Variation in floral sex allocation and reproductive success within inflorescences of Corydalis ambigua (Fumariaceae): pollination efficiency or resource limitation?

1 The variation of floral sex allocation with flower position within inflor escences was investigated in the spring ephemeral, Corydalis ambigua. Inves tment in female function (pistil), attraction (corolla) and nectar producti on decreased from bottom to top flowers, whereas male investment (stamen) d id not differ. 2 This self-incompatible species appears to set seeds as a result of visita tion by nectar robbing bumblebee queens. The tendency of bees to visit lowe r flowers first and then move upwards within an inflorescence should result in directional pollen flow from bottom to top flowers. 3 Naturally pollinated upper flowers set fewer seeds than intermediate and lower flowers due to pollen limitation. The lack of differences in seed set and seed mass per pod following artificial outcrossing indicated that reso urce limitation did not explain the variation in seed production of flowers in different positions. Pollen viability also did not differ significantly between flower positions. 4 A model of pollination was developed that incorporated the visitation pat tern of bumblebees and observed variations in nectar distribution between f lower positions. This predicted that receipt of outcross pollen would decre ase from bottom to top flowers, but that pollen export to other plants woul d not differ between flower positions provided that the pollen exchange rat e of pollinators was either small or positively correlated with nectar cont ent of each flower position. The observed pattern of floral sex allocation would then be parallel to relative success of pollen export and import betw een flower positions within inflorescences.