Considerations on overcoming intrasomatic selection during mutation breeding of vegetatively propagated plants

When seeds and vegetative tissues are irradiated, a competition occurs in meristematic tissues between lethally and sublethally affected cells on one hand and unaffected cells on the other hand to the advantage of the latter. This phenomenon was first recognized by FREISLEBEN and LEIN (1943) and termed intrasomatic selection by KAPLAN (1951) and diplontic selection by GAUL (1957). The work done on the nature of this phenomenon in seed-propagated plants is summarized along with suggested methods to reduce its intensity.The generally unencouraging results obtained so far in vegetatively propagated plants have been attributed to the more intensive operation of this phenomenon in this group of plants, consequent on the more complex nature of apical meristems and propagating materials. The work of ZWINTZSCHER (1959) in apples and BAUER (1957) in black currants have shown that it is possible to overcome its effects with suitable handling of irradiated materials.The radiogenetic studies done by the author in the potato have been reviewed in this context. Low intensity irradiation, dose fractionation and irradiation of physiologically dormant tubers gave higher survival value. The method of isolating sprouts from irradiated tubers and growing them, and scoring for mutations in sproutling progenies was found to increase mutation frequency considerably. Other methods proposed for minimizing the effects of intrasomatic selection in vegetatively propagated plants are chronic irradiation, neutron irradiation, chemical mutagenesis and lastly, mutagenesis of isolated single cells and growing them into whole plants.