An indoor screening method for improvement of freezing tolerance in alfalfa
Freezing tolerance is a determinant factor of persistence of alfalfa (Medicago sativa L.) grown in northern climates. Selection for winter hardiness in field nurseries is difficult because of the unpredictability of the occurrence of test winters allowing the identification of hardy genotypes. A method of selection entirely performed indoor in growth chambers and walk-in freezers has been applied for the identification of genotypes with superior freezing tolerance. Using that approach, cultivars recommended for growth in eastern Canada have been submitted to cycles of recurrent selection to generate populations potentially more tolerant to freezing (TF). Subsequent determination of the freezing tolerance of populations recurrently selected using plants acclimated to natural hardening conditions in an unheated greenhouse revealed a progressive increase in response to this selection approach. Field assessment of TF populations also showed better survival and forage yield than original cultivars at sites that experienced severe winter conditions. At stressed sites, a significant proportion of the variance in the yields of the populations was explained by freezing tolerance potential. Our results show that major increases in freezing tolerance (between 3 and 5°C) of alfalfa and better survival to severe winter conditions in the field can be achieved by screening for freezing tolerance under indoor growing conditions and intercrossing the selected plants.