Aim: The aim of the present study was to analyze the impact of different water regimes, by means of various levels of irrigation, on the physiological and agronomical behavior of an aromatic white grapevine (cv. Arvine). The consequences of the plant water status were followed through to chemical (aromatic precursors) and sensorial analysis of resulting wines.

Methods and results: Adult vines of Vitis vinifera L. cv. Arvine grafted onto 5BB were subjected to different water regimes (various levels of irrigation) during the growing season. Physiological indicators were used to monitor the plant water status [the predawn leaf (Ψ_PD) and stem (Ψ_STEM) water potentials and the carbon isotope composition (δ¹³C) in the must]. Gas exchange (net photosynthesis A_N and transpiration E), stomatal conductance (g_s), yield parameters, berry composition at harvest, analysis of potential grape aromatic properties (glycosyl-glucose G-G, precursor 3-mercaptohexanol P 3-MH) and the sensorial quality of wines were analyzed over a period of 8 consecutive years (2009-2016) on the Agroscope experimental vineyard in Leytron under the relatively dry conditions of the Rhône valley, in Wallis, Switzerland.

In non-irrigated vines, the progressively increasing water deficit observed over the season reduced the leaf gas exchange (A_N and E) and g_s. The intrinsic water use efficiency (WUE_i, A/g_s) increased over the season and was greater in vines that had suffered water restriction than in irrigated vines. The rise in WUE_i was correlated with an increase in δ¹³C in the must sugars at harvest. A decrease in plant vigor was observed in the water stressed vines over multiple years. Moderate to high water stress during fruit ripening lowered the contents of total and malic acidity in the musts and the content of yeast available nitrogen (YAN). On the other hand, contents in sugar and the aromatic precursor (P-3MH) in berries were not influenced by the vine water status. Values of G-G in berries increased with rising water stress in non-irrigated vines. Wines from plants subjected to water stress and to yeast available nitrogen deficiency (non-irrigated vines during hot and dry seasons) presented a less distinctive typicity, and developed a lower aromatic expression with a more bitter taste, than wines from non-stressed plants. Overall, and compared with stressed vines, the organoleptic characteristics and quality of Arvine wines coming from vines where no restrictions in water and nitrogen had been suffered during the growing season were better appreciated.

Conclusions: The vine’s physiological behavior (leaf gas exchange, plant vigor) and agronomic parameters (yield, berry composition), together with the quality of white aromatic Arvine wines, were strongly influenced by vine water regimes during the growing season.

Significance and impact of the study: Vine water status and must nitrogen contents are key factors in grape composition and in the sensorial quality of resulting aromatic white wines.

Keywords: water stress, gas exchange, water use efficiency (WUE), carbon isotope composition, berry composition, aromatic compounds, wine quality

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 % 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).

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 Arvine, 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.

Table

titre du tableau
	2009	2010	2011	2012	2013	2014	2015	2016	Long-term
January	109	11	22	57	21	42	55	110	51
February	28	29	7	0	59	79	11	104	47
March	23	27	14	5	29	5	63	19	42
April	37	8	5	51	45	29	12	38	35
May	25	120	43	52	83	34	123	76	49
June	40	15	40	37	24	17	34	45	54
July	87	73	69	51	52	106	35	46	58
August	16	45	22	65	30	87	78	27	57
September	18	22	42	52	45	15	14	14	44
October	11	14	34	39	67	30	29	32	52
November	68	36	2	53	95	44	42	75	52
December	108	70	168	152	17	42	4	0	64
Year	570	470	468	614	567	530	500	586	603

Table

titre du tableau
	2009	2010	2011	2012	2013	2014	2015	2016	Long-term
January	-2.7	-1.5	0.2	1.5	1.0	2.6	1.4	2.2	-0.1
February	1.0	1.5	2.9	-1.7	0.0	4.2	1.3	4.4	1.8
March	5.9	6.1	7.9	9.1	5.2	8.4	7.9	6.6	6.5
April	12.4	11.8	14.2	10.9	10.9	12.8	12.2	11.3	10.4
May	16.4	14.0	17.0	16.1	12.5	15.6	15.6	14.7	14.9
June	18.4	18.9	18.8	20.0	18.1	20.1	20.6	18.7	18.1
July	20.5	21.8	18.6	20.3	21.6	19.3	24.0	21.5	20.1
August	21.6	18.5	21.0	21.3	20.2	18.4	20.9	21.0	19.2
September	16.8	14.8	17.8	15.8	16.3	16.9	14.9	18.4	15.2
October	10.3	10.3	10.4	11.5	12.7	13.0	10.5	10.1	10.3
November	6.7	5.5	5.2	6.4	3.8	8.1	5.9	5.8	4.3
December	1.0	-0.6	1.9	0.6	0.4	2.7	2.2	-0.7	0.6
Year	10.7	10.1	11.3	11.0	10.1	11.7	11.5	11.2	10.1

Three 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.

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 composition (δ¹³C) of the must sugars was 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 δ¹³C 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).

Leaf gas exchange [net photosynthesis (A_N) and transpiration (E)], stomatal conductance (g_s) 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/g_s) and instantaneous water use efficiency (WUE_inst, A_N/E) were determined from single leaf gas-exchange measurements that related the net photosynthesis rate (A) either to the stomatal conductance for water vapor (g_s), termed WUE_i (Osmond et al., 1980), or to the leaf transpiration rate (E), termed WUE_inst.

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.

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, titrable 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. The aromatic potential of the Arvine grape variety was evaluated by the Glycosyl-Glucose (G-G) method, according to Williams et al. (1995), and the precursor 3-mercaptohexanol (P-3MH) was analyzed according to the Luisier et al. (2008) method on musts, after crushing and the addition of sulphites. 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).

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.

Tables 1 and 2 present monthly and annual rainfall and temperatures for the period under study, from 2009 to 2016. The experimental wine-growing area is situated in a relatively dry Alpine region where long-term mean rainfall (1981-2010) is around 600 mm per year. During the study period, rainfall amounts were a little below the long-term average, especially in 2010, 2011 and 2015 when measured precipitation was approximately 100-130 mm less per year. Particularly dry summer periods (August-September) were recorded in 2009, 2010, 2011, 2015 and 2016. The year 2014 was an exception when rainfall was higher in July (106 mm) and August (87 mm) than normal (1981-2010). Annual temperatures were greater by about 0.6 to 1.6°C than the long-term means (1981-2010), except for the years 2010 and 2013 when average annual temperatures were equivalent to the long-term figures. In comparison with the 30-year mean figures, particularly high summer temperatures (July-September) were recorded in the region in 2009, 2011, 2012, 2015 and 2016.

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.

Table

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

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.

Table

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