Freezing Tolerance of Winter Canola Cultivars is Best Revealed by a Prolonged Freeze Test

Abstract

Small but important differences in winter survival are known to exist between winter canola cultivars. The objective of this study was to compare a short‐term (lethal temperature for 50% plant kill [LT 50 ]) and a prolonged (lethal duration time for 50% plant kill [LD 50 ]) freeze test to identify differences in freezing tolerance of winter canola ( Brassica napus L. var. oleifera Metzg. and Brassica rapa L. var. oleifera Sink.) species and cultivars. Viability following each freeze test was determined by electrolyte leakage, plant survival, biomass of shoot regrowth, and root regeneration. Plant survival from the LT 50 (cooling rate 3°C h −1 ) and LD 50 (−8°C isothermal for up to 24 d) tests were poorly correlated, and only the LD 50 test was able to identify cultivar differences in freezing tolerance. Electrolyte leakage did not correlate with actual survival measurements in both freeze tests. Shoot regrowth was a more sensitive viability test than plant survival. Brassica rapa plants subjected to the LT 50 test showed greater shoot regrowth than B. napus plants, yet no cultivar differences were found within each species. In the LD 50 freeze test, no difference in shoot regrowth was detected between the species; however, cultivar differences were found within both species. Tolerance to prolonged freezing is critical for winter survival, and the LD 50 freeze test may allow more accurate screening for species and cultivars with improved freezing tolerance than the LT 50 freeze test.