Timing of seed germination in two alpine herbs on the southeastern Tibetan plateau: the role of seed dormancy and annual dormancy cycling in soil

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Germination timing strongly affects the probability of seedling survival and the conditions for subsequent plant growth and reproduction. For alpine species, timing of germination should be restricted to a period that ensures seedlings will attain a critical size during the short growing season and thus survive winter. Thus, we hypothesized that seeds of herbaceous alpine perennials germinate only at the beginning of the growing season in spring, when temperatures are low, and the seed dormancy mechanism helps synchronize the germination event to the beginning of the growing season. Seed traits and effects of light, temperature, cold stratification and GA3 on germination of Primula alpicola and Pedicularis fletcheri were tested in the laboratory and germination responses to temperature of seeds buried in the field at 4500 m a.s.l. and exhumed over a 2-year period were investigated. Fresh seeds were dormant, and cold stratification and GA3 overcame dormancy in seeds of both species and the light requirement for germination of P. alpicola seeds. Buried seeds of both species exhibited an annual dormancy/nondormancy/dormancy cycle. Seeds of P. alpicola and P. fletcheri have nondeep physiological dormancy and dormancy cycling. Dormancy cycling was coordinated with seasonal environmental conditions, and seeds germinated soon after snowmelt in late spring. This is the first comprehensive demonstration of regulation of germination timing by dormancy cycling in alpine plants.