Bumblebee visitation and seedset in Melampyrum pratense and Viscaria vulgaris: heterospecific pollen and pollen limitation.

Fruiting and seed set in two bumblebee-pollinated herbs, Melampyrum pratense L. (annual, Scrophulariaceae) and Viscaria vulgaris Bernh. (perennial, Caryophyllaceae) were studied on a dry meadow in southwestern Sweden in June 1986 and 1988. Both species produced seeds by self-fertilization. In Melampyrum (homogamous) fruiting and seed set by selfing were much lower than by natural pollination; in Viscaria (protandrous) fruiting by selfing and by natural polination were similar, but seed set per flower was lower by selfing than by natural pollination. Sequential hand pollinations increased seed set in Melampyrum, but not in Viscaria. Thus, the number of pollinations is important for high seed set in Melampyrum, and number of pollen grains deposited one pollination is important for high seed set in Viscaria. Late flowering resulted in the production of fewer seeds in both species, although the visitation rate in pure Viscaria stands was sufficient, because of limited resources. Pollen was the limiting resource in Viscaria, because hand pollination increased natural seed set. In Melampyrum pollen was limiting in 1988 but so were consumable resources, because the seedset decreased with time despite hand pollination. Pure stands of Viscaria had sced set similar to plants in mixed stands (with Melampyrum and Rhinanthus), although plants in mixed stands received fewer visits. Many seeds produced late in the season are the result of self pollination; emasculated Viscaria flowers had a very low seedset late in the season. Pollen loads containing approximately 50% heterospecific grains did not affect seed set in either species. Application of heterospecific (Lupinus) pollen to receptive Viscaria styles 6 h before conspecific pollen did not affect seed set.