Effect of freeze-thaw cycles on hard-seededness of alfalfa.

Freezing at an ultralow temperature and thawing was shown in 1930 to decrease hard-seededness of alfalfa but the method has never been applied commercially. Freezing equipment now exists that could be adapted for freezing batches of alfalfa seed. Thus this study was conducted to evaluate the effectiveness of freeze-thaw treatments to reduce hard-seededness of two alfalfa seed lots known to have hard seed. A study was also conducted to determine if freeze-thawing of seeds has any negative effects on subsequent plant growth. Freezing to -10°C was ineffective but freezing to -80°C was highly effective at decreasing hard-seededness. Freeze-thaw treatments effectively decreased hard seed content of seed lots of a purple flowered alfalfa (Medicago sativa L.) cultivar, Peace, and a yellow flowered alfalfa (Medicago falcata L.) cultivar, Anik. Hard seed content decreased exponentially with the number of freeze-thaw cycles. With a freeze thaw cycle consisting of 2 h exposure to -80°C and 2 or 18 h thaw at 20°C, it took 1.6 cycles to decrease Peace hard seed by 50% and 2.6 cycles to decrease Anik hard seed by 50%. The -80°C freeze-thaw treatments increased rate of germination based on total seed population, total germination, rate of seedling development, and number of seedlings established. Freeze-thawing does not affect nonhard seeds since the freeze-thaw treatment had no effect on the rate of seed germination when based only on the population of seeds germinating. Freeze-thaw treatment of seed had no effect on seedling vigor as measured by dry matter production. The freeze-thaw treatment is an effective method for decreasing alfalfa hard-seededness; thus, an economic analysis of its use for commercial application is required. Additional index words: Medicago, Germination, Seedcoat, Permeable, Temperature.