COMPARATIVE MORPHOLOGICAL-STUDY OF ZYGOTIC AND MICROSPORE-DERIVED EMBRYOS OF BRASSICA-NAPUS L. AS REVEALED BY SCANNING ELECTRON-MICROSCOPY

A comparative morphological study of microspore-derived (MD) and zygot ic embryos of Brassica napus L. was conducted, illustrating substantia l similarities in external morphology of these embryos throughout thei r development. Haploid embryos were produced from isolated microspores cultured on high molecular weight polyethylene glycol (PEG), replacin g sucrose as an osmoticum. Morphological changes during the time-cours e of microspore embryo development induced on PEG (25%) and sucrose (1 3%) are described in detail as revealed by scanning electron microscop y (SEM) and compared to the corresponding stages of zygotic embryos de veloped in ovulo. At the heart, torpedo and early cotyledonary stages, microspore-derived (MD) embryos on PEG closely resemble their zygotic counterparts. In contrast, the external morphology of embryos induced on high sucrose medium differs from that of PEG and zygotic embryos i ndicating that a high concentration of sucrose in culture has a morpho genetic effect on MD embryo development in B. napus. Fragments of the original pollen wall are regularly observed at the root pole region an d at the tips of suspensors in MD embryos throughout their development . This suggests that polarity in MD embryos might originate from struc turally polarized late uninuclear microspores and early bicellular pol len. (C) 1998 Annals of Botany Company.