Germplasm Supply, Propagation and Nursery Management of Miombo Fruit Trees - Indigenous Fruit Trees in the Tropics: Domestication, Utilization and Commercialization - page 354

grow more firmly. Their disadvantage is genetic variation that may lead to
variability in the performance of the scion of the grafted plant, especially when
the seedling is raised from an unknown seed source (Hartmann et al. , 1997).
Careful selection of the rootstock is necessary to reduce such variability. On the
other hand, clonal rootstocks are genetically the same, and all the plants in a
clone can be expected to have identical growth characteristics.
In both macro- and micropropagation, delayed, early graft incompatibility
or outright stock/scion rejection may occur as a result of adverse physiological
responses between grafting partners (Mng'omba et al. , 2007a, b), virus or
phytoplasma transmission, anatomical mismatching or poor craftsmanship of
the grafter, the phenomenon known as 'green finger,' and anatomical
abnormalities of the vascular tissue in the callus bridge (Macdonald, 1986;
Hartmann et al. , 1997).
19.4.2 Top-working
Top-working is a form of grafting done in the field by connecting tissues of a
scion from a desired tree to a growing tree. A fruit tree may be an undesirable
cultivar, such as an old, unproductive plant or cultivar, or one that is susceptible
to diseases. Top-working is done to extend the production cycle or to increase
resilience or the value of the cultivar. For indigenous fruit trees, top-working
could help to rejuvenate old trees and make them more productive. In addition,
certain trees that are dioecious may be unproductive because of a lack of male
(staminate) flowers. Such trees could be made more productive by introducing
male branches into an orchard with predominantly female (pistillate) stands and
vice versa.
Just as in grafting, top-working between and within the same family,
species and clone of a species is always feasible, but compatibility is greater for
clones (Hartmann et al. , 1997). Top-working has been successful in Uapaca
kirkiana at Makoka. Scions of superior trees were grafted onto growing Uapaca
kirkiana planted a few years earlier. Similarly, A. Mkonda et al. (unpublished)
obtained 65% graft take in in situ grafting of Uapaca kirkiana in the wild. This
technique would be particularly useful for improving the production quality of
seedlings of established trees on the farm. One major advantage of top-working
is that it will overcome problems associated with tree establishment and poor
survival.
19.4.3 Air-layering
Air-layering is a method of propagation performed directly on the branches of
growing parent trees in situ . It involves the rooting of cuttings in which
adventitious roots are initiated on a stem while it is still attached to the plant
(Hartmann et al. , 1997). The rooted stem (layer) is then detached and
transplanted, and becomes a separate plant on its own roots. The physical
attachment of the stem during rooting provides a continual supply of water,