Submitted to: Ecological Monographs
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 1, 2000
Publication Date: N/A
Interpretive Summary: A new assay and standard is presented that allows for reliable and accurate measurement of protein concentration from small amounts of pollen. This is applied to a survey of 377 plant species in 93 families, providing the first comprehensive overview of pollen protein concentration. The range is extreme, from merely 2.5% protein to 60% protein dry weight (or double that of dried legumes in our diet). Pollen protein concentration is found to be predictable by the genus and to some extent the family of plant to which a species belongs. Since bees depend solely on pollen for all of their dietary protein, their choice of pollen from such a broad range of possibilities is of great significance for bee reproductive success. The survey also points to promising plant species to serve as pollen hosts for bees whose populations are being "farmed" for agricultural pollination.
Technical Abstract: Pollen ranges from 2.5% to 60% protein. Most pollen proteins are likely to be enzymes that function during pollen tube growth and subsequent fertilization, but the vast range of protein quantity need not reflect only pollen-pistil interactions. Because numerous vertebrate and invertebrate floral visitors consume pollen for protein, protein content may influence floral host choice. Additionally, many floral visitors pollinate their host plants. If protein content influences pollinator visitation, then pollinators could select for increased protein content of host plants. We analyzed or gleaned from literature crude pollen protein concentrations of 377 plant species from 93 plant families. Using this data base, we compared pollen protein concentration with pollination mode, collection by bees, and distance from stigma to ovule after accounting for phylogeny through paired phylogenetic comparisons and a nested ANOVA including taxonomic rank. We found that zoophilous species were not statistically richer in pollen protein than anemophilous species, although the three most speciose anemophilous clades surveyed were generally poor in protein. We also found that bee-pollinated species were not richer in protein than other zoophilous species and that pollens collected by bees, regardless of pollination, were not richer in protein than pollens not collected by bees. Plant genera hosting specialist pollen-collecting bees did not have particularly protein-rich pollen. Both mass of protein per pollen grain and pollen grain volume were correlated with stigma-ovule distance.