Germination responses to temperature responsible for the seedling emergence seasonality of Primula sieboldii E. Morren in its natural habitat

Temperature requirements for the breaking of seed dormancy and germination in Primula sieboldii E. Morren and the annual surface‐soil temperature regime in one of its natural habitats were investigated in order to clarify the germination responses determining the seedling emergence seasonality of the species. In a grassland nature reserve in an abandoned flood plain of the Arakawa River, natural seedling emergence of the species was shown to be restricted to mid‐ to late‐spring before the closure of seasonal vegetational gaps, when the daily mean soil surface temperature reached about 15°C, accompanied by large daily fluctuations of about 10°C. Mature seeds collected in late June were never able to germinate at any constant temperature in the range of 8–40°C unless they had been previously subjected to moist‐chilling treatment. The proportion of seeds which were released from dormancy increased with increasing duration of the moist‐chilling treatment at 2°C, 70–85% of seeds becoming germinable at 16–28°C after 12 weeks of pretreatment at 2°C. The thermal time required for the germination of the thus‐pretreated seed population was 905–1690 Kh with a base temperature of around 5°C. Fluctuating temperatures between 24°C and 16 or 12°C had a remarkable dormancy‐breaking effect, inducing considerably quick germination in most of the seeds previously subjected to 2°C moist‐chilling for 8 weeks.