Characteristics of black medic (Medicago lupulina L.) seed dormancy loss in Western Canada

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Wilson, Leanne C.
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Cover crops are an important innovation in sustainable cropping systems. The successful use and management of black medic (Medicago lupulina L.) as a self-regenerating cover crop requires a better understanding of its physical seed dormancy. In order to break this seed dormancy, it appears that a low temperature 2-stage seed softening process is required. However, whether or not this 2-stage process is required for black medic seed softening in Western Canada is unclear. Also, the influence of the presence of a companion crop, medic population type, seed burial depth and seed production environment on black medic production and seed softening is unknown. Field and controlled environment studies were established in 2003 and 20O4 in an effort to address these questions. The results from a field study conducted in different prairie environments showed that although, seed production environment, the presence of a companion crop, and medic population affected the growth and development of the black medic plants, they did not affect initial seed dormancy. A second field study tested the effect of seed production environment, seed burial depth and population on seed softening. Results indicated that there was an effect of population on summer seed softening, which suggested that there were differences in seed dormancy between a population of black medic that had been subjected to selection pressures (e.g., herbicides, competition) for 12 years versus one that had not (Foundation stock). Therefore, this suggests that some genetic drift had occurred within the population. The results also indicated that there was an effect of seed burial depth on seed softening, with more seed softening occurring for the buried seed during winter/spring and more for the surface seed during summer, and these differences appeared to be somewhat linked to differences in soil temperature. Results of this field study suggested that black medic in Western Canada goes through a 2-stage softening process. Hence, a controlled environment study was established to test this hypothesis. Results from both studies confirmed that a 2-stage softening process is required for black medic softening in western Canada. Stage 1 requirements appear to be met by exposing seed to temperatrues between-5oC and 5oC for at least 4 weeks, while exposure of the seed to a low fluctuating temperature (e.g. 15/6oC) for a short period of time (i.e., approximately 4 days) appears to meet stage 2 requirements. In summary, this research has provided us with valuable information about black medic seed softening under Western Canadian conditions, which will hopefully lead to a better understanding of how to best manage and utilize black medic as a self-regenerating cover crop in a Western Canadian cropping system.