TREE MANAGEMENT - Tropical Fruits - page 67

Tissue culture
This approach ( in vitro propagation) uses micro-cuttings, which can be
meristems or portions of cells from dif erent parts of the plant (Fig. 4.1b). The
process involves two phenomena: dedif erentiation from its original tissue
condition to callus, followed by redif erentiation to generate all the tissues of a
whole plant, which involves somatic embryogenesis. In many crops and under
certain tissue culture protocols, somaclonal variations can be a problem.
Somaclonal variation is thought to be due to DNA changes in the cell nucleus
caused by tissue culture stress in the presence of plant growth regulators such
as auxin and cytokinin.
This in vitro approach is used for rapid and consistent reproduction of elite
or dii cult-to-propagate genotypes. Since the tissues are so small and tender,
they are very prone to rot or infection, and therefore they are fi rst surface-
sterilized then handled under sterile conditions in special growth chambers.
During the last three or four decades, several species have been propagated by
this system to ensure freedom of diseases and true-to-type plants. The cost per
plant is normally higher than asexually propagated plants, but many plants
can be generated quickly from one elite mother plant.
Until now, soft-tissue plants like pineapple, papaya, banana and plantain
have been commercially produced, while with trees some success has been
obtained with avocado, mango, Annona s and jackfruit. This propagation
method is essential if molecular biology gene transfer approaches are to be
used for tropical fruit tree improvement.
Layering
In layering a portion of a branch relatively close to the tip and still attached
to the tree is put in contact with a moist substrate and the stem below the
substrate is cinctured. New roots emerge into the medium, then the branch
portion with the new roots is separated from the mother plant, cutting below
the roots, and a new plant is obtained.
The most popular layering system for tropical fruit trees is air-layering or
marcottage. The leaves are removed from a portion of a branch located about
30-60 cm from the tip and a 3 cm ring of bark peeled of . The girdled area and
part of the stem with bark remaining are covered with a ball of moist sawdust
or peat moss. The ball is wrapped tightly with plastic sheet or aluminium
foil. Sometimes auxins can improve the rooting of these air layers. After 1-3
months, when the roots are visible and have turned from a white to cream
colour, the air layer can be harvested by cutting below the new rooted area.
The wrap is removed and the ball with roots transplanted into a pot or bag with
a substrate and held under medium shade. When this new plant shows signs
of growth, the bag or pot can be taken into full sun for some months until an
adequate size is attained for transplanting to the fi eld. Air-layered plants lack a
tap root and can be more easily blown over by strong winds, the same as plants
propagated by cuttings, but this is not a problem under normal fi eld conditions.