The Sainsbury Orchid Conservation Project:
Propagation for Conservation
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.
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 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.