Costs and Benefits of Flower Abscission and Fruit Abortion in Asclepias Speciosa

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Abstract
Costs and benefits associated with flower abscission (dropping of flower prior to postfertilization ovary enlargement) and fruit abortion (shriveling of ovary after postfertilization enlargement) are examined in Asclepias speciosa, in which °97% of the ovaries fail to develop into mature pods. The cost in terms of mineral nutrients of producing flowers and initiating pods was low relative to mature pod production as both abscised flowers and initiated pods that aborted containing °3.2% of the amount of N, P, K, or Mg in mature pods. Abortion of initiated pods sometimes entailed an additional loss of nutrients over that entailed in flower abscission, as large aborted pods contained two to seven times the amount of N, P, K, or Mg is abscised ovaries that did not initiate surplus pod development. The relatively low cost of producing surplus ovaries and initiating surplus pods is partially opposed by the benefits of selective pod maturation. Within umbels, smaller developing pods contained less N, P, K, and water than did larger pods of the same age, and slower growing pods had a lower percentage of N and, at certain stages, less P, K, and water than faster growing pods of the same size. Smaller pods within umbels also contained both fewer and lighter seeds. These results suggest that pods differ in ability to compete for limiting resources. Through selective elemination of the poorest competitors (the slower growing, smaller pods) at an early age, pod abortion may result in survival of the fittest offspring.