IN VITRO CULTURE AND CELLULAR BIOTECHNOLOGIES APPLIED TO PLANT BREEDING. PRESENT STATE AND PERSPECTIVES (WITH PARTICULAR REFERENCES TO PLANT DISEASE SELECTION).

• the natural breeding system of the species, and any artificial changes that can be induced in it;
• the type of variety to be marketed.

Without going into detail or noting every particular case, some general features can be noted which characterise the breeding of fruit trees.

Vegetative multiplication and particularly grafting and layering are very common and long established practices : the aim of selection is thus an individual rather than a family or parent lines for hybrids.

The classical system of scion and rootstock allows growers to exploit interactions, between two genetic entities which make particular contributions to adaptation to varying cultural conditions. This involves the creation of one physiological entity, which can not be sexually reproduced.

Interspecific hybridisation has for a long time been important in genetic improvement of fruit trees. A large resource of genetic variability is thus made available, but little is known of the genetic control of most characters.

Most breeding objectives involve the fruit:uniformity in ripening and in size, keeping and maturing qualities, resistance to damage by mechanical harvest and transport. Tree form and early bearing are also selection criteria but the yield and quality of the fruit take priority.

One last feature also involves variety structure. Grafting leads to complete genetic homogeneity of the upper parts of the trees in an orchard, with the well known risks of epidemics this involves.

The arsenal of methods available to the breeder is now complemented, by newer techniques, which all involve a release from the constraints of sexual multiplication. For different species, these new techniques can be applied at varying points in the breeding process :

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