THE IMPORTANCE OF LEAF FROST RESISTANCE TO THE WINTER SURVIVAL OF SEEDLING STANDS OF ALFALFA

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Abstract
Plants propagated from single plant selections of the alfalfa cultivars Saranac and Luna (Medicago sativa L.), Beaver (M. media Pers.) and Anik (M. falcata L.) were transplanted into the field in May and subjected to a range of freezing temperatures at 3-wk intervals during August and September. A portable field freezing chamber was used to study the influence of frost on leaf injury in the fall and winterkill following the severe 1977–1978 winter. Leaves of all plants had the capacity to harden during the fall, but the selection from Anik was consistently the most frost hardy. Temperatures ranging from −4 to −5 °C in mid-August caused 50% leaf injury to the selections of Beaver, Saranac and Luna, while in late September, temperatures ranging from −9.5 to −10.5 °C were required to produce similar amounts of injury. In the Anik selection, −6 °C in mid-August caused 50% leaf injury, but in late September −12.5 °C caused less than 35% injury. Thus, the Anik selection appeared to start hardening about 3 wk earlier than all other selections. In all plants, the potential for winter injury during the 1977–1978 winter increased as a result of leaf frost damage during mid-August and early September in 1977. The application of these results to the winter survival of seedling stands of alfalfa is discussed.

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