Chemical analyses of the pollen protein resources in entomophilous flowers

Over the last decades, populations of pollinators have been declining worldwide. Changes or decreases in floral resources, i.e. nectar and pollen, are recognised as one of the main causes of such decline. Our assumptions are that floral resources (nectar and pollen), available throughout the foraging season and of high nutritional quality (for example, high levels of proteins and amino acids in pollen) are essential to sustain pollinator populations. The first objective of this research was to find an easy, cheap and reproducible method to assess the protein resources, i.e. amino acids and polypeptides, present in the pollen. The second objective was to detect relationships between plant family membership and the protein content of pollen. We compared our results with previous data obtained with similar protocols. We analysed amino acid and polypeptide contents for 20 and 5 different plant species respectively, belonging to different genera and families. Our third objective was to assess the nutritional quality of those pollens. An amino acid score (AAS) was determined as a simple metric for the protein nutritional quality of the pollen. The amino acid profiles and polypeptide contents were different among the analysed plant species. Higher differences were detected within plant families than among families. Twenty of the 45 floral pollen species presented a low amino acid score, lacking in one to four amino acids, and were considered of poor nutritional quality for pollinators. The essential amino acids most often absent were isoleucine, leucine, methionine, and valine. Protein contents and nutritional qualities are discussed in the context of pollinator requirements and pollination service.