We explored the combined effects of seasonal variation in both pollinator assemblage and availability of pollen donors on pollination in a gynodioecious species, Sidalcea oregana ssp. spicata (Malvaceae). Hermaphrodites produced flowers with significantly larger petals but maintained fewer open flowers per inflorescence than females. Flowers of hermaphrodites produced 50% more nectar sugar in the 24 h after anthesis than the flowers of females. Nectar sugar production was also significantly and positively correlated with petal length. Pollinator visitation rates were influenced more by differences in petal length than by differences in flower number per inflorescence. Consequently, her-maphrodites experienced higher visitation rates on a per-flower basis. Female flowers tended to receive pollen at a lower rate than hermaphrodites, but remained in the receptive female-phase longer than hermaphrodites. On average, the length of the period of flower receptivity declined as pollen deposition rate increased. These opposing processes resulted in the sex morphs receiving equivalent levels of pollination. Seasonal variation in the rate of pollen reception was more strongly influenced by the efficiency of the available pollinator pool than by rates of visitation. Dramatic seasonal shifts in the composition of the pollinator assemblage and pollen availability were correlated with increased pollination intensity as the season progressed. Not only were more pollen grains received, but they arrived in a shorter period of time and the number of potential pollen donors (hermaphrodites) in-creased. These findings suggest that pollen competition in both sex morphs may be more intense late in the season.
Ashman, Tia-Lynn and Stanton, Maureen, "Seaonal Variation in Pollination of Sexually Dimorphic Sidalcea Oregana ssp. Spicata (Malvaceae)" (1991). An. Paper 54.
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