modify their performances under different environmental conditions [56]. Moreover, reducing the canopy size could provide another excellent emergency strategy for vineyards that face extreme drought [54]. Clones with smaller canopies and, as a consequence, a reduced transpiration surface (especially 360 and 373), substantially reduced vine water use under water stress, also releasing water tension in the xylem and saving water [53]. Interestingly, WS reduced TLA in low/medium-vigor clones (276, 360, 373) between 33-41% (compared to their controls), while high-vigor clones (especially 4, 94) did not reduce TLA under WS (Figure 9E). Despite an increased stomatal closure and reduced leaf area, \(\Psi_{s}\) was also substantially reduced in these vines (near-anisohydric behavior), contrary to the response that is frequently observed in anisohydric species [55; 57]. However, a tight regulation of \(\Psi_{1}\) is not necessarily associated with a greater stomatal control nor with a more constrained assimilation during drought [58]. Variation in (an)isohydry may result from a slight deviation in the balance between transpiration rate (controlled by stomatal aperture) and whole plant hydraulic conductance. Therefore, the drop in water potential may suggest that hydraulic conductance is limiting water transport on the path from the soil through the plant to the leaves [53; 59], and it is not enough to offset the water losses by transpiration. Indeed, a lower \(g_{s}\) may also be the result of hydraulic signalling, e.g., changes in plant hydraulic conductivity [60]. The higher anisohydric behavior (greater water stress) and greater water loss control (i.e., tighter stomatal regulation of \(E\) and reduced plant transpiratory surface) experienced by these low-vigor clones would indicate that they were the most sensitive clones to drought conditions and put into play more water conservation mechanisms under water stress. It is also noticed that clone 276 presented the highest sensitivity to water stress among all of the clones for several reasons: under water stress, it had a tighter stomatal regulation of transpiration (Figure 10A) and a significantly lower leaf N and Mg content (below optimum values, Table 4), which could have a higher effect on photosynthetic activity (approximately 40% of \(A\) reduction at the early morning and midday, Figure 4A,D), a greater yield reduction (48%, Figure 9A), and lower \(\Psi_{\rm s}\) (more negative), juice, and water percentages in the berries (2021) (Figure 3A and Table 10).