HABITAT-CORRELATED VARIATION IN SEED-GERMINATION RESPONSE TO CHILLINGIN PENSTEMON SECTION GLABRI (SCROPHULARIACEAE)

Members of Penstemon Section Glabri are common perennial herbs in habi tats from desert shrublands to subalpine meadows throughout the Interm ountain West. Germination response to moist chilling was evaluated for 36 populations belonging to 13 species within the section. Eleven spe cies (P. cyananthus, P. cyaneus, P. cyanocaulis, P. fremontii, P. laev is, P. leiophyllus, P. perpulcher, P. scariosus, P. speciosus, P. stri ctus and P. subglaber) had seeds that were largely dormant at harvest and responded positively to chilling, whereas two (P. ammophilus and P . commarhenus) had seeds that were largely nondormant. The proportion of seeds that could be rendered nondormant through chilling varied amo ng species, populations and years of collection; many middle elevation collections contained a large fraction nonresponsive to chill. Most c hill-responsive seeds germinated during prolonged chilling. Across spe cies, germination rate during chilling was significantly correlated wi th mean January temperature at the collection site for each of 3 yr of collection, i.e., collections from habitats with warmer winters germi nated more quickly in the cold than those from sites with colder winte rs. Similarly, the chilling period needed for maximal postchilling ger mination increased as collection site winter temperature decreased. Ex periments with common garden-grown seeds of six of the species showed that patterns characteristic of wild-collected seeds persisted in the garden, indicating a probable genetic basis for the differences. Adapt ive radiation in Penstemon Section Glabri has resulted in evolution of population-level germination syndromes that maximize the probability of establishment in a range of specific habitats.