Utilization of sophorolipids as biosurfactants for postemergence herbicides

Sophorolipids are carbohydrate-based, amphiphilic biosurfactants produced by several species of the Starmerella yeast clade. Most sophorolipids are partially acetylated sophorose sugars O-β-glycosidically linked to 17-L-hydroxy-Δ9-octadecenoic acid, where typically the acyl carboxyl group forms a 4″-lactone to the terminal glucosyl residue. Recently sophorolipids were discovered in which the sophorose is linked to the ω-carbon of the acyl group and occurs predominately in a non-lactone, anionic form. In this study we compared lactone sophorolipids produced by Starmerella (Candida) bombicola (Sb) and non-lactone sophorolipids produced by Candida kuoi (Ck) against a synthetic polyethoxylated tallowamine surfactant (POEA) which is used in commercial postemergence herbicides. When mixed with the lipophilic contact herbicide lemongrass oil (LGO), stable emulsions with Ck lasted longer than with either POEA or Sb. Phytotoxicity (as measured by fresh and dry weights and visual damage three days after spraying) to sicklepod (Senna obtusifolia) by the Ck/LGO and Sb/LGO mixtures were similar to a POEA/LGO mixture, while visual damage to corn (Zea mays L.) was increased by the addition of all of the surfactants. When applied together with the water-soluble herbicide phosphinothricin, the Ck/LGO and Sb/LGO treatments caused decreases in sicklepod dry weights and herbicide damage ratings (HDR) compared to phosphinothricin applied without a surfactant ten days after treatment. With corn, POEA and Ck applied with PT had the greatest reductions in fresh and dry weights, and HDR values. These results indicate that sophorolipids have excellent promise as natural surfactants for postemergence herbicides.