PLOIDY AND DIVISION EFFICIENCY OF PETIOLAR PROTOPLASTS OF BRASSICA-NAPUS

The cellular heterogeneity in the DNA content of petioles of plantlets cultured in vitro was examined by flow cytometry after density gradient centrifugation. Three different subpopulations of petiolar protoplasts were separated using a mannitol/NaCl gradient. The first or upper fraction contained a substantial proportion of protoplasts with 4C nuclei. The protoplasts had larger size, and displayed a higher division frequency. The third or lower fraction contained mostly 2C nuclei and had a lower cell division frequency, while the middle fraction with its division frequency approaches that of the lower fraction. Approximately 50% of the mitotic activity of cells in culture derived from protoplasts of the upper fraction. Cells from petioles of different ages also showed different extents of mixoploidy, with the older petioles having higher ploidy levels. Within the same petiole, cells from the proximal part contained higher ploidy levels than those from the distal part.