Nectar Production and Pollination of Asclepias exaltata

Nectar production and pollination of the herbaceous perennial milkweed Asclepias exaltata were studied in three populations in southwestern Virginia. Individual flowers were available to pollinators, chiefly large bees and wasps, an average of 6.2 days. Volumes, concentrations, and amounts of sucrose in nectar decreased as flowers aged. Most nectar was secreted overnight, and standing stocks increased in concentration from 20-30% in the morning to 40-60% in the afternoon. There were significant differences among plants in all components of nectar production, with sucrose amounts varying up to four-fold. The average number of pollinaria removed per flower was significantly greater in 1981 (1.38 ? 1.25; x ? s) than in 1982 (0.78 ? 0.09), as was the average number of insertions per flower (0.66 ? 0.93 versus 0.39 ? 0.66). Pollination efficiency, the ratio of insertions to removals, was nearly the same in 1981 (0.43 ? 0.66) as in 1982 (0.36 ? 0.59). Rates of pollination were positively correlated with nectar concentration. In addition, plants producing concentrated nectar matured more fruits and had higher percentage fruit-set than did plants with dilute nectar. These data from natural populations agree with general results of laboratory trials suggesting that bees prefer concentrated nectar and that bees can respond to differences in nectar quality among plants. The extremely specialized morphology of milkweed flowers is rivaled only by that of the orchids. Unlike the orchids, however, milkweeds have been studied intensively by plant reproductive biologists only within the past ten years. This is surprising in view of the "elegantly contrived . pollinating apparatus of asclepiads" and general agreement that "the entomophilous pollination of asclepiads provides one of the most fascinating chapters in biology" (Woodson 1954). The flower of Asclepias exaltata L. , like those of the other 107 North American species (Woodson 1954), consists of five showy, reflexed petals covering five smaller, greenish sepals (fig. 1). Two free, superior ovaries are joined by their styles to form a gynostegium with five lateral stigmatic surfaces. These surfaces are enclosed by the tightly abutting wings of adjacent anthers to produce five stigmatic chambers. Five coronal extensions from the base of the stamen (hoods), each of which usually contains an arching structure (horn), serve as reservoirs for nectar secreted by nectaries located within the stigmatic chambers (Galil and Zeroni 1965). There are five pollinaria, each of which consists of paired pollen sacs (pollinia) from adjacent anthers joined by translator arms to a corpusculum situated just above the slit or opening to the stigmatic chamber (Lynch 1977). Pollination is a two-stage process: 1) removal of a pollinarium occurs when a groove in the corpusculum catches on a bristle or other appendage of an insect and is forcibly pulled from the flower, and 2) insertion is effected when a pollinium lodges in a stigmatic chamber. The unique morphology of milkweed flowers makes it possible to quantify aspects of pollination and fruit-set that are difficult to study in other plants, whose small pollen grains are transported individually. Pollination rates can be scored directly and easily in the field by examining flowers and recording numbers of pollinaria removed. Removal rates give some indication of relative success of plants as male parents. Dissection of preserved flowers allows scoring of numbers of insertions, an index of relative female success. Several workers have taken advantage of these features in studies of the reproductive biology of milkweeds (e.g., Willson and Rathcke 1974; Wyatt 1976, 1978, 1980a, 1980b, 1981; Willson and Price 1977; Lynch 1977; Bertin and Willson 1980; Kephart and Heiser 1980; Kephart 1981; Fritz and Morse 1981; Morse 1982; Chaplin and Walker 1982; Pleasants and Chaplin 1983). Milkweeds also are favorable subjects for studies of nectar production because nectar collects in the hoods and can be removed with relative ease. The durable, relatively long-lived