The development of plant tissue culture and micropropagation in research laboratories and horticultural companies during the last 25 years has demonstrated that these techniques allow a far greater number of plants to be produced in a given time than can be achieved by conventional propagation methods. It was realised that these techniques could play an important role in the management of botanical collections and so it was decided to set up a micropropagation unit at Kew.
Opened in 1974, the Unit was given the task of propagating plants which are rare, endangered or difficult to propagate conventionally. The techniques used include micropropagation from vegetative material and the in vitro germination of seeds and spores.
Since the Unit was opened a wide range of plants have been micropropagated and expertise has been developed in the micropropagation of plants which are seldom, if ever, worked on elsewhere. A large element of empirical research is therefore employed.
At any one time, approximately 500 species from all over the world are being worked on. Most of these can be assigned to groups of related plants.
Fern species that are rare or difficult to propagate from spores by conventional methods are propagated in vitro. The techniques used reduce the risk of contamination with spores of other species and the associated problems of hybridisation.
When vigorous young sporophytes (the familiar 'frond' stage in the life cycle of ferns) have been produced, they are separated from the gametophyte (the small thallus stage) and transferred to fresh medium. When rooted, they are transferred to compost and kept at high humidity to avoid desiccation of the delicate fern fronds.
Successes with plants of conservation interest include Cyathea australis from New South Wales, Cibotium schiedei from Central America, Marattia purpurascens from Ascension Island and Angiopteris boivinii from the Seychelles.
A large number of carnivorous species have been grown from seed using in vitro techniques at Kew. The use of these techniques has kept problems due to low seed viability and susceptibility to fungal pathogens to a minimum.
Genera in the collection include Sarracenia, Nepenthes , Drosera, Pinguicula, Heliamphora, Dionaea and Cephalotus. Habitat destruction and over collection constitute serious threats to many species. Micropropagation is, therefore, a useful technique for increasing the stocks of some of these species in cultivation. Notable examples of successes in this work include Nepenthes rajah from Sabah, N. khasiana from India, and Sarracenia oreophila from Alabama.
Micropropagation of succulent plants is used at Kew for rescuing plants which fail to establish or which develop fungal or bacterial rots. This can be of particular importance where the species are undescribed or rare.
Groups that have been worked on successfully include Asclepiadaceae, Cactaceae, Crassulaceae, Aizoaceae and Euphorbiaceae. Shoot pieces containing one or more axillary buds are excised and placed on media containing growth regulators. These induce the buds to elongate and form shoots, which are cut up and used for further shoot production. The shoot tips are transferred onto rooting media and the rooted propagules potted up.
Examples of successes with rare species include Aloe polyphylla from Lesotho, Gasteria baylissiana from the Cape Province, and Huernia insigniflora from the Transvaal.
In nature most orchids form a symbiotic association with mycorrhizal fungi in order to help the plant obtain nutrition. This is due to the absence of endosperm in most orchid seeds.
Many tropical epiphytic and terrestrial orchids are grown from seed in vitro under sterile conditions at Kew (see sheet K14). The media used contain nutrients to sustain seedling growth which avoids the necessity for a mycorrhizal fungus.
Very high levels of germination can be achieved with good quality fresh seed. Thousands of seedlings can be produced from a single capsule. In comparison with many other families, weaning tropical orchids is fairly easy as they possess a relatively thick cuticle (reducing dehydration) in vitro.
Examples of successes with species of high conservation interest include Cymbidium rectum and Dendrobium spectatissimum from Sabah. Notable successes with plants of high horticultural interest include several species of Coryanthes and a leafless epiphyte, Dendrophylax funalis.
Temperate terrestrial orchids remain difficult to grow asymbiotically using these nutrient media devised for epiphytic orchids. For this reason, the Sainsbury Orchid Conservation Project (see sheet K15) was established in the Unit to raise seedlings using symbiotic techniques wherever possible. The emphasis has been on British orchids, to re-establish species such as Liparis loeselii and Cypripedium calceolus in their native localities.
A wide range of other rare and endangered species are propagated in vitro either from seed or vegetative material. Many of these are members of island floras and are thus particularly at risk. Projects on these floras include propagation of rare plants from St Helena, the Mascarene Islands, Hawaii, the Canary Islands and Socotra.
Successes include rare Limonium and Echium species from the Canary Islands, Trochetiopsis erythroxylon and T. melanoxylon from St Helena, Argyroxiphium kauense from Hawaii, Nesocodon mauritianus from Mauritius, Trichodesma scottii from Socotra and Impatiens gordonii from the Seychelles.
Other endangered species which have been propagated successfully include Cosmos atrosanguineus from Mexico (extinct in the wild but reduced to a sterile population in horticulture maintained by clonal propagation), Silene tomentosa from Gibraltar (believed to be extinct since 1985 until 3 plants were found in 1994) and Ceratonia oreothauma from Oman.
Cryopreservation is the storage of living material at or near the temperature of liquid nitrogen (-196oC). At this temperature, cellular processes are effectively stopped and the cells or organs are stored in a state of suspended growth free from pathogens or the risk of genetic drift. It is thus a valuable tool for plant genetic conservation and since 1992 the possibility of maintaining vegetative material of rare and endangered species as a base collection at Kew has been investigated.
Success has been achieved with Cosmos atrosanguineus, the Mexican Chocolate Flower. Other species are being evaluated and trials are taking place.
Successful micropropagation often produces more plantlets than are required by Kew. As a result, surplus plant material is distributed to other botanic gardens and institutes around the world. The plants are dispatched in vitro and so do not present any problems with international quarantine regulations.
Where conditions allow, some of this material will hopefully be used in reintroduction trials. Examples of species which have been sent back to their country of origin include Dendrobium spectatissimum from Sabah, and a range of plants from the Canary Islands.
Where successful germination of tropical orchid seed is achieved, the number of protocorms produced can be very high. Therefore a spirit collection has been formed. This should prove to be a useful tool for taxonomists, allowing them to study a stage of development which cannot normally be identified in the wild.
The Unit is developing a database system for storage and maintenance of references, protocols and contacts. The software used is BG-Base, originally developed for the management of botanic gardens collections.
Summarised information on micropropagation techniques developed at Kew, currently stored on a card index system, is being entered into a micropropagation protocols database. This will be used to answer enquiries, assist the work of the Unit and allow publication of methods. The Unit also has a large collection of reference material, much of which is now on the database.
For further information on the work of the Unit or specific enquiries about particular plants or groups please contact us on the address below.
Royal Botanic Gardens, Kew,
Surrey. TW9 3AB.