Variation in Pollen Limitation among Plants and Phenotypic Selection on Floral Traits in an Early-Spring Flowering Herb

Abstract

Female reproductive success in animal pollinated plants may be limited by the amount of suitable pollen that reaches stigmas, but little knowledge exists on how the extent of pollen limitation varies among plants of the same population. Here we ask; (I) if there is pollen limitation at the whole population level in a Norwegian population of an early spring blooming herb, Crocus vernus, (2) if so, if a variation in pollen limitation exists among plants, (3) over which plant traits this variation occurs, and (4) if there is phenotypic selection on plant traits caused by pollen limitation. Pollen limitation on seed number and seed:ovule ratio occurred at the whole population level. By measuring flowering time, plant size, and various aspects of floral display on both supplementary hand-pollinated plants and control plants, we found a significant relationship between reproductive success and petal size in the control plants, but not in plants that received additional pollen. Reproductive success of plants with white stigmas were more pollen limited than that in orange-stigma plants. It is likely that plants with small petals and white stigmas have lower chances of being recognised by pollinators (Apis mellifera), and that this causes the pollen limitation on reproductive success in such plants. Multivariate selection analyses showed significant linear selection on petal size in control plants, but not in extra-pollinated plants, suggesting pollinator mediated selection on petal size.