CELLULAR-CHANGES DURING HEAT-SHOCK INDUCTION AND EMBRYO DEVELOPMENT OF CULTURED MICROSPORES OF BRASSICA-NAPUS CV TOPAS

Brassica napus cv. Topas microspores, isolated and cultured near the t ime of the first pollen mitosis and subjected to a heat treatment of 2 4 h, can be induced to develop into haploid embryos. This is a study o f microspore structure during induction and embryo determination. Earl y during the 32.5 degrees C incubation period the nucleus moved away f rom the edge of the cell, and granules, 30 to 60 nm in diameter, appea red in the mitochondria and as a cluster in the cytoplasm. Cells divid ed symmetrically and at the end of the heat treatment, acquired the fe atures of induced bicellular structures described previously. The feat ures persisted as the cells divided randomly within the exine for 4-7 days following heat induction. Multicellular structures released from the exine underwent periclinal divisions resulting in protoderm differ entiation of the globular embryo, thus determining embryo development. The cytoplasm of early heart-stage embryos contains abundant polyribo somes. Non-embryogenic development was indicated by large accumulation s of starch and/or lipid and thickened cell wails or an unorganized pa ttern of cell division following release of the multicellular structur es from the exine. Embryogenesis is discussed in terms of induction, e mbryo determination and development.