Towards a Domestication Strategy for Indigenous Fruit Trees in the Tropics - Indigenous Fruit Trees in the Tropics: Domestication, Utilization and Commercialization - page 34

not common for tropical and subtropical tree species. However, from work on a
few tropical tree species it does appear that some generalizations are possible
(Leakey et al ., 1994; Leakey, 2004) and that through a modelling approach
(Dick and Dewar, 1992) it is possible to predict the best management options
for new species. It is, however, usually necessary to start out by propagating a
larger number of clones than is needed, as some will be lost while going
through the various rooting and multiplication cycles.
From the above, it is clear that, in addition to economic considerations, in
formulating a strategy for clonal forestry it is advisable to consider which forms
of propagation have the lowest risk of failure. It seems that the low-tech system
of rooting stem cuttings is the most robust.
The use of mature tissues
As trees grow they develop a gradient towards maturity (ontogenetic ageing)
and after a time reach a threshold above which the newly developing shoots
have the capacity to fruit and flower, while those below the threshold are still
juvenile. The transition from the juvenile to mature state is called a 'phase
change', and the coppicing of mature trees is generally regarded as the best
way to return to the juvenile state. Because of the difficulty in rooting cuttings
from mature tissues, the most commonly used vegetative propagation methods
for mature trees of horticultural and cash crops are grafting and budding
techniques (Hartmann et al ., 2002). Using these techniques requires skill, as the
close juxtaposition of the cambium in the scion and rootstock is necessary if
callus growth is to heal the wound and reconnect the vascular tissues. Failures
also result from dehydration of the tissues.
These techniques can, however, result in severe and often delayed
problems because of incompatibility between the tissues of the rootstock and
scion, in which graft unions are rejected and broken, sometimes after
5-10 years of growth. This is a form of tissue rejection, and it is less common
between closely related tissues (Jeffree and Yeoman, 1983). Mng'omba et al .
(2007a) have recently shown that graft incompatibility is caused by the
presence of p -coumaric acids, and that greater incompatibility can be expected
for heterospecific than for homospecific scion/stock combinations. Another
common problem with grafting and budding is that shoots can develop and
grow from the rootstock. If not carefully managed, growth from the rootstock
dominates that of the scion, which dies, resulting in the replacement of the
selected mature cultivar with an unselected juvenile plant.
Once mature tissues are successfully established as rooted propagules, be
they marcotts, cuttings or grafts, they can be used as stockplants for subsequently
harvested cuttings. With good stockplant management, good rooting treatments
and an appropriate rooting environment, mature cuttings from these stockplants
can usually be rooted easily. Nevertheless, it is clear that the need to propagate
from mature tissues does pose a severe constraint and challenge to domestication
strategies, especially of fruit trees.
In vitro culture techniques hold some promise of circumventing the
problems of maturation, as some rejuvenation of in vitro cultures has been