Controversies exist regarding the iso/anisohydric continuum for classifying plant water use strategies. Isohydricity has been argued to result from a plant-environment interaction, rather than an intrinsic property of the plant. Discrepancies remains regarding the degree of isohydricity () of plants and their threshold for physiological responses and resistance to drought. Thus, the aim of this study was to evaluate the isohydricity of the grapevine varieties Syrah and Carménère through a non-lethal water deficit progression from veraison on different locations: Cachapoal Valley (CV) and Maipo Valley (MV), in central Chile, and with different rootstock only in Syrah. For this purpose, the midday water potential (md) regulation and stomatal responses to drought, leaf traits related to pressure-volume curves, stomatal sensitivity to ABA, cavitation threshold and photosynthetic responses were assessed. A higher atmospheric water demand was observed in the CV compared to the MV, with lower md values in the former for both varieties. Also, the values in Carménère were 1.11  0.14 MPa MPa-1 and 0.68  0.18 MPa MPa-1 in the CV and MV, respectively, and in Syrah was 1.10  0.07 MPa MPa-1 in the CV and 0.60  0.10 MPa MPa-1 in the MV. Even though similar variations in between locations in both varieties were evident, Carménère plants showed a conserved stomatal response to md in both study sites, while those of Syrah resulted in a higher stomatal sensitivity to md in the site of lower . Besides the differences in seasonal weather conditions, it is likely that the different rootstock and clonal variability on each season in Syrah was able to induces coordinated changes in gs12 and osmotic potential at full turgor (0). On the other hand, irrespective of the , and given the similarity between the 0 and gs12 in leaves before drought, it seems that0 could be a convenient tool for assessing the md threshold values posing a risk to the plants, in order to aid the irrigation decision making in grapevines under controlled water deficit. Finally, water deficit in vineyards might be irreversibly compromising the photosynthetic capacity of leaves.

Xylem vessels are responsible for conducting water, in a metastable state, to the transpiring leaves. As tension increases, hydraulic failure may occur. At any given tension, the resistance of the xylem vessels to cavitation depends on their morphology, of which vessels diameter has been pointed out as relevant. Grapevines (Vitis vinifera L.) are often cultivated in Mediterranean climates and mostly under controlled water déficit conditions. Besides, a high variability in the stomatal sensitivity exists in different varieties. 'Cabernet Sauvignon' (CS) is recognized as isohydric, 'Syrah' (S) as anisohydric, and no clear information exist for 'Carménère' (C), an important grapevine variety for the Chilean wine industry. In the present study, xylem morpho-anatomical traits from stems in CS, S, and C, growing in a single vineyard, were analyzed, inferring their specific theoretical hydraulic conductivity. Also, the maximal hydraulic conductivity and vulnerability to cavitation, the latter expressed as the xylem water potential reducing a 50% loss in hydraulic conductivity (PLC₅₀) were assessed in stems collected from the field. Higher vessels diameter was found in S and C, followed by CS, in general ranging from 21 to 120 Lim, resulting in an expected proportional theoretical hydraulic conductivity of nearly 40 to 43 kg s^-1 m^-1 MPa^-1 in C and S, and merely 23 kg s^-1 m^-1 MPa^-1 in C. This values were similar -but not proportionally- to maximal actual hydraulic conductivity measured in stems with median values, in average, of 28, 17, and 13 kg s^-1 m^-1 MPa^-1 in S, C, and CS, respectively. Even though wider xylem vessels have been correlated with higher xylem vulnerability to cavitation, PLC₅₀ was significantly higher in S (-2.3 MPa) compared to C (-1.1 MPa), and even though xylem vessels diameter were similar between S and C, C was as vulnerable as CS. We found no such tradeoff between hydraulic efficiency and vulnerability.

Absorbed energy in excess of that used by photosynthesis induces photoinhibition, which is common in water deficit conditions, resulting in reductions in stomatal conductance. In grapevines, controlled water deficit is a common field practice, but little is known about the impact of a given water shortage on the energy transduction processes at the leaf level in relation to contrasting stomatal sensitivities to drought. Here, we assessed the effect of a nearly similar water deficit condition on four grapevine varieties: Cabernet Sauvignon (CS) and Sauvignon Blanc (SB), which are stomatal sensitive, and Chardonnay (CH) and Carménère (CM), which are less stomatal sensitive, grown in 20 L pots outdoors. Plants were maintained to nearly 94% of field capacity (WW) and 83% field capacity (WD). We have assessed plant water status, photosynthesis (A_N), photorespiration, A_N vs. PAR, AC_i curves, photochemical (qP) and non-photochemical (qN) fluorescence quenching vs. PAR, the photoprotective effectiveness of NPQ (qPd) and light interception by leaves. Photorespiration is important under WD, but to a different extent between varieties. This is related to stomatal sensitivity, maintaining a safe proportion of PSII reaction centres in an open state. Additionally, the capacity for carboxylation is affected by WD, but to a greater extent in more sensitive varieties. As for qN, in WD it saturates at 750 μmol PAR m⁻²s⁻¹, irrespective of the variety, which coincides with PAR, from which qN photoprotective effectiveness declines, and qP is reduced to risky thresholds. Additionally, that same PAR intensity is intercepted by WD leaves from highly stomatal-sensitive varieties, likely due to a modification of the leaf angle in those plants. Pigments associated with qN, as well as chlorophylls, do not seem to be a relevant physiological target for acclimation.