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Completed Project

Resource Allocation in the Andromonoecious Plant Polanisia dodecandra ssp. dodecandra

Jeremy Middleman ’18 and Mark Schlessman (Biology)

Andromonoecy is a sexual system characterized by the presence of bisexual and male flowers on the same plant. The Resource Saving (RS) hypothesis suggests that andromonoecious plants save resources on male flowers. This hypothesis predicts that all parts of male flowers would be smaller than those of bisexual flowers. In contrast, the Increased Pollen Donation (IPD) hypothesis suggests that male flowers may be better at donating pollen than bisexual flowers.  This hypothesis predicts that male flowers would have more pollen per anther, more stamens, and larger petals than bisexual flowers. Polanisia dodecandra is an andromonoecious plant whose flowers grow in zones of bisexual and male flowers, with bisexual flowers preferentially made when the plant is high on resources. We used floral size measurements to assess these contrasting, but not mutually exclusive hypotheses. Both a bisexual and a male flower were examined on each of 30 plants. The next day, short sections of grass shoots were placed over bisexual stigmas in order to prevent pollination. Paired t-tests were conducted for sepal length, petal length, total stamens, average stamen length, pistil length, ovary length, nectar sugar concentration, nectar volume, and corolla width. Bisexual flowers had statistically significantly longer petals, pistils and ovaries, larger nectar volumes, and wider corollas. These results support the RS hypothesis. Our data yielded only weak support for the IPD hypothesis: male flowers had more stamens and higher nectar sugar concentrations than bisexual flowers, but the data were not statistically significant. While this study examined size measurements of the flowers, biomass measurements of flowers may provide new insights. Future research is needed to know whether resources saved through male flowers are reallocated to fruit production.