Pollination is a key natural resource contributing billions of dollars annually to U.S. agriculture. A primary goal in promoting these valuable services is providing nutritious forage for managed and wild bees. However, factors affecting the production of floral rewards (pollen and nectar) are not well understood. It cannot be assumed that simply increasing the number of flowers in the landscape is adequate without considering how management practices may alter abundance or quality of food provided by those blooms. Soil health, including the soil microbiome, play an important role in promoting plant growth, health, and reproduction, and thus are expected to also impact pollen and nectar production, but this connection has yet to be empirically studied. Furthermore, widely used agricultural chemicals such as herbicides may also alter floral rewards. Using a series of controlled greenhouse experiments, we will compare plants grown with an intact soil microbiome to plants with the microbiome experimentally removed. We then will explore the effects of the herbicide glyphosate on floral rewards. Changes in plant vegetative growth, flower phenology and phenotype, quantity and nutritional quality of pollen and nectar, and chemistry of plant volatiles involved in pollinator attraction will serve as metrics to evaluate these effects. The results of this study will be relevant to a range of crops nationwide for assessment of how land and crop management strategies impact both plant and pollinator health and will open the door to development of potential new strategies for manipulation of soil microbes for promotion of nutritive pollinator forage.
Project objectives from proposal:
This project has two primary objectives.
1. Identify the effects of experimental removal of the soil microbiome on floral rewards by measuring flower number, size and phenology; pollen nutritional content and grain size; nectar sugar concentration; and plant organic volatile compounds in plants grown in sterile soil versus soil inoculated with a field collected community of bacteria and fungi.
Prediction 1.1: Plants grown in sterilized soil will bloom later and produce inflorescences with reduced number of flowers compared to plants grown in inoculated soil, but inflorescence number will not differ.
Prediction 1.2: Pollen produced by plants grown in sterilized soil will exhibit smaller average grain size and will differ in amino acid concentration and/or composition compared to pollen from plants grown in inoculated soil.
Prediction 1.3: Flowers of plants grown in sterilized soil will produce smaller volumes of nectar, and nectar may differ in sugar concentration compared to plants grown in inoculated soil.
Prediction 1.4: Plant volatiles will differ between plants grown in sterile soil and plants grown in inoculated soil
2. Examine the effects of glyphosate treatment on floral rewards by comparing flower number, size and phenology; pollen nutritional content and grain size; nectar sugar concentration; plant organic volatile compounds; and insect visitation between treated and untreated plants.
Prediction 2.1: Plants treated with glyphosate will exhibit delayed flowering time compared to control plants.
Prediction 2.2: Plants treated with glyphosate will differ in the volume and sugar content of nectar produced compared to control plants.
Prediction 2.3: Plants treated with glyphosate will produce pollen with smaller grain size and pollen crude protein and amino acid profiles will differ between treatment and control plants.
Prediction 2.4: Plant volatile organic chemical composition will differ between treated and untreated plants.
Prediction 2.5: Glyphosate treated plants will attract a lower abundance and diversity of flower visiting insects compared to control plants.
We have chosen sunflower (Helianthus annus) as a study plant because 1) it is grown both as a crop plant and bee forage plant in hedgerows and pollinator gardens, 2) it produces large quantities of pollen and is used as a food source by a wide range of native and managed bee species, 3) it has a composition flower with large inflorescence containing many flowers per plant, and 4) dwarf varieties able to be grown in growth chambers are available.