ANNUAL SEED DORMANCY CYCLES IN 2 DESERT WINTER ANNUALS

1 Freshly matured seeds of Eriogonum abertianum Torr. and Eriastrum di ffusum (Gray) Mason and those of both species exhumed after various pe riods of burial in the Chihuahuan Desert in Arizona, USA, were tested for germination in light and darkness at five thermoperiods. 2 Seeds o f Eriogonum were nondormant (ND) at maturity in autumn 1988, and thus 87% of them germinated in light at 15/6 and at 20/10-degrees-C. Seeds entered conditional dormancy (CD) in winter; germination at 20/10-degr ees-C decreased to 6% in light. However, a high percentage of the seed s were again ND by autumn 1990. Thus, like seeds of facultative winter annuals in humid, temperate regions, those of Eriogonum have an annua l CD/ND cycle, which is characteristic of annuals in unpredictable hab itats. 3 Seeds of Eriastrum were dormant (D) at maturity in May 1989. A portion of the seeds after-ripened during summer, and in October 65 and 38% of them germinated in light at 15/6 and 20/10-degrees-C, respe ctively. Most of the ND seeds re-entered dormancy during winter, but b y the following October 92% of them were ND. Thus, like seeds of oblig ate winter annuals in humid, temperate regions, those of Eriastrum exh ibit an annual D/ND cycle, which is characteristic of annuals in predi ctable habitats. 4 This is the first report of dormancy cycles in seed s of desert winter annuals.