Observation of dual function of nectaries in Ruellia radicans (Nees) Lindau (Acanthaceae)
(** Specialist Dieter C. Wasshausen has since determined Ruellia radicans (Nees) Lindau to be Polylychnis fulgens Bremek.
Reprinted with permission from the Torrey Botanical Society from Bull. Torrey Bot. Soc. 118 (2): 188-190. 1991.
The New York Botanical Garden, Bronx, NY 10458-5126
Gracie, C. (The New York Botanical Garden, Bronx, NY 10458-5126). Observation of dual function of nectaries in Ruellia radicans (Nees) Lindau (Acanthaceae). Bull. Torrey Bot. Club 118: 188-190. 1991.--The flowers of Ruellia radicans produce copious amounts of nectar which serves as an attractant for hummingbirds, the presumed pollinators. The annular nectaries surrounding the ovaries continue to secrete nectar after the corollas have fallen. Ants are attracted to the droplets of post-floral nectar which is substantially higher in sugar concentration than that produced at the time of anthesis.
Key words: Acanthaceae, Ruellia, nectar, ants.
In September 1989, observations were made on Ruellia radicans (Nees) Lindau (Acanthaceae) in non-flooded, moist forest surrounding Saül, French Guiana. Ruellia radicans has long (52-83 mm), tubular, red corollas (Fig. 1A) with no discernible aroma, produces nectar, and is visited (and presumably pollinated) by hummingbirds. Sugar concentration of the nectar, as measured on a weight per total weight basis with a handheld refractometer, ranged from 11.4% (one unusually low reading) to 24.0% (N =10, mean = 21.9%), a range consistent with the energetic reward and low nectar viscosity favored by hummingbirds (Baker 1975; Baker and Baker 1982). Nectar in excess of 10 ml accumulated in the bases of the corollas. Floral visitor activity was greatest between 0730-1100 hours. Several different species of hummingbirds visited the flowers, the most frequent of which was the long-tailed hermit (Phaethornis superciliosus). Flowers generally fell the day after anthesis.
Many large and small ants were observed on the inflorescences of R. radicans. The ants were primarily visiting the bases of the developing ovaries. It was observed that even after the corollas had fallen, droplets of nectar continued to be secreted by the pointed projections of the annular nectaries which surround the ovaries (Fig. 1B). The ants were consuming these droplets as rapidly as they were produced (Fig. 1C). When ants were excluded from inflorescences with young fruits, and nectar droplets were manually removed, each nectary would produce another droplet within an hour. Sugar concentrations in these post-floral nectar droplets ranged from 41% to 51%. The dramatic increase in sugar concentration might serve to make the small droplets of nectar a more attractive resource for the ants.
It has been well documented that ant foragers are effective protectors of plants which offer a reward of extrafloral nectar (Bentley 1982; Elias 1982; Elias and Gelband 1975; Faegri and van der Pijl 1979; Hölldobler and Wilson 1990; Inouye and Taylor 1979; Janzen 1966). In R. radicans, however, the production of nectar for attracting ant-guards occurs within the reproductive portion of the plant itself, rather than on the vegetative structures. Although other taxa have been shown to have, on their developing fruits, nectaries which secrete nectar to attract ants (e.g., Crescentia cujete L. (Elias and Prance 1978) and Campsis radicans (L.) Seem. (Elias and Gelband 1975)), the system operating in R. radicans differs from the above in that the same structure which produces nectar to attract pollinators during anthesis continues to function after corolla drop for the purpose of attracting ants. I hypothesize that the nectaries of R. radicans continue to secrete nectar after the corollas have fallen in order to attract ant-guards to defend the young fruits from predation by herbivores. Thus, the same nectary appears to serve both a pollination function and a post-floral, non-pollination function. The effectiveness of the visiting ants in deterring herbivores has not been assessed. However, virtually no damage to fruits was observed. Further research to determine if the ants are effective guards would be interesting.
BAKER, H. G. 1975. Sugar concentrations in nectar from hummingbird flowers. Biotropica 7(1): 37-41.
------AND I. Baker. 1982. A brief historical review of the chemistry of floral nectar. Chapter 4. In B. Bentley and T. Elias [eds.], The biology of nectaries. Columbia University Press, New York.
BENTLEY, B.L. 1982. Nectaries in agriculture, with an emphasis on the tropics. Chapter 7. In B. Bentley and T. Elias [eds.], The biology of nectaries. Columbia University Press, New York.
ELIAS, T. S. 1982. Extrafloral nectaries: Their structure and distribution. Chapter 6. In B. Bentley and T. Elias [eds.], The biology of nectaries. Columbia University Press, New York.
------AND H. GELBAND. 1975. Nectar: Its production and function in trumpet creeper. Science 189: 289-291.
------AND G. T. PRANCE. 1978. Nectaries on the fruit of Crescentia and other Bignoniaceae. Brittonia 30(2): 175-181.
FAEGRI, K. AND L. VAN DER PIJL. 1979. The principles of pollination ecology, 3rd rev. ed. Pergamon Press, Oxford.
HÖLLDOBLER, B. AND E. O. WILSON. 1990. The ants. Belknap Press of Harvard University Press, Cambridge, MA.
INOUYE, D. W. AND O.R. TAYLOR JR. 1979. A temperate region plant-ant seed-predator system: Consequences of extra floral nectar secretion by Helianthella quinquenervis. Ecology 60(1): 1-7.
JANZEN, D. H. 1966. Coevolution of mutualism between ants and acacias in Central America. Evolution 20(3): 249-275.