Galactans and cellulose in flax fibres: putative contributions to the tensile strength

International Journal of Biological Macromolecules

21 (1997) 179–188

Galactans and cellulose in flax fibres: putative contributions tothe tensile strength

Raynald Girault a, Francois Bert a, Christophe Rihouey a, Alain Jauneau a,Claudine Morvan a,*, Michael Jarvis b

a Uni6ersity of Rouen, Scueor Ura 203 CNRS, 76 821 Mont Saint Aignan cedex, Franceb Chemistry Department, Glasgow Uni6ersity, Glasgow G12 8QQ, UK

Received 21 August 1996; received in revised form 14 January 1997; accepted 10 February 1997

Abstract

The proton spin-spin relaxation time, T2, measured from solid-state NMR, indicates a greater rigidity for cellulosethan for the adhesive matrix between the microfibrils of flax ultimate fibres. Cytochemical and biochemical analysesallow the identification of: (1) EDTA-soluble RG I-polymers in the primary walls and cell junctions of fibres; (2) long1�4-b-D-galactan chains between primary and secondary wall layers; and (3) arabinogalactan-proteins throughoutthe secondary walls. These polymers in the adhesive matrix between microfibrils and/or cellulose layers ensure thatcracks propagate along the matrix rather than across the fibres and play an important role in allowing flax fibres toapproach the tensile strength of advanced synthetic fibres like carbon and Kevlar. © 1997 Elsevier Science B.V.

Keywords: Pectin; Hemicellulose; Solid-state NMR

1. Introduction

Flax fibre cells are several millimeters in lengthand 15–25 mm in diameter, with thick secondarycell walls [1–5]. Unlike cotton, the fibres of flaxcontain only 60–80% cellulose, the remaining ma-terial being rich in water-soluble polysaccharides,proteins and phenolics [6–12].

The native fibres of flax possess mechani-cal properties [13,14] (including both rigidityin tension and high tensile strength) that comevery close to those of some Aramid fibres, buta large part of their tensile strength is lost dur-ing the flax processing cycle [15]. Hence for abetter definition of the quality of flax fibres andlinen after transformation, a better understand-ing of the fibre structure is needed. The aim ofthis paper is concerned with the following ques-tions:

Abbre6iations: APGs, arabinogalactan-proteins; PGA, poly-galacturonic acid; RG I, rhamnogalacturonan of type I.

* Corresponding author. Tel.: +33 35 146751; fax: +33 35705520; e-mail: [email protected]

0141-8130/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved.

PII S 0141 -8130 (97 )00059 -7

R. Girault et al. / International Journal of Biological Macromolecules 21 (1997) 178–188180

1. What factors give flax fibres high tensilestrength?

2. Which water-soluble polymers, and at whichlevel, contribute to the breaking strength offlax fibres?

3. What is the rigidity of flax fibre polymers?

2. Methods

2.1. Plant material

Elementary flax fibres were obtained by me-chanical treatment (carding) of bundles extractedfrom scutched stems that had been field retted.These were a gift of Rubalin S.A. (Bacqueville enCaux, France). For biochemical analyses, fibreswere abundantly washed with chloroform-methanol (1v/1v) in order to solubilize the re-maining cell components and treated with thecalcium chelator EDTA-Na2 (0.5% m/v, 100°,2×1 h) so that the fibres consisted mainly ofsecondary cell-walls.

On the other hand, flax seeds (Linum usitatissi-mum L., var. Ariane, a gift from La CooperativeLiniere, Fontaine Cany, France) were germinatedon moist paper at 2291°C in the dark for 3 days.The average length of the 3-day-old hypocotylswas 9.892.9 mm (mean9S.D., n=100). Flaxseedlings were transferred onto a liquid mediumand cultivated under continuous light (0.032mmol of photon m−2s−1) at 2291°C for 7 days[16]. The average length of the 10-day-oldhypocotyl was then 34.594.2 mm. Fragments (1mm3) in volume were sampled for microscopyanalyses in the middle part of the 10-day-oldhypocotyl. The results were obtained from threeindependent experiments.

2.2. Specimen preparation for microscopy.

Samples were fixed in a mixture consisting of2.5% (m/v) glutaraldehyde in 0.1 M cacodylatebuffer (pH 7.2) for 90 min at room temperature.They were then rinsed in distilled water and post-fixed in an aqueous solution of 1% (m/v) osmiumtetroxide for 1 h in the dark. They were thendehydrated in alcohol series and finally embeddedin Spurr’s epoxy resin.

2.3. b-D-glucosyl Yari6 staining

After fixation, free-hand razor blade sectionswere washed in PBS buffer (8 g l−1 NaCl, 0.2 gl−1 KCl, 1.44 g l−1 Na2HPO4, 0.2 g l−1 KH2PO4,0.132 g l−1 CaCl2-2H2O, 0.1 g l−1 MgCl2-6H2O,adjusted to pH 7.3 with NaOH) and then incu-bated (1 h, 20°C) in, b-D-glucosyl Yariv reagent[17] (2 mg in 1 ml of 1.5% NaCl). After threewashings and overnight incubation in 1.5% NaCl,sections were examined.

2.4. Immunofluorescence labelling

Semi-thin sections (2.5–3.0 mm in thickness)were etched with a NaOH-methanol (6%, m/v)mixture for 16 min and washed several times indistilled water. They were incubated for 30 min inTTBS buffer (0.05 M Tris–HCl, pH 7.5, 0.15 MNaCl, 0.1% (m/v) Tween 20 and 0.5% (m/v)BSA). They were treated overnight with eitherpure JIM 7 [18], anti-PGA/RGI [19], LM5 [20]monoclonal antibodies or anti-gal6 polyclonal an-tibodies diluted 1/5, according to the authors [21],in TTBS with BSA (0.5%, m/v). Sections werewashed in TTBS, treated for 90 min with a sec-ondary antibody coupled to FITC (anti-rat IgGfor Jim 7 and LM5 and anti-rabbit IgG for antiPGA/RGI and anti-gal6, from Sigma). They werethen rinsed thoroughly with distilled water andmounted in Citifluor. To assess the specificity ofthe labelling, primary antibodies were omitted inthe procedure. Observations were made with aZeiss epifluorescence microscope (Axioskop) suit-ably equipped (exciter filter 485 nm, barrier filter515-565 nary). All photographs were obtainedusing black and white film (HP5, 400 ISO, llford)with an acquisition time of 40 s.

2.5. Nuclear magnetic resonance (NMR)spectroscopy

2.5.1. Solid state CP/MAS 13C NMRMeasurements of proton relaxation behaviour

were carried out on a Varian VXR-300 spectrom-eter with Teflon seals on the rotor end-caps toretain water. The effective proton field was cat 40kHz during both CP and data acquisition. The

R. Girault et al. / International Journal of Biological Macromolecules 21 (1997) 179–188 181

Hartmann-Hahn matching condition was adjustedindividually for each sample to allow for selectiveabsorption of proton radiofrequency energy bywater. A recycle time of 1 s allowed essentiallycomplete recovery of magnetisation betweenscans.

The proton spin-spin relaxation time T2 wasmeasured in a CP-MAS experiment modified bythe insertion of a variable delay Tg between theinitial proton 90° pulse and the contact pulse[22,23], Since proton T2 relaxation can be mod-elled as Gaussian for crystalline solids or expo-nential for more mobile materials, the decay ofsignal intensity with Tg was fitted to the equation:

S=S0 exp− (Tg/T2)n (1)

where S is the signal intensity, S0 is the signalintensity at Tg=0, and 1BnB2.

Gaussian behaviour corresponds to n=2 andexponential to n=1. This procedure allowed forrelaxation curves intermediate between these twoextremes.

2.5.2. Solution 13C NMRThe spectrum of the soluble sample was ob-

tained using 5 mm tube at 303 K in D2O (30 mg/0.7 ml) at 75.46 Mhz on a Bruker spectrometer.

2.6. Sugar composition

To estimate the total sugar compositon, includ-ing uronic acids, samples (1 mg) weremethanolysed (24 h at 80°C) and methylsilylated(4°C, overnight) in 1% trimethylchlorosilane inN,O-bis-(trimethylsilylfluoroacetamide) and anal-ysed by gas liquid chromatography on a DB 225capillary column (JW instruments) as previouslydescribed [24].

2.7. Sugar linkages

Samples (1 mg) were solubilized in 1 ml ofDMSO, frozen and methylated by incubationwith 200 mg of NaOH and 1 ml of methyl iodidefor 45 min at 4°C. The reaction was terminated byaddition of 2 ml of Na2S2O3 [25]. Methylatedpolymers were hydrolysed and acetylated accord-ing to the procedure of Albersheim et al. [26],

solubilized in dichloromethane and analysed bygas chromatography on a DB 225 capillarycolumn (JW instruments) as previously described[11].

2.8. High pH anion exchange chromatography(HPAEC)

A fraction of the alkali extract of fibres ob-tained after size exclusion chromatography onSephacryl S200 [11], was analyzed by HPAEC ona CarboPac PA1 column (250×4 mm, Dionex)using standard Dionex hardware and a lineargradient increasing from O to 1 M sodium acetatein 0.1 M sodium hydroxide over 40 min.

3. Results and discussion

3.1. What factors gi6e flax fibres high tensilestrength?

Flax is a composite fibre on two scales. On the1 mm scale, commercial flax fibres are made up ofbundles of long fibre cells (elementary fibres).Their inner or secondary walls are the regions therichest in cellulose, which essentially provides thetensile strength. These are divided by the outer,primary cell-walls and the middle lamella. On thenm scale, the secondary cell-wall is itself a com-posite of strong cellulose microfibrils embedded ina weaker matrix of other polymers. Unlike cotton,flax has its cellulose microfibrils running more orless longitudinally, in line with the axis of the celland the tensile stresses that it resists.

The theory of composite materials [27] cantherefore be useful in explaining the strength offlax and linen. A synthetic composite like carbon-fibre/epoxy is stronger than the fibrous compo-nent alone, because it avoids the problem ofbrittleness that results from flaws concentratingstresses in the rigid fibre material and growinginto major cracks. Thin fibres can have moreperfect crystallinity, i.e., fewer flaws, and if thematrix is weaker than the fibres it deflects anycracks into a longitudinal direction between them.The matrix must however, be strong enough totransmit stresses from one fibre to the next, and


its optimum strength is a compromise betweenstress transmission and crack deflection. Thelonger and thinner the fibres, the more surfacearea is available to transmit stresses and theweaker the matrix can afford to be without thefibres sliding apart.

Undamaged flax fibre cells have a high aspectratio (length/diameter) of about 100, and the as-pect ratio is still higher for cellulose microfibrilswithin their secondary walls. Thus we can expectthe water-soluble, non-cellulosic polymers in thematrix of the secondary wall to play an importantrole in allowing the elementary fibres to approachthe theoretical tensile strength of cellulose, whichat ca. 2 GPa is in the same range as advancedsynthetic fibres like carbon and Kevlar. The mid-dle lamella and primary wall, which are relativelyweak due to lack of longitudinally oriented mi-crofibrils [14], play a corresponding role in con-necting together the strong secondary cell-walls inlinen.

Even though cellulose is the strong componentof flax, therefore, it is important to know aboutthe non-cellulosic polymers since these allow thestrength of the cellulose to be effectively utilised.Specifically, we need information on the distribu-tion of polymers between the secondary wall, theprimary wall and the middle lamella, on themolecular rigidity of each polymer involved andon the way in which its rigidity is affected by, forexample, hydration.

3.2. Which water-soluble polymers, and at whichle6el, can increase the breaking strength of flaxfibres?

Fig. 1 shows indirect immunofluorescence im-ages of transverse sections of hypocotyl of 10day-old seedlings, which have been previouslyshown to develop cellulosic fibres [23]. When us-ing: (A) JIM 7, a monoclonal antibody raisedagainst highly esterified pectins [18]; (B) the poly-clonal anti RG I i PGA [19]; and (C) the poly-clonal anti-gal6 [26] specific to a 1�6-b-D

galactotetraosyl group, labelling is obtainedthroughout the thickness of the developing sec-ondary wall. On the other hand, with the mono-clonal antibody LM5 [27] specific to 1�4-b-D

galactan, two thin lines are visualised (Fig. 1D)adjacent to the primary and secondary cell walls.

Rhamnogalacturonans of type 1 [28] with short1�4-b-D galactan side chains have been previ-ously shown to be abundant in the primary cellwalls as well as in the cell junctions of flax fibres[8,9,11,29,30] and would be good candidates forthe anti RG I/PGA labelling. We have also previ-ously shown [11] that the NaOH extract from thesecondary cell-walls of flax fibres shows very highpercentages of galactose. Table 1 shows that thehigher molecular-mass polymers of this extract(S200-1–S200-3) mainly consist of long 1�3-b-D

galactan chains. There are also lesser amounts oflateral chains of �6-b-D-galactose either on 1�4-b-D or 1�3-b-D galactan chains, which proba-bly account for the labelling with the polyclonalantigal6 antibody. As Gorshkova et al. [12] alsofound in fibre-rich strips peeled from flax stems,these galactans only accounted for a few percentof the total galactan content. Consequently, theywere not further studied. On the contrary, galac-tan chains in low molecular mass polymers (S200-6), that represent the major part of the extract,contain mainly 1�4-b-D chains with few terminalgalactose residues, suggesting longer chains ofgalactan than the side chains of the RG I ex-tracted with EDTA-Na2, prior to NaOH [9,11].From the 13C-NMR spectrum of this fraction andthe assignments, based on those of flax [9] andonion pectins [31], the well resolved signals i.e.,105.8 ppm for the C1 and 79.1 p.p.m. for the C4of of 4-b-galp-(1-\ unit confirm the methylationanalysis (Fig. 2A). In addition, HPAEC-PADprofile (Fig. 2B) indicates that these galactans arerelatively homogenous in size. However, the elu-sion in the inclusion volume of S200 and theabsence of calibration in HPAECPAD did notallow us to determine the molecular mass pre-cisely below 10 000. Besides, the monoclonal anti-body LM5 is very specific to this flax b1-4galactan (Knox, personal communication) whichexplains the high labelling of fibres with LM5antibodies.

Otherwise the level of glucose, without signifi-cant amount of xylose or mannose associatedwith high molecular mass polymers (S200-1), sug-gests the presence of some water-soluble glucans.


Fig. 1. Indirect immunofluorescence images of transverse sections of 10 day old flax seedling labelled with different antibodies. (A)JIM 7, a monoclonal antibody raised against highly esterified pectins [24]. Bar 13 mm; (B) the polyclonal anti RG I/PGA [25]. Bar15 mm; (C) the polyclonal anti-gal6 [26] specific to a b1-6 galactotetraosyl group. Bar 13 mm; (D) the monoclonal antibody LM5[27] specific to b1-4 galactan. Bar 13 mm; F: Fibre, P: cortical parenchyma, X: xylem.

In the case of the low molecular-mass polymers(S200-5), large amounts of mannose together withglucose indicate some glucomannans similar tothose described by McDougall [10].

Fig. 3A shows indirect immunofluorescence im-ages of transverse sections of hypocotyls of 10day-old seedlings, when using the monoclonalMac 207, specific to arabinogalactan-proteins(AGPs), more generally present along the plas-malemma [32]. These results lead us to the conclu-sion that some AGPs are present in the secondary

walls of flax fibres. Indeed, we were able to stainfibres of mature stems of flax with the b-glucosylYariv reagent specific to AGPs (Fig. 3B). Theidentification of 1�3-b-D and 3,6-b-D galactans,as well as proteins in alkali extracts of ultimatefibres are data consistent with such a hypothesis.Also, Gorshkova et al. [12], have indicated thatfibre-rich phloem sheets from flax contain type-llAGs. Considered together, the cytochemical andbiochemical data allow us to point out severalpolymers that might be associated with either


Fig. 2. Characterization of the oligomeric fraction S200-6 obtained after fractionation of NaOH extract of flax fibres on SephacrylS200. (A) 13C NMR spectrum (30 mg/0.7 ml in D20, using 5 mm tube at 303 K); (B) HPAEC-PAD chromatogram on CarboPacPA1 column (250×4 mm, Dionex).


Table 1Composition of fractions obtained after treatment of fibres with NaOH and size exclusion chromatography on Sephacryl S-200

S200-5 S200-6Fraction S200-1 S200-2 S200-3 S200-4

90.90.5Yielda 1.3 3.6 3.2 0.5

Molar % sugar compositionb

— 4.3—GalU —— —2.7 3.8Rha 2.4 4.2 5.6 3.8

5.2 2.3Ara 2.3 2.9 9.4 1.7—— 0.6Xyl 1.9— 1.2

1.417.0Man 2.9 4.7 2.4 2.920.08.3Glc 4.623.7 5.0

76.1 79.8 58.0Gal 88.268.7 82.0

Gal linkages9t-Galp-(1-\

57-\3-Galp-(1-\ 94 9330-\4-Galp-(1-\ 7 30 80

6 6-\6-Galp-(1-\ 4100 64-\3,6-Galp-(1-\ 5

-\4,6-Galp-(1-\ 2 131.9 0.4Proteinc 1.3 0.5 5.00.7

The NaOH extract was obtained as described in Experimental, neutralized with HCI and submitted to size excision chromatographyon Sephacryl S200 as previously described [11]. As no dialysis was run prior to the chromatography, oligomers could be fractionatedand were recovered mainly in fraction S200-6. Otherwise fractions S200-1 to S200-5 were similar to those previously described [11].a The yield was the percentage of the mass recovered after fractionation.b Sugar composition was expressed as molar %, 100% being the sum of acid and neutral sugars.c Proteins were estimated using the Bradford reaction, in mg per g of lyophilised polymer.

microfibrils or fibres: (1) EDTA-soluble RG l-polymers in the primary walls and cell junctionsof fibres [11]; (2) 1�4-b-D galactans betweenprimary and secondary wall layers; and (3) AGPsthroughout the secondary cell walls. Their rigidityhas been estimated as follows:

3.3. Rigidity of polymers within flax fibres

The molecular rigidity of polymers within thecell wall can be estimated by solid-state NMRrelaxation methods [33,34]. The 13C solid-stateNMR spectrum of flax fibres is dominated bycellulose [35], but the presence of small amountsof residual pectin (101 ppm), hemicellulosic galac-tan and/or mannan (82 ppm) and protein (42ppm) can also be inferred (Table 2). The speed ofdecay of proton magnetisation by the spin-spinprocess, measured by the rate constant T2, de-pends on the amount of thermal motion which theprotons undergo and hence on the rigidity of thepolymers in which they are incorporated. Spin-

spin decay of magnetisation in rigid, highly crys-talline polymers such as cellulose follows aGaussian curve with time constant ca. 10 ms,whereas in hydrated primary cell walls T2 decay isexponential and the time constant typically 20–30ms.

The results of a proton T2 experiment on partlyhydrated flax fibres show that the small amountsof pectin, galactans and galactan-protein were lessordered and rigid than the cellulose, but morerigid than the non-cellulosic polysaccharides ofhydrated primary cell walls from other plants(Table 2). The proton T2 values of the pectins(primary wall) and galactans (secondary wall andinterface) appeared to be similar. With primarycell walls, it is normal to find from the proton T2

and other relaxation parameters that hydrationhas little effect on cellulose and hemicelluloses buttransforms the pectic matrix from a brittle glass toan almost fluid gel. The hydrated pectins of theprimary wall in flax fibres were apparently ratherless mobile than this but it may be assumed that


Fig. 3. (A) Indirect immunofluorescence images of transverse sections of 10-day old flax seedling labelled with the monoclonalantibody Mac 207 [32]. Bar 13 mm; (B) Transverse section of mature stem of flax labelled with Yariv antigen [17] Bar 20 mm; F:Fibre, P: cortical parenchyma, X: xylem.

the effect of hydration is qualitatively similar, i.e.,the matrix becomes much less rigid. It may beconcluded that the primary wall/middle lamella

has suitable characteristics to act as a weak ma-trix between secondary walls, that within the sec-ondary wall the galactanproteins or galactans can


act as a weak matrix between cellulose microfi-brils, and that on both of these scales of com-posite architecture the rigidity and brittleness ofthe matrix are likely to be greatly increased bydrying. These conclusions have implications forthe processing and utilisation of flax.

4. Conclusion

Originating from procambial cells in the pro-tophloem, flax fibres have to be: (1) retted inorder to free the bundles from the surroundingpectin-rich sheets of epidermial and cortical cells;(2) scutched in a process that helps to dissociatethe fibres from the woody central cylinder; (3)hackled; and (4) degummed and/or bleached, be-fore being spun. Throughout this processing cycle,the fineness of the technical fibres increases astheir cross-section decreases from that of the bun-dle (of the order of 10 000 mm2) to that of theelementary fibre (between 150 and 500 mm2).

Unfortunately, a large part of the mechanicalproperties of flax fibres, especially the tensilestrength, is lost during these microbiological,chemical and mechanical processes. The two mainreasons are as follows:

1. Retting and chemical treatments solubilize theadhesive matrix of fibres and microfibrils in anuncontrolled way, which, without any mechan-ical treatment, reduces the breaking strengthfrom 5.0 to 1.4 N mm-2 dry fibres [36].

2. Mechanical treatments (i.e., scutching, hack-ling, combing and spinning) increase the fre-quency and the area of imperfections such asfolds, cracks, dislocations that preexist at thesurface of the fibres [15].

This decreases the fibre strength in two ways:by shattering large areas of matrix so that thefibres can slide apart, or, as previously observedby Satta [37] using SEM, by initiating morecracks that can run across fibres when the sur-rounding matrix remains rigid. These modes offracture can be distinguished by the appearance ofthe fracture planes, respectively clean and frayed,using light microscopy if the ‘fibres’ are fibre cellsand EM if they are cellulose microfibrils.

We may conclude that a good knowledge of themorphological structure of flax fibres is required ifas much as possible of the native physico-chemi-cal behaviour of flax fibres is to be transferredinto high-quality linen.

Acknowledgements

This work has been supported by the RegionalCouncil Of Normandy. We thank Prof. P. Ler-ouge, from the University of Rouen, URA 464CNRS for Dionex experiments, and Dr G. Cham-bat from CERMAV-CNRS, France for valuablediscussions during the writing of the manuscript.We also thank EPSRC for NMR spectrometertime and Dr D Apperley for obtaining thespectra.

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Galactans and cellulose in flax fibres: putative contributions to the tensile strength

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