Effects of abscisic acid on somatic embryo maturation of hybrid larch

Abstract

Somatic embryos of hybrid larch (Larix x lepto-europaea) which had been matured for 4 weeks on maturation medium, developed normally on medium supplemented with 60 μM 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 level, differences between both treatments were visible only after 2-3 weeks of maturation. At around this time, abnormal development becomes evident macroscopically: ABA- minus embryos remain rather stubby as opposed 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 latter is mainly due to cell expansion and not division. After 4 weeks of maturation, ABA-minus embryos begin to elongate in the hypocotyl region, and precocious germination was observed frequently. Again, these morphogenetic events were largely due to abnormal timing of cell expansion. Histochemically, storage proteins were found only in somatic embryos matured for 4 weeks with ABA. This observation is in line with results obtained by total protein analysis, yielding significantly lower total protein contents in ABA-minus embryos both on a fresh weight and a per embryo basis after 4-5 weeks of maturation. Deposition of starch grains mainly in the cortex tissue of the hypocotyl region was observed within 2 weeks of maturation in varying amounts regardless of ABA supply. Polyphenols, in particular catechins and proanthocyanidins, were present in all embryos from the very onset of development. They were localized preferentially in the proximal suspensor cells and the basal region of the embryo. However, accumulation of polyphenols was generally much more pronounced in embryos matured without ABA, indicating a lack of biochemical regulatory competence in those embryos.