Table

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Rootstock	Shoot weight (g m-1)	Yield (kg m-2)	Sugar (g L-1)	Acidity (g L-1)	Tartaric acid (g L-1)	Malic acid (g L-1)	YAN (mg N L-1)
41B MGt	40.7 c	0.7 b	240.3 a	6.8 a	6.4 a	1.9 b	107 b
Riparia Gloire	42.5 bc	0.6 b	232.6 a	7.2 a	6.3 a	2.6 a	175 a
101-14 MGt	49.2 abc	0.6 b	239.8 a	7.0 a	6.1 a	2.5 a	168 a
3309C	56.9 ab	0.7 b	237.8 a	7.2 a	6.4 a	2.6 a	198 a
Kober 5BB	63.7 a	0.9 a	241.7 a	7,3 a	6.5 a	2.6 a	173 a

Means followed by the same letter do not differ at p < 0.05 by Fisher’s LSD test. Means followed by different letters in the same column differ significantly at p < 0.05 by Fisher’s LSD test.

Carbon isotope composition (δ¹³C) measured on sugars in must at ripeness ranged from –24.81‰ for vines grafted onto 3309C to –23.28‰ for vines grafted onto 41B MGt (Table 2). The differences were not statistically significant (P = 0.19).

The anatomical analyses revealed that most xylem vessels had a cross-sectional area lower than 1000 µm², both stems and petioles (Tables 4 and 5). Petioles of Pinot noir grafted onto 41B MGt had the highest proportion of vessels with a cross-sectional area smaller than 500 µm² and the lowest proportion with a cross-sectional area higher than 1001 µm² (Table 4).

Table

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Frequency (%) of vessels per cross-sectional area (µm2) category
	200–500	501–1000	1001–1500	1501–2000	2001–2500	>2500
41B MGt	48.36 b	30.02 a	7.17 b	1.39 c	0.14 b	0.05 c
Riparia Gloire	38.67 a	31.92 a	14.42 a	5.25 b	1.59 ab	0.98 ab
101-14MGt	39.35 a	30.14 a	13.19 a	6.11 ab	2.13 a	0.60 abc
3309C	37.92 a	32.04 a	14.86 a	6.25 ab	2.41 a	0.56 bc
Kober 5BB	36.30 a	31.57 a	15.42 a	6.81 a	2.27 a	2.45 a

Means followed by different letters in the same column differ significantly at p < 0.05 by Fisher’s LSD test.

For most category sizes, there was no difference among 101-14 MGt, Riparia Gloire, Kober 5BB and 3309C. In the category size above 2500 µm², most differences were observed between vines grafted onto 41B MGt and Kober 5BB, but for a very low frequency.

Table

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Frequency (%) of vessels per cross sectional area (µm2) category
	200–500	501–1000	1500–3000	3001–5000	5001–10,000	>10,000
41B MGt	21.99 ab	27.67 a	12.39 a	9.74 a	14.61 a	8.36 cd
Riparia Gloire	32.50 b	25.04 a	9.12 a	7.63 a	10.28 a	7.04 d
101-14 MGt	20.83 a	24.32 a	13.24 a	10.93 a	14.44 a	11.62 bc
3309C	15.90 a	28.70 a	10.86 a	10.16 a	17.33 a	16.65 a
Kober 5BB	23.61 ab	23.10 a	12.64 a	9.31 a	14.03 a	14.21 ab

Stems of vines grafted onto Riparia Gloire and 3309C were characterised by the highest and lowest frequency of small vessels (200–500 µm²), respectively, whereas there was no significant difference for the other rootstocks (Table 5). The highest frequency of vessels number with a cross-section area higher than 10,000 µm² was observed for vines grafted onto 3309C and Kober 5BB compared to 41B MGt and Riparia Gloire. There was no similar pattern among rootstocks between smaller (200–500 µm²) and larger stem vessels (>10,000 µm²) (Table 5).

An inverse correlation was found between S_Ψ and K _petiole while shoot weight, petiole vessel size higher than 2000 µm² and stem vessel sizes higher than 10,000 µm² seemed not to be affected by S_Ψ (Table 6). On the other hand, increased shoot weight and petiole vessel size higher than 2000 µm² were associated with increased K _petiole . A/gs measured at harvest was also significantly correlated with δ¹³C (Table 6).

Table

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Variables	SΨ(Mpa d-1)	K petiole (mmol m-2 s-1MPa-1)
	Equation Regression	R2	P-value	Equation Regression	R2	P-value
Kpetiole	y = 15.83-0.52x	0.82	0.03	-	-	-
Shoot weight	y = 68.09-1.41x	0.68	0.08	y = 23.97+2.86x	0.92	0.01
Petiole vessel size >2000 µm2	y = 5.27-0.21x	0.54	0.16	y = 4.63+1.77x	0.78	0.04
Stem vessel size >10,000 µm2	y = 17.53-0.47x	0.45	0.21	δ13C (‰)
				Equation Regression	R2	P-value
A/gs				y = 417.05+14.24x	0.87	0.02

The use of drought-tolerant rootstocks has been considered a low-cost adaptive strategy to cope with the decrease in soil water availability in viticultural areas predicted under the context of global change (Fraga et al., 2013; van Leeuwen and Darriet, 2016). This study reports data recorded during one season regarding rootstock genotype effects on ecophysiological and agronomical responses to water deficit of field-grown Pinot noir under cool climatic conditions and how these effects could be related to vigour. In comparison to references, rainfalls were slightly higher during the spring and summer of 2017, but both seasons were considered the third warmest ones since records began in 1864 (MétéoSuisse database). This higher mean temperature recorded during the 2017 growing season associated with the water shortage imposed by soil coverage of the experimental plot with waterproof plastic, equivalent to the year 2009 situation (Spring et al., 2016), contributed to simulating stressful climatic projections for a non-irrigated and well-established vineyard.

Studies regarding scion-rootstocks interactions under water deficit in cool-climate countries have not been well explored as most published data have been carried out under very hot and dry summer conditions such as usually recorded in the Mediterranean (Koundouras et al., 2008; Romero et al., 2018) and Australian (Soar et al., 2006a; Stevens et al., 2010) vineyards. Moreover, most of these studies were carried out in deficit-irrigated vineyards. Our findings showed that, in a non-irrigated vineyard of a cool country such as Switzerland, invigorating rootstocks favoured the water supply to the scion and it was associated with some features such as increased xylem hydraulic conductivity, high nitrogen uptake and probably also due to larger root system.

The soil coverage used in this experiment promoted a progressive water deficit throughout the growing season, as revealed by the reduction in Ψ_pd and_{Ψstem. According to the literature, the water deficit intensity observed during the ripening period was classified from weak to moderate (Ψpd = –0.3 to –0.5 MPa; Ψstem= –0.9 to –1.1) and severe (Ψpd <–0.6 MPa; Ψstem=< –1.4 MPa) depending on rootstocks and physiological indicators of vine water status (Deloire et al., 2004; van Leeuwen et al., 2009). Based on Ψstem values, water deficit could be classified as severe for vines grafted onto 41B MGt, moderate for vines grafted onto 101-14 MGt and Riparia Gloire and weak for vines grafted onto 3309C and Kober 5BB. Based on Ψpd values, only two levels of water deficit could be defined with severe water deficit for 101-14 MGt and 41B MGt (Ψpd <–0.6 MPa) and mild (–0.3 to –0.5 MPa) for Riparia Gloire, Kober 5BB and 3309C, confirming that Ψstem is amore sensitive indicatorof water status than Ψpd (Choné et al., 2001; Patakas et al., 2005; Souza et al., 2009). SΨ was shown to be a useful parameter for estimating and integrating water stress in different crops (Meyers, 1988), including grapevine (Souza et al., 2005a). Indeed in this study, SΨ followed similar patterns as Ψstem values for the different rootstock combinations. The SΨ values of Pinot noir grafted onto 3309C and Kober 5BB were similar to values found for Moscatel and Castelão cultivars under full irrigated conditions (100%ETc) in a Mediterranean vineyard during the driest year, while 41B MGt and 101-14MGt induced values near to deficit irrigated Castelão (50%ETc) as shown by Souza et al. (2005a). These results indicate that Pinot noir, even under a non-irrigated vineyard in a cool climate region, expressed a lower intensity of water stress than other field-grown cultivars under warmer and drier conditions.}

Rootstocks had a more pronounced impact on vine water status than on the gas exchange of Pinot noir. The differences among rootstocks for gas exchange traits were hardly significant, except during the last weeks of the ripening period depending on variables and time of measurements. In general, at later stages of ripening, Kober 5BB and 3309C exhibited higher A, gs and E, mainly as compared to 41B MGt during the afternoon. A, gs and E decreased for all rootstocks from the onset (166DOY) to the last date of measurements (247DOY), with a more pronounced decrease before the 210DOY when Ψ_pd values were still above –0.4 MPa and Ψ_stem above –1.0 MPa. Nevertheless, differences among rootstocks appeared to be more pronounced afterwards when Ψ_pd and Ψ_stem further decreased. Grapevines have been considered resistant to drought mainly due to the early and efficient stomatal control to regulate water losses during mild to moderate water stress (Chaves et al., 2010). Even under water deficit characterised as severe (Ψ_pd =_{–0.7 MPa) (240 and 247 DOY), the values of mid-morning gs were lower than 0.10 mol m^-2 s^-1 in vines grafted onto 41B MGt and 101-14 MGt only at 240DOY due to the higher VPDL (2.3 kPa). However, at 247 DOY, all rootstocks induced values of mid-morning gs higher than 0.10 mol m^-2 s^-1 when VPDL was lower (1.8 kPa), indicating still active gas exchanges and the absence of metabolic limitations of photosynthesis as previously suggested (Medrano et al., 2002).}

Gs was significantly correlated with Ψ_pd (excepted for Kober 5BB) and with VPDL, suggesting control by both the soil water status (dependent on the rootstocks) and the evaporative demand of the atmosphere (independent of the rootstock) (Davies et al., 2002; Chaves et al., 2016). However, the relationships of gs versus VPDL and Ψ_pd suggested a similar stomatal response among rootstocks, even though it was previously shown that the sensitivity of gs to VPDL and Ψ_pd could be affected by the scion cultivar and rootstock (Soar et al., 2006a,b; Tramontini et al., 2013). In our study, in some few days, A, gs and E were higher for the more vigorous vines (higher stem weight) grafted on 3309C and Kober5BB, in agreement with the observations of Soar et al. (2006a), who reported, in an irrigated vineyard, that the most vigorous rootstocks such as Teleki 5C and Ramsey induced the highest gs in Syrah. According to these authors, rootstock effects on canopy gas exchange would be initially driven by differences in root capacity to extract soil water, mediated via dynamic variation in physiological-active leaf abscisic acid (ABA).

Along the water deficit progression, an increase in intrinsic water use efficiency (A/gs) was reported for all rootstocks, agreeing with previous authors (Souza et al., 2005a; Chaves et al., 2016; Zufferey et al., 2017). Leaf ageing along the growing season could also affect A/gs. The highest A/gs has been observed in vines grafted onto 41B MGt, mainly due to the lowest stomatal conductance induced by this rootstock during the days where significant differences among rootstocks were found. In contrast to gas exchange techniques that provide measurements of photosynthesis rates at a single point in time, δ¹³C measured on must integrate the ratio of net photosynthesis to water transpired over the growing season. It has been shown to be a useful tool for evaluating vine water use efficiency over the ripening period (Gaudillère et al., 2002; Souza et al., 2005a; Zufferey et al., 2017). Although δ¹³C was correlated with mid-morning A/gs measured at harvest, the rootstocks could not be discriminated by δ¹³C, probably due to the small range of δ¹³C values among rootstocks (van Leeuwen et al., 2009). It could also be related to a short period of water deficit and small differences in the level of water stress intensity among rootstocks, while in other studies, the authors found larger differences in vine water status among water treatments (Souza et al., 2005a; Zufferey et al., 2017).

A pronounced effect of rootstock on K _petiole of Pinot noir leaves was observed. The seasonal average of K _petiole was closely correlated with S_Ψ, petiole xylem vessel frequency higher than 2000 µm² and shoot weight, suggesting a link between petiole hydraulic conductivity, petiole xylem anatomy, stem vigour and water status. K _petiole was high for Kober 5BB and 3309C as compared to other rootstocks. 101-14 MGt was characterised by a higher K _petiole and a greater frequency of xylem vessels with a larger cross sectional area in the petiole than 41B MGt. It was previously shown that petiole specific hydraulic conductivity (K _petiole ) of grapevine cultivars could be affected by vine water status (Zufferey et al., 2017; Dayer et al., 2017) and our data show that it can also be affected by rootstock genotype, probably in a relationship with conferred vigour.

Shoot vigour was significantly correlated with the frequency of the largest vessels in the stem. It supports the findings reported by Santarosa et al. (2016) for grapevine and by Gonçalves et al. (2005) for sweet cherries, which show that vigorous rootstocks affect the size of scion xylem vessels. Atkinson et al. (2003) also reported that the stem hydraulic conductivity of grafted apple trees was increased by invigorating rootstock in relation to xylem anatomical changes. In peach, xylem vessel characteristics of the dwarfing rootstock genotypes appear to influence tree growth directly rather than through an effect on the xylem vessel size of the scion (Tombesi et al., 2010).

Although the negative relationship between shoot weight and S_Ψ was not significant, drought responses of field-grown Pinot noir appeared to vary according to differences in shoot vigour induced by rootstock over the growing season. Vines grafted onto high vigorous rootstocks (3309C and Kober 5BB) were characterised by higher Ψ_pd and Ψ_stem compared to 41B MGt, Riparia Gloire and 101-14 MGt, which induced a lower shoot weight and more negative Ψ_pd and Ψ_stem. These observations agree with those of Souza et al. (2009), who showed that vigorous rootstocks promoted water status and vegetative vigour of Syrah vines. They also agree with the results reported by Spring et al. (2016) under rainfed conditions, suggesting that rootstocks conferring a high vegetative vigour appear to limit the appearance of water deficit in vineyards.

The lowest shoot vigour measured in vines grafted onto 41B MGt was probably not explained by water deficit as the largest water status differences on Ψ_pd appeared only after the 2 August once shoot growth was largely completed and because of the absence of correlation between S_Ψ and shoot vigour. The absence of a relationship between S_Ψ and frequency of vessel sizes larger than 2000 µm² in petiole and larger than 10,000 µm² in stem also suggests that vascular anatomy was not affected by water deficit but most probably related to differences of vigour promoted by rootstocks. The reduced stem xylem vessel size observed for the less vigorous vines can decrease the shoot hydraulic conductivity and the efficiency of the water flux in vine shoots under a soil water restriction, as demonstrated by Pagay et al. (2016).

The lowest chlorophyll index values measured in vines grafted onto 41B MGt at the beginning of the season, under not-yet limiting water conditions, indicate that shoot vigour was also reduced due to low nitrogen uptake rather than water limitation. This is also in agreement with the lower yeast assimilable nitrogen of grape must observed for this rootstock. It is well known that nitrogen is one of the most important nutrients for vegetative development because it is a component of proteins, enzymes, chlorophyll and nucleic acids (Taiz and Zeiger, 2004) and, therefore, under low N supply, vine shoot growth can be compromised (Keller, 2010; van Leeuwen et al., 2018). As root system development is correlated with vegetative vigour (Southey and Archer, 1988), it may be hypothesised that a reduced root system development for the vines grafted onto 41MGt may have impaired both nitrogen and water uptake. In addition, the negative effect of water deficit on nitrogen uptake and content in leaves and berries could have further limited YAN content in grape must of vines grafted onto 41B MGt (Spring et al., 2012; Zufferey et al., 2017). Low vigour may also explain the lowest malic acid content for this combination, probably due to an increased berry exposure to sunlight, favouring malic acid degradation by rising in temperature as shown by other authors (Souza et al., 2005b; Santos et al., 2007; Zufferey et al., 2017). A direct effect of drought level on malic acid content has also been reported occasionally (Kizildeniz et al., 2018).

There is no accordance in the literature regarding the degree of drought tolerance among these five rootstocks ranging from low to highly drought-sensitive depending on scions and experimental conditions (Serra et al., 2014; Ollat et al., 2016; Spring et al., 2016). Under semi-arid conditions, Romero et al. (2018) reported that Monastrell vines grafted onto 41B MGt exhibited a lower vigour and yield than vines grafted onto 1103P, without any differences in vine water status. It was also shown that Sultana vines grafted onto 41B MGt grown in pots showed a reduced root system development and shoot biomass in comparison to vines grafted onto Kober 5BB (Yıldırım et al., 2018). Our field study brings additional data to show that of the responses to water restriction of Pinot noir vines were affected by the vigour conferred by rootstocks. Although 41B MGt was supposed to be the most lime tolerant rootstock among the five studied, it did not perform well in these alkaline conditions. Kober 5BB and 3309 provided the slowest water deficit progression throughout the growing season, probably due to a better water supply supported by a larger and deeper root system. From a practical point of view, our one-year data contribute to support that the use of vigorous rootstocks could avoid the need for supplemental irrigation in vineyards during the periods of water scarcity. Nevertheless, experiments should be replicated to identify the most relevant traits to characterise drought responses induced by different rootstocks under field conditions to be used in breeding programs for a cool climate region.