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 free zing tolerance. A highly significant correlation was obtained between the m inimum temperature tolerated by fully cold-hardened seedlings and the field survival index for 36 winter wheat cultivars with freezing tolerance varyi ng from -13 degreesC to -23 degreesC. On the basis of their long-term field survival under cold stress, these cultivars represent two separate genotyp ic groups, semi-cold-hardy (Group A) and very cold-hardy (Group B). The cor relation coefficient between minimum survival temperature and winter surviv al for the semi-hardy genotypes was not significant, although it was signif icant for the hardy genotypes (coefficient of determination was 25.9%). How ever, the minimum survival freeze test did not differentiate genotypes that varied widely in field survival. In comparing the very hardy winter genoty pes (e.g., Norstar, Alabaskaja, Roughrider, etc.), no significant correlati on was observed between either minimum survival temperature or crown moistu re content. The freezing tolerance of 33 winter wheat genotypes was compare d for seedlings naturally cold acclimated and for seedlings grown either in soil or hydroponically and hardened in a controlled environment chamber. O n average, soil-grown seedlings, cold acclimated in a controlled environmen t were more freezing tolerant than seedlings acclimated naturally or grown hydroponically and acclimated in a controlled environment. Several semi-win ter-hardy genotypes attained a freezing tolerance equivalent to that of ver y hardy winter genotypes when acclimated in a controlled environment chambe r. Thus, it is possible to overestimate the freezing tolerance of seedlings acclimated in a controlled environment.