Freezing and desiccation tolerance of imbibed canola seed

Abstract

Depending on the environmental conditions, imbibed seeds survive subzero temperatures either by supercooling or by tolerating freezing‐induced desiccation. We investigated what the predominant survival mechanism is in freezing canola (Brassica napus cv. Quest) and concluded that it depends on the cooling rate. Seeds cooled at 3°C h−1 or faster supercooled, whereas seeds cooled over a 4‐day period to −12°C and then cooled at 3°C h−1 to−40°C did not display low temperature exotherms. Both differential thermal analysis and nuclear magnetic resonance (NMR) spectroscopy confirmed that imbibed canola seeds undergo freezing‐induced desiccation at slow cooling rates. The freezing tolerance of imbibed canola seed (LT50) was determined by slowly cooling to −12°C for 48 h, followed with cooling at 3°C h−1 to −40°C, or by holding at a constant −6°C (LD50). For both tests, the loss in freezing tolerance of imbibed seeds was a function of time and temperature of imbibition. Freezing tolerance was rapidly lost after radicle emergence. Seeds imbibed in 100 μM abscisic acid (ABA), particularly at 2°C, lost freezing tolerance at a slower rate compared with water‐imbibed seeds. Seeds imbibed in water either at 23°C for 16 h, or 8°C for 6 days, or 2°C for 6 days were not germinable after storage at −6°C for 10 days. Seeds imbibed in ABA at 23°C for 24 h, or 8°C for 8 days, or 2°C for 15 days were highly germinable after 40 days at a constant −6°C. Desiccation injury induced at a high temperature (60°C), as with injury induced by freezing, was found to be a function of imbibition temperature and time.