Pollen Germinability of Asclepias exaltata: Effects of Flower Age, Drying Time, and Pollen Source

Abstract

Using in vitro pollen germination experiments, we quantified the effects of flower age, drying time, and pollen source on germinability of pollinia of Asclepias exaltata. Germinability varied greatly among plants; some plants produced pollinia that germinated with twice the average success of other pollen sources. Flower age had comparatively little effect on pollen germinability, although germination rates for 3and 4-day-old flowers were slightly less than for 0-, 1-, and 2day-old flowers. Pollinia that were removed from flowers and allowed to dry lost viability relatively rapidly. After 24 hours, their germinability was reduced by more than 50%. Thus, even though the durable covering of milkweed pollinia may reduce the rate of desiccation and loss of germinability, pollen dispersal distances should still be largely restricted to distances that pollinators travel in a 24-hour period. The Asclepiadaceae and Orchidaceae are unique among dicots and monocots, respectively, in that transfer of pollen between plants involves discrete packets (pollinia). In milkweeds, this packaging of pollen appears to increase the efficiency of pollination (Wyatt 1976). Morse (1982) suggested that the durable covering of milkweed pollinia might allow a long residence time on pollinators, which would promote long-distance dispersal of pollen. This assumes that pollinia are highly resistant to desiccation and are slow to lose viability following their removal from flowers. To test this assumption, we performed in vitro pollen germination experiments. These experiments quantified the effects of flower age, drying time, and pollen source on germinability. The species selected for these tests was Asclepias exaltata L., a perennial herb commonly called poke milkweed. It occurs in moist forests and meadows in the eastern United States from Maine to Georgia and westward to Minnesota and Iowa (Woodson 1954). At maturity the plants typically produce 1-2 stems, each of which bears 3-4 umbels from the upper nodes. Each umbel is composed of about 20 flowers. Flowering occurs in June and July, and the flowers are pollinated by a variety of large flying insects, including Hymenoptera, Lepidoptera, and Diptera. We report elsewhere on nectar production, pollination, and other aspects of the reproductive biology of A. exaltata (Wyatt and Shannon 1986; Shannon and Wyatt