Submitted to: Crop Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/25/2014
Publication Date: 3/31/2014
Publication URL: http://handle.nal.usda.gov/10113/60593
Citation: Vaughn, S.F., Behle, R.W., Skory, C.D., Kurtzman, C.P., Price, N.P.J. 2014. Utilization of sophorolipids as biosurfactants for postemergence herbicides. Crop Protection. 59:29-34.
Interpretive Summary: Nearly all postemergence herbicides are formulated with adjuvants that increase their toxicity to target weeds by increasing absorption of the herbicides. The most important of these adjuvants are surfactants which break the surface tension of the plant’s surface and also act as emulsifiers to keep the herbicides in solution. Sophorolipids are a group of naturally-occurring surfactants (termed biosurfactants) produced by certain yeast species. These sophorolipids normally exist in a ring-like chemical structure called a lactone. Our research group has recently identified a species of yeast that produces sophorolipids which exist in a linear chemical form similar to synthetic surfactants. When combined with the organic herbicide lemongrass oil, the linear sophorolipids were superior to the lactone sophorolipids as emulsifiers, and increased the herbicidal activity of the lemongrass oil in a similar manner to a synthetic surfactant. When combined with the water-soluble herbicide phosphinothricin, the herbicidal activity of the linear sophorolipid was greater on corn plants than with the lactone sophorolipids. These results indicate that sophorolipids have excellent promise as natural surfactants for postemergence herbicides.
Technical Abstract: Sophorolipids are carbohydrate-based, amphiphilic biosurfactants produced by several species of the Starmerella yeast clade. Most sophorolipids are partially acetylated sophorose sugars O-ß-glycosidically linked to 17-L-hydroxy-delta9-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 omega-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 both sophorolipid/LGO mixtures was similar to a POEA/LGO mixture, while visual damage to corn (Zea mays L.) was greatest with the Ck/LGO mixture. When applied together with the water-soluble herbicide phosphinothricin, all surfactants caused decreases in sicklepod dry weights and HDR values compared to phosphinothricin applied without a surfactant ten days after treatment. With corn, POEA and Ck applied with PT had the greatest reductions in dry weights, although HDR values were only higher with the POEA/PT mixture. These results indicate that sophorolipids have excellent promise as natural surfactants for postemergence herbicides.