EFFECTS OF ABSCISIC-ACID ON SOMATIC EMBRYO MATURATION OF HYBRID LARCH

Somatic embryos of hybrid larch (Larix x leptoeuropaea) which had been matured for 4 weeks on maturation medium, developed normally on mediu m supplemented with 60 mu M ABA, but abnormally on medium with no ABA. A comparative structural and histochemical investigation was carried out on these two types of mature embryos. At the light microscope leve r, differences between both treatments were visible only after 2-3 wee ks of maturation. At around this time, abnormal development becomes ev ident macroscopically: ABA-minus embryos remain rather stubby as oppos ed to the more cylindrically shaped ABA-plus embryos. Whereas somatic embryos matured with ABA consist of densely cytoplasmic cells showing a high rate of cell division, ABA-minus embryos are largely made up of expanded and highly vacuolate cells, indicating that growth in the la tter is mainly due to cell expansion and not division. After 4 weeks o f maturation, ABA-minus embryos begin to elongate in the hypocotyl reg ion, and precocious germination was observed frequently. Again, these morphogenetic events were largely due to abnormal timing of cell expan sion. Histochemically, storage proteins were found only in somatic emb ryos matured for 4 weeks with ABA. This observation is in line with re sults obtained by total protein analysis, yielding significantly lower total protein contents in ABA-minus embryos both on a fresh weight an d a per embryo basis after 4-5 weeks of maturation. Deposition of star ch grains mainly in the cortex; tissue of the hypocotyl region was obs erved within 2 weeks of maturation in varying amounts regardless of AB A supply. Polyphenols, in particular catechins and proanthocyanidins, were present in all embryos from the very onset of development. They w ere localized preferentially in the proximal suspensor cells and the b asal region of the embryo. However, accumulation of polyphenols was ge nerally much more pronounced in embryos matured without ABA, indicatin g a lack of biochemical regulatory competence in those embryos.