THE EFFECTS OF ABSCISIC-ACID ON THE MATURATION OF BRASSICA-NAPUS SOMATIC EMBRYOS - AN ULTRASTRUCTURAL-STUDY

A comparison of embryos, cultured for increasing periods of time with and without abscisic acid (ABA), was undertaken to investigate, at the ultrastructural level, the influence of this growth regulator on the maturation of rapeseed (Brassica napus) somatic embryos. In the absenc e of ABA, the embryos germinated precociously while lipid bodies (LB), which were not numerous, soon degraded, as revealed by a depletion pr ocess associated with the appearance of morphologically mature glyoxys omes and an increase in the number of mitochondria. Moreover, a lack o f protein bodies indicated that storage protein accumulation was not i nitiated under these conditions. On the contrary, the addition of ABA (10 muM) induced marked modification of embryo metabolism. Indeed, ABA completely prevented precocious embryo germination and inhibited lipi d reserve catabolism. Moreover, the formation of small vacuoles and pr oliferation of rough endoplasmic reticulum in their vicinity suggested the onset of storage protein accumulation. After 15 days in the prese nce of ABA, the embryos contained abundant lipid and protein bodies. N evertheless, these somatic embryos were not exactly the same as their mature zygotic counterparts since differences were found in chloroplas ts, amyloplasts, and nuclear structures. These observations suggest th at additional factors might be required to obtain fully mature somatic embryos.