INVITRO FREEZING TOLERANCE IN RELATION TO WINTER SURVIVAL OF RAPESEED CULTIVARS

Winter survival is a complex trait dependent on a number of parameters, including the morphological and physiological characteristics of the plant, soil conditions, and weather fluctuations. Freezing tolerance of the plant is a major factor determining winter survival and can be assessed indirectly by measuring ion leakage from leaves exposed to freezing temperatures. This study was conducted to determine the relationship between in vitro freezing tolerance and winter survival in rapeseed. Ten rapeseed cultivars (Brassica napus and B. rapa) were subjected to an in vitro controlled freeze and assayed for ion leakage both before and after cold acclimation for three weeks. In addition, these same cultivars were fall-seeded at two field locations and their winter survival evaluated. Freezing tolerances determined by the in vitro freezing assay increased after acclimation, and cultivars differed significantly for freezing tolerance before and after acclimation. The increase in in vitro freezing tolerance of the cultivars after acclimation was correlated significantly with winter survival (r = 0.824.85, P < 0.05), and, therefore, this in vitro assay could be a useful predictor of field results. One exception to this correlation was the B. napus cultivar Santana, which had a large capacity for acclimation but did not survive in either field location. These results suggest that freezing tolerance is an important, but not the only, component of winter survival.