In vitro culture of higher plants as a tool in the propagation of horticultural crops.

In vitro culture of higher plants is the culture, under sterile conditions, of plants, seeds, embryos, organs, explants, tissues, cells and protoplasts on nutrient media. This type of culture has shown spectacular development since 1975, resulting in the production and regeneration of viable individuals of many plant species. In addition, since 1980 there has been an explosion in the development of genetic manipulation and biotechnology. Various types of cultures and their applications are discussed: embryo culture, orchid seed culture, meristem culture, cloning of crops (single-node culture, meristem culture, axillary branching, adventitious shoot formation, callus culture, single cell and protoplast culture), anther and microspore culture, culture of ovules and excised flowers, and protoplast culture. Vegetative propagation in vitro (also called micropropagation) will be dealt with in greater detail since it has yielded results which are of enormous importance in horticulture. A recent survey in the Netherlands has shown that the number of commercial tissue culture laboratories in this country has increased from 28 in 1983 to 50 in 1986; the total number of plants cloned in vitro increasing from 21 to 43 million over the same period. The widespread use of in vitro cloning is an indication of its many advantages. However, there are also disadvantages. The criteria desired for the success of in vitro cloning can be itemized as follows: no induction of mutations; the starting material must be disease-free, or disease-free material must be produced by meristem culture; the transfer from test tube or container to soil should not be too difficult; the regeneration ability should not be lost; the technique should be economically viable and not too complicated (synthetic seeds!), otherwise it will be rejected. Woody species (trees and shrubs) are far more difficult to clone in vitro than herbaceous plants; the reasons for the differences are given. In vegetative propagation and propagation by seeds, in vitro culture can also be a tool to: obtain disease-free plants; transport disease-free plant material; increase the genetic variation (by somaclonal variation and genetic manipulation); induce haploids; separate chimeras; obtain tetraploids; store plant material in gene banks where it can be kept disease-free under conditions which limit growth and development