Effect of maturation stage, storage time and temperature on seed germination of Medicago species

Seeds of M. arabica, M. orbicularis and M. lupulina were harvested at two stages of maturation: immature (30 days after anthesis) and mature seed (after dissemination). Germination percentage and speed of germination (as an estimation of seed vigour) were evaluated at five temperatures (5, 10, 15, 20, and 25°C) after four storage periods (0, 120, 240 and 360 days after harvest). Mature seeds were scarified. Germination was greatly influenced by the stage of seed development in all the three studied Medicago. Mature seed showed 100% of hard seed, while scarification determined full germination, showing that dormancy of mature seeds must be entirely ascribed to hard coat. In immature seeds germination occurred without removing the seed coat, reaching germination up to 90%, indicating that hard coat dormancy establishment occurs after the acquisition of germination capability. Immature seed viability significantly decreased throughout the storage period. Germination was negatively influenced by temperature of 5°C in M. Lupulina, while M. Arabica showed a better adaptation to lower temperature than the other two species. Final germination was not affected by temperature in mature seed, but the vigour expression clearly indicates that for all the species the best temperature is 20°C. During ripening of Medicago seeds, there is a window in which the germination capability is already achieved and dormancy is not yet imposed by the coat.