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
nondeep physiological dormancy (PD). In the buried seed study,
dormancy-break occurred in summer (June to 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 to 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 one year, and it may be a bet-hedging strategy in the
rainfall-unpredictable cold desert environment.