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.