Translocation and Absorption of Glyphosate in Flowering Sicklepod (Senna obtusifolia)

Authors: Walker, Eric; OLIVER, LAWRENCE - UNIVERSITY OF ARKANSAS

Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/10/2007
Publication Date: 6/1/2008
Citation: Walker, E.R., Oliver, L.R. 2008. Translocation and Absorption of Glyphosate in Flowering Sicklepod (Senna obtusifolia). Weed Science. 56:338-343

Interpretive Summary: Glyphosate, the main ingredient of Roundup herbicide, kills many weeds when applied to young, small, actively growing plants. Although the chemical is not as effective on older, larger weeds, it will reduce the number of seed produced by some weeds when applied during flowering. Sicklepod, a weed that reduces the yields of corn, cotton, soybean, and other crops, responds to glyphosate in these ways. Since little is known about how glyphosate is absorbed and distributed throughout sicklepod plants that are flowering and how the chemical actually reduces sicklepod seed production, research was performed to determine these things. The application of traceable radioactive glyphosate to flowering sicklepod showed that although half of the glyphosate remained in the leaflet that received the chemical application and most of the remaining glyphosate in the plant moved downward toward the roots, a small but significant amount of glyphosate accumulated in the buds and flowers. This, combined with plant stress caused by the toxic effects of glyphosate, caused the sicklepod to shed the buds and flowers, the plant parts from which the seeds develop. These results may be used to reduce the number of problematic weed seed in the soil, thus reducing the number and severity of future weed infestations in crops. Also, the results may also be used to develop more effective strategies to reduce weed seed production.

Technical Abstract: Sicklepod is a competitive and prolific weed that emerges throughout the crop season. Glyphosate applications at sicklepod flowering have been shown to greatly reduce seed production, although there is limited information on glyphosate translocation in flowering weeds. Therefore, a laboratory study was conducted to document the absorption and translocation of 14C-glyhosate in flowering sicklepod. Flowering sicklepod plants were treated with four 1-µl droplets of 14C-glyphosate spotting solution, and 14C-glyphosate absorption and translocation was quantified by scintillation spectrometry. Absorption and translocation of 14C-glyphosate in flowering sicklepod were similar for both 0.21 and 0.42 kg ae ha-1 glyphosate. Although the treated leaflets retained approximately 50% of the recovered 14C-glyphosate, movement of the herbicide was both acropetal and basipetal, with the highest 14C-glyphosate concentrations in the shoot below the treated leaf and the roots. By 96 HAT, sicklepod buds and flowers had abscised, but analysis of the structures revealed the accumulation of 2% of the recovered 14C-glyphosate. Based on the results of the study, the effects of glyphosate accumulation in the buds and flowers combined with plant stress associated with the primary and secondary effects of glyphosate result in bud and flower abscission, drastically reducing sicklepod seed production.