Towards a Domestication Strategy for Indigenous Fruit Trees in the Tropics - Indigenous Fruit Trees in the Tropics: Domestication, Utilization and Commercialization - page 31

(Leakey, 1985; Hartmann et al ., 2002), including grafting, stem cuttings,
hardwood cuttings, marcotting (air-layering), suckering and in vitro techniques,
such as meristem proliferation, organogenesis and somatic embryogenesis
(Mng'omba et al ., 2007a, b).
The decision to pursue the clonal opportunities offered by vegetative
propagation necessitates the formulation of a strategy, as there are a number of
factors associated with the process of cloning that need to be considered. These
factors relate to: (i) the methods of propagation; (ii) the level of technology that
is appropriate; and (iii) the effects of using juvenile or mature tissues (Leakey,
1991; Leakey and Simons, 2000).
2.2.1 Methods of propagation and the cloning process
The principal reason for cloning is to take advantage of its ability to capture
and fix desirable traits, or combinations of traits, found in individual trees. By
taking a cutting or grafting a scion onto a rootstock, the new plant that is
formed has an exact copy of the genetic code of the plant from which the tissue
was taken. In contrast, following sexual reproduction seedlings are genetically
heterogeneous, each seed having inherited different parts of the genetic codes
of its parent trees, with segregation of genes among the progeny. Vegetative
propagation is thus both a means of capturing and utilizing genetic variation
and of producing cultivars to increase productivity and quality (Mudge and
Brennan, 1999; Leakey, 2004). Vegetative propagation results in the formation
of clones (or cultivars), each of which retains the genetic traits of the original
tree from which cuttings or scions were collected. Both on-farm and on-station
approaches to tree domestication can involve vegetative propagation and
clonal selection, but with a growing interest in the participatory domestication
of agroforestry trees (Leakey et al ., 2003; Tchoundjeu et al ., 2006). There is
now great interest in vegetative propagation using stem cuttings (Leakey et al .,
1990; Shiembo et al ., 1996a, 1996b, 1997; Mialoundama et al ., 2002;
Tchoundjeu et al ., 2002) and grafting for Uapaca kirkiana and Strychnos
cocculoides (Akinnifesi et al ., 2006) and Sclerocarya birrea (Holtzhausen et al .,
1990; Taylor et al ., 1996). Vegetative propagation gives the tree improver the
ability to multiply, test, select from, and utilize the large genetic diversity present
in most tree species. It should be noted that, contrary to some misguided
opinion, vegetative propagation does not in itself generate genetically
improved material. Only when some form of genetic selection is employed in
tandem with propagation will improvement result.
To capture the first asexual propagule from proven and mature field trees
that have already expressed their genetic traits, it is necessary to use either
grafting/budding or air-layering techniques. Alternatively, coppicing can be
used to produce juvenile material. The latter is preferable for clonal timber
production, whereas the former is more suitable for fruit trees (Leakey, 1991).
Grafting produces many more individuals with less effort than air-layering,
although many more individuals can be produced from stem cuttings if the tree
resprouts copiously after coppicing. Once clonal stocks have been obtained, the