Germplasm Supply, Propagation and Nursery Management of Miombo Fruit Trees - Indigenous Fruit Trees in the Tropics: Domestication, Utilization and Commercialization - page 356

be an effective method of propagation (Shackleton, 2004). In South Africa,
Shackleton (2004) reported that farmers plant Sclerocarya birrea using truncheons
(25%), transplanting wildings (31%) or seeds directly (44%). Likewise, truncheons
of Adansonia digitata sprout readily and roots when pushed into the soil. Although
propagation of Sclerocarya birrea by truncheons has been successful, this method
has poor commercial prospects (Taylor et al. , 1996).
Many miombo fruit trees fail to undergo active rooting because of low
genetic and physiological capacity for adventitious root formation, which limits
the effectiveness of propagation with cuttings. Mng'omba (2007) and
Mng'omba et al. (2007a, b) have alluded to the possibility of phenolic
compound formation at the cut surface, limiting vegetative propagation - in
grafting this might lead to a weak union or incompatibility between scion and
stock; when propagation is being attempted from cuttings it might cause the
development of calluses without rooting. Excessive formation of calluses has
also been indicated as a reason for poor rooting of cuttings in Uapaca kirkiana
and Sclerocarya birrea in southern Africa (Jaenicke et al. , 2001).
19.4.5 In vitro micrografting
Tissue culture is an aseptic laboratory procedure that requires unique facilities and
special skills. In vitro regeneration involving the use of relatively small propagules,
referred to as explants, cultured under controlled environments in small containers
of a nutrient medium in which all nutrients required for growth are provided
artificially within an aseptic environment (Hartmann et al. , 1997; Mudge and
Brennan, 1999). The multiplication process may involve the stimulation of normal
(non-adventitious) elongation and/or the branching of the original explant
(shoot/root), and/or the de novo induction of adventitious organs (shoots and/or
roots), including but not limited to axillary or nodal shoots (Mudge and Brennan,
1999). Shoot culture involves using growth regulators during a multiplication stage
to promote repeated cycles of shoot elongation and/or branching, followed by
repeated cycles of subdivision and reculture on fresh medium; the proliferated
roots are then rooted as microcuttings either in vitro or ex vivo . Therefore, it is not
feasible for use by small-scale farmers in decentralized nursery systems.
In the past, it was thought that tissue culture was inappropriate for small-
scale farmers, and less emphasis has been placed on it in domestication work
compared with other propagation methods. Recently, in vitro micropropagation
has been pursued as one of the methods that should be exploited for the mass
and rapid vegetative multiplication of superior priority indigenous fruit trees
(Akinnifesi et al. , Chapter 8, this volume). Reasons for using tissue culture
(Hartmann et al. , 1997) include: (i) micropropagation involves very small
amounts of plant tissue; (ii) somatic embryogenesis is possible; (iii) embryo
rescue is possible; (iv) anthers (microspores) can be cultured to produce
haploid plants for breeding; (v) micrografting is possible; and (iv) tree crops can
be improved. In addition, it makes possible the mass multiplication of specific
clones, the production of pathogen-free plants, and the clonal propagation of
parent stock for hybrid seed production (Hartmann et al. , 1997).