Floral Biology of the federally threatenedApios priceana(Fabaceae)

Abstract

Apios priceana B. L. Rob. is federally threatened and little is known about its pollination biology. At the Redstone Arsenal population, Alabama, we studied its breeding system, determined major floral visitors and their pollination effectiveness, and explored pollen limitation of fruit and seed production. Fruit initiation was low (3.9%) for cross-pollinated flowers but only marginally greater than for self-pollinated or nonpollinated flowers (none initiated a fruit). Most floral visits (97%) were by bees with 43% of visits by large bees (> 1.5 cm in length) and 51% by medium bees (1–1.5 cm in length). Keel petals of A. priceana flowers enclose the style and stamens: flowers must be “tripped” for pollination to occur. Large and medium bees were equally effective in tripping flowers, whereas small bees (< 1 cm in length) did not trip flowers. Success of large and medium bee species (several Bombus species and Megachile sculpturalis Smith) in initiating legumes after a single floral visit ranged from 17–35% but did not differ significantly among species. Pollen supplementation showed legume production was pollen-limited: 24% of supplemented flowers produced fruits versus 6% of nonsupplemented flowers. Seed counts revealed a marginally nonsignificant trend for more seeds in legumes from pollen-supplemented flowers. We conclude that this rare plant species is dependent on medium and large bees for pollination, and that reproduction is partially pollen-limited. Conservation efforts should include measures to protect bee pollinators of this species to ensure that managed populations of this rare plant continue to reproduce sexually.