Seed coat impermeability in the common annual legume pasture species of Western Australia

Abstract

Impermeable or hard seeds of 21 common annual legume pasture species of Western Australia were exposed for six months to a constant 15¦C and to daily fluctuating temperatures of 15¦ - 30¦C, 15¦- 40¦C, 15¦- 50¦C, 15¦- 60¦C, and 15¦- 70¦C. At a constant 15¦C no softening of impermeable seeds took place with any species. With the fluctuating temperatures, the treatments capable of causing softening varied with the species and there were cultivar differences within species. All Trifolium species softened significantly after three months at 15¦ - 400C; some softened at 15¦ - 30¦C. Hard seeds of Lupinus varius required a fluctuation of 15¦ - 60¦C before softening commenced. For most species, once the critical temperature maximum for softening had been reached, the subsequent rate or total extent of softening in a set time interval increased as the maximum temperature of the daily fluctuation increased. Four species of Medicago all showed extreme impermeability compared with the Trifolium species. The extent of softening was relatively small at 15¦ - 500C and did not increase as the maximum temperature of the fluctuation was increased from 50¦C to 70¦C. The failure of Medicago species to regenerate and establish effectively in many of the cooler south coastal districts of Western Australia is attributed in part at least to this extreme hardseededness. In these districts daily surface soil 4emperatures fluctuate between 15¦ - 45¦C during the dry summer period.