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Floral morphological changes and reproductive success in deer weed (Lotus scoparius, Fabaceae)¹

²Department of Biological Science, California State University, Fullerton, California 92834; and ³Department of Ecology and Evolutionary Biology and the Department of Botany, University of Tennessee, Knoxville, Tennessee 37996

Pollination-related and time-dependent floral morphological changes occur in a diverse set of angiosperm taxa and appear to be particularly common in species occupying resource-limited environments. In deer weed (Lotus scoparius), such floral modifications include a color change from yellow to orange and a folding of the banner petal down over the keel. These changes are rapidly induced by pollination, but will also occur much more slowly without pollination. Orange flowers typically lack nectar and pollen. We examined the reproductive success of these plants to test the hypothesis that retention of orange flowers increases pollinator visitation rate and fruit set while reducing costs to the pollinators. All of the common species of bee pollinators that visited deer weed easily distinguished between yellow and orange flowers at close range and preferentially probed yellow flowers. Retention of orange flowers by these plants resulted in a higher frequency of pollinator visits and a higher fruit set per flower than plants that lacked orange flowers. The number of flowers visited by each pollinator was lower on plants with a mixture of yellow and orange flowers, suggesting that the presence of orange flowers may reduce selfing. The possible selective pressures involved in the evolution of these mechanisms and their relation to stressful environments are also discussed.

Key Words: Fabaceae • floral display • Lotus scoparius ssp. scoparius • pollinator visitation • postpollination changes • reproductive success • self-pollination

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