Leakey, R.R.B.

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   Chapter (non-refereed)    Leakey, R.R.B.. 1981 Physiological approaches to the conservation and improvement of Triplochiton scleroxylon - a West African timber tree. In: Last, F.T.; Gardiner, A.S., (eds.) Forest and woodland ecology: an account of research being done in ITE. Cambridge, NERC/Institute of Terrestrial Ecology, 105-109. (ITE Symposium, 8).    Copyright © 1981 NERC  This version available at http://nora.nerc.ac.uk/7058/   NERC has developed NORA to enable users to access research outputs wholly or partially funded by NERC. Copyright and other rights for material on this site are retained by the authors and/or other rights owners. Users should read the terms and conditions of use of this material at http://nora.nerc.ac.uk/policies.html#access    This document is extracted from the publisher’s version of the volume. If you wish to cite this item please use the reference above or cite the NORA entry                          Contact CEH NORA team at  nora@ceh.ac.uk 24. PHYSIOLOGICAL APPROACHES TO THECONSERVATION AND IMPROVEMENT OFTRIPLOCHITON SCLEROXYLON—A  WESTAFRICAN TIMBER TREER.R.B. LEAKEYTriplochiton scleroxylon  K. Schum. is an importanttimber tree of West Africa (Plate 16), whosegenetic resources have been severely depleted bythe clearance of high forest for: (i) usable timber,(ii) the needs of shifting agriculture, and (iii)the construction of roads, towns, etc. Althougha pioneer species, natural regeneration of  T.scleroxylon  is spasmodic because in most years fewseeds are produced. Even when flowering isprolific, many fruits are destroyed by pests andpathogens, eg  Apion  and  Mycosyrinx  spp (Jones,1975b). In addition, seeds remain viable for only afew months (Bowen & Jones, 1975), so restrictingthe possibilities of commercial plantings.40c.)'6 20(A)80(B)In 1970, the UK Ministry of Overseas Developmentestablished 2 projects on  T. scleroxylon,  jointlyaimed at the conservation and improvement ofthis species. One is based at the Forestry ResearchInstitute of Nigeria (F RIN) and the other atITE Edinburgh. The former is primarily concernedwith (i) the natural distribution and phenologyof  T. scleroxylon  in West Africa (Jones, 1975a),(ii) the collection of seed and establishmentof a g'ene bank, (iii) fruit maturation and factorsaffecting seed storage (Jones, 1976; Bowen &Jones, 1975; Howland & Bowen, 1977), (iv)techniques for vegetative propagation (Howland,1. Vegetative propagation1051975a,  b, c)  to provide alternative planting stockand (v) field growth of clonal plantations (Howlandet al.,  1977). While colleagues in Nigeria have beenfocussing on these more immediately practicalaspects, the ITE team has investigated the funda-mentals of some of the physiological mechanismscontrolling (i) root initiation in cuttings, (ii) apicaldominance and the control of branching, which isof importance to the maintenance of stockplantsand the early selection of superior clones, and(iii) flower initiation and seed production, vital toa breeding programme. In this way it is hoped thatthe overall programme, pure and applied, is properlybalanced and may be regarded as a model for theconservation and improvement of other potentiallyimportant tropical tree species.Having evolved conditions favouring the rooting ofT.  scleroxylon  cuttings in tropicalised glasshousesin Edinburgh (Leakey  et al.,  1975) 2 aspectshave been studied in detail:(C)Control 8  40 200  Control 8  40 200 Control 8 40 200Log Auxin (NAA+ IBA) Concentration (pg/cutting)Fig. 48  Effects of different concentrations of a 50:50 mixture of NAA and IBA (auxins) on the rooting ofleafy single-node cuttings of  T. scleroxylon.(A) An easily propagated clone, relatively insensitive to different auxin concentrations(B) Clone whose rooting is considerably enhanced by a wide range of auxin concentrations(C) Two different clones  (40 and 0) requiring specific auxin applications with different optimumconcentrations.1.1 Inherent variation in relation to rootingability and responses to rooting hormonesHowland (1975b) found that applications of auxins(NAA—naphthalene acetic acid: IBA, indole-3-butyric acid) were usually unnecessary forrooting. Subsequently, however, it has becomeapparent that rooting ability varies widely withinT. scleroxylon.  Some clones root well with orwithout auxins while within a group of poorrooters the responses to auxins may be (a) virtuallyindependent of their concentrations or (b) concen-tration specific (Figure 48). Clones such as 8021106are more responsive to IBA whereas others (eg 8036)respond similarly to IBA and NAA. For practicalpurposes, the recommended application for T.scleroxylon cuttings is now a 50:50 mixture ofNAA & IBA at 40 pg/cutting, or a 'quick-dip' in0.4% alcoholic solution. This application enhancesrooting and increases the number of roots percutting.\ Leaf retainedX BudRemovedBud & Leafe Removed\ LateralShoot(a)Nodes 3-4,9-10. Nodes 3-4,9-10. Nodes 3-4,9-10. Nodes 9-10. Nodes 3-4.CLONE 67 27 * 50 89  if r CL 0 N E 81 868038 8049% cuttings rooted (week 10) % cuttings rooted (week 8)Fig. 49 Effects of node position and competition between shoots on percentage rooting of leafy single nodecuttings of Triplochiton scleroxylon. *Lateral shoots removed as part of treatment and thereforenot available for later rooting1.2 Identification of some stockplant effects on illuminated, rooting percentages of cuttings fromrooting apical and basal shoots were similar (Figure 49b).Cuttings taken from different parts of the samestockplant have different rooting abilities, theoverall percentage rooting also reflecting thephysiological condition of the stockplants. Theseeffects may be ephemeral or persistent, the latterbeing generally attributed to 'phase-change',the change from juvenility to maturity, whichprobably includes many physiological processes.Experiments with T. scleroxylon have tested theeffects of stockplant orientation, soil nutrientstatus, stockplant size, numbers of shoots/plants,position of shoots on mainstems and the removalof stem apices from stockplants. In all these experi-ments, cuttings from basal shoots were always theeasiest to root. Nonetheless, the degree of successwas dependent upon the supply of soil nutrientsand the degree of competition fora gravity sensitivefactor, originating in stockplant roots, whichstimulates root initiation in cuttings. The rootingsuccess of cuttings was inversely proportional tothe number of shoots per stockplant. Experi-ments which limited growth to particular lateralshoots of decapitated stockplants support thehypothesis that competition for a root factoraffects their subsequent rooting ability (F igure 49a).Furthermore shoots only 5-6 nodes apart hadmarkedly different rooting abilities even when thelevel of inter-shoot competition was constant(Figure 49a). This result, however, can probably beattributed to an effect of light, for, when equally(b)TABLE 33 Effects of different day lengths on the growthof vegetatively propagated plants of Cordiaalliodora of mainstem and lateral shoot origin,after 5 weeks.Origin of cuttingsPlant height (mm) Leaf area (cm2)All these factors are therefore contributing toshort-term carry-over or 'c' effects unrelated topersistent phase-change, but of considerableimportance to the development of stockplantmanagement systems aimed at the production ofuniform planting stock. The degree of undesirabletraits such as plagiotropism (Plate 17) and reducedvigour (Table 33) incorporated in cuttings esta-blished from badly-managed stockplants requiresfurther investigation.100820Seed Lot  A2  4 6 0 2 4 6Weeks After DecapitationFig. 50  Genetic variation in bud growth of  T. scleroxylon  seedlings following decapitation at n9de 10,4 months from germination (3 seedlings per seed lot A, 0 and 0; A, • and •).2. Clonal selectionFOrest plantings of clones derived from seedcollected from the entire natural range of  T.scleroxylon,  from Zaire in the east to Sierra Leonein the west, have been made by colleagues inNigeria. Meanwhile attempts have been made inEdinburgh to develop a rapid test for predictingbranching habit of mature trees, the test beingbased on the way in which lateral shoots developafter the removal of the dominating mainstemapex. In the event it was found that the responsesof seedlings from the same mother tree differedgreatly—in some, apical dominance was quicklyre-established with relatively few buds becomingactive whereas in others with weak apical dominancemany buds flushed, at least temporarily (Figure50). These differences were subsequently shown tobe inherent.Apical dominance is affected by many environ-mental and physiological variables which need tobe standardized for predictive tests. These include:2.1 Numbers of leaves per plant and theirpositionsFewer lateral shoots extended on undefoliatedplants than on those whose leaves were eitherpartially or fully removed (Table 34);TABLE 34 Effects of partial and complete defoliation onthe activity (%) of buds remaining when the2 uppermost nodes were removed from  T.scleroxylon  plants.Seed Lot B1072.2 The amount of stem removed when decapita-tingWhen different amounts of stem were cut from auniform batch of trees, the percentage of buds108r14 401914080602000(a) Plants of uniform  height  cut  to different  numbers  of nodes.(b) Terminal bud and uppermost node removed from plantsof different sizes.22 4 6 8 10Weeks4 6 8 10WeeksFig.  51 Effects of decapitation at different heights on bud activity in T. sc lerox y I on.No. OfNodesRemainingAfterDecapitation102022$No. OfNodesRemainingAfterDecapitation91622which became active did not vary appreciably(Figure 51a). On the other hand when plants ofdifferent heights were cut to the same level thepercentages of active buds were more variable(Figure 51b).2.3 Nutrient statusAlthough other species can be released fromapical dominance by the application of fertilisers(McIntyre, 1977), effects on bud activity ofdecapitated plants of  T. scleroxylon  have not beenclearly demonstrated. Numbers of lateral budsflushing after the removal of apical meristemsseemed to be independent of nutrient status butsubsequently dominance was asserted sooner inplants with fewer nutrients.It should soon be possible to calibrate results fromsuch predictive tests with clones planted in Nigeria.3. Flowering and flower inductionIn nature,  T. scleroxylon  rarely flowers before it is15-20 years old, When, because of its size, controlledpollination is already almost impossible. Grafts andcuttings from mature trees have flowered in theforest nursery in Nigeria, and controlled pollina-tions resulted in the production of viable seed.Cuttings from mature trees have flowered andfruited in tropicalised glasshouses in Edinburgh,but more importantly 12 young trees, of 2-3years old, have also flowered. Seeds from self-pollinations did not germinate, whereas those fromcontrolled cross-pollinations did. Stocks of theseclones with a propensity to flower are beingincreased so as to make a thorough examination offlower induction, recognising that the techniquesused on temperate trees (Longman, 1975) may beinappropriate.4. ConclusionTo date, many aspects of vegetative propagationand some of flower induction have been investi-gated for  T. scleroxylon.  There is now some hopethat this species will be sustained and that suppliesof its timber 'Obeche' will be safeguarded. Alreadyextensive experimental plantings have been made.Improvement by the combination of sexual andasexual propagation (Longman, 1976; Bowen  et al.,1977; Longman  et al.,  1979) requires that the keyto flower induction in young trees be foundyielding progeny which would contribute bothto the gene bank and to commercial forestry.Such an approach could also benefit many other'endangered' tropical tree species.References109Bowen, M.R., Howland, P., Last, F.T., Leakey, R.R.B.& Longman, K.A. 1977.  Triplochiton scleroxylon:  itsconservation and future improvement.  For. Genet. Resourc.Int, no. 6,  38-47.Bowen, M.R. & Jones, N. 1975. Preliminary results on thegermination, drying and storage of seed and fruits ofTriplochiton scleroxylon  (K. Schum.).  Proc. Symp. Variationand Breeding Systems of  Triplochiton scleroxylon  K.Schum., lbadan, Nigeria,  74-81.Howland, P.  1975a. Vegetative propagation methods forTriplochiton scleroxylon  K. Schum.  Proc. Symp. Varia-tion and Breeding Systems of  Triplochiton scleroxylon  (K.Schum.), lbadan, Nigeria,  99-109.Howland, P.  19756. Variation in rooting of stem cuttings ofTriplochiton scleroxylon  K. Schum.  Proc. Symp. Variationand Breeding Systems of  Triplochiton scleroxylon  (K.Schum.), lbadan, Nigeria,  110-124.Howland, P.  1975c. Current management techniques forraising  Triplochiton scleroxylon  K. Schum. from stemcuttings.  Proc. Symp. Variation and Breeding Systems ofTriplochiton scleroxylon  (K. Schum.), lbadan, Nigeria,125-129.Howland, P.  &  Bowen, M.R.  1977.  West African hardwoodsimprovement project research report 1971-1977.  Triplo-chiton scleroxylon  K. Schum. and other West Africantropical hardwoods.  Lagos: Federal Department of Forestry.Howland, P., Bowen, M.R., Ladipo, D.O.  &  Oke, J.B.  1977.The study of clonal variation in  Triplochiton scleroxylon  K.Schum. as a basis for selection and improvement.  IUFROWorkshop, Working Parties S2 - 02 - 8 and S2 - 03 - 1.Brisbane, 1977.Jones, N. 1975a. The distribution of  Triplochiton sclero-xylon  K. Schum. with special reference to Nigeria,  Proc.Symp. Variation and Breeding Systems of  Triplochitonscleroxylon  (K. Schum.), lbadan, Nigeria,  8-18.Jones, N. 1975b. Observations on  Triplochiton scleroxylonK. Schum. flower and fruit development.  Proc. Symp.Variation and Breeding Systems of  Triplochiton scleroxy Ion(K. Schum.), lbadan, Nigeria,  28-37.Jones, N. 1976. Some biological factors influencing seedsetting in  Triplochiton scleroxylon  K. Schum. In:  Tropicaltrees: variation, breeding and conservation,  edited byJ. Burley and B.T. Styles, 125-134. London: AcademicPress.Leakey, R.R.B., Chapman, V.R. & Longman, K.A. 1975.Studies on root initiation and bud outgrowth in nine clonesof  Triplochiton scleroxylon  K. Schum.,  Proc. Symp.Variation and Breeding Systems of  Triplochiton scleroxylon(K. Schum.), lbadan, Nigeria,  86-92.Longman,  K.A. 1975. Tree biology research and plantpropagation.  Proc. Int. Plant Propagators Society, 25,219-236.Longman,  K.A. 1976. Conservation and utilization ofgene resources by vegetative multiplication of tropicaltrees. In:  Tropical trees: variation, breeding and conser-vation,  edited by J. Burley and B.T. Styles, 19-24. London:Academic Press.Longman,  K.A., Leakey, R.R.B., Howland, P. & Bowen,M.R.  1979. Physiological approaches for utilizing andconserving the genetic resources of tropical trees.  Proc.World Consultation on Forest Tree Breeding, 3rd, Canberra,1977, vol 2, 1043-1054.McIntyre, G.I. 1977. The role of nutrition in apical domi-nance. In:  Integration of activity in the higher plant,edited by D.H. Jennings, 251-273. Cambridge: CambridgeUniversity Press.

Physiological approaches to the conservation and improvement of Triplochiton scleroxylon - a West African timber tree

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Physiological approaches to the conservation and improvement of Triplochiton scleroxylon - a West African timber tree

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