Dormancy release in Lolium rigidum seeds is a function of thermal after-ripening time and seed water content

Dormancyrelease in seedsof Lolium rigidumGaud. (annual ryegrass) was investigated in relation to temperature and seed water content. Freshly matured seedswere collected from cropping fields at Wongan Hillsand Merredin, Western Australia. Seedsfrom Wongan Hillswere equilibrated to water contentsbetween 6 and 18% dry weight and after- ripenedat constant temperatures between 9 and 50degreesC for up to 23 weeks. Wongan Hillsand Merredin seedsat water contentsbetween 7 and 17% were also after- ripenedin full sun or shade conditions. Dormancywas tested at regular intervals during after- ripeningby germinating seedson agar at 12-h alternating 15degreesC (dark) and 25degreesC (light) periods. Rate of dormancyrelease for Wongan Hills seedswas a positive linear functionof after- ripeningtemperature above a base temperature (T-b) of 5.4degreesC. A thermal after- ripeningtime model for dormancyloss accounting for seedmoisture in the range 6-18% was developed using germination data for Wongan Hills seedsafter- ripenedat constant temperatures. The model accurately predicted dormancyrelease for Wongan Hills seedsafter- ripenedunder naturally fluctuating temperatures. Seedsfrom Merredin responded similarly but had lower dormancyat collection and a faster rate of dormancyrelease in seedsbelow 9% water content.