The comparative biology of the two closely-related species Penstemon tenuiflorus pennell and P. hirsutus (L.) Willd. (Scrophulariaceae, Section Graciles): II. Reproductive biology

Flowering pattern, flowering phenology, and mating system of the closely-related, interfertile species Penstemon tenuiflorus and P. hirsutus are the same. Flowers are protandrous, but there is some overlap of anther dehiscence and stigma receptivity. Additionally, flowers of different sexual phases are scattered throughout an inflorescence. Primary pollinators of both species were Bombus pennsylvanicus and B. bimaculatus. Outcrossed plants of P. tenuiflorus (studied in both field and greenhouse) and of P. hirsutus (studied in greenhouse only) produced significantly more seeds than selfed plants. In a greenhouse study, bagged, self-pollinated plants of both species produced significantly more seeds than bagged, nonmanipulated ones. However, although number of seeds produced by bagged, self-pollinated plants of P. tenuiflorus in a field study (P. hirsutus not studied) was greater than that produced by bagged, nonmanipulated plants, the differences were not significant. In both species, there was little or no difference in weight or in percent viability in outcrossed vs. selfed seeds, and very few or no seeds were produced by bagged, emasculated flowers. In the field, number of seeds produced by nonbagged, nonmanipulated and by nonbagged, cross-pollinated (by hand) plants of P. tenuiflorus did not differ significantly (P. hirsutus not studied). Neither percent seed germination nor seedling-juvenile growth parameters differed between selfed vs. outcrossed progeny of P. tenuiflorus (P. hirsutus not studied). These results suggest that both species use a mixed mating system consisting of xenogamy, geitonogamy, and autogamy; agamospermy is unimportant. Further, seed set in P. tenuiflorus is not pollinator limited (P. hirsutus not studied), and the species does not appear to exhibit inbreeding depression in seed weight, in seed germination, or in several growth parameters. However, in both species, selfed flowers produced significantly fewer seeds than outcrossed flowers.