industrial propagation by rooted cuttings of mature selected clones

Mass production by rooted cuttings of mature selected clones of Hevea brasiliensis in SoGBnursery facilities.Photograph A. Masson.

Aurélien MassonJean-Marc JulienLuc Boedt

SoGB, SOCFIN groupestate 01BP365San PedroCôte d’Ivoire

Industrial propagation by rooted cuttings

of mature selected clones of Hevea brasiliensis

B O I S E T F O R Ê T S D E S T R O P I Q U E S , 2 0 1 3 , N ° 3 1 7 ( 3 ) 51PROPAGATION D’HÉVÉA / LE POINT SUR…

RÉSUMÉMULTIPLICATION INDUSTRIELLE PARBOUTURAGE DE CLONES MATURESD’HEVEA BRASILIENSIS

L’importance de l'hévéa (caoutchouc), Heveabrasiliensis, en tant que culture de rente necesse d'augmenter justifiant de s'intéresserà de nouvelles techniques de clonage plusefficaces que le greffage (écussonnage) tradi-tionnellement utilisé pour la productionindustrielle de matériel de plantation de qua-lité supérieure. Les bonnes performances surle terrain (croissance rapide, haut rende-ment) des hévéas produits par embryoge-nèse somatique n'ont été constatées jusqu'àprésent qu'à l'échelle expérimentale. La pro-pagation de masse in vitro par embryoge-nèse somatique ou microbouturage declones d'hévéas sur leurs propres racinesreste pénalisée par un manque de réactivitéde la plupart des génotypes sélectionnés etpar des coûts de production prohibitifs. Faceà cette situation, la propagation par boutu-rage de clones matures sélectionnés issus demicropropagation in vitro a été tentée par laSoGB en Côte d’Ivoire comme une alternativepossible à l’utilisation exclusive des tech-niques in vitro. Les deux clones maturesindustriels, A (70 ans) et B (53 ans), ontd’abord été rajeunis in vitro par embryoge-nèse somatique puis micropropagés en plusgrand nombre par microbouturage. Aprèsacclimatation, les microboutures enracinéesin vitro ont été rempotées dans des pots indi-viduels pour être gérées de manière intensivecomme pieds-mères destinés au bouturage.Après 3 semaines en conditions horticolesadéquates, les taux d’enracinement obtenuspour les boutures des clones A et B ont étérespectivement de 74,6 % (1203/1613) et76,5 % (198/259). Les racines adventicesnéoformées étaient généralement vigou-reuses. A l’issue d’une phase d'acclimatationréussie, les boutures se sont développées defaçon conforme pour atteindre 4 mois plustard une hauteur de 25-30 cm suffisantepour être plantées au champ. En sus d’uneplus grande vigueur et conformité sur le ter-rain, les clones issus de bouturage peuventêtre produits plus rapidement, sur des sur-faces plus réduites à moindre coût et dansdes conditions de travail plus faciles par rap-port aux plantes issus d’écussonnage. Desanalyses en cours devraient permettre d'éta-blir les avantages comparatifs des bouturespar rapport aux plants écussonnés en ce quiconcerne d’autres caractères à fort impactéconomique tels que le rendement de latex.

Mots-clés: Hevea brasiliensis, embryo-genèse somatique, bouture, clones, rajeu-nissement, production industrielle , pied-mère, rentabilité.

ABSTRACTINDUSTRIAL PROPAGATION BY ROOTEDCUTTINGS OF MATURE SELECTED CLONESOF HEVEA BRASILIENSIS

The importance of Hevea brasiliensis (rubbertree) as a cash crop keeps increasing war-ranting the development of new and moreefficient techniques than the bud-graftingtraditionally used for mass producing supe-rior planting material. In vitro production ofrubber trees by somatic embryogenesis andthe good field performance (fast growth,high yield) of the resulting emblings havebeen reported for years, but so far only on anexperimental scale. In vitro mass productionof self-rooted rubber clones by somaticembryogenesis or microcuttings is hinderedby a lack of responsiveness of most of theselected genotypes and by prohibitive pro-duction costs. Given this situation, macropropagation by rooted cuttings following invitro propagation of mature selected cloneswas attempted by the SoGB estate in Côted'Ivoire as a possible alternative to in vitrotechniques only. Two industrial matureclones, clone A (70yrs-old) and clone B(53yrs-old), were first rejuvenated in vitro bysomatic embryogenesis then micropropa-gated in greater numbers by microcuttings.After acclimatization, these in vitro-rootedmicrocuttings were potted in individual con-tainers to be managed intensively as stockplants to produce macro cuttings. After3 weeks under suitable rooting conditions inthe nursery, rooting rates of 74.6%(1203/1613) for clone A and 76.5%(198/259) for clone B macrocuttings wereobtained. The cuttings produced vigorousand taproot-like adventitious roots. Aftersuccessful acclimatization, all the rootedcuttings developed true-to-type and4 months later reached a sufficient height of25-30 cm to be field planted. In addition tohigher field vigor and true-to-typeness, cut-ting-derived rubber tree clones can be pro-duced more quickly, on less nursery space atlower cost and in easier working conditionsthan traditionally bud-grafted plants. Furtherinvestigations are underway for assessing atthe field level the comparative advantages ofrooted cuttings versus grafted clonal off-spring with respect to traits of major eco-nomic importance such as latex yield.

Keywords: Hevea brasiliensis, somaticembryogenesis, rooted cuttings, clone,rejuvenation, mass production, stock plant,cost efficiency.

RESUMENPROPAGACIÓN INDUSTRIAL POR ESTAQUILLAS DE CLONES MADUROSDE HEVEA BRASILIENSIS

La importancia de la hevea (árbol del cau-cho), Hevea brasiliensis, como cultivocomercial no cesa de aumentar, lo que justi-fica el interés de nuevas técnicas más efi-cientes que el injerto (en escudo) tradicio-nalmente empleado para la producciónindustrial de material de cultivo de calidadsuperior. El buen desempeño en el campo(crecimiento rápido y alto rendimiento) delas heveas producidas por embriogénesissomática, llevan años observándose, perohasta ahora sólo a escala experimental. Lapropagación masiva in vitro por embriogéne-sis somática o microestaquillas de clones dehevea de sus propias raíces, está obstaculi-zada por falta de reacción positiva de partede la mayoría de los genotipos selecciona-dos y por costos de producción prohibitivos.Ante esta situación, la propagación de esta-quillas de clones maduros seleccionadosprocediendo de micopropagación in vitro,ha sido llevada a cabo por la compañíaSOGB de Côte d'Ivoire, en tanto que posiblealternativa al uso de técnicas exclusiva-mente in vitro. Los dos clones madurosindustriales, A (70 años de edad) y B(53 años de edad), fueron primero rejuvene-cidos in vitro por embriogénesis somática yluego micropropagados en mayor númeropor microestaquillas. Una vez aclimatadas,las microestaquillas enraízadas in vitro setrasplantaron en macetas individuales parasu manejo intensivo como plantas madresdestinadas a producir estaquillas. Al cabode tres semanas en condiciones adecuadasde cultivo, las tasas de enraizamiento logra-das fueron: 74,6 % (1203/1613) para elclon A y 76,5 % (198 /259) para el B. Las raí-ces adventicias neoformadas resultarongeneralmente vigorosas. Tras el éxito de suaclimatación, todas las estaquillas tuvieronun desarrollo favorable y cuatro meses des-pués alcanzaron una altura de 25-30 centí-metros, lo suficiente para su trasplante encampo. Además de mayor vigor y mejor con-formación en plantación, los clones reprodu-cidos por estaquillas pueden multiplicarseen vivero con mayor rapidez, menor espacio,fáciles condiciones de trabajo y costos infe-riores a lo que requieren las plantas proce-dentes de injerto. Estudios en curso debe-rian poder establecer ventajas comparativasde plantas procedentes de estaquillas frentea las producidas por injerto en lo que serefiere a factores de gran importancia econó-mica tales como el rendimiento en látex.

Palabras clave: Hevea brasiliensis, embrio-génesis somática, estaquillas, clones, reju-venecimiento, producción industrial, plantamadre, rentabilidad.

A. Masson, J.-M. Julien, L. Boedt

52 B O I S E T F O R Ê T S D E S T R O P I Q U E S , 2 0 1 3 , N ° 3 1 7 ( 3 )

FOCUS / RUBBER TREE PROPAGATION

Introduction

Hevea brasiliensis Muëll. Arg. (rubber tree) is a speciesbelonging to the Euphorbiaceae family and originated fromthe South American Amazonian forest. Being the mainsource of natural rubber, it has been domesticated and intro-duced in many countries of Africa and South East Asia to beexploited in large estates since the end of the 19th century.This expansion was warranted by an increasing need fortyres and the development of new ways of transportation.Rubber plantations were originally established with unse-lected seedlings resulting in high heterogeneity and very lowyield (300 to 600 kg of dry rubber (DR) per ha per year). Sincethe 1920s, bud-grafted clones have been used in breedingprograms and have contributed to a dramatic yield increase(2,500 to 3,500 kg of DR/ha/y). The “Wickham” populationexported from Brazil to Asia via Kew Gardens in 1876 hasbeen the first and only gene pool used in H. brasiliensisbreeding, until recent genetic enrichment by other sources(CLEMENT-DEMANGE et al., 2007). Genetic improvementthrough conventional breeding is hampered by a long imma-ture period, an insufficient fruit production and a high levelof heterozygosity. Development of superior H. brasiliensisclones remains a long term and high-investment process:RRIM600 which is currently the most widespread cloneworldwide was obtained by manual pollinations (TJIR1 xPB86) in 1937 at the Rubber Research Institute of Malaysia.

Bud grafting was used for clonal mass propagation ofselected clones as an alternative to propagation by rooted cut-tings. Experiments were conducted in 1952 at the Firestoneestate, Liberia (LEVANDOWSKY, 1959) on thousands ofcuttings collected from bud-grafted clonal stock plants regu-larly rejuvenated by intensive hedging practices. The clonesoriginated from the 1920s breeding programs and were about30 yr-old from seed germination. These cuttings were set forrooting at different dates between 1952 and 1954 and after 75days on average, rooting success varied from 10 to 75%according to the clones. Among the various factors tested i.e.application of exogenous rooting substances, urea sprays, useof various rooting substrates, the genotype had the greaterinfluence on the rooting ability of the cuttings. Despite theseencouraging results, experiments on clonal propagation of rub-ber trees by rooted cuttings stopped prematurely and empha-sis was given to tissue culture for attempting to producingclones on their own roots. Noteworthy success has beenobtained by in vitro somatic embryogenesis but, although welldemonstrated (MONTORO et al., 2012), this technique is stillrestricted to a few genotypes produced in limited numbers.Moreover prohibitive cost remains a major hindrance to itslarge-scale utilization despite a strong and persisting interestfrom the planters. Recent findings have confirmed that indus-trial clones, when produced after in vitro rejuvenation on theirown roots, grow faster and yield more latex than those derivedfrom budding (XIONTING et al., 2001; NAYANAKANTHA &SENEVIRATNE, 2007; DIBI et al., 2010; MONTORO et al., 2012).

This situation, the increasing pressure on availableland and demand for higher and earlier returns haveprompted SOCFIN group to reconsider the possibilities ofmass production of Hevea clones by rooted cuttings.Research was done in SoGB estate facilities, Ivory Coast, andthe method and results constitute the purpose of this paper.

Rejuvenating the mature selectedgenotypes by somatic embryogenesis

Direct or primary somatic embryogenesis (CARRON &ENJALRIC, 1985) was first used for rejuvenating the maturegenotypes: the emblings obtained from the integuments ofthe fruits have the same genotype as the mother tree(grafted selected clone) and are by definition ontogeneticallyjuvenile (LARDET et al., 2009; MONTEUUIS et al., 2011).

Immature fruits were harvested from 4 yrs-old bud-graftedtrees of the industrial mature clones A and B – respectively 70and 53 yrs-old since seed germination – at SoGB estate in2008, then sent to the Laboratory for Applied In Vitro PlantBiotechnology of University Ghent in Belgium. Applying theCARRON and ENJALRIC (1985) somatic embryogenesis protocol,153 emblings were obtained for clone A and 91 for clone B.These emblings were further micropropagated by axillary bud-ding to produce microcuttings. These were rooted in vitro withaverage success rates of 63.3% (752/1,188) for clone A and64.5% (178/275) for clone B, then shipped to SoGB estatefacilities for acclimatization to ex-vitro conditions in 2012.

Getting responsive stock plants

After a 3 day-long airfreight shipment, the in vitro rootedmicrocuttings arrived in SoGB nursery facilities in GrandBereby, Ivory Coast, and were immediately potted individuallyin 200 cm3 (50mm wide x 150 mm high) containers filled withcoconut peat to be acclimatized during 6 weeks in a green-house equipped with a shade and mist-system giving nearly95% relative humidity (RH) and 75% shade. These conditionswere progressively reduced to reach ambient humidity and50% shade at the end of the acclimatization process (photo-graph 1). After acclimatization, 77% (581/752) of the micro-cuttings of clone A and 78% (139/178) of clone B survived

Photograph 1.Acclimatization of the in vitro plantlets in SoGB nursery facilities.Photograph A. Masson.


Once acclimatized, the rooted microcuttings were hard-ened outdoors under a suitable watering regime and sprayedwith appropriate foliar fertilization and phytosanitary treat-ments. They were initially protected from direct sun exposureby a 50% shade net that was removed after 1 month. Plantswere ready to be suitably managed as stock plants as soon asthe bark at the base of the trunk was lignified (photograph 2),roughly 6 months after the acclimatization process started.

For stock plant establishment, 500 microcutting-derived plants of clone A and 100 of clone B were individu-ally potted in 3000 cm3 containers (156 mm wide x 200 mmhigh) filled with coconut fiber and placed in lines on benchesfor easy maintenance, sanitation treatments and harvestingof macro cuttings. Spacing of the stock plants was 30 cm x30 cm giving 10 plants per m². The objective was to producethe more shoots that can be rooted per stock plant. Theseconsisted of newly produced soft shoots with 2 to 4 fullydeveloped leaves and a resting terminal bud in their upperpart, as illustrated in photograph 3. Fifteen shoots of thistype to be used as cuttings on average could be producedmonthly per m² of stock plants. These shoots were harvestedevery 6 days, during the 4 months from February to May2013 so that the rooted cuttings could be planted from Juneto September, at the beginning of the rainy season (from endof May to October with average rainfall of 1 150 mm for thisperiod vs1 600 mm per annum on the plantation site).

Rooting and development of cuttings

Immediately after collection from the stock plants, theleafy shoots were trimmed to 15 to 20 cm long terminal cut-tings with half of the leaf surface cut in order to lower eva -potranspiration (photograph 3). The cuttings were theninserted into 200 cm3 (50mm wide x 150 mm high) contain-ers filled with coconut peat and maintained under the samegreenhouse conditions as detailed previously for theacclimatization of the microcuttings. The first adventitiousroots appeared 15 days later and after 15 more days thecuttings were hardened off in the same conditions asdescribed above for the microcuttings.

Of the cuttings initially set for clone A, 74.6% (1 203/1613) and 76.5% (198/259) for clone B were successfullyrooted and acclimatized applying these protocols. Theadventitious roots formed were vigorous and showed astrong positive geotropism (photograph 4). After a2.5 month-long raising period under standard nursery con-ditions, the rooted cuttings resumed new growth, develop-ing new leaves to reach a sufficient height of 25 to 30cm tobe field planted (photograph 5).

Discussion

For the present, this work is the first demonstrationthat industrial clones of Hevea brasiliensis can be masspropagated by rooted cuttings with success rates exceeding70%. This can be considered as a real breakthrough for thishighly economically important species.

Until now, bud grafting has been the only means ofmass clonally propagating mature selected rubber treegenotypes. For rubber, as with other tree species, grafting isliable to induce graft incompatibility symptoms such as:within clone variability, reduction of vigor and of latex pro-duction, anticipated mortality and bark necrosis syndrome(HARTMANN et al., 1997; CHRESTIN et al., 2004). The failureof the few experiments conducted during the 1950’s on thepropagation of industrial mature clones by rooted cuttings(NAYANAKANTHA & SENEVIRATNE, 2007) was very likelydue, similarly to other tree species, to the inhibitory effect ofageing on adventitious rooting ability (BORCHERT, 1976;HACKETT, 1985). This was supported by the observation thatrubber tree cuttings root more easily when collected fromjuvenile than from mature donor plants (MUZIC, 1958;OLIVER & MONTEUUIS, 2009; MONTEUUIS, personal com-munication). The determining factor of our success was thecapacity first, to physiologically rejuvenate the matureselected genotypes by somatic embryogenesis. This wasmade possible by Hevea brasiliens is being one of the veryfew trees species for which somatic embryos can beobtained from maternal tissues of mature genotypes.However, direct somatic embryogenesis was preferred tolonger protocols in order to reduce the risks of somaclonalvariations, or even mutations. These can be increased byprolonged exposure to non-optimal culture media, which

Photograph 2.Acclimatized microcutting-derived plant of clone A ready to be managed as stockplant. Photograph A. Masson.



may also induce maturation symptoms (DUNSTAN et al.,1995; VON ADERKAS & BONGA, 2000; MONTORO et al.,2010). The possibility of further amplifying by micropropa-gation the limited number of emblings obtained per geno-type resulted in greater numbers of in vitro rejuvenatedplants produced which could then be used as stock plants.This is a real asset as the ability of Hevea brasiliensis to bein vitro propagated by somatic embryogenesis and micro-propagation is strongly genotype dependent (NAYANAKAN-THA & SENEVIRATNE, 2007). Such genotype-related effi-ciency of the in vitro methods and the constraintsassociated with the operational use of these methods them-selves should not be underestimated. In this regard, thepossibility to physiologically rejuvenate mature selectedgenotypes to produce stock plants with a high rootingcapacity without resorting to tissue culture needs to be seri-ously investigated. An encouraging indication is that thecuttings rooted in nursery conditions could be used as newstock plants with similar responsiveness as the in vitro onesfrom which they were derived. Mass clonal propagation byrooted cuttings of mature selected genotypes of teak and ofsome eucalypt species and hybrids has been proven to besuccessful using nursery techniques only, even for physio-logically rejuvenating the mature material, which remainsthe fundamental requisite (MONTEUUIS et al., 1995; SAYA etal., 2008). Rooting rates of more than 75% for cuttings frommature Hevea brasiliensis genotypes were unexpected

based on previous experiments and must therefore be con-sidered as very encouraging. This demonstrates the impactof suitable techniques – and not necessarily the mostexpensive and sophisticated ones – on successful true-to-type propagation by rooted cuttings of a plant material con-sidered till then as strongly recalcitrant. This high respon-siveness for adventitious rooting allowed for rooting thecuttings directly in the containers saving manipulations androoting substrate resulting in higher cost efficiency.

Mass production of rooted cuttings in nursery condi-tions required more equipment, with special mention of areliable and automatic mist system, than for grafting. Butthe higher cost of the facilities needed is offset by numerousadvantages associated with the use of rooted cuttings com-pared with grafted plants, as described below.

Photograph 3.Suitable shoot to be used as a cutting.Photograph A. Masson.

Photograph 4.Cutting rooted after 1 month.Photograph A. Masson.


The propagation process is much shorter

Industrial clones can be produced by rooted cuttingsfor field planting within 4 months vs 10 months for bud-grafts: 7 months are needed to get the sufficiently deve -loped rootstocks from seed, then 3 more months for thegrafted plants to be ready for field planting.

More plants can be produced on a much smaller nursery area

Projections in our site conditions based on an activityperiod of 4 months indicate a requirement of 100-150 m2 forstock plants and 50 m2 for rooting, acclimatization and nurs-ery raising to produce enough cuttings to plant 10 ha at adensity of 550 trees/ha (ratio 1:1,000) vs 1,200 to 1,400 m2

of budwood garden and a nursery raising area of 800 m2,i.e. 8 times more for planting the same area with graftedplants (that are raised in bigger 50 x 15 cm containers).

The differences between the two systems are likely tobe more important under heavier rainfall in absence of dryseason as rooted cuttings can be produced and fieldplanted year round.

Cuttings-derived plants require less manpower than bud-grafted ones

Two rounds of grafting are required to get similar prop-agation success rates (74-75%) as by rooted cuttings. Alsocuttings rooted directly in small and reusable containersplaced in racks of 60 units are definitely easier to handlethan grafted plants, as illustrated in photograph 6. Despitethe relatively small volume of medium in the rooted cuttingscontainers, growth in the field is rapid (photograph7).Grafted plants are produced in much bigger (60 x 15 cm)and heavier plastic bags, requiring larger planting holes.These plastic bags cannot be re-used after planting in con-trast with the containers used for the cuttings.

Photograph 5.Four-month old rooted cutting of clone A ready to be field planted.Photograph A. Masson.

Photograph 6.Container-grown rooted cuttings are more convenient to handle than grafted plants in poly bags.Photograph A. Masson.



Cuttings-derived Hevea brasiliensis clones are expected to be more vigorous and produce more latex

than the same bud-grafted clones

XIONTING et al. (2001) reported superiority in averagestem girth and dry rubber yield of 9.1 to 35.2 % and of 29.9to 46.3 % respectively from microcutting-derived clones com-pared to the same clones produced by bud-grafting. Morespecifically, RRIM600 rooted microcuttings trees had an aver-age trunk volume 14.36 % bigger and produced 32.5 % moredry rubber than bud-grafted trees of the same clone at thesame age. These results show that self-rooted juvenile clonesi.e microcuttings and cuttings-derived clones, are consideredto be superior in growth and production.

Further investigations on mass propagation by rootedcuttings of H. brasiliensis industrial clones must be pur-sued, focusing more particularly on the following issues: ▪ The applicability of the protocols developed to a widerrange of clones and of environments, especially underhigher levels of precipitation and in areas with no distinctdry season.▪ The possibility to succeed using nursery techniques exclu-sively, even for physiologically rejuvenating the matureclones without resorting to in vitro methods. ▪ Advanced field observations of the rooted-cuttings-derivedclones compared with the same clones traditionally pro-duced by bud-grafting, with special consideration for eco-nomically important traits such as vigor, within clone varia-tion and latex yield.

AknowledgementsAuthors are very grateful to Dr Olivier Monteuuis (CIRAD) forhis help in writing this paper.Authors would like to thank Prof. Stefaan Werbrouck andJoris De Wilde (University of Ghent, laboratory for applied invitro plant biotechnologies) and SoGB staff for their opera-tional support, particularly MM. Olivier Martin (GeneralManager) and N’Da Kouame Gilbert (assistant).

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Photograph 7.Rooted cutting of clone A one month after planting at SoGB estate.Photograph A. Masson.


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industrial propagation by rooted cuttings of mature selected clones

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