Introduction

The vine water status derives from numerous pedoclimatic factors (soil properties, soil water reserves, mesoclimate (Seguin, 1975), from the rootstock’s genetic characteristics (root density, for example) and grape variety (drought sensitivity), and from cultivation practices (cover crop, leaf-fruit ratio), including irrigation in some cases. Restrictions in water supply during the season may affect plant growth (Chaves et al., 2010), the canopy’s photosynthetic capacity, yield components (floral initiation, the rate of fruit set, berry weight …) and the quality of grapes and wines (van Leeuwen and Seguin, 1994). Stomatal regulation of leaf transpiration provides a short-term response of controlling water losses in vines when edaphic water stress and/or atmospheric demands (water vapor deficit in the air, VPD) rise sharply during the day (Lovisolo et al., 2016). By limiting water flux in the plant (lowered transpiration) and canopy development (decreasing leaf area by leaf fall), the plant is able to maintain its water status and stabilizes the leaf water potential above the threshold required for avoidance of hydraulic rupture (cavitation phenomena with development of air bubbles in vessels) in cases of high water stress (Tyree and Sperry, 1988).

The effects of drought on yield components depend on the timing, the intensity and the duration of water deficit (Deloire et al., 2004). Water stress early in the season leads to a smaller number of berries per cluster; severe stress developing between the fruit set and veraison period causes a reduction in berry weight (Ojeda et al., 2001) and increases the heterogeneity of berry size. No water stress, combined with high soil water availability, often develops vigorous plant growth and increases the risk of fungal diseases, altering the harvest quality (increased yield, increased competition in the sugar allocation between the vegetative and reproductive vine parts, dilution phenomena, and reduced wine color). On the other hand, a progressively increasing and moderate water stress (after fruit set) leads to a slow stop in plant growth around the veraison, promoting the production of berries richer in sugars, anthocyanins and phenol compounds, with less acids (van Leeuwen et al., 2009; Zufferey et al., 2017). For the elaboration of high-quality white wines, it is well known that vines should experience no mineral nutrient deficiencies, particularly nitrogen deficiencies (Bell and Henschke, 2005; Lacroux et al., 2008). For this reason, vines should not be exposed to a water deficit that is too severe (Choné et al., 2006; Deluc et al., 2009) which may result in a loss of aromatic compounds and wine quality (Reynolds et al., 2010; Reynard et al., 2011; Verdenal et al., 2012).

The Arvine grape variety, cultivated mainly in the Swiss canton of Wallis, is well appreciated for the character and the complex fruity and floral bouquet of its wines (Spring et al., 2014). This aromatic typicity arises from sulphur compounds of the thiol family and in particular 3-mercapto-hexanol (Fretz et al., 2005), whose aromas are evocative of grapefruit, lemon, rhubarb and exotic fruits. These aromas are linked to glutathione and cysteine (P-3MH) in musts and liberated in volatile form during alcohol fermentation (Tominaga et al., 2000).

In this study, the impact of water supply to vines on plant behavior was analyzed by creating different water regimes during the growing season. The experiments included three levels of irrigation: a deficit irrigation that compensated approximately 30&nbsp% of evapotranspiration potential (ETP) from flowering to the onset of ripening (veraison); no water supply (no irrigation, rain-fed vines) throughout the whole season; and water stress imposed by covering the soil with a plastic, impermeable, non-reflecting sheet during the growing period (April-October). The vine water status was monitored by using physiological indicators, such as leaf and stem water potentials and carbon isotope composition in the must sugars at harvest. The influence of the water regime on the vine physiology (gas exchange, plant vigor, leaf and berry mineral nutrition) and agronomical behavior (yield components) and the quality of the resulting wines was observed in adult vines of the cultivar Arvine over a period of eight consecutive years on the experimental Agroscope vineyard, which is located in a relatively dry alpine region of Switzerland (Valais).

Materials and methods

Results and discussion

2. Plant water status

Seasonal monitoring of vine water status was carried out by means of predawn leaf water potential measurements (Ψ_PD) throughout the study period 2009-2016 (Figure 1). In irrigated vines from bloom to fruit ripening, no water stress was recorded, and values of Ψ_PD varied between -0.05 and -0.25 MPa throughout the season and according to the year. Non-irrigated vines, however, showed a gradual increase in water stress from the end of July (DOY 2010), with Ψ_PD values fluctuating between -0.3 and -0.5 MPa, indicative of moderate water stress.

Figure 1. Seasonal evolution of the predawn leaf water potential (Ψ_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&nbsp% level of probability. Arrows indicate start of irrigation. Arvine, Leytron (Switzerland), 2009-2016.

An exception was noted for the year 2014 when any manifestation of water stress was absent for the whole season. Soil-covering associated with no irrigation resulted in moderate to high water stress towards the end of berry ripening (values of Ψ_PD < -0.5 MPa), especially during the hot, dry summers of 2009, 2011, 2015 and 2016. Water stress remained more moderate during the summers of 2010, 2013 and 2014 due to the cooler temperatures and regular rainfall in July and August, which meant lower water vapor deficits (VPD) and a reduced rate of soil water depletion. The measurement of Ψ_PD in darkness provides an estimate of vine water status when leaf transpiration is greatly reduced, and thus reflects the soil water availability (edaphic stress) in the wettest part of soil colonized by roots (Améglio et al., 1999). When measured during the hottest hours of the day (solar midday), Ψ_STEM allows not only an estimation of edaphic stress, but also of climatic stress resulting from the evaporative demand of the air (i.e. VPD). These two physiological approaches of Ψ_PD and Ψ_STEM provide valuable information concerning the supply of water to vines (Choné et al., 2001, van Leeuwen et al., 2001a; van Leeuwen et al., 2001b; van Leeuwen et al., 2009). Values of Ψ_PD and of Midday Ψ_STEM are notably correlated (R² = 0.85, P < 0.01) throughout the 2009-2016 study period.

A correlation (R² = 0.81, P < 0.01 and R² = 0.80, P < 0.01, respectively) was observed between these two indicators of vine water status, Ψ_PD et Ψ_STEM, recorded during the fruit-ripening to harvesting period, and the carbon isotopic composition (δ¹³C) in musts at harvest-time (Figure 2A, B). Values of δ¹³C varied from -23 ‰ (high water stress) in non-irrigated vines, to -27 ‰ (no water stress) in irrigated vines. Measurement of C isotopic composition in musts has the advantage of providing an integral overview of the vine water regime during the phase of sugar accumulation, that is, between the time of fruit-ripening and harvesting (Gaudillère et al., 2002).

Figure 2. Relationship between the predawn leaf water potential (Ψ_PD, A), the stem xylem water potential (Ψ _STEM , B) measured during the period of veraison-harvest and the relative C isotope composition (δ¹³ C) in must sugars at harvest for different irrigation treatments.

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

3. Physiological behavior in response to progressive water deficit

3.1 Water and gas exchange relationships

Figure 3 shows the impact of the different water regimes, estimated by night-time Ψ_PD (Figure 3A), on stomatal conductance g_s (Figure 3B), net photosynthesis A (Figure 3C), transpiration E (Figure 3D), mesophyll resistance r_m (Figure 3G) and water use efficiency WUE (Figure 3 E, F).

Figure 3. Changes in the predawn leaf water potential (Ψ_PD, A), stomatal conductance (g_s, B), net photosynthetic rate (A_N, C), transpiration rate (E, D), instantaneous water use efficiency (WUE_inst, E), intrinsic water use efficiency (WUE_i, F) and mesophyll resistance (r_m, G), during the 2015 season.

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Means ± SE for eight leaves. Arvine, Leytron (Switzerland).

Taking the 2015 season as an example, increasing water stress observed in non-irrigated vines was accompanied by a decrease in g_s, A and E. The values of g_s measured were below 100 mmol m^-2s^-1 at the end of the season in non-irrigated vines with covered soils, corresponding to half of the rates observed in well-watered vines (irrigated vines). From the end of July (DOY 2010) net photosynthesis was reduced by 30 to 50 % in vines suffering from water restrictions, in comparison with irrigated vines, where levels of almost 5 μmol CO₂ m^-2s^-1 were reached by the end of the season. In addition, leaf transpiration decreased by nearly a half during grape ripening in vines with plastic-covered soils and no irrigation, compared with the well-watered vines. Under increasing water stress conditions, the plant can respond rapidly by stomatal regulation of gas exchange (photosynthesis and transpiration), a process which has been well described by many studies (Chaves et al., 2007; Chaves et al., 2010, Medrano et al., 2002; Medrano et al., 2003). Effectively, stomatal control of transpiration allows a rapid response to edaphic and atmospheric water deficits, while managing risk of hydraulic rupture in vessels which, in extreme cases, could lead to embolism (Lovisolo et al., 2002, Zufferey et al., 2011).

Under the conditions of moderate to strong water stress of the present study (non-irrigated vines with plastic-covered soils), net photosynthesis dropped off less markedly than g_s during the growing season. Consequently, intrinsic water use efficiency (WUE_i, A/g_s) rose during the growth period, stabilizing at the end of fruit ripening. WUE_i was higher in vines which were stressed during ripening, compared with well-watered vines. As previously demonstrated by numerous studies in Mediterranean climates (Schultz 1996, Chaves et al., 2007; Pou et al., 2008, Flexas et al., 2007, Tomas et al., 2014), the WUE_i tends to increase under drought conditions. However, it has been noted that, when water stress becomes very high, there is a reduction in WUE_i (Prieto et al., 2010), suggesting an increase in mesophyll resistance, as observed in the present study, thus reducing assimilation (Flexas et al., 2007). Moreover, a correlation between WUE_i and δ¹³C in grape sugars (results not presented) would confirm findings from other research (de Souza et al., 2005a, de Souza et al., 2005b; Bchir et al., 2016). Non-irrigated vines demonstrated higher δ¹³C values than irrigated vines without water stress. This result is probably associated with a lower discrimination with regard to ¹³C and to enrichment of ¹³C in musts, which correlates with an increase in WUE_i. 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, recent studies (Spangenberg et al., 2017, Spangenberg and Zufferey, 2018; Spangenberg and Zufferey, 2019) have shown 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.

The decline in g_s and in A_N during the growth season was followed by an almost linear decrease in E. In this way, the A_N/E ratio, named instantaneous water use efficiency (WUE_inst), diminished over the season (Figure 3E) but remained, nevertheless, practically identical among the different vine water regimes. Stomatal conductance g_s is sensitive not only to water stress but also to environmental factors, such as, for example, the water vapor deficit of the air or VPD (Prieto et al., 2010; Zufferey and Smart, 2012). A rise in VPD generally leads to the progressive closure of stomata but an increase in E, resulting finally in a fall in WUE_inst. The rise in VPD tends to diminish WUE_inst over a wide range of water deficits as a result of decreasing g_s and A_N, but not the WUE_i. (Schultz and Stoll, 2010). Under conditions of water stress and high VPD, the opposite tendency between WUE_i et WUE_inst is sometimes observed, which was the case in this study.

Mesophyll resistance (r_m) increased during the growing season (Figure 3G), whatever the vine water status. Water stress did, nevertheless, accentuate r_m, showing that resistance increased to CO₂ transfer from sub-stomatal cavities towards carbon fixation sites or chloroplasts (Flexas et al., 2002). The increase in r_m coupled with the decrease in g_s in non-irrigated vines subjected to water stress, leads to a fall in leaf photosynthesis, as was observed in this study.

3.2 Mineral supply and pruning weight

The leaf content of N, P, K and Ca had a tendency to diminish with rising water stress, especially in the non-irrigated vines with plastic-covered soils when compared with well-watered vines (Table 3); this tendency was not, however, significantly different in the study period. The leaf content of Mg did however increase with water stress, and in a significant way. These results confirm those obtained from other grape varieties under the same study conditions (Zufferey et al., 2017, 2018). No significant differences in leaf chlorophyll content was found among the different water regimes imposed on vines. The content of yeast available nitrogen in musts (YAN), however, dropped as water stress increased in non-irrigated vines with plastic-covered soils (Table 3, Figure 4).

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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&nbsp	Foliar analysis (% D.M)					N-tester	YAN (mg N/L)
	N	P	K	Ca	Mg
Irrigated vines	2.58a	0.26a	1.04a	4.29a	0.29a	524a	231a
	(±0.15)	(±0.02)	(±0.07)	(±0.17)	(±0.02)	(±15)	(±14)
Non irrigated vines	2.46a	0.25a	0.97ab	4.14a	0.33ab	509a	215ab
	(±0.17)	(±0.02)	(±0.05)	(±0.15)	(±0.02)	(±13)	(±12)
Non irrigated vines + plastic covered	2.35a	0.22a	0.84b	4.10a	0.36b	483a	189b
	(±0.16)	(±0.03)	(±0.07)	(±0.12)	(±0.02)	(±15)	(±15)

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

Figure 4. Relationship between the predawn water potential (Ψ_PD) and the yeast available nitrogen (YAN) in the must at harvest for different irrigation treatments.

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

A relationship between plant water status, measured by Ψ_PD during the period between fruit-ripening and harvesting, and the YAN (R² = 0.62, P < 0.01) was highlighted: the latter decreased as water stress rose. The reduced availability of soil water and the increased plant water stress caused weakening of leaf transpiration, the process which drives minerals uptake (including nitrogen) at the root level and their subsequent transport through the canopy transpiration flow (Gonzalez-Dugo et al., 2010). Nitrogen mineralization and soil microbial activity are both negatively influenced by the soil drying conditions (Celette and Gary, 2013). Studies, carried out in different wine-growing “terroirs” by Reynard et al. (2011), have also brought to light the major role played by water supply to vines on the nitrogen content in leaves and berries. Verdenal et al. (2012) further observed that water and nitrogen supplies constitute a marker of the «&nbspterroir&nbsp» effect, related to wine quality of Arvine wines, under pedoclimatic conditions of the Swiss canton Wallis.

Plant vigor, estimated by pruning weight in winter, was greatly influenced by the vine water status (Table 4). Water-stressed vines (non-irrigated vines with plastic-covered soil) showed the lowest weights of pruned wood, compared with irrigated vines. When compared with well-watered vines (irrigated vines), the loss in pruned wood weight reached 19&nbsp% on average in the 8 years of study in non-irrigated vines with plastic-covered soil, and 5&nbsp% in non-irrigated vines (rain fed).

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

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&nbsp	Pruning weight (g/vine)								Mean
	2009	2010	2011	2012	2013	2014	2015	2016	2009-2016
Irrigated vines	643a	650a	640a	696a	644a	626a	540a	479a	615a
	(±45)	(±27)	(±25)	(±37)	(±27)	(±30)	(±22)	(±17)	(±37)
Non irrigated vines	555ab	630a	574b	647a	638a	585a	546a	489a	579a
	(±25)	(±30)	(±27)	(±34)	(±30)	(±30)	(±25)	(±22)	(±30)
Non irrigated vines + plastic covered	503b	590a	407c	566b	490b	453b	506a	474a	500b
	(±24)	(±20)	(±30)	(±40)	(±20)	(±27)	(±25)	(±27)	(±35)

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

3.3 Yield components and must characteristics at harvest

Arvine bud fertility (number of inflorescences per shoot) remained uninfluenced by vine water regimes on average over the 8-year study period (Table 5). Water stresses suffered by non-irrigated vines with plastic-covered soils had no impact on the fruiting of buds. The replenishment of carbon (starch) and nitrogen reserves was most likely to have been sufficient from one year to the next during the study period, in both irrigated and non-irrigated vines (Zufferey et al., 2015). The yield regulation was carried out identically between the different irrigation treatments, where 5 clusters per plant were removed. In non-irrigated vines with plastic-covered soils, subjected to the highest water restrictions, lower berry weights were observed than from irrigated vines, or non-irrigated vines where water stresses were weak to moderate (Table 5). Finally, vine water status had no impact on cluster weights and yield per m² of soil (and per vine) at harvest time (Table 5). The water stress, which made its appearance relatively late during the ripening period in non-irrigated vines, generally had no major effect on yield (Ojeda et al., 2001; Zufferey et al., 2018), in contrast with early water deficits during the season (before veraison) (Ollat et al., 2002).

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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&nbsp	Bud fertility (inflo/shoot)	Cluster removal (-x clusters per vine)	Berry weight (g)	Cluster weight (g)	Yield (kg/m2)
Irrigated vines	1.8a	-5.2a	1.3a	240.4a	0.93a
	(±0.05)	(±0.1)	(±0.1)	(±15)	(±0.13)
Non irrigated vines	1.8a	-5.2a	1.2a	225.2a	0.91a
	(±0.04)	(±0.2)	(±0.1)	(±20)	(±0.12)
Non irrigated vines + plastic covered	1.8a	-5.2a	1.0b	208.4a	0.86a
	(±0.05)	(±0.2)	(±0.1)	(±21)	(±0.10)

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

Table 6 presents must characteristics at harvest time, such as sugar content, total acidity, tartaric and malic, pH and values of glycosyl-glucose (G-G) and of aromatic precursor (3 mercapto-hexanol, P-3MH). The vine water regime did not exert any major influence on sugar contents, nor on the pH in must. Malic acid content and total acidity in musts were found to be lower from vines where moderate to high water stress had been experienced (non-irrigated vines), compared with well-watered vines (irrigated vines). During the hot and dry years of 2009, 2011 and 2015, the water stress observed in non-irrigated vines with plastic-covered soils led to the yellowing and loss of leaves from the basal zone of shoots: consequently, the microclimate of the cluster zone was modified, and clusters were more exposed to direct sunlight and berry temperatures rose. Such conditions may well have led to the degradation of malic acid in berries and a reduction in total acidity of musts (Ruffner, 1982a; Ruffner, 1982b). Conversely, tartaric acid increased with rising water stresses.

Table 6. Harvest characteristics: sugar content, pH, titratable acidity (total acidity, tartaric and malic acid), glycosyl glucose (G-G) and aromatic precursor (P-3MH) in must.

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&nbsp	Sugar (g/L)	pH	Total acidity (g/L)	Tartric acid (g/L)	Malic acid (g/L)	G-G (mg/L)	P-3MH (mg/L)
Irrigated vines	239a	3.07a	10.9a	8.1a	4.8a	41.1a	48a
	(±11)	(±0.05)	(±0.4)	(±0.3)	(±0.5)	(±3.1)	(±4)
Non irrigated vines	239a	3.06a	10.4a	8.3a	4.0ab	43.6ab	47a
	(±10)	(±0.04)	(±0.3)	(±0.2)	(±0.4)	(±2.5)	(±3)
Non irrigated vines + plastic covered	237a	3.04a	9.7b	8.7b	2.9b	47.8b	48a
	(±8)	(±0.05)	(±0.3)	(±0.2)	(±0.6)	(±2.2)	(±4)

Letters: statistically significant at the 5&nbsp% level of probability. Averages 2009-2016, Arvine, Leytron (Switzerland).

The aromatic potential of the Arvine grape variety was evaluated by global analysis of glycosylate compounds (G-G method) and direct quantification of the precursor 3-mercaptohexanol (P-3MH) in musts. G-G values were higher in non-irrigated vines where a moderate to strong water stress had been experienced, than in irrigated vines. Identical results had been obtained by Dienes-Nagy et al. (2016) in a study over several years and in varying climatic conditions: in hot and dry seasons, the G-G content of musts was higher than in cooler, rainy years. Observation of P-3MH concentration in musts, however, showed no significant differences between irrigated and non-irrigated vines. Furthermore, the quantity of P-3MH in musts may be influenced by the presence of Botrytis cinerea on grapes (Thibon et al., 2011, Spring et al., 2014), this fungus may provoke over-production of P-3MH in this case. During the present trial period, Botrytis cinerea was inexistent, whatever the year or the water regime imposed on vines.

In wines, values of pH, total acidity and tartaric acid were very similar among the different irrigation treatments, and no significant differences were observed (results not presented).

4. Wine tasting

Figure 5 presents sensorial analysis notes conferred on wines (noted from 1 to 7), carried out a few weeks after bottling, and covering the main wine characteristics: wine bouquet (quality of aromas), structure, bitterness and global appreciation of the wine.

Figure 5. Evaluation of wine quality by tasting according to different sensory variables: olfactive (bouquet), gustatory (structure, bitterness) and overall impression for the different irrigation treatments.

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The notation scale ranges from 1 (poor) to 7 (high). Letters indicate statistical significance at the 5&nbsp% level of probability. Arvine, Leytron (Switzerland), 2009-2016.

The quality of the Arvine wine bouquet (typicality and finesse of aromas) was judged to be slightly less attractive in wines whose grapes came from non-irrigated vines, where a strong water stress had been experienced, particularly in the years 2010 and 2011, compared with well-watered (irrigated) vines. Taking into consideration the whole period, however, differences were not always significant. Poorer supplies of nitrogen to musts (concentration of yeast available nitrogen in musts below 200 mg/L, Spring et Lorenzini, 2006), which were observed in water-stressed vines, most certainly influenced bouquet appreciations, and also the perception of bitterness and astringency on the palate. Indeed, wines from water-stressed vines and from musts deficient in yeast available nitrogen (non-irrigated vineyards with plastic-covered soils) were distinguished by higher notes of bitterness, in comparison with vines well supplied with water and nitrogen (irrigated vines). These results corroborate the conclusions reached by Verdenal et al., (2012) with regard to the importance of water and nitrogen supplies to Arvine vineyards and their impact on the wine quality from different pedo-climatic conditions. The global appreciation – or hedonic impression – of wines tasted in the present study was judged to be superior in wines coming from irrigated vines, well-supplied with nitrogen, as opposed to vines with high water deficits and yeast available nitrogen deficiency in musts. This was especially true for the hot and dry years, such as 2009, 2010 and 2011. Research led by Spring et al., (2014) into the effects of nitrogen supply and the characteristics of Arvine wines, demonstrated that available nitrogen content in musts, below 180-200 mg/L, brought about a decrease in aromatic precursor concentrations in must (P-3MH) and in aromas in wines. The YAN content observed over the years of the present study remained on average above 180 mg/L, including non-irrigated vines with plastic-covered soils. In this study, P-3MH content was equivalent in musts from both irrigated and non-irrigated vines.

In white cultivars, a low to slightly moderate water stress benefits the development of aromatic components (Choné et al., 2006; Reynolds et al., 2010). A severe or sometimes even moderate water stress can have a negative impact on white wine quality, particularly affecting the finesse and the typicality 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).

Acknowledgments

The authors wish to express their warm gratitude to the viticulture and the wine technology and analysis teams for their excellent work and invaluable collaboration.

References

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