Induction of freezing tolerance in microspore-derived embryos of winter Brassica napus.

Freezing tolerance was induced in microspore derived embryos of winter Brassica napus cv. Jet neuf by the addition of ABA or mefluidide to the culture media during embryogenesis. Survival after freezing was estimated by culture of frozen-thawed embryos to plantlets. A higher freezing tolerance (50% survival at -15°C) was induced when 50 μM ABA or 3.2 μM mefluidide was incorporated initially into the medium during embryogenesis at 25°C followed by culture at 2°C for 3 weeks. When embryos were induced in the absence of ABA or mefluidide and maintained at 2°C for even as long as 12 weeks a lower degree of freezing tolerance (10% survival at -15°C) was obtained. Plants regenerated from embryos hardened maximally by a combination of either ABA or MFD with low temperature did not require further vernalization for flowering.