Apoplastic and Symplastic Pathways of Atrazine and Glyphosate Transport in Shoots of Seedling Sunflower

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Abstract
[14C]Atrazine (2-chloro-4-[ethylamino]-6-[isopropylamino]-s-triazine) and [14C]glyphosate (N-[phosphonomethyl]glycine) were xylem fed to sunflower shoots at 100 micromolar for 1 hour in the light, then placed in the dark at 100% relative humidity for 1, 4, 7, or 10 hours. The distribution of atrazine and glyphosate between shoot parts, in the leaves, and between the apoplast and symplast of the leaf was determined. The apoplastic concentrations and distribution patterns of atrazine and glyphosate in the leaves were evaluated using a pressure dehydration technique, our results were compared to the previously reported distribution patterns of the naturally occurring apoplastic leaf solutes, and the apoplastic dye PTS (trisodium 3-hydroxy-5,8,10-pyrenetrisulfonate). The pattern of atrazine and glyphosate distribution in the shoot, and between the leaf apoplast and symplast, was found to reflect the potential of these herbicides to enter the shoot symplast. The results of this study are discussed with respect to current theories of xenobiotic transport in plants, and have been found to be consistent with the intermediate permeability hypothesis for xenobiotic transport.