Proximate and Ultimate Causes of Low Fruit Production in Asclepias Exaltata

In Asclepias exaltata L., a hermaphroditic milkweed, fruit production is constrained primarily by the availability of nutrient resources, as shown by the increased survival of fruits after removal of competing fruits and after application of fertilizer. The results of other tests exclude genetic-developmental incompatibilities, fruit herbivory, and insufficient pollination as major causes of low fruit production. Removal of half the flowers in each inflorescence had little effect on fruit production but decreased pollinarium removals by half, suggesting that one reason why few flowers mature fruits is that many flowers are produced primarily to enhance male reproduction. It also raises the possibility that fruit abortion is an adaptation that allows plants to invest resources more profitably in male reproduction, but tests of this hypothesis were inconclusive.