The variability in the xylem architecture of grapevinepetiole and its contribution to hydraulic differences
UriHochbergA,B, AsfawDeguA,B, TanyaGendlerB, AaronFaitBandShimonRachmilevitchB,C
AAlbert Katz International School, Beer-Sheva, Israel.BThe French Associates Institute for Agriculture and Biotechnology of Drylands (FAAB), the Jacob BlausteinInstitutes for Desert Research, Ben-Gurion University of the Negev, 84990 Sede Boqer, Israel.
CCorresponding author. Email: [email protected]
Abstract. Grapevine cultivars possess large variability in their response to water availability, and are thereforeconsidered as a good model to study plant hydraulic adjustments. The current research compared the petiole anatomyof two grapevine (Vitis vinifera L.) cultivars, Shiraz and Cabernet Sauvignon, in respect to hydraulic properties. Hydraulicdifferences between the cultivar petioles were tested over 3 years (2011–2013). Anatomical differences, hydraulicconductivity and embolism were tested under terminal drought conditions. Additionally, xylem differentiation underwell watered (WW) and water deficit (WD) conditions was compared. Shiraz was shown to possess larger xylem vesselsthat resulted in a significantly higher theoretical specific hydraulic conductivity (Kts), leaf hydraulic conductivity (Kleaf)and maximal petiole hydraulic conductivity (Kpetiole). Under WD, smaller vessels were developed, more noticeably inShiraz. Results confirmed a link between petiole hydraulic architecture and hydraulic behaviour, providing a simplemechanistic explanation for the higher transpiration rates commonly measured in Shiraz. Smaller xylem vessels inCabernet Sauvignon could imply on its adaptation to WD, and explains its better performances under such conditions.
Additional keywords: anatomy, anisohydric, embolism, hydraulic conductance, isohydric, Vitis vinifera.
Received 18 June 2014, accepted 9 November 2014, published online 12 December 2014
Introduction
The water potential of a leaf (Yl) depends mostly on theequilibrium between the leaf ability to conserve its water andthe plant capacity to supply it with water. Although the formeris determined partly by leaf stomatal conductance, the latteris determined by the soil water availability and the whole-plant hydraulic resistance. These factors are affected by vesselarchitecture, pit morphology, chemical signalling and aquaporinexpression (Prado and Maurel 2013). Because of the largenumber of parameters and their variability in different plantsspecies, the mechanism in which hydraulic conductivity isdetermined and its effect on Yl are not clear.
Grapevines are known for their hydraulic vulnerability owingto their long wide vessels (Choat et al. 2010). Additionally,different cultivars vary in their response to water availability(Chaves et al. 2010) and therefore present a good model to studyplant hydraulic adjustments. Past studies have highlighted theimportance of several biological mechanisms underlying theintra-species variability in hydraulic behaviour including:aquaporin expression (Vandeleur et al. 2009; Moshelion et al.2014), vessel architecture in respect to embolism susceptibility(Schultz 2003), and chemical signalling, and stomata regulation(Soar et al. 2006; Brodribb andMcAdam 2013; Tramontini et al.2014). Under low water potentials, grapevines are susceptibleto xylem embolism that can result in a fatal hydraulic failure
(Sperry et al. 1987). As part of their avoidancemechanism, underlow water availability grapevines were shown to have a reducedvessel area (Lovisolo and Schubert 1998). Large vessels havehigher conductivity but are more susceptible to cavitation.Differentiation to a specific vessel size is a trade-off betweenthe xylem conductivity and its resistance to embolism (Tyreeet al. 1994). In addition to vessel area, inter vessel pitting alsoplays a significant role in cavitation susceptibility (Wheeleret al. 2005).
In a comparison of several grapevine organs (namely petioles,shoots and roots), petioles were found to be highly susceptibleto embolism, and establishment of petioles percent loss ofconductivity (PLC) curves showed that under low waterpotentials (–1.5MPa) there is a conductivity loss of up to 90%(Lovisolo et al. 2008; Zufferey et al. 2011). Since differentgrapevine cultivars showed a large variability in theirhydraulic conductivity, transpiration and susceptibility toembolism (Bota et al. 2001; Schultz 2003; Alsina et al. 2007),it was hypothesised that they will also display large variabilityin the petiole’s xylem vessel architecture (Schultz 2003).
In the present study we continued (following Hochberget al. 2013a) the water deficit trials of Shiraz and CabernetSauvignon cultivars, which differ significantly in theirhydraulic behaviour (Chalmers 2007; Tramontini et al. 2014).Xylem architecture of the cultivars was compared with
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hydraulic conductivity and embolism susceptibility. A possiblemechanism for the determination of leaf water potential issuggested.
Materials and methods
Hydraulic architecture differences between the grapevine (Vitisvinifera L.) cultivars Shiraz and Cabernet Sauvignon were testedin three trials, conducted over 3 subsequent years (2011–2013).In the first season, hydraulic behaviour and anatomy of each ofthe cultivars were characterised, through comparison of 6plants for each cultivar� treatment combination, as describedby Hochberg et al. (2013a). In the second year, we repeated thecultivar comparison conducted over the first year with increasednumber of replicates (eight), but additionally, the anatomicaldifferences of each cultivar in response to irrigation treatmentswere measured. In the third year, in order to test the effects ofthe anatomical differences measured in the first two seasons, wecompared the percent losses of conductivity (PLC) for each ofthe cultivars.
Plant material, growth conditions and irrigation treatments
Experiments were conducted in the early summer. In each ofthe experiments grapevine cultivars, Shiraz and CabernetSauvignon, grafted on Richter 110, were grown in greenhouseconditions. In the first and second seasons, 1-year-old vines wereused. In the third season, the plants from the second season(2 years old) were used. To have a detailed description ofPLC over a large range of Yl, the vines were supplementedwith 20 vines (of the same age and genotype) of each cultivar.Throughout all experiments, greenhouse temperatures were keptbetween 26� 2.5 and 17� 1.5�C day and night respectively. Tomaintain plant in vegetative state flowers were removed. Plantswere grown in 9 L plastic pots filled with 8 L of potting media(RAM8, Tuf, Merom Golan, Israel) that includes slow releasefertiliser. Additional fertiliser 30 g plant–1 (Multigan, Ecogan,Israel) was supplemented twice annually. Two weeks after budburst plants were shaped to a single shoot that was trainedvertically by being tied to a stick. Until the beginning of theexperiment (50 days post bud break, ~15 leaves) irrigation tofield capacity was given twice a week.
WW plants were irrigated to field capacity every 4 days. TheWD treatment was different in each of the years. In thefirst year, WD treatments were applied by halting irrigationuntil first signs of wilting were observed (described byHochberg et al. 2013a). In the second year, to allow leafdifferentiation under WD conditions, WD treatment wasapplied by maintaining a soil volumetric content (q) ofbetween 5–10% for 3 weeks (according to Pou et al. 2012).These q values were shown to inflict a reversible water stress(gs ~0.05mol H2O m–2 s–1) at previous experiments (data notshown). At that time, the average minimum leaf water potentials(measured at midday) of both cultivars were at approximately–1.15MPa. After 1 month at q of between 5–10%, irrigationwas stopped for 1 week until wilting was observed. In thethird year irrigation treatments were similar to the first year,with the supplemented vines (20 per cultivar) giving a treatmentidentical to the WD. Those supplemented vines were destroyed
for PLC measurement while dehydrating to achieve a widerange of Yl.
Water relations: water potential (C), transpiration (E) andcalculated leaf hydraulic conductivity (Kleaf)All measurements were performed on fully expanded, sunexposed, mature leaves between 1200 and 1430 hours. Stemand leaf water potential (Ys, Yl, MPa) were measured using apressure bomb chamber (Arimad 3000, MRC, Holon, Israel)duringmidday. In the first two seasons, two leaves per plant weremeasured (one for eachYs,Yl) and in the third season, three leavesof Yl were averaged. Measurements were performed on fullyexpanded, sun exposed, mature leaves. Two hours beforeexcision (for Ys) or immediately before excision (for Yl), aplastic bag was placed over the leaf lamina to prevent rapidwater loss. Each leaf was excised from the shoot using a sharprazor blade and then placed into the pressure chamber with thepetiole protruding from the chamber lid. The chamber waspressurised using a nitrogen tank, and Yl was recorded whenthe initial xylem sap was observed emerging from the cut end ofthe petiole.
Leaf transpiration (E) was measured using the LI-6400portable photosynthesis system (Li-Cor, Lincoln, NE, USA).Measurements were taken at midday on the same leaves thatwere immediately after excised for Yl. and averaged across allreplicates. A constant light intensity of 1000 photosyntheticphoton flux density (PPFD) with a CO2 concentration of400mmolmol�1 was used.
Kleaf was estimated from the relationship between the ratesof single-leaf transpiration and stem–leaf water potentialdifference. Kleaf = E/(Ys – Yl) (Sperry and Pockman 1993).
Xylem architecture, theoretical specific hydraulicconductivity (Kts) calculation, leaf area
The xylem architecture of the petioles was characterised in thefirst and second year of the experiment. In the first year weaveraged three petioles per plant of six Shiraz and six CabernetSauvignon vines. In the second year, to study the effect ofwater stress on xylem architecture, two petioles per plant weresampled: one petiole that reached maturity before the initiationof the water treatments, and a second that developed under WDconditions. To assure development under WD conditions, onlyleaves that were smaller than 4 cm2 before the initiation of thewater treatment were sampled. After sampling, cross-sectionswere processed essentially as described by Rewald et al.(2012). The sections were fixed for 48 h in a 0.5 : 0.5 : 9solution of formaldehyde, acetic acid and ethanol (70%)respectively. Tissue sections were dehydrated using a gradedethanol series (50, 70, 95 and 100%, 30min each) followedby immersion in tert-butanol (8 h) and embedded in ParaplastPlus (Leica, Peterborough, UK). After hardening, cross-sections8mm thick were cut with a rotation microtome (RM2235, Leica,Nussloch, Germany). Cross-sections were collected on glassslides and placed on a warming tray (40�C, 3 h). The tissuesections were de-paraffinised in xylene (33�C, 10min),rehydrated (ethanol 100, 95, 70 and 50%, 5min each) andstained with safranin solution (0.5 g safranin, 50mL ethanol
B Functional Plant Biology U. Hochberg et al.
(95%), 50mL distilled water). Finally, the sections were washedin water until no more stain could be removed.
Slides were examined through anAxio ImagerA1microscopewith led illumination (Zeiss, Goettingen, Germany) andphotographed with an AxioCam HRC camera (Zeiss) (Fig. 1).Images were analysed using image analysis software (Digimizerversion 4.2.5, Ostend, Belgium). Since petioles are nearlysymmetrical, half of the vessels (later extrapolated to thewhole petiole) were manually (i.e. without an automatedalgorithm) labelled, and their areas were measured using theappropriate function of the software. The frequency – comparedwith the total vessels number – of each vessel category wascalculated and averaged across petioles from the same treatmentand cultivar. Based on the radius of each vessel, theoreticalhydraulic conductivity (Kt; mmolmMPa�1 s�1) was calculatedwith the modified Hagen–Poisseuille’s law described by (Tyreeand Ewers 1991):
K t ¼ pr128h
Xn
i¼1
ðd4i Þ; ð1Þ
where d is the radius of the vessel in meters, r is the fluid density(assumed to be 1000 kgm–3) andh is the viscosity (assumed to be
(a) (b)
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WW
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Sh Cs
Xylem vessels
Fig. 1. Petioles cross-sectionofShiraz (Sh) (a, c) andCabernet Sauvignon (Cs) (b, c) after 1monthofwellwatered (WW) (a, b) or water deficit (WD) (c, d) irrigation treatment.
2011 2012 2013
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Fig. 2. Leaf water potential (Yl) from three growth season (2011, 2012,2013). Shiraz (Sh) and Cabernet Sauvignon (Cs) after 1 month of wellwatered (WW) or water deficit (WD) irrigation treatment. Data representsmeans� s.e. of 5–8 plants. Different letters represent significant difference(P< 0.05) between columns.
Xylem architecture of grapevine petiole Functional Plant Biology C
1� 10–9MPa�s). The theoretical specific hydraulic conductivity(Kts) was calculated by normalising Kt to the leaf area (LA):Kts =Kt/LA.
LA was calculated using Image J (Abràmoff et al. 2004)from the scanned image of each leaf.
Measurement of petiole hydraulic conductivity (Kpetiole)and embolismAll measurements were conducted under greenhouse conditionsbetween 1200–1530 hours, when embolism was reported to bestable (Lovisolo et al. 2008; Pou et al. 2008; Zufferey et al.2011). Three mature petioles per plant 4–6 cm in length weremeasured and averaged. To avoid xylem adaptations to drought,measurements were conducted on leaves that reached maturitybefore the initiation of the water treatments. Hydraulicconductivity and embolism were measured using the HPFM-P-G3 high pressure flow meter (Dynamax, Houston, TX, USA)exactly as described by (Lovisolo et al. 2010), a methodspecifically designed for grapevine petioles. Stems were bentto a large water container to allow the leaves to be placed underdouble distilled water (DDW) for a relaxation period of 2min(Wheeler et al. 2013). Using a sharp razor blade, leaves werecut and connected to the HPFM. 15mM KCL solution waspressurised at 40 kPa into the petiole until a stable flow wasachieved (between 2–10min). Initial conductivity at 40 kPa (Ki)was logged and the petiole was pressurised at 5 kPa s–1 to a finalpressure of 600 kPa for 3min. After 3min the pressure wasreduced back to 40 kPa and after stability was achieved thefinal conductivity was measured (Kmax). The petiole maximalhydraulic conductivity was normalised to the segment length (L)and LA as follows: Kpetiole = Kmax�L/LA. The normaliseddifference between the Ki and Kmax represents the degree ofembolism which can be quantified as the percent loss ofconductivity. PLC (%) = 100� (1 – Ki/Kmax).
Statistical analysis
Statistical tests were performed using SPSS (Chicago, IL, USA)ver. 21. Null hypothesis were rejected at P< 0.05. Data wastested for normal distribution using Shapiro-Wilk test andhomogeneity of variance using Bartlett’s test. Tukey’s HSDtest and Student’s t-test were performed to determine possiblestatistically significant differences between means. To testthe differences between the PLC curves, we have adjustedlogistic sigmoidal model to the PLC values using Yl as a fixedexplanatory variable. To test for the potential difference in thePLC curves of the two cultivars, we have added to the model
Table 1. A comparison of xylem properties between Shiraz (Sh) and Cabernet Sauvignon (Cs) in 2011 and 2012Significant differences between means are indicated: P< 0.05, n= 5–8
2011 2012Sh WW Cs WW Significant Sh WW Cs WW Significant
Average s.e. Average s.e. difference Average s.e. Average s.e. difference
Petiole area cross-section (mm2)
5.38 0.43 3.78 0.32 – 6.88 0.55 5.70 0.43 –
No. xylem vessels 148.04 12.03 107.92 8.51 * 155.20 13.11 122.71 4.25 *Average vessel area (mm2) 800.49 44.18 634.76 43.88 * 663.56 150.59 496.85 109.10 –
Leaf area (cm2) 101.77 10.69 90.97 6.22 – 140.85 27.23 153.66 7.07 –
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Fig. 3. Hydraulic conductivity of Shiraz (Sh) and Cabernet Sauvignon(Cs) in three growth season (2011, 2012, 2013). Presented are: (a) leafspecific theoretical conductivity according to the Hagen–Poisseuille’s law(Tyree and Ewers 1991) (Kts); (b) petiole maximal hydraulic conductivity(Kpetiole) measured with the HPFM-P-G3 (Dynamax, Houston, TX, USA)according to Lovisolo and Tramontini (2010) (c) leaf conductivitycalculated according to Ohms law analogy (Kleaf). Different lettersrepresent significant difference (P< 0.05) between cultivars.
D Functional Plant Biology U. Hochberg et al.
the cultivar and the interaction term between Yl and thecultivar. The model coefficients were then determined using ageneralised linear model. A rejection of the null hypothesisthat both cultivar respond similarly was based on theestimation of the interaction term value.
Results
Comparison of midday leaf water potential (Yl), after 1 monthin terminal drought conditions, resulted in similar leaf waterpotentials for each cultivar and treatment over threeconsecutive years. In both cultivars well watered (WW) plantsmaintained Yl of –0.5 to –0.6MPa. A month of WD treatmentresulted in significantly different water potentials for eachcultivar. The Yl of shiraz was approximately –2.2MPa, while
that of Cabernet Sauvignon was between –1.36 and –1.63MPa(Fig. 2).
In order to test possible explanations for those differencesin hydraulic characteristics, and in light of the respectivelyattributed contribution to hydraulic architecture (Schultz2003), a comparison of Shiraz and Cabernet Sauvignon xylemarchitecture was conducted (Table 1). Although both cultivarshad similar leaf area, all other parameters measured, such aspetiole cross-section area, number of xylem vessels and averagevessel area were higher in Shiraz as compared with CabernetSauvignon in both the first and second season (Table 1; Fig. 1).Calculations of the theoretical specific (to LA) conductivity(Kts) for each cultivar according to the Hagen Poiseuilleequation, revealed a twice as large Kts for Shiraz as comparedwith that of Cabernet Sauvignon in both seasons (Fig. 3a). In the
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Fig. 4. The xylem architecture of Shiraz (Sh) and Cabernet Sauvignon. (Cs)Vessel distribution (a, c) and leaf specific theoreticalconductivity (Kts) of different vessel area categories (b, d) from twogrowth season, 2011 (a, b) and 2012 (c, d). Data representmeans� s.e.of 5–8 plants. Significant differences between cultivars are indicated: P< 0.05.
Table 2. A comparison of xylem properties between well watered (WW) and water deficit (WD) Shiraz (Sh) and Cabernet Sauvignon (Cs) vinesSignificant differences between means are indicated: P< 0.05, n = 5–8
Sh WW Sh WD Significant Cs WW Cs WD SignificantAverage s.e. Average s.e. difference Average s.e. Average s.e. difference
Petiole area cross-section (mm2) 4.38 0.54 3.65 0.33 – 4.92 0.36 4.58 0.22 –
No. xylem vessels 100.67 8.96 103.60 8.64 – 99.43 8.46 98.57 4.49 –
Average vessel area (mm2) 640.81 217.87 475.07 146.07 – 472.72 94.98 474.80 121.82 –
Leaf area (cm2) 158.84 8.71 105.86 8.61 * 127.31 11.31 117.72 10.69 –
Kts (mmolm–1 s–1MPa–1) 18.86 5.6419 14.067 2.939 – 12.407 2.7525 11.227 1.985 –
Xylem architecture of grapevine petiole Functional Plant Biology E
first season the Kts of Shiraz was 30 while that of CabernetSauvignon was 14.4, and in the second season the Kts ofShiraz was 25.4 and that of Cabernet Sauvignon was 12.6(mmolm–1 s–1MPa). In accordance with Kts, petiole hydraulicconductivity Kpetiole was higher in Shiraz (4.5 as compared with3.9mmolm–1 s–1MPa) (Fig. 3b) and calculated leaf hydraulicconductivity (Kleaf) was significantly higher in Shiraz in 2011(19.4 as compared with 11.9mmolm–2 s–1MPa), but not in 2012(12.9 as compared with 11.7mmolm–2 s–1MPa) (Fig. 3c).
Counting and analysing all vessels revealed a significantlydifferent vessel distribution between the cultivars (Fig. 4).Generally, the vessel distribution of both cultivars had a highfrequency peak at a vessel area of 400–600mm2 and a secondsmaller peak at 1000–2000mm2. As compared with CabernetSauvignon, Shiraz had smaller frequencies of small vessel
(<600mm2) and higher frequencies of large vessels (>600mm2)resulting in a more distinct bimodal distribution. Kts calculationaccording to the Hagen–Poisseuille’s equation demonstratedthe importance of the theoretical conductivity of the largervessels, and emphasised Shiraz higher theoretical capability toconduct water to the leaf. Similar vessel distribution patternsand their differences between the cultivar repeated themselvesin both seasons (Fig. 4).
When observing the vessel modification in response to waterstress, it appears that Shiraz underwent larger modifications ascompared with Cabernet Sauvignon. LA, petiole cross-section,average vessel area, and Kts decreased under WD conditions inShiraz, but not in Cabernet Sauvignon (Table 2). In Shiraz, WDtreatment resulted in smaller vessels as compared with the WWplant (as was previously reported by Lovisolo and Schubert
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Fig. 5. Water stress effect on xylem architecture of Shiraz (Sh) and Cabernet Sauvignon (Cs). Vessel distribution(a) and leaf specific theoretical conductivity (Kts) of different vessel area categories (b) from the 2012 growth season.Petioles were differentiated under (WW) or water deficit (WD) irrigation treatment. Data represents means� s.e. of5–8 plants. Significant differences between cultivars are indicated: P< 0.05.
F Functional Plant Biology U. Hochberg et al.
(1998). At most small vessel categories (<600mm2) WD plantshad higher frequencies, whereas at most large vessel categories(<600mm2) WD had lower frequencies (Fig. 5).
Establishing PLC curves resulted in a significantly differentbehaviour for each of the cultivars (Fig. 6). Models showed(Cs – Ln (1/y) – 1) =2.49–0.13x; Sh – Ln((1/y) – 1)= 1.6–0.08x)the significant effect of the cultivar (c2 = 8.6, d.f. = 1, P = 0.003),water potential (c2 = 8.6, d.f. = 1, P = 0.003) and the interactionterm between cultivar and water potential (c2 = 10.63, d.f. = 1,P = 0.001) Maximal PLC values were 41% for CabernetSauvignon and 36% for Shiraz.
Discussion
The experiment compared the xylem vessels distribution oftwo grapevine cultivars, Shiraz and Cabernet Sauvignon.Results revealed the high architectural variability within Vitisvinifera specie that give rise to mechanistic models explainingvarietal differences in hydraulic conductivity and embolismformation. The hydraulic differences between Shiraz andCabernet Sauvignon were shown in three consecutive years’measurements (Fig. 2). As was anticipated (Tuzet et al. 2003),the near anisohydric behaviour of Shiraz (detailed descriptionin work by Hochberg et al. 2013a) was accompanied by higherhydraulic conductivity. The larger petiole area, number ofxylem vessels and the average xylem vessel area in Shirazresulted in a twice as large Kts. As expected (Tyree andZimmermann 2002), Kts values overestimated the actualKpetiole but provided a simple mechanistic explanation for thedifferences between the cultivars. Compared with CabernetSauvignon, higher Kts in Shiraz correlates with the higher Kleaf
values measured in 2011, the higher Kpetiole values measuredin 2013 (Fig. 3), and previous findings that characterised thehigher transpiration rates (up to 70%) measured in Shiraz as
comparedwithCabernet Sauvignon (Chalmers 2007; Tramontiniet al. 2013, 2014).
It is well established that water stress causes a decrease inthe area of grapevine vessels and xylem hydraulic conductivity,35% and 72%, respectively, when Yl = –0.8MPa (Lovisolo andSchubert 1998;Lovisolo et al. 2002). Smaller vessel size preventsexcessive water loss by reduction of xylem conductivity andmay help preventing the occurrences of embolism (Lovisoloet al. 2010), as smaller vessels are less susceptible tocavitation (Tyree and Dixon 1986; Davis et al. 1999). Inaccordance with these findings, a lower average vessel sizewas measured under WD condition in Shiraz but not inCabernet Sauvignon. Compared with Shiraz, the significantlysmaller vessels of Cabernet Sauvignon under WW conditions(resembling that of Shiraz after adaptation to WD) suggestsits preadaptation to drought. This can explain the measuredcultivar PLC differences that are reinforced by a recentlypublished work by Tramontini et al. (2014). As both xylemdifferentiation through auxin counteracting and embolismrepair through reduced transpiration are mediated by abscisicacid (ABA) (Lovisolo et al. 2008; Shkolnik-Inbar and Bar-Zvi2010), it is possible that ABA signalling is linked with the twophenomena observed. This assumption is further supportedby Shiraz lower abscisic acid level in the sap as comparedwith Cabernet Sauvignon (Hochberg et al. 2013b; Tramontiniet al. 2014).
Smaller vessel size and embolism formation are linked withtranspiration (E) through hydraulic conductivity (Sperry andPockman 1993). Nonetheless, the measured reduction in vesselsize and Kts (up to 25% in Shiraz), and the maximal PLCmeasured (up to 50% even when wilting had occurred),explain only part of the substantial reduction in transpirationunder WD conditions to less than 20% of values measuredunder WW conditions (Hochberg et al. 2013a). Thesemeasurements highlight the importance of other mechanismssuch as aquaporins and stoma regulation in water balancemanagement (Chaves et al. 2010). Our results imply thatgrapevine have more conductive area than necessary to copewith water loss through stomata, and thus embolism should notbe a limiting factor for survival.
Not even a single petiole (out of 150 measured) exceededPLC of 67% even though several publications measured suchvalues in petioles of other grapevine cultivars (Lovisolo et al.2008; Zufferey et al. 2011). A recent publication revealed thatwhen cutting xylem under tension there is a risk of inflictingcavitation (Wheeler et al. 2013). In contrast, Trifilo et al.(2014) pointed out that releasing the tension throughrehydration of the xylem might result in a fast embolismrepair. Other inconsistencies between trials, such as the initialpressure (for measuring Ki) and segment length, stresses thatthe methodology for analysing grapevine PLC should beimproved, possibly through reference technology that canmonitor embolism in intact vessels (Cochard et al. 2014). Thevalues measured in this experiment were similar to thosemeasured with NMR by Choat et al. (2010) in grapevine stems.
The question whether larger vessels promote lower Yl, (aswas displayed by Shiraz) – possibly through increased PLC –
is still under debate. A strong support for this assumptionarises from a recent study by Netzer et al. (2013). These
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Fig. 6. Percent loss of conductivity (PLC) curves of Shiraz (Sh) andCabernet Sauvignon (Cs). Sigmoidal regressions of PLC against leaf waterpotential (Yl) established according to measurement from the third growthseason (2013). PLC was measured using the HPFM-P-G3 (Dynamax,Houston, TX, USA) according to Lovisolo and Tramontini (2010). Datarepresent means� s.e. of 3–6 plants. Curves are significantly different(P< 0.05).
Xylem architecture of grapevine petiole Functional Plant Biology G
authors created xylem architecture variability in CabernetSauvignon grapevines through the application of high- andlow-irrigation treatments in an early developmental stage. Lowirrigation treatment had 10% lower xylem diameter resulting in25% lower Kts. When uniform irrigation treatments were appliedto all vines in the second part of the season, vines with largervessels reached significantly lowerY, which the authors assumedto origin from differential cavitation occurrences. Nonetheless,other publications reported no correlation between vessel sizeand Y (Shelden 2008; Tramontini et al. 2012), pointing thatthe contribution of other hydraulic mechanisms should not beoverlooked. Even though grapevines were categorised as eithernear iso/anisohydric according to their hydraulic behaviour, it isnot likely that a single characteristic determines this behaviour.The large variability in grapevines hydraulic behaviour wasattributed to a large number of biological mechanisms(reviewed by Chaves et al. (2010) of which the knowledge ontheir interaction, to a large extent, is still missing. Even thoughseveral publications suggested a possible interaction betweenthese factors (Parent et al. 2009; Shatil-Cohen et al. 2011),a holistic unification is still needed for a more deterministichydraulic model.
To conclude, the present study shows the high variability ingrapevines xylem architecture (Schultz 2003) and highlightedits importance in respect to hydraulic conductivity, embolismand water deficit response. The modulation of petiole xylemarchitecture is a fundamental part of a plethora that adds upto many biological mechanisms that have been suggested tocontribute to the large variability between grapevine cultivars(Chaves et al. 2010).
Acknowledgements
Wewould like to thank DrMerav Seifan for her assistance with the statisticalanalysis and Professor Herve Cochard for his comments. The work was donewith the support of the IsraelMinistry of Agriculture, grant no. 857–0614–09,and it was funded in part by Research Grant no. IS-4325–10 from BARD theUnited States – Israel Binational Agricultural Research and Development.
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Xylem architecture of grapevine petiole Functional Plant Biology I
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