Germplasm Supply, Propagation and Nursery Management of Miombo Fruit Trees - Indigenous Fruit Trees in the Tropics: Domestication, Utilization and Commercialization - page 352

19.4 Vegetative Propagation
Vegetative (asexual) propagation is defined as the regeneration of a new
individual from a portion (ramet) of a stock plant (ortet) by a process involving
mitotic cell division, and the subsequent regeneration of complementary cells,
tissues and or organs, or an entire plant, to replace those missing from the
ramet (Mudge and Brennan, 1999). This can be effected by grafting, cutting,
air-layering and shoot micropropagation.
Although many miombo fruit tree species can be propagated by seed,
progeny obtained in this way may not be true to type. Natural variability in trees
propagated through seedlings is a major bottleneck in fruit orchards. Sexual
outbreeding through cross-pollination has evolved as the dominant natural
reproductive strategy (breeding system) in trees because it ensures genetic
recombination, heterozygosity, and concomitant seedling variation on which
natural selection may act to bring about adaptation favourable to survival
(Mudge and Brennan, 1999). The domestication of normally outcrossing species
depends on increasing uniformity because it makes selection of improved
genotypes possible. Genetic gains associated with deliberate selection of superior
genotypes of normally outcrossing species can be captured immediately, via
cloning of selected individual putative trees (Simons and Leakey, 2004; Leakey
and Akinnifesi, Chapter 2, this volume), thereby avoiding the loss of the selected
trait in subsequent seedling generations due to segregation of alleles (Mudge and
Brennan, 1999). Therefore, vegetative propagation techniques are indispensable
for the capture and multiplication of the phenotypic variation expressed by
superior individuals of miombo fruit trees with desirable characters.
Vegetative propagation could also help in shortening the period to first fruit
as most miombo indigenous fruit trees may start fruiting in 12-16 years when
propagated by seed (Taylor and Kwerepe, 1995). It is also possible to select
fruits for seedlessness. This aspect has not been researched for miombo fruits,
but it may be worthwhile in future. Vegetative propagation of trees can be done
using a variety of techniques, which include grafting, budding, air-layering,
rooting juvenile stem cuttings and in vitro tissue culture.
19.4.1 Grafting
The history of improvement of many tropical fruit tree crops around the world,
such as mangoes, citrus and avocado, indicates that grafted stock is the best
way to effect improvements rapidly. Because there is evidence that phenotypic
fruit traits are similar to genotypic characteristics (Ndugu et al. , 1995) and can
be fixed by vegetative propagation (Simons, 1996; Mudge and Brennan, 1999;
Simons and Leakey, 2004), there is a good case for phenotypic selection of
desirable fruit traits (Akinnifesi et al. , 2006).
Fruit and nut trees have been grafted because: (i) it is difficult to propagate
important clones by cuttings; (ii) the grafted crops are superior and have high
value; (iii) it is also possible to benefit from improved rootstocks (double-
working), when carefully selected, and from changing cultivars of certain