Alfalfa seed germination tests and stand establishment: The role of hard (water impermeable) seed

Abstract

Freeze thaw scarification has been observed to increase the germination rate of alfalfa (Medicago sativa L.) seed containing a large proportion of hard (water impermeable) seed in a 7-d laboratory germination test; however, a comparable increase in plant density is not always seen in the field. To investigate this discrepancy, a field experiment with untreated and scarified seed was carried out using cultivars with high (Apica, 35%; Barrier, 32%) and low (Apollo II, 1%; WL316, 0.3%) percentages of hard seed. Plants were counted at the three-trifoliate-leaf and 10% bloom stages in 1992, the planting year, and at 10% bloom in 1993. In the field, effects of scarification were seen only at the 10% bloom stage in 1992, increasing the plant densities of high hard seed cultivars by 17% while decreasing those of low hard seed cultivars by 10%. Two laboratory experiments were also done to determine the effect of temperature, lighting (light, shade, dark) and media (on blotter, in soil) on the germination of hard seed. Germination was highest at alternating 35/5 °C and higher at 28/14 °C than at constant 20 °C. At 20/–10 °C most germinated seed died. Differences among germination media and lighting treatments were small. Scarification may possibly have some negative effect on the quick-germinating fraction of the seed in the field but the net effect for cultivars with a high hard seed content appears positive. Some seed identified as hard in a 7-d test at constant 20 °C likely germinate and contribute to an alfalfa stand in the field when soil temperatures are higher. Thus the discrepancy previously observed between the laboratory and field results may possibly be accounted for in part by a negative effect of scarification and also by differences in temperature. Key words: Medicago, germination, seed coat, temperature