1. Study site and plant material

The experiments were conducted from 2009-2016 at the Agroscope experimental station in Leytron, Switzerland (46°10’ N, 7°12’ E; 485 m asl), which is located in an alpine valley. The planting material was the cultivar Chasselas (clone 14/33-4), grafted onto Vitis berlandieri x Vitis riparia cv. Kober 5BB rootstock. Vines were trained in the Guyot system (vertical shoot-positioned) with a planting density of 5500 vines ha^-1 (planting distance: 1.8 x 1.0 m). Six shoots per vine were maintained. The experimental site in Leytron lies on very stony (peyrosol > 60% large elements, stones, blocks and gravel) and deep (> 2.5 m vine root depth) soil with a water-holding capacity estimated at 150 mm. Monthly rainfalls and temperatures from 2009 to 2016 (and long-term averages from 1981 to 2010) from the meteorological station in Leytron are presented in Tables 1 and 2.

2. Irrigation treatments

Three different irrigation treatments were established. In the first treatment, 9 L/m² of soil (16 L/vine) was drip-fed weekly from bloom (~150 DOY) to fruit ripening (~215 DOY). This level of irrigation (deficit irrigation, DI) corresponded to an approximate weekly compensation of 30% ETP. The second treatment applied no irrigation throughout the entire growing season (plants were rain fed). The third treatment also involved no irrigation; in addition, waterproof and non-reflecting plastic sheeting was placed on the soil from bloom (~150 DOY) to harvest (~280 DOY) to eliminate the infiltration of water from precipitation events. The trial was conducted using 40 plants per treatment, which were set out in four split-plot randomized blocks of ten vines each.

3. Measurements of plant water status and relative carbon isotope composition (13C)

Predawn leaf water (Ψ_PD) and midday stem water (Ψ_STEM) potentials were measured using a pressure chamber (Scholander et al., 1965) according to Turner (1988). Ψ_PD was measured between 0400 and 0500 in the morning, in complete darkness, on eight mature, undamaged and non-senescent leaves. Midday Ψ_STEM measurements were performed between 1400 and 1500, when evapotranspiration was at a maximum. Midday Ψ_STEM values were determined for eight leaves bagged with a plastic sheet and covered with aluminum foil to stop transpiration at least one hour before the measurement (Fulton et al., 2001).

The stable carbon isotope compositions (¹³C) of the must sugars and leaf sugars (leaf carbon isotope signature) were determined at harvest at the Stable Isotopes Laboratory of the University of Lausanne by elemental analysis-isotope ratio mass spectrometry (EA-IRMS) using a Carlo Erba 1108 elemental analyzer connected to a Thermo Fisher Scientific (Bremen, Germany) DeltaV mass spectrometer. The stable isotope composition was reported as ¹³C values per mille (‰), with deviations of the isotope ratio relative to known standards as follows: δ = [(R_sample − R_standard) R_standard] × 1000, where R is the ratio of heavy to light isotopes (¹³C/¹²C). The R_standard value for ¹³C in Vienna Pee Dee Belemnite limestone is 0.0112372 (Deléens et al., 1994).

4. Leaf gas exchange measurements

Leaf gas exchange [net photosynthesis (A) and transpiration (E)], stomatal conductance (gs) and mesophyll resistance (r_m) were measured on healthy, fully expanded, mature and non-senescent leaves well exposed to direct sunlight (PFD > 1800 μmol m^-2s^-1) from June to mid-October. Eight leaves per irrigation treatment were measured in the morning (1000) on days with clear skies. Gas exchange was measured using a portable photosynthesis system (LI-6400, LI-COR Inc., Lincoln, NE, USA). R_m was calculated as r_m = (C_i-Γ)/A, where C_i is the intercellular partial pressure of CO₂, Γ is the CO₂ compensation point (corresponding to 50 ppm for adult leaves with a leaf temperature between 20-25°C, Schultz, 1996), and A is net photosynthesis. The intrinsic water use efficiency (WUE_i, A/gs) and instantaneous water use efficiency (WUE_inst, A/E) were determined from single leaf gas-exchange measurements that related the net photosynthesis rate (A) either to the stomatal conductance for water vapor (gs), termed WUE_i (Osmond et al., 1980), or to the leaf transpiration rate (E), termed WUE_inst.

5. Xylem embolism assessment

Xylem embolism, induced by the presence of air bubbles in vessels, was measured using the method introduced by Sperry and Pockman (1993) and established by Cochard et al. (2000). The principle of the XYL’EM device (Xylem Embolism Meter, Bronkhorst Instructec, Montigny-les-Cormeilles, France) is to accurately measure the water flow (F; mmol s^-1) entering the petiole of a cut leaf exposed to hydrostatic pressure (P; MPa) using a high-resolution liquid mass flow meter. For the measurement, shoots were cut at the base under water and were held under water for approximately 20 min to promote xylem tension relaxation; then, the petioles of the apical zone of these shoots were excised under water and connected to the XYL’EM apparatus. The initial hydraulic conductance (K_init) was determined with a hydrostatic pressure gradient of approximately 3 to 4 kPa. Distilled and degassed water was used as the perfusion liquid for all measurements. To measure the maximum conductance (K_max), the petioles were then flushed twice over a period of 2 min with water pressurized at 0.15 MPa. The percentage loss of conductivity (PLC), known as the vulnerability to cavitation or embolism, was calculated as 1 – (K_init/ K_max). The shoot Ψ_STEM was monitored at regular intervals throughout the day and immediately before petiole sampling. The XYL’EM was interfaced with a computer to automatically log the data. PLC figures were constructed using the average values obtained from four petioles and as many measurements representing Ψ_STEM. PLC versus Ψ_STEM curves were used to determine vulnerability to cavitation, expressed as _PLC50, and the xylem water potential inducing a 50% loss in hydraulic conductivity. _PLC50 was estimated by fitting a sigmoid PLC curve to the Ψ_STEM data as follows: PLC = 100/[1+ exp (a (Ψ_stem – _PLC50))], where “a” represents the slope of the curve at the point of inflection.

6. Leaf and berry mineral nutrition, pruning weight measurements

A foliar analysis was performed to determine the levels of leaf nitrogen (Kjeldahl method), potassium, phosphorous, calcium and magnesium. The samples consisted of 25 leaves gathered in the cluster zone at veraison. Leaves with petioles were washed, over-dried, ground and analyzed. The leaf chlorophyll index was measured using an N-tester apparatus (Yara, Nanterre, France) on adult leaves situated at the middle of shoots. Yeast available nitrogen (YAN) was estimated by NIR spectroscopy (WineScan®, FOSS NIRSystems, USA). This method quantifies the N-compounds of juice available to yeasts during fermentation. YAN corresponds to the concentration of ammonium ions and primary amino acids, excluding proline. In winter, the total weight of the pruned vine shoots was recorded (six shoots per vine), representing ten plants per replicate and 40 plants per treatment.

7. Yield components, berry composition and wine analytics and testing

Bud fertility (number of clusters per shoot) was observed each year shortly before flowering. At harvest, 50 berries per replica were randomly selected and weighed to determine the berry weight. The yield per plant (kg/vine) divided by the number of clusters per plant enabled the average weight of the clusters at harvest to be estimated. The fruit composition parameters at harvest included soluble solids content (g/L), pH, titratable acidity (g tartrate/L) expressed as tartaric acid (g/L), and malic acid (g/L). Microvinification (60 kg of grape) was conducted in an identical fashion for all irrigation treatments by the same winemaker. Wine composition was assessed at bottling by using infrared spectroscopy. A sensory analysis was conducted two months after bottling by a panel of 12 experienced tasters. The panelists rated the intensities with 16 sensory attributes, including wine appearance, bouquet and palate. Wines were evaluated on an unstructured line scale from 1 (no perception) to 7 (very intense perception).

8. Statistical analysis

The significance of each treatment was evaluated with analysis of variance (P < 0.05) followed by a single-comparison Newman–Keuls test using XLSTAT 2011.2.04 (Addinsoft, Paris, France). The linear and non-linear regressions of Ψ_PD and Ψ_STEM and the different physiological parameters were determined using SigmaPlot software (version 13.0) and were statistically analyzed using the SigmaStat program package.

1. Climatic conditions

There were large fluctuations in annual average rainfall and temperature throughout the study period from 2009 to 2016 (Tables 1, 2). Generally, the annual precipitation rates were slightly below the long-term average (1981-2010) of 600 mm: the lowest rainfall measurements were noted for the years 2010, 2011 and 2015, when annual rainfall was 100 to 130 mm lower than the 30-year mean. In addition, relatively dry periods in the months of August and September were observed in 2009, 2010, 2011, 2015 and 2016. The year 2014 was marked by unusually high summer rainfall levels. The annual temperatures were 0.6 to 1.6°C higher than the 30-year average, with the exception of the years 2010 and 2013, when the annual temperatures were identical to those recorded for the period of 1981-2010. The summer temperatures (July-September) were particularly hot in 2009, 2011, 2012, 2015 and 2016, relative to the long-term values.

2. Plant water status

The seasonal courses of Ψ_PD, recorded in the growing seasons from 2009 to 2016, highlighted the variations in the vine water regime as a function of year and irrigation level (irrigated and non-irrigated vines, Figure 1). No water stress was observed in the vines that had been watered from bloom to veraison, for which values of Ψ_PD oscillated between -0.05 and -0.25 MPa. The non-irrigated vines showed Ψ_PD values that became increasingly negative from the end of July (DOY 2010). These values reached -0.3 to -0.5 MPa during the fruit-ripening period, reflecting the presence of moderate water stress. Only in 2014, when there was heavy summer rainfall, was no water stress recorded. In the non-irrigated vines with soil covered by a plastic waterproof sheet, severe water stress (values of Ψ_PD < -0.5 MPa) was observed at the end of the growing season, especially during the hot, dry summers of 2009, 2011, 2015 and 2016. The water stress remained moderate in 2010, 2013 and 2014, due to cooler summer temperatures and the occasional heavy rainfall event (Tables 1, 2), which resulted in lower ETP values and less depletion of soil water resources.

The Ψ_PD and Ψ_STEM values recorded between veraison and harvest correlated well (R² = 0.92, P < 0.01 and R² = 0.90, P < 0.01, respectively) with the ¹³C values in the musts at harvest (Figure 2). The ¹³C values varied from -22.5 ‰ (high water stress) in the non-irrigated vines to -27.0 ‰ (no water stress) in the irrigated vines. A correlation between Ψ_PD and Ψ_STEM (R² = 0.90, P < 0.01) recorded at solar midday was also found, with a vapor pressure deficit (VPD) of approximately 2.5 to 3.5 kPa during the eight-year study period (results not presented).

The different physiological indicators used in this study (Ψ_PD, Ψ_STEM and ¹³C) gave a good picture of the vine water regime during the growing season, as has been previously shown in earlier studies (Choné et al., 2001; van Leeuwen et al., 2009). These indicators often correlate well among each other (van Leeuwen et al., 2001a; van Leeuwen et al., 2001b). The Ψ_PD and the Ψ_STEM data enable the evolution of the plant water status to be followed throughout the growing season. On the other hand, ¹³C, which is analyzed in must sugars at harvest, presents an integrative approach to the water regime that prevailed during the sugar-accumulating phase in berries, that is, from the onset of fruit ripening (veraison) to harvest (Gaudillère et al., 2002). The ¹³C values in must sugars can be used as an indicator of soil water availability in the vineyard (de Souza et al., 2005a ; de Souza et al., 2005b).

Figure 1. Seasonal evolution of the predawn leaf water potential (PD) for different irrigation treatments. Arrows indicate irrigation onset.

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Means ± SE for eight leaves. Letters indicate statistical significance at the 5% level of probability. Chasselas (Switzerland), 2009-2016.

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Figure 2. Relationship between the predawn leaf water potential (_PD, A), the stem xylem water potential (Ψ_STEM, B) measured during the period of veraison-harvest and the relative C isotope composition (¹³C) in must sugars at harvest for different irrigation treatments. Means ± SE. Chasselas, Leytron (Switzerland), 2009-2016.

3. Physiological behavior in response to progressive water deficit

Water and gas exchange relationships

The different water regimes, measured by the Ψ_PD of the irrigated and non-irrigated vines (Figure 3 A), led to consequences affecting leaf gas exchange, especially net photosynthesis (Figure 3 C), transpiration (Figure 3 D), stomatal conductance gs (Figure 3 B), mesophyll resistance (Figure 3 G) and water use efficiency WUE (Figures 3 E-F). A reduction in gs, A and E during the growing season of 2009 was measured (presented as an example) in response to increasing restrictions on soil water availability. In the non-irrigated vines, where the soils were covered by an impermeable plastic sheet, gs reached very low levels during the season, which corresponded to a residual conductance g_res close to 30 mmol m^-2s^-1. Compared to that of irrigated vines, the net photosynthesis of non-irrigated vines dropped by 40 to 50% from the end of July (DOY 2010), reaching levels of approximately 5 mol CO₂ m^-2s^-1. In the summer, the transpiration rates of the non-irrigated vines dropped sharply in response to moderate to severe water stress, decreasing from 6 to 1.5 mmol H₂O m^-2s^-1 at the end of the season. The leaf senescence observed in September also partly explains the progressive reduction in gas exchange (A et E) during fruit ripening. The leaf stomatal conductance and gas exchange values had a good correlation with the plant water status (Ψ_PD values) in the period from bloom to the mid-ripening of the berries; correlation coefficients up to 0.88 (P < 0.01) for A, 0.90 (P < 0.01) for E and 0.92 (P < 0.01) for gs were recorded (results not presented in the present article).

The stomatal control of photosynthesis and transpiration in periods of progressively developing water stress has been widely documented by various studies (Chaves et al., 2007; 2010). The gradual closure of stomata indeed offers a rapid and effective response to soil water deficits and to increases in VPD, where the latter coincides with the drying of the soil. Stomatal regulation constitutes a short-term adaptation, enabling plants to save water by reducing leaf transpiration and simultaneously limiting the risk of vessel embolism phenomena and the rupture/failure of the hydraulic conductivity in the plant’s vascular system (Lovisolo et al., 2002).

The instantaneous water use efficiency (WUE_inst), which expresses the ratio between photosynthesis A and transpiration E, tended to increase during the growing season and was identical in both the irrigated and non-irrigated vines (Figure 3 E). Intrinsic water use efficiency (WUE_i), on the other hand, which is calculated by the A/gs ratio, was higher in the non-irrigated vines than in the irrigated vines (Figure 3 F). Under the conditions of the present study, as water stress increased, a greater decrease in gs than in photosynthesis was observed, leading to a higher WUE_i in the non-irrigated vines than in the irrigated vines. Some studies, however, have shown that, in very severe drought situations, WUE_i can decrease (Prieto et al., 2010), suggesting an increase in mesophyll limitations (increase in r_m), which, in turn, lowers the rates of photosynthesis (Flexas et al., 2007). Furthermore, a correlation between the WUE_i and ¹³C in the berry sugars was observed over the eight-year study period (Figure 4), thus confirming previous observations (Medrano et al., 2005; Zufferey et al., 2017). The increase in WUE_i was linked to that of ¹³C in the must sugars. Non-irrigated vines presented higher ¹³C values (less negative), probably associated with a weaker discrimination against ¹³C and ¹³C enrichment in must sugars. The latter has been correlated with a rise in the WUE_i of non-irrigated vines compared to well-watered vines. In addition, differences in leaf and fruit isotope signatures were observed (results not presented) under conditions of water stress (Bchir et al., 2016): the ¹³C in leaves reflects not only the leaf ratio of Ci/Ca and the WUE of the current growing season but also the C assimilated in the previous year and its allocation during spring growth. There is generally an enrichment of ¹³C in berries compared to leaves due to the import of carbon assimilates from the leaves after the onset of ripening, when water stress is high (stomata closed), thus supplying carbon assimilates enriched in ¹³C. Lastly, a recent study (Spangenberg et al., 2017) showed that the analysis of carbon isotopes, undertaken in the solid residues and volatile organic components of wine, constitutes a very effective tool for estimating variations in the water status of plants and soils in different vineyards.

Among all treatments, the mesophyll resistance (r_m) was greatest at the end of the season in the non-irrigated vines with sheet-protected soils (Figure 3 G). Water stress increases the resistance to the transfer of CO₂ from leaf substomatal cavities to the chloroplasts, where CO₂ fixation sites and photosynthetic enzymes are situated (Flexas et al., 2002). The rise in r_m associated with the decrease in gs under water stress lowers the availability of CO₂ in the chloroplasts and, consequently, the rate of photosynthesis in the leaves, as observed in the present study.

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Figure 3. Changes in the predawn leaf water potential (_PD, A), stem water potential (Ψ_STEM, B), stomatal conductance (gs, C), mesophyll resistance (r_m, D), photosynthetic rate (A, E), intrinsic water use efficiency (WUE_i, F), transpiration rate (E, G) and instantaneous water use efficiency (WUE_inst, H) during the 2009 season. Means ± SE for eight leaves. Chasselas, Leytron (Switzerland).

Figure 4. Relationship between the intrinsic water use efficiency (WUEi) and the relative C isotope composition (13C) in must sugars at harvest for different irrigation treatments.

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Means ± SE. Chasselas, Leytron (Switzerland), 2009-2016.

Cavitation vulnerability

A curve representing cavitation vulnerability in leaf petioles was established by relating the PLC with Ψ_STEM (Figure 5). PLC increased progressively with water stress, reaching approximately 30%, with a Ψ_STEM value of -1.0 MPa. The 50% threshold of PLC was observed at Ψ_STEM values of -1.25 MPa. In non-irrigated vines with plastic-covered soils, values close to 70-80% PLC were noted when water stress levels were very high (Ψ_STEM < -1.5 MPa). In the watered vines, with Ψ_STEM values remaining above -0.8 MPa (weak water stress), no cavitation or very low cavitation values (below 10%) were recorded. The rain-fed vines presented an intermediary situation. An earlier study (Zufferey et al., 2011) demonstrated the sensitivity of cv. Chasselas to embolism under very high water stress conditions during the growing season, as was the case for the non-irrigated vines in the present work. The progressive stomatal control of transpiration, followed by the reduction of hydraulic conductivity in protective « fuse » organs, such as petioles, allows the _leaf value to remain above a critical threshold, which, up to a certain point, prevents hydraulic rupture and embolism. The petiole hydraulic conductivity in the non-irrigated vines indeed decreased progressively as water stress intensified over the season (results not presented). The hypothesis that hydraulic and chemical signals significantly contribute to differences in preservation or water stress behaviors among vine varieties appears increasingly likely, especially in isohydric and anisohydric plants (Tardieu and Simonneau, 1998; Pantin et al., 2013). In particular, the stomatal and hydraulic conductances of the Chasselas cultivar seem to be sensitive to soil water availability and, therefore, to Ψ_PD, compared to the Pinot noir vine grown in the same experimental plot in Leytron (Switzerland); the latter appears to be more sensitive to VPD variations (Spring, 1997; Zufferey et al., 2017). Based on the perception of water stress by leaves and roots, different grapevine varieties react differently to water deficits. Some recent studies have attempted to better comprehend the relationships between the stomatal regulation of gas exchange, hydraulic conductivity in roots and shoots, aquaporin expression levels and hormonal signals (mainly ABA, Stoll et al., 2000) under water stress conditions (Coupel-Ledru et al., 2017). Hydraulic segmentation, whereby the risk of hydraulic rupture is shared among plant organs, may also play an important part in the sensitivity of plants to embolism phenomena during drought. The greater hydraulic resistance measured in shoots than in petioles (Lovisolo et al., 2007) and the anatomical characteristics of vessels (Pagay et al., 2016) may, in addition, contribute to preventing the spread of embolism by reducing cavitation vulnerability in the perennial structure of plants.

Figure 5. Vulnerability to cavitation (PLC, percent loss of conductivity) versus stem water potential (ΨSTEM) for grapevine leaf petioles measured in situ for different irrigation treatments.

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Chasselas, Leytron (Switzerland), 2012-2015.

Letters indicate statistical significance at the 5% level of probability. Averages 2009-2016, Leytron (Switzerland).

Mineral supply and pruning weight

Table 3 presents the analysis of leaf mineral content (N, P, K, Ca and Mg) and the chlorophyll index values (N-tester) at veraison and the available nitrogen in the must (YAN) at harvest. The leaf nitrogen content tends to decrease with increasing water stress: leaf N levels were the lowest in non-irrigated vines with plastic-covered soil. No significant difference in the leaf P, K, Ca and Mg contents of the irrigated and non-irrigated vines was observed. Identical results were obtained from the Pinot noir grape variety under the same study conditions (Zufferey et al., 2017). The leaf chlorophyll index remained similar in all irrigation treatments. YAN levels decreased as water stress increased, particularly in non-irrigated vines with plastic-covered soil (Table 3, Figure 6). In water-stressed vines, the reduction in the canopy transpiration flow due to progressive soil drying (non-irrigated vines) and the leaf fall over the growing season reduces the soil mineral uptake, especially the uptake of nitrogen, as noted by Gonzalez-Dugo et al. (2010). Nitrogen mineralization and soil microbial activity are also negatively influenced by soil drying conditions (Celette and Gary, 2013). Studies from Reynard et al. (2011), which were carried out in different wine-growing “terroirs”, additionally highlighted the depressive effect of water deficit, whether climatic and/or edaphic, on the nitrogen content in leaves and berries.

Plant vigor, estimated by pruning weight, was influenced by the water supply to vines (Table 4). Pruning weights were found to be lower in the non-irrigated vines with plastic-covered soils suffering drought in hot summers (years 2011, 2015, 2016) than in the irrigated vines. An intermediate situation was observed for the rain-fed vines. Compared with the irrigated vines, a pruning weight decrease of nearly 20% was observed in the non-irrigated vines with plastic-covered soils, and a decrease of 2% was observed in the non-irrigated vines (rain fed).

Table 3. Mineral supply to the vine: leaf nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg) content at veraison; leaf chlorophyll index (N-tester measurements) at veraison; and yeast available nitrogen (YAN) content in the must at harvest.

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	Foliar analysis (% D.M.)					N-tester	YAN (mg N/L)
	N	P	K	Ca	Mg
Irrigated vines	2.38a	0.25a	1.10a	4.07a	0.31a	586a	170a
Non-irrigated vines	2.32a	0.23a	1.11a	4.05a	0.31a	585a	155ab
Non-irrigated vines + plastic covered	2.11b	0.19a	0.95a	3.91a	0.33a	551a	122b

Means ± SE. Chasselas, Leytron (Switzerland), 2009-2016.

Figure 6 - Relationship between the predawn water potential (PD) and the yeast available nitrogen (YAN) in the must at harvest for different irrigation treatments.

Table 4. Pruning weight (g/vine) per year and the 2009-2016 mean.

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Letters indicate statistical significance at the 5% level of probability. Averages 2009-2016, Leytron (Switzerland).

Table 5. Yield components: bud fertility (number of inflorescences per shoot), cluster removal per vine, berry and cluster weight at harvest, and yield (per m2).

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			Pruning weight (g/vine)							Mean 2009-2016
	2009	2010		2011	2012	2013	2014	2015	2016
Irrigated vines	646a	650a		690a	889a	686a	581a	686a	679a	689a
Non-irrigated vines	638a	635a		570b	834a	713a	610a	714a	704a	677a
Non-irrigated vines + plastic covered	618a	596a		460c	608b	550b	561a	571b	497b	557b

Letters indicate statistical significance at the 5% level of probability. Averages 2009-2016, Leytron (Switzerland).

4. Wine tasting

Figure 7 presents the results of the wine tasting, which occurred several weeks after bottling, for major wine characteristics: wine bouquet (sharpness and quality of aromas), wine structure, wine bitterness, and global appreciation of the wine. Globally, the vine water regime had little influence on the quality and organoleptic characteristics of the Chasselas wines, with the exception of the hot, dry year in 2009, when the quality of wine from the irrigated vines, which had not experienced water stress, was more appreciated. Bitterness was generally more noticeable in wine from non-irrigated vines that had experienced drought than in wine from well-watered vines. No significant difference was recorded for the bouquet and wine structure during the study period, whether from irrigated or non-irrigated vines. Concerning the global appreciation of wines (hedonic impression), those obtained from the irrigated vines were appreciated slightly more than those obtained from the non-irrigated vines. This was due to the perception of less bitterness in the wines from well-watered vineyards than in the wines from non-irrigated vineyards. Progressive and moderate water stress, beginning at fruit ripening, favors the enrichment of sugars, anthocyanins and phenol compounds in the berries of red cultivars (Kennedy et al., 2002; van Leeuwen et al., 2009; Zufferey et al., 2017). On the other hand, in white cultivars, a low to slightly moderate water stress benefits the development of aromatic components (Reynolds et al., 2010). A severe or sometimes even moderate water stress can have a negative impact on white wine quality, particularly affecting the sharpness of aromas and the perception of bitterness (van Leeuwen and Vivin, 2008; Spring and Zufferey, 2011). Furthermore, high water stress is often accompanied by a reduction in the available nitrogen content of must. According to the vineyard, year, vintage, and grape variety, this reduction may impair the aromatic expression (Tominaga et al., 2000), the quality and the characteristics of white wines (Peyrot des Gachons et al., 2005; Reynard et al., 2011).

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