## 4 Discussion
### Clones Show Different Hydraulic Behavior, Stomatal Regulation, and Vine Water Use
Indeed, when irrigation water was reduced in 2021 (Table 2), vines from clone 4 continued to systematically maintain a greater leaf gas exchange and total leaf area, and a better vine water status than clones 94 and 188 (Table 3), which indicates a greater drought tolerance and suggests that this clone can be more adequate for rainfed conditions or more severe DI strategies. Clones Differ in Their Photosynthetic Ability and Their Sensibility to Atmospheric VPD and Soil Water Deficit  
The significant and close relationships between \(g_{s}\) and \(A\) (positive), and between \(g_{s}\) and \(A\)/\(g_{s}\) (negative) indicate that differences in \(A\) among clones come mainly from differences in stomatal control (Figure 8 A-D). In addition, the gradual linear reduction of Ci as water stress developed and stomatal closure occurred (Figure 8E,F) would also indicate that the decreased CO\({}_{2}\) availability in the mesophyll was mainly due to stomatal limitation (by stomatal closure) in this range of moderate water stress. In grapes, the photosynthetic machinery appears to be very tolerant to mild, and even medium, levels of water deficit, and it seems that non-stomatal limitations become dominant only when grapevine \(g_{s}\) falls below 0.05 mol m\({}^{-2}\) s\({}^{-1}\)[67], something that was rarely observed in this study (Table 3). Our results support that there is a high degree of co-regulation in the plant to cope with water deficits through their stomata, thus providing multiple layers of regulation to balance water loss and CO\({}_{2}\) assimilation in dry environments and under moderate water stress [54]. However, a more detailed analysis about the specific \(A\)/\(g_{s}\)-\(g_{s}\) and \(A\)-\(g_{s}\) relationships established for each genotype (Figure 8A-D) indicated that for the same \(g_{s}\) value (mainly at high \(g_{s}\) range), \(A\) was systematically higher in clones 372, 276, or 188 (Figure 8B) (in this order, with greater slopes, 63.20, 58.71, 41.84, respectively, data not shown) than in clone 4 (lower slope, 36.45), suggesting also that some variability exists in the photosynthetic capacity among clones, regardless of the stomatal control. The absence of differences in N, Ca, or P content of leaves among clones does not indicate differences in photosynthetic machinery [27] (e.g., chlorophyll content), but could rather be related to differences in leaf CO\({}_{2}\) diffusion capacity (not \(g_{s}\), but mesophyll conductance), or in the biochemical properties of the leaf (higher Rubisco content or more efficient Rubisco) [11; 68]. However, leaf content of Mg did increase with more severe water stress (clone 360). These results confirm those obtained from other grape varieties [69]. At veraison, although N, P, K, Ca, Cu, B, and Zn values were low in all of the clones and close to the lower limits of the optimum ranges proposed for whole grape leaves in irrigated vineards [70], there were no leaf deficiencies of these elements in this low-input vineyard. Clone 4 also maintained a higher and more constant Ci than low-vigor clones at early morning and midday as stomatal closure occurred (276, 372, 360, higher slope), and, for the same \(g_{s}\) value, Ci was higher in clone 4 than in other clones (Figure 8E,F). Photosynthetic intensity is related to Ci [27], so a greater Ci in clone 4 suggests that there is a greater stomatal aperture and CO\({}_{2}\) availability in the carboxylation sites and could explain the greater \(A\). Moreover, high-vigor clones (especially clone 4) were also more efficient in the photosynthetic nitrogen use (NUE\({}_{\text{ph}}\)) than low-vigor clones.