South African Avocado Growers’ Association Yearbook 1981. 4:22-26
PLANT PROPAGATION THROUGH TISSUE CULTURE
P ALLAN DEPARTMENT HORTICULTURAL SCIENCE, UNIVERSITY OF NATAL, PIETERMARITZBURG INTRODUCTION In plant propagation various methods are used, depending on the objective. To reproduce a botanical species of plant, seedlings are generally used even if there is a certain amount of variation amongst the daughter plants. This variation, which results from the combination of different genes from the two parent plants may be quite acceptable in the propagation of certain ornamental plants where one simply wishes to reproduce the general species characteristics. However in many cases one is interested in reproducing certain specific characteristics of a plant that are of great importance from a horticultural point of view, e.g. the colour of a rose flower, or the shape, size and texture of a specific fruit. Where one has a group of plants which all have identical desirable characteristics that can be reproduced when propagated, the group is referred to as a cultivated variety or cultivar (Anon, 1969). Some cultivars e.g. of annual flowers and vegetables come true-to-type when grown from seed because their chromosome pairs have fairly uniform genes (homozygous). By culling off-type plants before flowering, the desirable characteristics of the 'line' are retained when it is propagated by seed. Many other plants, especially perennials, have very variable gene-pairs (i.e. they are heterozygous) and seedlings grown from them do not reproduce the horticulturally desirable characteristics. In such cases it is essential to propagate the cultivar by some asexual or vegetative method to retain the desirable, characteristics, in the daughter plants. This is possible with plants where, in the normal process of cell division, the exact genetic constitution (genotype) is duplicated in every daughter cell. With plants it is also possible to induce the differentiation of different organs from other parts of the plant, e.g. roots from stems, and buds on roots. Thus there are numerous methods of vegetative propagation varying from naturally detachable structures (bulbs & corns), through many kinds of cuttings to methods of budding and grafting — all of which lead to the production of genetically uniform plants i.e. a clone. However a danger inherent in all methods of vegetative propagation is that if the plants become infected with virus diseases the virus that is contained within the cells, can be propagated along with every daughter plant. Besides the undesirable visual symptoms
that may arise from virus infection, the growth and production of infected plants can be severely depressed. Hence the importance of the 'Superplant' scheme in the deciduous fruit industry and the Citrus Improvement Programme, where material free of specific viruses is utilized as far as is possible. Another problem in vegetative propagation is that with some plants only a few daughter plants can be produced in a year or more. Hence it is very difficult to produce large numbers of a new clone in a short period of time to supply an anticipated demand. PLANT TISSUE CULTURE From a horticultural point of view the most exciting developments involve the rapid clonal propagation of plants that are possibly free of infection by a harmful virus. Both these aspects are of vital interest to nurserymen. The potential was soon exploited commercially overseas and today there are many nurseries in the USA & Europe that have their own tissue culture laboratories for the clonal propagation of specific plants on a vast, previously undreamt of, scale. Researchers have calculated that it is possible to. produce thousands of plantlets, within a single year from one original piece (explant) of a plant, by repeated sub culturing. While it is possible to obtain plants free of a specific disease through tissue culture, it is a fallacy to believe that all tissue-cultured plants are...
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