Semiannual dormancy cycling results in two seedling cohorts of annual species in the cold desert of Central Asia

Our aim was to quantify the life-history strategies of the cold desert annual species Alyssum linifolium and Tetracme quadricornis (Brassicaceae), with particular emphasis on the seed stage. Freshly matured seeds were tested for germination over a range of alternating temperature regimes in light and in dark, and the effects of seed coat scarification, cold stratification, dry after-ripening and GA₃ on dormancy-break and germination were determined. Seeds were buried in the field at 0 (surface), 2 and 5 cm and germination of exhumed seeds tested at monthly intervals for 2 years. Seedling emergence of both species was monitored in the cold desert, and survival to maturity of plants from autumn- and spring-germinated cohorts was determined. Most fresh seeds were dormant, and dormancy was broken by all four treatments tested. In the early stages of dormancy-break, seeds germinated to low percentages over the range of temperatures, and with additional dormancy-break germination percentages increased. Thus, seeds have Type 6 non-deep physiological dormancy. In the buried seed study, dormancy-break occurred in summer (June–August), and germination peaked in late summer. By late autumn (November), all non-germinated seeds had re-entered dormancy. During snowmelt in late winter (February–March), some dormancy-break occurred, and low percentages of seeds of both species germinated. In the field, seeds of both species germinate in autumn and in spring, with more seeds of A. linifolium germinating in autumn than in spring and more seeds of T. quadricornis germinating in spring than in autumn. A portion of plants from both seedling cohorts of both species survived to maturity (set seeds) with more spring- than autumn-germinating plants doing so for both species. Thus, both species behave as facultative winter annuals. Synthesis. Seeds of A. linifolium and T. quadricornis have two dormancy cycles per year. In one cycle, dormancy is broken via warm temperatures in summer and in the other one via cold stratification in late winter. Semiannual dormancy cycling results in two germination cohorts in 1 year, and it may be a bet-hedging strategy in the rainfall-unpredictable cold desert environment.