Factors modifying frost tolerance of legume species

Variability in seedling death of legumes because of spring frost in the USA and Canada is associated with several factors. Experiments were conducted to evaluate factors that modify frost tolerance of seedling legumes. Experiment 1 was comprised of two hardenings, three freezing temperatures, eight legume species, and four growth stages. Experiment 2 had three temperatures, four durations of temperature, and four legume species. Experiment 3 included three soil types, two soil water levels, and two legume species. At each growth stage, legume seedlings were placed in a programmable freezing chamber at 3degreesC and temperature decreased/increased 1degreesC h(-1) to and from a minimum freezing temperature. Duration of minimum freezing temperature was 1 h for Experiments 1 and 3 and varied according to the treatment in Experiment 2. Hardening increased seedling survival up to 40% over unhardened seedlings across growth stages and species. Forage legumes were more frost tolerant than soybean [Glycine max (L.) Merr.] and field pea (Pisum sativum L.) at all temperatures. Increase in duration of freezing temperature decreased the frost tolerance of all species when freezing temperature was near the LT; (temperature that kills 50% of seedlings). Seedling survival of both alfalfa (Medicago sativa L.) and soybean was greater in light-textured soil than the heavy-textured soil with soil water at field capacity. However, one-third of field capacity soil water allowed greater seedling survival in the heavy-textured than the light-textured soil. The results suggest that the factors studied should be considered to assess the frost tolerance of legume seedlings.