In some crop plants, the genetic differences between plants derived from callus and suspension cultures (discussed in Volume 2) are considerable, and are sufficient to have attracted the interest of plant breeders as a new source of selectable variability. However, plants obtained from callus lines with a high degree of morphogenic competence, appear to be much more uniform genetically. Care must be taken though to see that primary explants are not taken from plant tissue likely to be endopolyploid. Subsequent exposure to high levels of growth substances such as 2,4-D should also be avoided as far as possible. Genetic stability of plants from highly competent callus cultures may be assisted by the continual presence of superficial meristems. As mentioned previously in Section 3.2.2, these probably repress shoot formation from cells within the callus mass (Hussey, 1983).
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