From a quantitative point of view, indirect embryogenesis does provide an efficient method of micropropagation; the same is not true of direct embryogenesis when it is unaccompanied by the proliferation of embryogenic tissue. Although plants can be regenerated from embryos directly initiated in vitro, and may be present in sufficient numbers for limited plant production in breeding programmes, the numbers of primary embryos per explant will usually be inadequate for large scale cloning. To increase the number of somatic embryos formed directly on immature zygotic embryos of sunflower, Freyssinet and Freyssinet (1988) cut larger zygotic embryos into four equal pieces.
Additional embryos are generally unwanted: they are frequently joined one to another as twins or larger groups so that abnormal seedlings with multiple shoots develop from them. The presence of accessory embryos can also impede the growth of the primary somatic embryo. Growth then becomes asynchronous and normal seedlings may not be obtained unless the adventive embryos are removed.
However it has been suggested that accessory embryos might be used for micropropagating some species. Perhaps this is a method of micropropagation which will be developed more in the future? Examples of where it has been successful are:
• Juglans regia. (McGranahan et al.1988b);
• Medicago sativa (Lupotto 1986).
As mentioned earlier embryogenesis has a great potential for mass propagation, however, all adventitious techniques do still have the associated problem of the lack of clonal stability. Therefore the commercial application of this technology remains limited except, perhaps, where embryos arise directly from parental tissue.
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