Propagation of Orchids from Seeds
Orchids are generally difficult to grow from seed, because in nature they require a symbiotic fungus. They have been grown from seeds at Kew for more than 30 years. In 1974 the Micropropagation Unit was set up to make laboratory techniques available for a wide range of plants including orchids. These methods allow the propagation of many orchids without a symbiotic fungus.
Using these techniques, the orchid collection at Kew is augmented every year by seedlings of rare and decorative species. Seeds come from plants at Kew and other sources, including botanic gardens, expeditions and private growers. All known information is recorded (donor, provenance, harvest date, condition of seeds etc.) and herbarium voucher specimens are made where possible. At present there are approximately 180 species in culture, many of which are known to be rare in the wild.
For in vitro techniques, it is preferable to harvest capsules before they split because the seeds from intact capsules do not need to be sterilised. Only the outside of the capsule is treated. It is opened in the laminar flow bench, and the seeds which are naturally sterile are sown. Better results are obtained in this way for many species, especially in Cypripedium and Paphiopedilum.
Seeds from split capsules are often collected under warm, moist conditions and may be contaminated by fungi or bacteria. It is therefore important to dry them after harvest. Seeds are placed in a desiccator at room temperature until sowing. This method is adequate for short periods, but for longer term storage, seeds are separated from any capsule debris and, after drying, are kept at 4°C. Seeds should, however, be sown as soon as possible. These seeds are normally sterilised using sodium hypochlorite, but calcium hypochlorite can also be used. Small quantities of seeds are sterilised in filter paper packets for 10 minutes, and then rinsed in sterile water. They can then be transferred to the medium on the unfolded paper or transferred to the surface of the medium from the paper.
Culture conditions
Seedlings have been grown on many different media, and the best medium varies between species. All the media used contain a carbohydrate source, a range of mineral salts and agar, and many also contain vitamins, amino acids, growth regulators or plant extracts such as banana pulp or potato extract. The media are sterilised in an autoclave. The main culture room conditions are a 16 hour photoperiod and a temperature range of 22-25°C. Some seeds are germinated in the dark or in low light.
The developing seedlings need to be subcultured several times until they are large enough to be transferred to the glasshouse. Well grown seedlings are removed from the flasks and established in the glasshouse environment. This process is known as 'weaning' and care has to be taken to ensure that the seedlings suffer as little stress as possible until they have hardened off, as the leaf cuticle is often poorly developed in in vitro culture. They are potted in either an epiphytic or a tropical terrestrial seedling compost as appropriate.
Some successes at Kew
A capsule of Dendrobium spectatissimum, a rare and striking species was collected in Sabah. The seeds germinated successfully on Vacin and Went (VW) medium and the seedlings grew very quickly. They were transferred to a modified medium as soon as they were big enough to handle and after a further 16 weeks the first batch were potted up. Seedlings have been returned to Sabah and distributed to other botanic gardens as well as being incorporated into the collection at Kew.
Peristeria elata, a beautiful species sometimes known as the dove or Holy Ghost orchid, is the national flower of Panama. Seeds donated to Kew germinated on a range of media. After nearly two years in culture, the seedlings were large enough to be potted up and many have been distributed to botanic gardens. Growers in Britain with facilities for growing this tall terrestrial species were also offered plants.
Seeds of the endangered species Clowesia rosea were sent to Kew from Oaxaca, Mexico. They grew best on VW and on half strength Murashige and Skoog (MS) media when these were supplemented with activated charcoal. Strong plants are now thriving in the glasshouses at Kew and have been distributed to other gardens and growers. Other successes with Mexican orchids include several attractive species of Encyclia.
Cymbidium rectum was thought to be extinct until rediscovered recently in Sabah. It differs from C. bicolor in having an upright raceme, which is an unusual characteristic in this section of the genus. The habitat where a few plants were rediscovered has since been clear-felled and it is likely that the species is now extinct in the area. Seeds germinated and the resulting seedlings grew most vigorously on VW or MS medium with activated charcoal. As with the other species discussed above, this Cymbidium has now been widely distributed.
Gynoglottis cymbidioides is the only species in this curious genus. It resembles Coelogyne in habit, but has small white flowers of distinct structure in a long, many flowered raceme. It occurs in Sumatra at about 1550 m altitude, and was unknown in cultivation until seed capsules collected in 1988 were sent to Kew. The seed capsules had not split on arrival at Kew and the seeds germinated within two weeks of being sown. Seedlings grew well in vitro and have been distributed in flasks to other institutions with facilities for growing plants on. Some plants are now growing in the glasshouses at Kew.
Sphyrarhynchus schliebenii, a very rare orchid from tropical East Africa was hand pollinated at Kew, and the capsule was harvested five months later. Seeds were sown on twelve different media, and although they germinated on all of them, VW with charcoal proved to be the best. After two years in culture, and after five subcultures, plants were ready to be weaned. This species flowered whilst still in vitro, just before weaning.
These success stories are only a sample of the orchid propagation carried out at Kew. The work is increasingly in volved with conservation, and propagated plants will be returned to their place of origin, or distributed to other botanic gardens and elsewhere.
