Effect of the environment in which seeds are grown and softened on the incidence of autumn seed softening in two species of annual medics

Abstract

The incidence of autumn seed softening, which is believed to offer some protection against false breaks of season, was investigated under a range of environmental conditions in three medic cultivars over three years of softening. Newly ripened pods of burr medic (Medicago polymorpha L.) cvv. Serena and Santiago, and barrel medic (M. truncatula Gaertn.) cv. Cyprus, that were grown at three sites were transferred to four seed softening sites in December. Their progress of seed softening was determined in March, April and June. The seed softening process was also studied in a range of laboratory treatments. Seed growing environments were more important than seed softening environments in determining rates of seed softening in all cultivars. However, the distribution of softening between summer and autumn was generally consistent between the growing and softening environments. More than half the annual softening of Serena and Santiago seeds occurred between the first week of March and June in each of the first three years. A smaller proportion of Cyprus seeds softened between March and June in the first and third years than was the case with the other cultivars. In the first year of softening most of the Serena and Santiago seeds that went on to soften between March and June were already present as latent soft seeds (impermeable seeds that soften rapidly once subjected to appropriate temperature fluctuations) in March, as determined by short-term treatment at 35/10¦C. There were fewer latent soft seeds present at this time in later years that responded to this same treatment, suggesting that the longer-term hard seeds had other temperature requirements for the final stage of seed softening. High correlations (r2 = 0.90) were obtained between percent seed softening of the three cultivars in the field in the first year and after laboratory treatment with diurnal temperature fluctuations of 60/15¦C for 16 weeks, followed by four diurnal cycles of 35/10¦C. Autumn seed softening is sufficiently consistent under a wide range of environmental conditions to be a worthwhile selection criterion in medic improvement programs.