The influence of gamma irradiation on dry seeds of lucerne, cv. Wairau

Lucerne (Medioago sativa L. 2n=32) is one of the chief forage crop plants and has an increasing importance in the New Zealand farming system. The natural variability in lucerne is considerable. This is mainly because of its polyploid nature and the fact that lucerne is largely an allogamic, self-incompatible species. Brock (1970a and 1972) has pointed out the difficulty regarding the expression and utilization of mutations in cross-fertilizing species and has questioned the usefulness of induced mutations in such species unless it could be clearly established that natural variability has been fully utilized. Since there is no indication of lack of variability in lucerne but rather difficulty in handling it, the use of radiation to induce further variability for plant breeding cannot be justified at present. However, a good case for studying the responses of lucerne to radiation can still be made, partly because little is known about the reactions of this species, and partly in preparation for possible future demand for induced mutations. Biological damage and mutation frequency both increase with increasing levels of radiation. A compromise must therefore be reached between the desired mutation frequency and the degree of biological damage that can be tolerated. Gregory (1968) has shown that the frequency of radiation-induced mutants increases with decreasing magnitude of effect, and this increase in frequency is strongly correlated with the increase in probability that a given change would effect an improvement. Research on the relationship between polyploidy and radiosensitivity carried out so far has shown that, in general, increasing degrees of ploidy are accompanied by increasing radioresistance. With this in mind, an inquiry into the properties of populations of lucerne produced after seed irradiation seemed appropriate and desirable