Absorption and translocation of glyphosate in glyphosate-resistant cotton as influenced by application method and growth stage

Abstract

The influence of herbicide placement and plant growth stage on the absorption and translocation patterns of 14C-glyphosate in glyphosate-resistant cotton was investigated. Plants at four growth stages were treated with 14C-glyphosate on a 5-cm2 section of the stem, which simulated a postemergence-directed spray (PDS) application, or on the newest mature leaf, which simulated a postemergence (POST) application. Plants were harvested 3 and 7 d after treatment and divided into the treated leaf or treated stem, mature leaves, immature leaves and buds, stems, roots, fruiting branches (including the foliage on the fruiting branch), squares, and bolls. The PDS versus POST application main effect on absorption was significant. Absorption of 14C-glyphosate applied to stem tissue was higher in PDS applications than in POST applications. Plants receiving PDS applications absorbed 35% of applied 14C-glyphosate, whereas those receiving POST applications absorbed 26%, averaged over growth stages at application. Absorption increased from the four-leaf growth stage to the eight-leaf stage in POST applications but reached a plateau at the eight-leaf stage. Plants with PDS applications showed an increase in absorption from the four- to eight- to twelve-leaf stages and reached a plateau at the 12-leaf stage. Translocation of 14C-glyphosate to roots was greater at all growth stages with PDS treatments than with POST treatments. Herbicide placement did not affect translocation of 14C-glyphosate to squares and bolls. Squares and bolls retained 0.2 to 3.7% of applied 14C-glyphosate, depending on growth stage. Separate studies were conducted to investigate the fate of foliar-applied 14C-glyphosate at the four- or eight-leaf growth stages when harvested at 8- or 10-leaf, 12-leaf, midbloom (8 to 10 nodes above white bloom), and cutout (five nodes above white bloom, physiological maturity) stages. Thirty to 37% of applied 14C-glyphosate remained in the plant at cutout in four- and eight-leaf treatment stages, respectively. The concentration of 14C-glyphosate in tissue (Bq g−1 dry weight basis) was greatest in mature leaves and immature leaves and buds in plants treated at the four-leaf stage. Plants treated at the eight-leaf stage and harvested at all growth stages except cutout showed a higher concentration of 14C-glyphosate in squares than in other plant tissue. Accumulation of 14C-glyphosate in squares reached a maximum of 43 Bq g−1 dry weight at harvest at the 12-leaf stage. This concentration corresponds to 5.7 times greater accumulation of 14C-glyphosate in squares than in roots, which may also be metabolic sinks. These data suggest that reproductive tissues such as bolls and squares can accumulate 14C-glyphosate at higher concentrations than other tissues, especially when the herbicide treatment is applied either POST or PDS during reproductive stages (eight-leaf stage and beyond).

Effects of Glyphosate Application Timing and Rate on Sicklepod (Senna obtusifolia) Fecundity1

“…Both cotton and sicklepod produce multiple reproductive sinks at various locations on the plant. Pline et al (2001) studied absorption and translocation patterns in 4-L and 8-L cotton using 14 Cglyphosate. When flowering was initiated, Pline et al (2001) found that cotton began translocating more glyphosate to the fruiting structures rather than to the roots.…”

“…These negative effects on reproductive structures may be enhanced because of the translocation of glyphosate from source leaves to metabolic sinks such as reproductive organs, where it accumulates (Pline et al 2001;Viator et al 2003).…”

Translocação do glyphosate em biótipos de azevém (Lolium multiflorum)

“…Os resultados foram apresentados em gráficos, utilizando-se médias observadas e desvios-padrão. (Bariuan et al, 1999;Hetherington et al, 1999;Hall et al, 2000;Kirkwood et al, 2000;Reddy, 2000;Satchivi et al, 2000;Sharma & Singh, 2001;Pline et al, 2001). Em Sorghum halepenses e Elytrigia repens a absorção variou de 7 a 74% durante os períodos de 48 e 120 horas de exposição (Mcwhorter et al, 1980;Devine et al, 1983).…”

Genetically modified organisms and risks of their introduction

“…However, there are reports that following glyphosate treatment of glyphosate-resistant varieties of sugar beet, these plants accumulated toxic glyphosate metabolites [80]. More than that, the reproductive tissues of glyphosate-resistant cotton were shown to accumulate very high levels of the herbicide, 0.14 to 0.48 mg/g [81]; such concentration in food would be deadly (in the United States, the tolerable amount of residual glyphosate and its toxic metabolites in foodstuffs is 0.02 mg/kg dry wt). Regrettably, the data on the residual concentrations of herbicides in resistant plant varieties are not found in the published descriptions and other documentation.…”