© CSIRO 2003 10.1071/FP02175 1445-4408/03/030345 Functional Plant Biology , 2003, 30, 345–352 www.publish.csiro.au/journals/fpb CSIRO PUBLISHING Dormancy release in Lolium rigidum seeds is a function of thermal after-ripening time and seed water content Kathryn J. Steadman A,C , Andrew D. Crawford A and Robert S. Gallagher A,B A Western Australian Herbicide Resistance Initiative, Faculty of Natural and Agricultural Sciences, University of Western Australia, Crawley, WA 6009, Australia. B Present address: Department of Crop and Soil Science, Washington State University, Pullman, WA 99161-6420, USA. C Corresponding author; email: ksteadman@agric.uwa.edu.au Abstract. Dormancy release in seeds of Lolium rigidum Gaud. (annual ryegrass) was investigated in relation to temperature and seed water content. Freshly matured seeds were collected from cropping fields at Wongan Hills and Merredin, Western Australia. Seeds from Wongan Hills were equilibrated to water contents between 6 and 18% dry weight and after-ripened at constant temperatures between 9 and 50°C for up to 23 weeks. Wongan Hills and Merredin seeds at water contents between 7 and 17% were also after-ripened in full sun or shade conditions. Dormancy was tested at regular intervals during after-ripening by germinating seeds on agar at 12-h alternating 15°C (dark) and 25°C (light) periods. Rate of dormancy release for Wongan Hills seeds was a positive linear function of after-ripening temperature above a base temperature (T b ) of 5.4°C. A thermal after-ripening time model for dormancy loss accounting for seed moisture in the range 6–18% was developed using germination data for Wongan Hills seeds after-ripened at constant temperatures. The model accurately predicted dormancy release for Wongan Hills seeds after-ripened under naturally fluctuating temperatures. Seeds from Merredin responded similarly but had lower dormancy at collection and a faster rate of dormancy release in seeds below 9% water content. Keywords: dormancy loss, seed moisture, weed biology, thermal after-ripening time model. Introduction Originally introduced as a pasture species, the winter annual Lolium rigidum Gaud. (annual ryegrass) has become the primary weed in cropping regions of southern Australia (Gill 1996; Powles and Bowran 2000). Plant persistence in the Mediterranean-type climate that prevails across much of the region is reliant on seeds with effective dormancy mechanisms that prevent untimely germination during sporadic summer rainfall events, but allow germination during autumn and winter months when rainfall is sufficient to sustain plant growth and development. Dormancy in annual ryegrass seeds at physiological maturity tends to be variable, with the amount of germination (at 20–25°C) reported in Western Australian collections ranging between 0% (Chapman et al. 1999), 40–50% (Gramshaw 1972; Pearce 1973) and 30–70% (Wiesner and Grabe 1972). Dormancy release occurs during the hot, dry summer months on the soil surface, and the least-dormant seeds germinate with the break-of-season rainfall. Some seeds lose dormancy and emerge later in the growing season in competition with the crop, and a further proportion remains dormant in the seedbank for future years. With widespread resistance to many herbicides common in annual ryegrass (Preston 2000), its management is an increasing problem in southern Australian cereal cropping systems. Improved physiological understanding of behaviour, and particularly dormancy (Forcella et al. 2000), of annual ryegrass seeds will improve emergence prediction and be useful in develop- ing alternative management options. Temperature is the major environmental parameter influ- encing dormancy release in most species, with warm temperatures usually promoting dormancy loss in dry seeds of winter annuals in a process termed ‘after-ripening’ (Baskin and Baskin 1976, 1986, 1998). For example, rate of dormancy release is faster at warmer temperatures in the annual grasses Hordeum vulgare L., Avena fatua L. and Bromus tectorum L. than at cooler temperatures (Favier and Woods 1993; Foley 1994; Bauer et al. 1998). However, Abbreviations used: DRR, dormancy release rate; G, germination percentage; RH, relative humidity; t, after-ripening time; T , after-ripening temperature; T av , average temperature; T b , base temperature for dormancy release; WC, water content; θ, thermal time.