MICROPROPAGATION OF WOODY SPECIES - STATE OF THE ART ON IN VITRO ASPECTS

The validity of clonal propagation for woody species is unrefutable. The greatest use of tissue culture plants has been in fruit trees for the production of clonal rootstocks (particularly peaches in Europe). Self rooted cultivars hold great promise for the future if early field evaluation results are confirmed by longer term evaluations (Zimmerman, 1986). In forestry there is agreement concerning the principle, but cost price is a serious handicap for practical acceptance.

Notwithstanding a lot of improvement in micropropagation of woody plants, trees have not yet attained the reputation of being model systems. Quite often we still do not know how to scale up a micropropagation procedure from the laboratory to the industrial level (Bonga & Durzan, 1987).

Progress in the micropropagation of woody plants is likely to mirror to some extent, predictions made for herbaceous species. However the techniques available for woody species are at a much earlier stage of development (Yeoman, 1986). But since the beginning of this decennium we have witnessed remarkable advances in the micropropagation of woody species, because many problems have been identified, but this does not necessarily mean that (an) adequate solution (s) have (s) been found.

The factors to be considered when microculturing a plant tissue can be grouped into five broad categories: [1] tissue (genotype, source and history); [2] media (minerals, hormones and other organics, supporting agents); [3] environment (light, temperature, gases, vessels); [4] timing (subculture period, dosage); [5] interactions between the above factors (McCown & Sellmer, 1987). Until recently the approach of micropropagation has mainly been that of good chief cook. It is only in the last decennium that micropropagation in general, and of woody species particularly has been considered from a more physiological point of view and in a more analytical way. The most important results of this new orientation will be reviewed.

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