Symbiotic propagation from seeds
Seeds are used in propagation, in preference to tissue culture,
in order to maintain genetic diversity and because some seeds
may be stored for later use. Orchid seeds are dust-like and
consist of an embryo of only 100-200 cells within the seed
coat. Unlike many other seeds, they contain very few food
reserves (endosperm) to enable the seed to germinate. In orchid
seeds, the nutrients required for germination are provided
by a mycorrhizal fungus with which it forms an association.
This can be mimicked in the laboratory by sowing sterilised
seeds on agar plates containing ground oats and inoculating
the plates with a culture of an appropriate fungus.
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Fungal isolates are obtained by dissecting roots of actively
growing orchid plants, a non-destructive process. The fungus
is found as intracellular fungal coils (pelotons), linked
via root hairs to the soil. The exact process of symbiosis
is poorly understood, but it is thought that the fungus digests
organic materials and that the resulting nutrients are transferred
to the cells of the orchid by simple diffusion and digestion
of the fungus by the orchid host. Following penetration of
the seed-coat and embryo by the fungus, in the first stage
of germination the seed-coat splits as the embryo expands
to form a protocorm with rhizoids. The orchid seems to be
able to control the rate of infection, and when protocorms
are transferred to fresh medium, no further inoculum of fungus
is required to sustain growth. The cultures are kept in the
dark for germination and the early stages of protocorm development,
and then moved into the light when the leaf shoots begin to
turn green. Further development of plants from protocorms
may be quite rapid. For example, the germination of seeds
of the green-veined orchid (Anacamptis morio) can occur
after 6 weeks and healthy green shoots are produced after
3 months. Plantlets of the autumn lady's tresses (Spiranthes
spiralis) form tubers and are ready to wean into the glasshouse
within a year of sowing if a symbiotic fungus is used. Plants
of the fen orchid have been raised using a fungus from the
Project's collection and planted out on former sites in the
fens of Norfolk.
Asymbiotic propagation
Asymbiotic media have been developed using chemicals and
plant extracts to imitate the nutrients supplied by the fungus.
Germination on asymbiotic media can take several months, and
years may be required for plants to reach a size comparable
to that achieved in a few months using symbiotic methods.
No appropriate fungus has been found in the roots of the
monkey orchid (Orchis simia), one of the plants protected
by legislation in Britain. However several hundred seedlings
have been raised by sowing the seeds on a nutrient medium.
A modification of this technique has been used for the lady's
slipper orchid. Immature seeds from green capsules are sown,
as trials with mature seed have been unsuccessful. The natural
development of the Cypripedium shows an adaptation
for survival in a cool climate and this is exploited in its
propagation. The seeds are sown on media containing mineral
salts and amino acids and then chilled. After a few months,
protocorms form, which then develop roots and leaf buds. The
first leaves are produced after a further period of chilling.
Thousands of seedlings have now been raised from seeds of
the British plant and are being planted at former sites. This
is carried out in collaboration with the Species Recovery
Programme of English Nature.
Elsewhere in Europe this species is more widespread and
observations of these populations have been useful in selecting
re-establishment sites. Plants in the Living Collection at
Kew, of North American origin , are being used to investigate
factors which influence germination.
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