2.4.2.1 Medicago
Cold tolerance is typically associated, among populations, with level of winter dormancy, suggesting that active winter growth is physiologically incompatible with cold tolerance. Yet Daday (1964) observed otherwise. He studied geographic races of Medicago sativa L. (lucerne or alfalfa). Among the races, he found the expected picture of a close association between level of winter dormancy and cold tolerance, admittedly not with severe winters. But, in the F2 and F3 generations of an interpopulation diallel cross that association broke down. This independence of inheritance of the two traits conflicted strongly with accepted doctrine, yet no attempt to ascertain its underlying evolutionary significance appears to have been published. We postulate that the observed winter dormancy was in large measure a defence against browsing pressure, rather than being a directly climatic adaptation. Grazing pressure would be intense in winter, and very depleting of the plants’ resources if they were making active growth, presumably leading to selection for winter dormancy. With severe winters, however, dormancy would likely be physiologically obligate, but that would not preclude breeding to extend autumn growth (Castonguay et al. 2006). Our hypothesis could be experimentally probed by studying the effect of winter grazing pressure on persistence of plants that maintained varying levels of winter growth. With suitable material available that should be readily feasible. Assembling suitable material might be a challenge, and an area of ground would need to be available. Actual grazing would require livestock, but grazing might be simulated.