Role of warm stratification in promoting germination of seeds of Empetrum hermaphroditum (Empetraceae), a circumboreal species with a stony endocarp

The broad objective of this research was to define the role of warm (≥15°C) stratification in breaking dormancy in seeds with stony endocarps that require warm-plus-cold (～0°-10°C) stratification for germination. This question was addressed using seeds (true seed + endocarp, hereafter called seeds) of Empetrum hermaphroditum. Only 2-5% of freshly matured seeds collected in September and October at five sites in Sweden germinated in light at daily alternating temperature regimes of 15°/6°, 20°/10°, and 25°/15°C. Dormancy was not due to impermeability of the stony endocarp surrounding each seed, and embryos did not grow prior to radicle emergence. Thus, seeds did not have physical, morphological, or morphophysiological dormancy. Long periods of either cold stratification (20 or 32 wk) or warm stratification (16 wk) resulted in a maximum of 22-38 and 10% germination, respectively, in light at 25°/15°C. After 12 wk warm stratification plus 20 wk cold stratification, 83-93% of the seeds germinated in light at the three temperature regimes. For a cold stratification period of 20 wk, germination increased with increase in length of the preceding warm stratification treatment. Gibberellic acid (GA(3)) promoted germination of 77-87% of the seeds. Based on dormancy-breaking requirements and response to GA(3), 62-78% of the seeds had intermediate physiological dormancy; the others had nondeep physiological dormancy. Contrary to suggestions of several other investigators that warm stratification is required to make the endocarp permeable to water via its breakdown by microorganisms, our results with E. hermaphroditum show that this is not the case. In this species, warm stratification is part of the dormancy-breaking requirement of embryos in seeds with intermediate physiological dormancy.