Submitted to: Weed Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/19/2003
Publication Date: 2/25/2004
Citation: Price, A.J., Pline, W.A., Wilcut, J.W., Danehower, D. 2004. PHYSIOLOGICAL BASIS FOR COTTON TOLERANCE TO FLUMIOXAZIN APPLIED POSTEMERGENCE-DIRECTED. Weed Science. 52:1-7. Interpretive Summary: Cotton growers typically include postemergence-directed herbicides in their weed control program. Flumioxazin (trade name Valor) is being evaluated for use as a postemergence-directed herbicide, and registration is anticipated for the 2004 growing season. However, research has shown that if flumioxazin contacts green stem cotton at time of postemergence-directed application, significant cotton injury can occur. As cotton matures, injury observed on cotton stems from flumioxazin postemergence-directed treatments decreases. Due to injury concerns, research was conducted to investigate potential causes of observed injury. Results showed that cotton tolerance to flumioxazin applied postemergence-directed is likely due to the development of a bark layer on more mature cotton. Results showed that older cotton was more efficient at metabolizing flumioxazin compared to younger cotton. Therefore, these cumulative factors may contribute to reducing observed localized injury in older cotton treated with flumioxazin postemergence-directed. If flumioxazin is registered for postemergence-directed application, these data suggest restricting application to cotton that has gained a bark layer on the lower stem.
Technical Abstract: Previous research has shown that flumioxazin, a herbicide being developed as a postemergence-directed spray (PDS) in cotton, has the potential to injure cotton less than 30 cm tall if the herbicide contacts green stem material due to rain splash or misapplication. In response to this concern, young cotton plants (five-leaf) with chlorophyllous stems as well as older cotton plants (16-leaf) with mature bark were treated with a PDS containing flumioxazin plus crop oil concentrate (COC) or non-ionic surfactant (NIS). Stems of treated plants and untreated plants at the respective growth stage were cross-sectioned and then magnified and photographed using bright-field microscopy techniques. More visible injury was evident in younger cotton. Also, there was a decrease in treated-stem diameter and an increase in visible injury with use of COC compared to NIS in younger cotton. The influence of plant growth stage and harvest time on the absorption, translocation, and metabolism of 14C-flumioxazin in cotton was investigated. Total 14C absorbed at 72 hours after treatment (HAT) was 77, 76, and 94% of applied at 4-, 8-, and 12-leaf growth stages, respectively. Treated cotton stems at all growth stages and harvest times contained higher concentrations (Bq/gram tissue dry wt.) of 14C than any other tissue. Flumioxazin metabolites made up less than 5% of the radioactivity found in the treated stem. Due to the undetectable levels of metabolites in other tissues when flumioxazin was applied PDS, flumioxazin was then applied foliar to determine if flumioxazin transported to the leaves may be metabolized. In foliar-treated cotton, flumioxazin metabolites in the treated leaf of four-leaf cotton totaled 4% of the recovered 14C 72 HAT. Flumioxazin metabolites in the treated leaf of 12-leaf cotton totaled 35% of the recovered 14C 48 HAT. These data suggest that differential absorption, translocation, and metabolism at various growth stages, as well as the development of a bark layer, are the bases for differential tolerances of cotton to flumioxazin applied PDS.