Reproductive ecology of Conradina verticillata Jennison, a rare, endemic mint of the Cumberland Plateau

The Cumberland rosemary, Conradina verticillata Jennison, is an endemic, woody mint of the Cumberland Plateau of eastern Tennessee and extreme southern Kentucky. It grows along fast-moving streams on seasonally inundated sand and gravel bars and reproduces primarily asexually by rooting at the nodes. Seedlings are rare. Its reproductive biology was studied at three populations along three streams during the spring of 1992 and 1993. Although seed production was high and consistent among populations, only two percent of 1992 seeds germinated in lab studies and only one seedling was found in the field. Seeds collected in 1993 were dissected and only 7.5 percent contained fully developed embryos. Although self pollen germinates on stigmas and initiates seed production, it is much less likely than cross pollen to produce fully-developed seeds. The poor production of fully developed seeds in the field may be partly attributed to receiving self pollen from pollinators; the likelihood of a flower receiving self pollen is probably influenced by clonal spread, but the extent of clonal spread has not yet been determined. Reports of general triploidy in the species were not supported by this study; the limited information that is presently available on wild populations indicates that the species is diploid.

The main pollinators of the three study populations were medium to large polylectic bees of the genera Bombus, Apis, Anthophora, Synhalonia, and Lassioglossum. Skippers (Hesperiidae) and syrphid flies (Syrphidae) were common visitors but inefficient pollinators. Pollinator abundance and identity varied among sites, among weeks of the flowering season, and within sites between years. Differences between years may be partially attributed to different environmental cues for host and pollinator: Conradina flowering is triggered by photoperiod while emergence time of some pollinators is triggered by temperature. Although seed production does not appear to be limited by the number of flower visits, individual insects, through the maintenance of foraging routes in a pollinator-scarce habitat, may play substantial roles in pollinating a population.