Agriculture Reference
In-Depth Information
resulting shoots can either be used to provide rooted cuttings for transplanting
to the field, additional material for clonal seed orchards, or scions for grafting
onto suitable rootstocks. Evidence from simple tests suggests that the majority
(probably more than 90%) of tropical trees are amenable to propagation by
juvenile stem cuttings (Leakey et al. , 1990). In southern Africa, some high-
priority indigenous fruit trees are propagated by grafting (e.g. U. kirkiana ,
Strychnos cocculoides , Sclerocarya birrea , Adansonia digitata and Vitex
mombassae ), while others (e.g. S. birrea ) are easily propagated by large,
leafless, hardwood cuttings or stakes/truncheons. Parinari curatellifolia is easily
propagated by root cuttings.
2.2.2 Appropriate technology
Low-cost non-mist propagators have been developed for the rooting of leafy stem
cuttings. These do not require electricity or running water and are extremely
effective, meeting the needs of most tree improvement projects in developing
countries in both the moist and dry tropics (Leakey et al. , 1990). In
technologically advanced countries, mist or fogging systems are available for the
rooting of cuttings. Alternatively, where laboratory facilities are available in vitro
culture techniques can be used, but these require highly trained staff and regular
power supplies, and are capital-intensive. In many cases, in vitro techniques have
resulted from empirical testing of different media and plant growth regulators. As
a result, inadequate knowledge about the long-term effects of treatments on the
field performance has caused some major failures of field performance. In the
same way, the vegetative propagation of mature tissues by marcotting, or air-
layering, requires a lower level of skill than grafting and budding. The low-
technology options are especially appropriate if the participatory approach to
domestication is the preferred strategy. In this situation, farmers vegetatively
propagate their best trees to create selected cultivars.
2.2.3 Choice between mature and juvenile tissues
An advantage often ascribed to propagation from mature tissues is that by the
time the tree is mature it has demonstrated whether or nor it has superior
qualities. However, it is not always easy to take advantage of this proven
superiority, as propagation by mature stem cuttings is notoriously difficult. This
is in contrast with the rooting of juvenile tissues, which is typically easy.
Another important advantage of propagating mature tissues is that they are
already capable of reproductive processes, and so will flower and fruit within a
few years, reducing the time before economic returns start to flow. Plants
propagated from mature tissues will also have a lower stature. On the other
hand, timber production requires the vigour and form associated with juvenile
trees, making propagation by cuttings attractive and appropriate. For timber
trees, propagation from mature trees is generally limited to the establishment of
clonal seed orchards within breeding programmes.
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