A re-evaluation of controlled freeze-tests and controlled environment hardening conditions to estimate the winter survival potential of hardy winter wheats

To identify superior winter-hardy winter wheat genotypes it is essential to have a reliable screening method that can detect small differences in freezing tolerance. A highly significant correlation was obtained between the minimum temperature tolerated by fully cold-hardened seedlings and the field survival index for 36 winter wheat cultivars with freezing tolerance varying from -13 degreesC to -23 degreesC. On the basis of their long-term field survival under cold stress, these cultivars represent two separate genotypic groups, semi-cold-hardy (Group A) and very cold-hardy (Group B). The correlation coefficient between minimum survival temperature and winter survival for the semi-hardy genotypes was not significant, although it was significant for the hardy genotypes (coefficient of determination was 25.9%). However, the minimum survival freeze test did not differentiate genotypes that varied widely in field survival. In comparing the very hardy winter genotypes (e.g., Norstar, Alabaskaja, Roughrider, etc.), no significant correlation was observed between either minimum survival temperature or crown moisture content. The freezing tolerance of 33 winter wheat genotypes was compared for seedlings naturally cold acclimated and for seedlings grown either in soil or hydroponically and hardened in a controlled environment chamber. On average, soil-grown seedlings, cold acclimated in a controlled environment were more freezing tolerant than seedlings acclimated naturally or grown hydroponically and acclimated in a controlled environment. Several semi-winter-hardy genotypes attained a freezing tolerance equivalent to that of very hardy winter genotypes when acclimated in a controlled environment chamber. Thus, it is possible to overestimate the freezing tolerance of seedlings acclimated in a controlled environment.