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M O D U L E 4 – A G R O F O R E S T R Y T R E E M U L T I P L I C A T I O N

Vegetative propagationHannah Jaenicke

IntroductionThe first part of this contribution lists reasons for using vegetative propagation approaches in tree domestication and discusses some general aspects of clonal propagation. The second part is dedicated to the description of vegetative propagation techniques, and starts with an outline of the physiological aspects involved in root and shoot regeneration. It ends with a brief list of possible propagation experiments. An appendix with short summaries of the main techniques is included.

Part IReasons for vegetative propagation

Vegetative propagation of plants is their reproduction from vegetative organs: stems, roots, leaves, buds, even single cells. Through vegetative propagation or cloning, exact copies of the ‘mother plant’ are produced, a process that can – in theory – be repeated indefinitely. The phenomenon of vegetative propagation is based on the ‘omnipotence’ of plant cells, meaning that any plant cell, unlike most animal cells, has the potential to regenerate into a functioning organism1. Vegetative propagation, as opposed to sexual propagation (by seed), offers a range of benefits in tree domestication as well as in conservation efforts (compare also with information provided in Module 3).

(1)The most important benefit is the capturing of genetic diversity. As sexual reproduction results in a constant recombination of genes, large genetic diversity has developed in wild tree populations which have not undergone any systematic improvement effort. By vegetative propagation, this diversity, or rather: particularly interesting traits, can be

1 Recent advances in modern biotechnology have now led to the possibilities of cloning animals as well, though only under laboratory conditions and in exceptional cases.

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‘captured’ and used both for the improvement of the species and for the conservation of important traits. Vegetative propagation methods are often used to speed up the process of domestication.

(2)Farmers experience often that when they plant the seed of a superior plant, for example a fruit tree with especially tasty fruits, the seedling does not perform up to their expectations. This is because the mother tree presents only half the genetic make-up of the seeds in its fruits, and the pollen has come from an unknown source. Thus the seeds have a different gene combination than the mother tree. The ability to produce tasty fruits was not inherited. In such a case of an ‘outbreeding’ species, vegetative propagation can help to provide an exact copy of the mother tree from which the seed was harvested and therefore retain the desired fruit quality in the following generation.

(3)Many tree species are dioecious, that is, there are separate male and female trees. Farmers are usually more interested in the female, fruit- or seed-bearing individuals and need only a few male trees around it for pollination. It is difficult if not impossible to predict in small seedlings whether they are female or male. In these cases, the vegetative propagation of the female trees could help increase the number of fruit-bearing trees on a farm.

(4)Another good opportunity for vegetative propagation is when the seeds are recalcitrant and/or only a low germination rate is achievable. Also the number of seeds produced may be low in some cases when flowering and fruiting occurs not every year or when fruit set is poor.

(5)A similar reason is that of parthenocarpy. Some species can produce fruits that contain no seeds (e.g., banana, fig). Whereas this is an advantage for human consumption, these species have to be propagated through vegetative means.

(6)In some cases, the genetic base of a species is so severely threatened, that only very few, if any, seed producing trees are available. We know the examples of several tree species in Madagascar of which only few individuals survive. A particularly important species is Prunus africana, which is harvested for its valuable bark that has medicinal properties against prostate disorders. The trees are felled to strip the bark off. Most of the trees now available are juvenile and they will be felled before they are able to produce seeds. No natural regeneration is possible.


(7)Many tree species reach their reproductive stage very late. By grafting or layering it is possible to short-cut this process and reach maturity within a few years. This allows farmers to market fruits earlier and realise an income more rapidly. These methods are also used to establish clonal seed stands.

(8)Vegetative propagation also has merit when low genetic ‘noise’ is required for experimentation that would otherwise be obstructed when using seedling material.

Dangers of vegetative propagation

Clonal propagation offers thrilling opportunities for research and development. Harvesting and using the existing diversity might help to improve the diversity of cultivated land as well as health and income of the rural people. However, a word of caution needs to be said. We know that any particular genotype has a very limited adaptation to its environment. It is well adapted only to the environment it developed in or ‘was made for’. Therefore, monoclonal plantations have no chance to respond to sudden environmental changes, like insect attacks or drought.

The use of clonal plant material poses a high direct economic risk to the farmers if they rely solely on one variety. A vivid example of the dangers of large-scale vegetative propagation is the increasing pest and disease infestation of bananas in small-holder farms in East Africa threatening their existence. As banana is a parthenocarpic fruit and no seeds are produced, genetic improvement needs to rely on the few remaining wild relatives of banana to introduce new genes into the genepool. The alternatives are high inputs in pesticides, as done by the commercial companies.

It is recognized that there is need for information within the communities about the potential deterioration of improved clonal material within a few generations when open pollinated. The options for continuous high performance are to continually buy new improved material (for example, as with hybrid maize) or to take on vegetative propagation to maintain the good germplasm that was released. Here, however, lies a big danger: in a clonal mixture, some clones will be more vigorous and some easier to propagate than others - within a possibly short period of time a clonal mixture could have deteriorated to a few clones if not carefully composed.

Thought needs to be put into a strategy of release of clonally produced plant material. It is of eminent importance for a clonal improvement programme to concentrate not only on the development of one clone but to develop several clones of similar phenotype but different genotype. The more specific or rare a certain trait is, the more difficult this will be to attain. Leakey and Simons (1999) discuss


the strategy to continuously release mixtures of improved or superior clones and to back up clonal propagation activities with breeding and selection of new germplasm. This strategy certainly will help to reduce the danger of monoclonal cultures.

Another, frequently ignored, problem is the potential spread of diseased material. It is very easy to inadvertently take cuttings from diseased plants, or to graft diseased material onto a healthy rootstock. As these plants are then planted elsewhere, potentially very dangerous virus diseases can spread over large distances. A common example is the suite of citrus diseases (for example, greening, seedling yellow, citrus chlorosis or tristeza) caused by viruses and bacteria, which are commonly transferred by both grafting tools and biological vectors, such as sucking insects. In warm climates, the trees can often outgrow the disease and appear healthy, however, when planted in harsher environments, the diseases show and the productivity of the trees is much reduced.

Other potential problems can be a deformed root system, when a cutting is unable to develop a strong taproot, and also the fact that the desired quality traits are often not linked with pest and disease resistance, hence many ‘superior’ producers perform poorly when attacked by pests or diseases.

Naming and certification of cultivars

Clonal selections represent cultivars which can be registered with the relevant national bodies. A very important first step is to select an appropriate and catchy name for the new cultivar. Care needs to be taken to give it an exciting and positive connotation, something that makes people want to grow that particular cultivar. Good examples of registered crop or ornamental cultivars are ‘Morning Glory’, ‘Sunburst’, ‘White Wonder’, ‘Money Maker’ etc. Local names or words are preferred to allow identification of the local farmers with the cultivar.

Registration of new cultivars has to follow the requirements of the relevant national body. The registration process can be lengthy and may require legal assistance. It is important to point out that registration of a cultivar means its protection against piracy and as such a protection of the intellectual property of the local communities which have contributed to the development of such a new cultivar. ICRAF’s policy on Intellectual Property Rights (ICRAF, 1997) states clearly that as a research body receiving public funding, the institution will only seek intellectual property protection for clones if necessary to ensure effective delivery to farmers.


Part IIPhysiology of vegetative propagation

insert here figure 1 - picture of stem transect

The basis of vegetative propagation is the fact that plant cells can regenerate and differentiate into functional tissue. During the life of the plant, the cambium constantly produces cells that differentiate into the phloem towards the outside, and the xylem towards the inside of the trunk. At wounding, an undifferentiated cell mass, or callus, is produced, primarily to seal and heal the wound. When particular hormone concentrations are present, these cells can also differentiate into functioning root and shoot tissue.

During the life of a plant, it develops from a juvenile, vigorous seedling to a mature, flower and fruit bearing, less vigorous adult. This ‘ontogenetic’ change is more or less permanent and is an important factor for success or failure of vegetative propagation. In mature tissue the ability to produce adventitious roots has often declined and therefore makes the rooting of cuttings very difficult. Within a plant, both juvenile and mature phases can be present at the same time with the juvenile part being located usually at the bottom and the mature part near the crown. Re-juvenation, or rather ‘re-invigoration’, can be achieved by coppicing or hedging a tree. The newly produced vigorous shoots have a higher ability to produce roots when taken for propagation.

Material taken for propagation usually retains its present attributes of juvenility or maturity. When taking mature material, e.g. from the crown of a tree, it is therefore possible to produce small plants that flower and fruit soon after propagation. At the same time, a juvenile cutting can maintain its vigour. These ‘tricks’ are extensively used in horticultural practice.

insert here figure 2 - new pics on gradients of maturity in trees (I – III)

Plants produce substances which regulate shoot and root growth, flowering, leaf abscission and other metabolic processes. The substances are commonly known as plant hormones or plant growth regulators. The most important ones for vegetative propagation are the root promoting auxins, the cell growth stimulating cytokinins, and the stem elongating gibberellins. All of these substances have an array of effects, which sometimes can be antagonistic. For example, auxins applied to cuttings stimulate the development of roots, but at the same time suppress shoot development. Plant growth regulators are commercially available and are used in horticulture. However,


since they can be expensive, locally available alternatives are being sought. As an example, coconut milk contains high levels of cytokinins. It is therefore an important ingredient in nutrient media for micropropagation.

Methods of vegetative propagation

Vegetative propagation methods have been developed and used for centuries. This explains why there is an abundance of different methods. Especially in temperate regions, vegetative propagation has been a most important tool in the domestication of fruit species and very special methods have been developed for the different species (Hartmann et al., 1997; Macdonald, 1986). In contrast in the tropics, relatively few fruit species have received attention with regard to their vegetative propagation (Garner et al., 1976), but emphasis has been put on timber species, notably Eucalyptus and tropical pines (see for example, IUFRO, 1988).

The most appropriate or promising vegetative propagation techniques for tropical tree species are described in the following paragraphs. These are: cuttings, grafting and budding, layering and micropropagation. Other techniques such as division of bulbs, tubers, corms and rhizomes are usually limited to the production of ornamentals and herbaceous species.

Cuttings

Cuttings are pieces of shoots or roots that can develop adventitious roots and shoots. Propagation by cuttings is a relatively easy method which can provide a large number of propagules. Therefore it is considered a good method for clonal propagation of forest trees (IUFRO, 1988) where large numbers of planting stock are required. Longman and Wilson (1993) have produced a very good manual for the rooting of cuttings of tropical trees using low-cost techniques.

Plant species differ in their ability to develop roots from cuttings, depending on a variety of factors, some of which are physiological, some morphological. Physiological factors include the balance of the different plant growth regulators and the nutritional and water status of the plants; morphological factors include the stage of lignification, and the formation of a sclerenchymatous fibre sheath around the phloem.

The propagation of tropical trees by cuttings or stakes is not a new concept and has been done by farmers with a variety of species for centuries, but it has received considerable attention in the scientific community only in the last few decades. Improved propagation environments have helped to increase the success rates for a number of tropical timber species and the needs for greater uniformity in


plantation forestry has also sped up the development of low-cost methods for the production of large numbers of plants.

Grafting

Grafting is the combination of a root system (rootstock) with an appropriate crown part (scion). Grafting techniques have three main purposes:

(1)to propagate species otherwise not easily propagated

(2)to substitute a part of a plant with that of another, and

(3)to join plants that were specifically selected for different purposes (e.g. disease resistance, adaptability to certain environmental constraints, productivity).

There are many grafting and budding methods, some very specific for a particular species. They depend on the size and shape of the scions and the place on the rootstock onto which they are grafted; they can be carried out at various times throughout the year; both roots and shoots can be manipulated. Grafting methods are labour intensive and need good training before they can be successfully applied. However, grafting has become the most used way to improve high-value fruit trees, such as citrus, mango, or avocado.

Grafting combines two independent individual plants, and produces one functioning individual. The process of graft union formation is regulated by a complicated balance of plant hormones and enzymes. Five stages have been defined (Hartmann et al., 1997):

insert here figure 3: pic on grafting process

(1)Lining up the vascular cambiums of rootstock and scion. The cambial cells of both partners will produce new parenchyma cells in the process of wound healing. These new cells must be in close proximity to each other to guarantee later a successful new formation of the vascular vessels. It is therefore important that scion and rootstock are aligned so that the cambium layers meet.

(2)Wounding response. As both scion and rootstock have been cut in the process of graft preparation, cells have been destroyed. These dead cells are being walled off to prevent entry of pathogens into the wound. A necrotic layer forms that will later disappear. Undifferentiated callus tissue is produced from the uninjured adjacent cells.


(3)Callus bridge formation. As the newly formed parenchyma cells fill the space between scion and rootstock, a ‘cement’ or binding material consisting of pectins, carbohydrates and proteins is produced by the callus cells. At this stage, parenchyma cells start showing more cytoplasmatic activity.

(4)Wound-repair xylem and phloem. The wound-repair xylem is usually the first to bridge the graft union, the phloem follows, then a vascular cambium layer forms between the systems of both scion and rootstock. These new elements form directly by differentiation of callus. The parenchyma cells of both scion and rootstock adjacent to the callus bridge differentiate into new cambium cells within two to three weeks after grafting. They then proceed further through the callus bridge, until a continuous cambial connection has been formed.

(5)Production of secondary xylem and phloem. The newly developed cambium starts developing new secondary xylem towards the inside and phloem towards the outside and therefore finalizes the vascular connection between the scion and the rootstock. It is essential that this stage is completed before the scion starts to develop leaves, otherwise it will die of desiccation.

There are, as with any other methods, also limitations to grafting. Apart from higher costs involved than with other methods, there are physiological limitations. For example, graft incompatibilities arise when rootstock and scion do not readily join, or when after a while tissue rejection processes start. These can even be several years later (need a reference here on P. caribaea in Queensland mentioned by Tony).Callus overgrowth and joint breakage can be observed. In order to produce successful and sustainable grafts, compatible clones have to be identified. This can be a large-scale and resource-consuming process but it is necessary to avoid disappointments. Graft incompatibilities occur more frequently the more distant the relationship between scion and rootstock. Generally, there are few problems with grafts between members of the same species, however the combination of different genera from the same family is usually unsuccessful, although some successful examples exist: Trifoliate orange (Poncirus trifoliata, Rutacea) is the common rootstock for orange (Citrus sinensis, Rutacea).

Layering

insert illustration on air layering

Layering is the development of roots on a stem that is still attached to the parent plant. Methods of layering are the oldest used for vegetative propagation. The Chinese developed airlayering some 4-5,000 years ago – the method is therefore often referred to as


‘Chinese layering’. Although the layering techniques are quite simple, their commercial use today is mainly restricted to very specific applications. The reason for this is that the productivity of layers is far smaller than that of cuttings. For the use in the domestication of tropical trees, however, they offer interesting possibilities.

There are three ‘classes’ of layering methods, depending on the soil and stem relationship:

(1)soil is mounded to the stem (mound layering, trench layering)

(2)stems are lowered into the soil (simple layering, tip layering)

(3) stems are not covered by soil (air layering)

Micropropagation

Micropropagation or in-vitro propagation differs from traditional vegetative propagation in that all stages of plant development can be manipulated under controlled environments. There are a number of different methods grouped under the term micropropagation, most commonly: meristem culture, axillary shoot cultures, embryo culture. They differ in the plant part that is used to culture cells and then later to differentiate into functioning plants with the help of carefully composed cocktails of plant nutrients and hormones.

The advantages of micropropagation lie in the production of large numbers of clonal plants, the production of pathogen-free stock, and the propagation independent of seasons. Its disadvantages include the high costs involved for facilities, trained personnel and specialized techniques, and the delicacy of the operation in which contamination with pathogens can cause high losses in a very short time. Micropropagation techniques are therefore only used for species with a high commercial value which is projected to remain high for many years.

Special forms of vegetative propagation

In some species, embryos are produced not by meiosis and fertilization, but by asexual processes. These effects are known as apomixis. Propagules from such plants contain the same genome as the mother plant and as such are clones. There are some plant species that only produce apomictic embryos, others both sexual and apomictic embryos. Apomixis offers interesting opportunities for research and production of selected clones. However, for most tropical tree species, we do not know whether or not they can produce apomicts.

Selection of propagation methods

If the propagation methods are not limiting, there is the general rule: the more valuable the tree and its products, the more elaborate the propagation methods. Or in other words: If you need a large number of low-value trees, don’t bother with complicated propagation


methods; however, if the products are highly valued, as, for example, are most fruits, then a labour-intensive method can be economical.

In plantation forestry, cuttings are the preferred source of vegetative propagules. They allow for large numbers of uniform plants that can later be harvested mechanically, for example for paper and pulp production. The propagation from rooted cuttings is relatively cheap and allows the plant to grow on its own roots, however it is not easily possible with all tropical trees.

In contrast, fruit trees are not necessarily harvested mechanically. Their primary product, the fruits, is highly valued and merits higher investments in the planting stock. Therefore, a more expensive, slower but more sophisticated propagation method can be used. Since the trees are valuable and might have a slow rotation time, the development of long-lived combination plants with, for example, disease resistant roots and high-quality fruits can have merits. The use of tissue culture can be advisable where the required technology is available or could easily be provided, where the appropriate methods are established, and where the market demands are high enough to justify the high initial investment costs of such an operation.

For domestication purposes, any method might be applied. If a domestication project is to be started, probably techniques like layering will be used to get a starting stock of material from adult trees. If seedlings are available, rooted cuttings from these juvenile plants can be used to gain a larger number of plants to start with. Table 1 gives a few examples for species and vegetative propagation techniques that have been tried with them.

Table 1: Examples for vegetative propagation with agroforestry trees

MethodCuttings Grafting/budding Layering MicropropagationAcacia tortilis (Dick and East, 1992)Bauhinia rufescens (Tchoundjeu, 1996)Cedrela odorata (Diaz et al., 1992)Gmelina arborea (Diaz et al., 1991)Milicia excelsa (Ofori et al., 1996)Prosopis africana (Tchoundjeu, 1996)Terminalia spinosa (Newton et al., 1992)

Myrciaria dubia (Narazas, 1992)Pouteria sapota (Umaña, 2000)

Pausynistalia johimbe (Z. Tchoundjeu, pers. comm.)Prunus africana (Z. Tchoundjeu, pers. comm.)

Grevillea robusta (Rajasekara, 1994)Sesbania bispinosa (Yan-Xiu et al., 1995)


Experimentation for vegetative propagation

Vegetative propagation research can include technical experiments, such as testing the necessary hormone level required for optimal rooting of cuttings, evaluating the optimal grafting technique for a new species or determining the appropriate substrate to be used for rooting of air layers. They can also cover much wider aspects of understanding the reasons of ageing and ways to overcome them through appropriate tree management, such as shading, light quality changes or coppicing. More strategic research will be concerned with understanding the effects of releasing limited numbers of clones on natural stands of the same species or evaluating farmers’ responses to new cultivars. For further information about this topic consult Jaenicke and Beniest (2001).


References and further readingDiaz E.R.A., Salazar, R. and Mesén F. 1991. Rooting of young cuttings of Gmelina arborea. Silvoenergia 49. CATIE. 4 pp.

Diaz E.R.A., Salazar, R. and Mesén F. 1992. Rooting of young cuttings of Cedrela odorata. Silvoenergia 51. CATIE. 4 pp.

Dick, J.McP. and East, K. 1992. Influence of light quality on growth and rooting of Acacia tortilis, a tree species of semiarid East Africa. Nitrogen Fixing Tree Research Reports 10: 97-101.

Garner RJ, Chaudhri SA and staff of Commonwealth Bureau of Horticulture and Plantation Crops. 1976. The Propagation of Tropical Fruit Trees. Horticultural Review No. 4. Commonwealth Bureau of Horticulture and Plantation Crops. East Malling, Kent, UK 566 p.

Hartmann HT, Kester DE, Davies FT and Geneve, RL. 1997. Plant Propagation. Principles and Practices. 6th edition. Prentice Hall, New Jersey.

International Centre for Research in Agroforestry (ICRAF). 1997. Intellectual Property Rights. Policy Guidelines Series 10. 4pp.

International Union of Forest Research Organisations (IUFRO). 1988. Genetic improvement of tropical trees. 150 pp.

Jaenicke H. and Beniest J. (eds.) 2001. Vegetative Tree Propagation in Agroforestry. Training Materials. ICRAF ca 130 pp.

Leakey, R.R.B. 1992. Enhancement of rooting ability in Triplochiton sclerocylon by injecting stockplants with auxins. Forest Ecology and Management 54: 305-313.

Leakey, RRB and Simons, AJ. 1999. When does vegetative propagation provide a viable alternative to propagation by seed in forestry and agroforestry in the tropics and sub-tropics? In: Problem of Forestry in tropical and Sub-tropical Countries - The Procurement of Forestry Seed - The Example of Kenya, H. Wolf and J. Albrecht (Eds.), Contributiones Biologicae Arborum, 7, Ecomed Publishers, Landsberg/Lech, Germany, 21pp.

Longman KA and Wilson RHF. 1993. Rooting Cuttings of Tropical Trees. Tropical Trees: Propagation and Planting Manuals Vol. 1. Commonwealth Science Council, London, UK. 137 p.


Macdonald B. 1986. Practical Woody Plant Propagation for Nursery Growers. Timber Press, Portland, Oregon, USA. 669 p.

Narazas, R.M.E. 1992. Propagación del Camu-Camu (Myrciaria dubia) por injerto. Instituto Nacional de Investigación Agraria y Agroindustrial (INIAA) Serie Técnica. Informe Técnico 0.2/2.3- No. 1. 17 pp.

Newton, A.C., Muthoka, P.N. and Dick J.McP. 1992. The influence of leaf area on the rooting physiology of leafy stem cuttings of Terminalia spinosa Engl. Trees 6: 210-215.

Ofori, D.A., Newton, A.C. and Leakey, R.R.B. 1996. Vegetative propagation of Milicia excelsa by root cuttings. Journal of Tropical Forest Science 9(1): 124-127.

Rajasekaran, P. 1994. Production of clonal plantlets of Grevillea robusta in in vitro culture via axillary bud activation. Plant Cell, Tissue and Organ Culture 39: 277-279.

Tchoundjeu, Z. 1996. Vegetative propagation of Sahelian agroforestry tree species: Prosopis africana and Bauhinia rufescens. In: Dieters, M.J., Nikles, D.G., Harwood, C.E. and Walker, S.M. (eds.) Tree Improvement for Sustainable Tropical Forestry. Proceedings QFRI-IUFRO Conference. Caloundra, Queensland, Australia. 27 October – 1 November 1996.

Umaña, C. 2000. Injertación del zapote (Pouteria sapota (Jacq.) H.E. Mooere & Stearn). Centro Agronómico Tropical de Investigación y Enzeñanza (CATIE). Serie Técnica. Manual Técnico No. 45. 15 pp.

Yan-Xiu, Z., Harris P.J.C. and Dun-Yi, Y. 1995. Plant regeneration from protoplasts isolated from cotyledons of Sesbania bispinosa. Plant Cell, Tissue and Organ Culture 40: 119-123.


Figure 1: transect through a tree trunk

periderm (bark)cortexphloemvascular cambiumsexondary xylempith


Figure 2: Gradients of maturity in trees

Gradients of maturity in trees III

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mature

Gradients of maturity in trees II

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re-invigoration (coppicing)

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Gradients of maturity in trees I

young, juvenile, vigorous seedling

old, non-vigorous, mature tree

ontogenetic age (maturity)

time


Figure 3: The grafting process

Stages 1 & 2:lining up of

cambium and wounding response

Stage 3:callus bridge

formation

Stage 4:wound-repair

xylem and phloem develop

Stage 5:secondary xylem and

phloem develop

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phloem develop

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Appendix-vegetative propagation overviewDefinitions and glossaryparthenocarpy The production of fruits without the

development of seeds. From Greek ‘parthenos’: virgin.

cambium Cell layer from which vascular tissue is being developed. Responsible for the increase in girth of plants.

ontogenesis The origin and development of an individual. From Greek ‘ont-‘: being, and ‘genesis’: birth.

sclerenchyma The woody tissue found in a plant, formed from lignified cells and usually providing support. From Greek ‘skleros’: hard and ‘egkhuma’: infusion.

vegetative (asexual, clonal) propagation The formation of a new genetically identical individual plant from a portion (ramet) of a stock plant via regeneration of, or fusion with, the missing organ(s).

cutting propagation The formation of a new plant through regeneration of missing organ(s) on a detached plant part.

grafting and budding The formation of a new plant through fusion (graft union formation) of a detached plant part (scion) with all or part of a plant that includes a new root system (root stock).

layering The formation of a new plant through regeneration of new roots on a portion of the shoot of an intact plant, followed by detachment of the newly rooted shoot to give a new intact plant.

suckering The formation of a new plant through regeneration of a new shoot system on the root of an intact plant, followed by detachment of the new shoot plus root to yield a new intact plant.

division The formation of a new plant through separation of an intact plant into 2 or more pieces, each of which includes some portion of the shoot and root systems of the original plant.


Advantages and disadvantagesAdvantages

clonal offspring are genetically identical to parent plant, i.e., true-to-type.

vegetative propagation is the only method of propagating certain plants which have seedless fruits such as banana, pineapple, some citrus varieties (e.g., Washington Navel orange).

usually more rapid than sexual (seed) propagation.

Disadvantages

generally more skill and labour-intensive than seed propagation.

possibility of transmitting viruses in stock plants to vegetatively propagated offspring.

clonal plantations might not be adapted to sudden environmental changes, (e.g. pest attacks, drought).

Cutting Propagation

A detached plant organ (stem, leaf or root) or combination of organs (shoot, a stem with a bud, with or without a leaf) is capable of regenerating new (adventitious) missing organs under appropriate environmental conditions.

Advantages

easiest and often least expensive method of vegetative propagation.

because the plant propagated from a cutting is on its own root system, the complex stock-scion relationships (including possible incompatibilities) that exist in a grafted plant are avoided.

Disadvantages

some plants are difficult or impossible to root from cuttings (e.g., Acacia sp.).

benefits derived from using grafted rootstocks cannot be exploited (size control, etc.).


Grafting and BuddingDefinitions

Grafting: joining of 2 or more plant parts so that they grow together and function as a single unit.

Budding (bud grafting): any grafting technique involving a scion consisting of a single bud; the term “grafting” usually applies to techniques in which the scion is a shoot section with several buds.

Scion: the upper portion of the grafted pair, which develops into the shoot system of the new plant.

Rootstock (stock, understock): the lower portion that includes (but is not necessarily limited to) the root system.

Uses of Grafting and Budding

propagation: useful technique for perpetuating and multiplying some plant clones that cannot be easily or conveniently multiplied by other vegetative means.

use of benefits conferred by specific rootstocks:

- size control (e.g., Malling series apple rootstocks, trifoliate orange for citrus);

- pest and disease resistance;

- tolerance to high soil moisture, low temperature, salinity or alkalinity.

top working:

- top working refers to the practice of grafting a scion high up into the canopy of an established tree;

- unproductive, shy bearing trees or those bearing poor quality fruits can be improved by top working (usually by cleft grafting) a superior quality scion to replace the unproductive canopy (e.g., mango);

- top working may be performed to add a pollinator branch to a self infertile fruit variety.

repair: useful for repairing the damaged trunk of a tree (bridge graft) or an injured root system (inarch graft).

graft indexing: the presence of viral diseases in a tree can be confirmed by grafting a suspect plant (scion) onto a sensitive


indicator plant of a related species (stock) which exhibits viral symptoms more clearly than the scion species.

Principles

Compatibility: stock and scion must be genetically compatible (if they are closely related enough to hybridize sexually, they will probably form a compatible graft union).

Cambial alignment: vascular cambium of the scion must be in intimate contact (or very close proximity) with the vascular cambium of the stock.

Timing: grafting operation must be done at a time when the stock and scion are in the proper physiological stage. In many cases, grafting is performed when scion buds are dormant and rootstock is just resuming growth. This avoids excessive desiccation associated with newly flushing growth of the stem.

Limitations

Sometimes the failure of a grafted tree to survive can be observed because stock and scion are insufficiently genetically related. This is termed “incompatibility”.

a. Primary Incompatibility: The union is completely unsuccessful from the start, and the scion dies quickly.

b.Secondary or Delayed Incompatibility:

the survival and success rates (graft union formation) are lower than the normal.

decline in vegetative growth, appearance of shoot die-back and general ill-health of the plant.

premature death of the grafted plant.

marked differences in growth rate or vigour of scion or stock.

bulging, callus-like overgrowth at the graft union.

Layering

The regeneration of new roots on a shoot of an intact plant, followed by detachment of the newly rooted shoot to give a new intact plant.

Layering may occur naturally when a shoot comes in contact with moist soil, or in the case of epiphytes. It may be induced deliberately by forcing a branch into contact with moist soil.


Advantages

may succeed with species difficult to propagate by cutting or grafting because water and carbohydrates from the layered plant’s own intact root system are continuously translocated to the zone of root formation; hence water and nutrient stress are avoided.

relatively low level of skill, minimal equipment and facilities required.

Disadvantages

labour intensive, not easily mechanized.

multiplication rate from a single stock plant is relatively low compared to other vegetative propagation techniques, since stock plants must be well spread out and relatively few rooted layers can be obtained from each.

soil-borne pathogens might accumulate in the stockplant area and make expensive soil fumigation necessary.

Stimulation of root development

by wounding or girdling: to disrupt the phloem, causing accumulation of endogenous auxin and carbohydrates, both of which promote rooting.

by etiolation: layered stems are often placed in darkness by covering (air layering) or burying (tip layering) which induces rooting.

by moisture: covering or burying (as above) also encourages moisture and/or humidity to accumulate, which promotes rooting.

Suckers

Suckers are new shoots which regenerate from adventitious buds initiated on the root system of an established plant. Suckers can be dug up along with a portion of attached root system and transplanted to a new location.

Trees which have the natural habit of suckering tend to be easy to propagate from root cuttings, e.g., Casuarina sp.

Offsets and slips

These are shoots which arise from axillary buds at the basal portion of the main stem or trunk of a mother plant.

(1)e.g., pineapple has slips and “suckers” (actually lateral shoots)

(2)e.g., date palm has offsets.


In some cases, rooting of offsets occurs when they are attached to the mother plant (hence a form of layering), but more typically they are detached and planted out before rooting occurs (a form of cutting).

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