Why Do Flowers of a Hummingbird-Pollinated Mistletoe Face down?

1. Pendant flowers are common among hummingbird-pollinated plants. A downward orientation of the flower or inflorescence could represent an adaptation to avoid either flower flooding or direct pollen losses from anthers or stigmas under rainy conditions. 2. We studied the adaptive significance of this trait experimentally in Tristerix corymbosus Kuijt (Loranthaceae), a mistletoe native to the temperate forests of southern South America. We applied three treatments: (i) natural pendant inflorescences; (ii) inflorescences tethered to face up; and (iii) inflorescences tethered to face down (as a control for tethering). We also considered natural exposure to rain as a second factor. 3. The treatments did not differ significantly in either nectar volume or concentration. Flowers exposed to rain for most of their lives contained more diluted nectar than those that remained dry, but this result did not depend on either inflorescence or flower orientation. 4. We found significantly fewer pollen tubes in styles of flowers from inflorescences tethered to face up than in flowers receiving the other two treatments, but this could not be attributed to a direct effect of rain exposure. Inflorescence orientation did not affect either the number of pollen grains left in anthers or seed set. No strong evidence was found for differential visitation by hummingbirds in relation to a flower's angle. 5. The results of this work support neither the flower-flooding nor the pollen-protection hypothesis. However, a flower's orientation may affect the extent of within-flower self-pollination or the efficiency of pollen transfer from a hummingbird's bill onto a flower's stigma.