The relationship of Helianthus atrorubens and Helianthus silphioides in Tennessee

The relationship of Helianthus atrorubens L. andH. silphioides Nutt. in Tennessee was investigated using data from morphology, floral flavonoid constituents, and seed germination.

Analysis of morphology by cluster analysis and principle coordinate analysis indicated two morphological groups with some intermediate specimens. The groups were separated by principle coordinate analysis on the basis of phenotypically stable characters. Half of the intermediate specimens were members of four populations collected in central Tennessee eventually identified as H. silphioides.

Flavonoids forming ultraviolet nectar guides on ray flowers of both species were studied. Anthochlor flavonoids were found to be consistently present in sampled specimens of H. atrorubens but were absent in individuals sampled from most populations of H. silphioides. Three out of four central Tennessee populations comprised of intermediate specimens exhibited absence of anthochlors in sampled individuals.

Seed germination of the species was tested to determine whether there were interspecific differences in dormancy and temperature response after stratification. H. silphioides seed were found to be dormant after 4 weeks dry storage while seed of H. atrorubens germinated slowly to near maximum. After various stratification treatments and incubation at two alternating temperatures, seed of H. silphloides germinated significantly slower than H. atrorubens seed.

On the basis of these data, the species are recognized as morphologically and, for the most part, chemically distinct in Tennessee. Intermediacy of central Tennessee populations based on morphology was not confirmed by evidence from floral flavonoid profiles. The relationship of these populations to the two species cannot be determined from this study. Differences in seed germination characteristics suggest that the species may have different germination strategies that are related to habitat preference and range limitation.