Habitat-Correlated Variation in Seed Germination Response to Chilling in Penstemon Section Glabri (Scrophulariaceae)

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Abstract
Members of Penstemon Section Glabri are common perennial herbs in habitats from desert shrublands to subalpine meadows throughout the Intermountain West. Germination response to moist chilling was evaluated for 36 populations belonging to 13 species within the section. Eleven species (P. cyananthus, P. cyaneus, P. cyanocaulis, P. fremontii, P. laevis, P. leiophyllus, P. perpulcher, P. scariosus, P. speciosus, P. strictus 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 among species, populations and years of collection; many middle elevation collections contained a large fraction nonresponsive to chill. Most chill-responsive seeds germinated during prolonged chilling. Across species, germination rate during chilling was significantly correlated with mean January temperature at the collection site for each of 3 yr of collection, i.e., collections from habitats with warmer winters germinated more quickly in the cold than those from sites with colder winters. Similarly, the chilling period needed for maximal postchilling germination increased as collection site winter temperature decreased. Experiments 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. Adaptive 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.

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