Frost tolerance of ten seedling legume species at four growth stages

Spring frost in cooler regions periodically kills seedling legumes and makes replanting necessary. Experiments were conducted in the growth chamber to determine freezing tolerance of 10 legume species at four growth stages and to determine the freezing temperature that kills 50% of seedlings (LT50) for each species under temperatures more commonly found in the field. Four temperatures (-2, -4, -6, and -8degreesC), four seedling ages (1, 2, 3, and 4 wk after planting), and 10 legume species [alfalfa (Medicago sativa L.), red clover (Trifolium pratense L.), sweetclover (Melilotus officinalis Lam.), alsike clover (T. hybridium L.), white clover (T. repens L.), sainfoin (Onobrychis viciifofia Scop.), pinto bean (Phaseolus vulgaris L.), navy bean (Phaseolus spp.), soybean [Glycine max (L.) Merr.], and field pea (Pisum sativum L.)] were included. Hardened (vernalized) seedlings were placed in a programmable freezing chamber at 3degreesC and the temperature decreased or increased 1degreesC h(-1) to or from a minimum-freezing temperature. Pinto and navy beans were the least tolerant, soybean and field pea were moderate, and forage legumes were the most tolerant to freezing temperature. The LT50 was -3.25 to -3.5degreesC for. dry beans, -4.5degreesC for soybean and field pea, and -6.3 to -7.4degreesC for forage legumes. One-week-old seedlings had the highest tolerance to freezing temperature when close to the LT50, compared with older seedlings. However, this tolerance at 1 wk of age disappeared when temperature was lower than the LT50 of the species. The prediction equation and LT50 for each species have potential for predicting seedling stand loss after a spring frost, realizing that field validation of these is impossible.