Alfalfa Winter Hardiness: A Research Retrospective and Integrated Perspective*

Abstract

Abstract Insufficient cold hardiness is a major impediment to reliable alfalfa (Medicago sativa L.) production in northern regions experiencing harsh winter conditions. Numerous studies have documented the morphological and physiological traits associated with the acquisition of freezing tolerance and winter survival in alfalfa. Use of this information as selection criteria to breed cultivars with superior winter hardiness has thus far been met with limited success. This can be attributed to many factors including: the large number of traits affecting winter survival; the multigenic nature of most traits, large environmental interactions, and an undesirable linkage between acquisition of freezing tolerance and fall growth cessation (fall dormancy). In the last two decades, the advent of molecular biology and quantitative genetic techniques has markedly increased our knowledge of the molecular and genetic bases of superior alfalfa winter hardiness. Our understanding of the mechanisms underlying the perception of the low temperature signal and its transduction into morphological and physiological responses leading to cold hardiness has progressed, but still remains fragmentary. Current evidence indicates that cold hardiness of alfalfa relies on tolerance to extensive freeze‐induced desiccation. Low temperature‐induced accumulation of soluble sugars and stress‐related translation products were found to be, in some instances, more abundant in cold‐tolerant cultivars and to be under some level of genetic control. Limited stability of these traits and conflicting reports on their relationship with freezing tolerance preclude their adoption as molecular screening tools. The development of robust screening techniques will require a more complete knowledge of the genetic bases of freezing tolerance. Heritability estimates suggest that independent selection for winter hardiness, freezing injury and autumn growth is possible, and that winter hardiness and autumn growth could be manipulated independently. This creates the opportunity to develop high‐yielding cultivars with improved winter hardiness. A screening test for freezing tolerance performed under controlled conditions recently led to the development of populations with increased freezing tolerance and led to significant improvement in alfalfa winter survival. Unique genetic material, combined with novel gene discovery approaches, could be lead to the identification of genetic polymorphisms associated with freezing tolerance in alfalfa and pave the way to marker‐assisted selection. Based on the current knowledge, we propose a conceptual framework for the genetic determination of cold adaptation of alfalfa.