SEED-GERMINATION TIMING PATTERNS IN INTERMOUNTAIN PENSTEMON (SCROPHULARIACEAE)

Regulation of seed germination timing is an important component of pop ulation persistence, especially for short-lived plants in seasonal env ironments with unpredictable year-to-year weather variation. Seed germ ination patterns were examined in the laboratory for seeds from 135 po pulations belonging to 38 species and 13 sections of the genus Penstem on (Scrophulariaceae), from habitats ranging from warm desert to alpin e tundra. Seeds of most species were dormant at dispersal and required a period of moist chilling to become germinable. Response to chilling was related to probable chilling duration at the collection site. Pop ulations from habitats with severe winters produced seeds with long ch illing requirements, while those from habitats with mild winters produ ced seeds with short chilling requirements. Populations from midelevat ion habitats produced seeds with intermediate chilling requirements bu t with a sizeable fraction whose dormancy was not broken by chilling o f any length. Another pattern observed mostly in warm desert populatio ns was little primary dormancy at autumn temperatures combined with in duction of a fraction into secondary dormancy by short chilling. Speci es with a wide habitat range included populations with contrasting ger mination patterns. Parallel habitat-correlated patterns were observed in different sections. Most species showed germination patterns that c ombined predictive mechanisms with potential for carryover of a persis tent seed bank. Results of common garden experiments suggested that ge rmination differences had a strong genetic basis both among population s and among plants in a population. Adaptive radiation in the genus Pe nstemon has thus included the evolution of habitat-specific germinatio n timing strategies in multiple lineages within the genus.