The genus Potamogeton comprising some 100 species worldwide is one of the most diverse groups of aquatic vascular plants. Despite their ecological importance as a food source and habitat for zooplankton, invertebrates, fish, and ducks, little is known about the breeding system and reproductive strategies of the majority of species.Potamogeton amplifolius is one of the few widespread species of Potamogeton found throughout the midwest and often produces abundant seed. As part of a longer term goal of understanding the correlation between critical habitat requirements and reproductive success in pondweeds, we undertook a study of inflorescence architecture and it’s association with reproductive success in P. amplifolius. The primary objectives of the study were: 1) to determine rates of reproductive success 2) to determine if positional effects within the inflorescence explain seed weight variation and patterns of differential seed set 3) to determine causal mechanisms of seed set variation among inflorescences. A total of 130 infructescences were sampled from Deep Lake in Northwest Indiana in late September 1997 just prior to time of seed abscission. Average seed weight for each infructescence was obtained on a microbalance. For a subset of infructescences (n=50) individual seed weight distribution was examined relative to, carpel number and location within a flower, and flower position on the inflorescence. Data analysis was performed using SYSTAT. Number of flowers per inflorescence, percentage seed set and average seed weight were found to vary significantly among infructescences. Seed weight was found to vary significantly by carpel location within flowers and by flower position on the inflorescence. Despite marked variations in seed weight very few aborted embros were observed. Even the smallest ovaries examined had embryos but highly reduced endosperm. The significance of the results will be discussed relative to phenology and architecture of the inflorescence of Potamogeton amplifolius.

Key words: aquatic plant, inflorescence, positional effects, Potamogeton, reproductive success