Agriculture Reference
In-Depth Information
will have to be budded or grafted onto the desired type. The rootstock may
have dif erent vigour or simply will provide a root for a species or variety that is
dii cult to root by cuttings or air layers.
Certifi ed planting material is meant to ensure that the desired variety is
being obtained and it is free of specifi c diseases. Care needs to be taken that
the planting material purchased is free of pests, nematodes and diseases that
could contaminate areas where these problems do not exist.
Main propagation methods
Sexual or seed propagation
Propagation by seed is regarded as the most 'natural', but frequently the
new plants are not identical to the mother plant and thus variability occurs.
The only seed-propagated plants that are identical to their mother plants are
those that have been self-fertilized for several generations so that the genetic
arrangement has become uniform (homozygous), and they are called pure
lines, such as rice or wheat varieties. This seldom happens with fruit trees,
which are mostly cross-pollinated and show variability when propagated by
seeds.
In some species, the seed is exactly the same as the mother plant. The
condition is called apomixes, where an embryo develops in the absence of
fertilization by pollen. Apomictic seeds occur in mangosteen, achachairu and
jaboticaba, with vegetative embryos and no sexual embryo being present in
the seed. Mango varieties of the Indochina-Philippine group (see Fig. 10.5)
and most citrus species' seeds also have a sexual embryo, and several embryos
can arise from the nucellar tissue of the ovary that surrounds the embryo
sac. Where you have embryos from nucellar tissue, often the sexual embryo
is suppressed by these surrounding asexual embryos. These propagation
methods are considered clonal, with seeds being used.
Seeds are used commercially to propagate herbaceous, short-lived crops
such as papaya, cocona, naranjilla, tree tomato, yellow and purple passion
fruit, giant passion fruit, sweet granadilla, banana passion fruit, cape goose-
berry and some others, such as coconut and pejibaye palm (
Bactris gassipaes
).
Variability is very low since the crops are very homogeneous and the best fruits
are selected to obtain seed. In the case of papaya, hybrids, a cross between two
selected parents or a selected variety like 'Maradol' and 'Kapoho' are used.
For selected varieties, hermaphrodite plants are self-fertilized to obtain the
same variety.
The other important use of seeds in fruit crop production is to obtain
rootstocks on to which the commercial varieties will be budded or grafted.
Normally seeds for rootstocks are highly variable, leading to the propagation of
rootstocks by cuttings or layers so that they are genetically identical and there
is no variability in the orchard. Rootstocks derived from apomictic embryos are
genetically identical to the mother plants.
