Contrasting seed germination patterns for intracontinental disjuncts of Heuchera (Saxifragaceae) in North America: the eastern temperate H. parviflora and western montane H. cylindrica

Abstract

Heuchera parviflora var. parviflora grows primarily in southeastern North America and Heuchera cylindrica var. cylindrica in northwestern North America. Seeds of H. parviflora were collected in Kentucky in November 1995 and 1996 during the dispersal stage of the life cycle, and those of H. cylindrica were collected in Washington in August 1996. The purpose of the study was to compare the germination patterns of these two intracontinental disjunct species of Heuchera. During 2–12 weeks of incubation over a range of thermoperiods (15:6, 20:10, 25:15, 30:15, 35:20°C (12 h : 12 h)) that simulated habitat temperatures, fresh H. parviflora seeds germinated to 62–87% at 20:10 – 30:15°C in light. Cold stratification at 5°C effectively broke conditional dormancy in H. parviflora seeds: 76–96% germination took place during 2 weeks of incubation at 15:6 – 35:20°C in light following 12 weeks of cold stratification in light. In contrast, fresh H. cylindrica seeds germinated to 69–94% at 15:6 – 25:15°C in light during 2–12 weeks of incubation in light, but cold stratification at 5°C did not improve germination. Peak germination of H. parviflora seeds sown in November 1996 in an ambient-temperature greenhouse occurred in early March 1997, whereas that of H. cylindrica seeds sown in September 1996 occurred in late October 1996. Adaptive divergence of these species apparently has occurred with respect to their seed germination ecology. Their dormancy and germination characteristics are similar to the many other species growing in the same vegetation types.Key words: adaptive divergence, Heuchera cylindrica, Heuchera parviflora, physiological dormancy, Saxifragaceae, seed germination.