THE POLLINATION BIOLOGY OF CLETHRA ALNIFOLIA L. (CLETHRACEAE) (ESSENTIAL OIL, CHEMICAL ECOLOGY, FLORAL ODOR)

Most pollination studies have focused on the insects attracted to flowers and not on the chemical relationship between the flowers and insects. Conversely, most analyses of floral odors have identified the volatile components but ignored insects attracted to the flowers. How insects are attracted to flowers is significant for the pollination of agricultural crops and in understanding the evolutionary relationship between insects and angiosperms. This is the first study of pollination biology in the Clethraceae, a family of some 75 species. Insects were collected from Clethra alnifolia L. flowers for four summers. Pollen loads on insects were examined for type of pollen and percent Clethra pollen. Inflorescences were given various treatments to determine the importance of apomixis and self-, wind and insect pollination. Nectar was collected from protected flowers. Sugars in it were analyzed by thin layer chromatography. Amino acids were analyzed by high performance liquid chromatography. An extract of whole inflorescences was made using hexane and methanol. Components of the extract were identified by their Kovats retention indices on Carbowax 20M and OV-102 capillary columns compared to those of standards, and by their mass spectra.;The sugars in the nectar were sucrose, glucose and fructose, with sucrose present in larger amounts than the other two. An unidentified, higher molecular weight compound was also present. Sixteen amino acids were present in the nectar in low amounts. Thirty-three compounds were identified in the solvent extract of the flowers. The ten largest components identified were: anethole, benzyl benzoate, cinnamyl alcohol, eugenol, 2-heptadecanone, 2-heptanol, cis-3-hexenol, methyl hexadecanoate, methyl isoeugenol, n-nonanal dimethyl acetal and phenylethyl alcohol.;The odor profile of C. alnifolia is similar to that of other flowers which attract bees, in that it contains anethole, phenylethyl alcohol, benzyl alcohol, linalool, eugenol and geraniol.;Based on their abundance and behavior on the flowers, the most important pollinators were Bombus spp., especially B. impatiens L. Bumblebees collect Clethra pollen by buzzing the flowers. Apis mellifera L. was also attracted in large numbers, but cannot vibrate the flowers for pollen. A small beetle, Cantharis nigriceps LeConte, was also common on the flowers.