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

Abstract

ABSTRACII'4ClAtrazine (2-chloro4-lethylaminol-6-Iisopropylamino-s-triazine) and [4Clglyphosate (N-Iphosphonomethyllglycine) 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 aoplast 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 PTIS (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.

Surfactant-Increased Glyphosate Uptake into Plasma Membrane Vesicles Isolated from Common Lambsquarters Leaves

“…They suggested that the negative charges associated with the cell wall and the negative membrane potential of the plasma membrane repel the anionic glyphosate molecule. Jachetta et al (1986) compared the uptake and translocation of atrazine and glyphosate and observed that glyphosate had limited access to the symplast of sunflower stem tissue when compared to the nonpolar atrazine. They attributed this difference to their relative lipid permeability .…”

Plasmodesmata: Symplastic Transport of Herbicides Within the Plant

“…In a simplified way, the movement of an herbicide within the plant can be accomplished by two main routes: apoplast and symplast. Apoplast is a group of dead cells -including cell walls, intercellular spaces, and xylem -which form a continuum where water and solutes have the ability to move (Jachetta, 1986). Symplast is defined as the total mass of living cells in a plant, which forms a long and complex net along the plant both through phloem and through direct connections between neighboring cells that are usually in the same organplasmodesmata (Hay, 1976).…”

“…In this situation, the herbicide moves freely among cells of the same domain because, in essence, "a single cytoplasm occurs" between cells of the same domain (Crawford & Zambryski, 2001). Penetration of these herbicides in a single cell enables their distribution to all other cells belonging to the same symplastic domain (Jachetta et al, 1986). In addition, plasmodesmata may have significant participation in translocation of molecules that have pKa (dissociation constant) below the pH of the xylem, which is approximately 5.5; it may also assist symplastic movement of these molecules by other routes besides phloem (Vidal, 2002).…”

“…In most plants, translocation of glyphosate is typically fast as it is essential for herbicidal activity. After penetration in leaves, glyphosate may be translocated both by phloem sieve tubes, which also involves plasmodesmata, and cell-tocell in the same symplastic domain via plasmodesmata, reaching all cells of the domain quickly (Franz et al, 1997;Jachetta et al, 1986). In fact, both translocations are complementary and non-competitive due to the fact that plasmodesmata act on loading and unloading the phloem (Sowinski et al, 2003).…”

“…Even with this behavior, absorption and translocation of glyphosate was less affected by the pH than sulfentrazone. The role of plasmodesmata in glyphosate translocation is known, indicating that they act not only in conjunction with vascular system, but also in a semiautonomous way (Jachetta et al, 1986). These characteristics are particularly important for the translocation of soil-applied herbicides where xylem has an important -but not exclusive -role in the translocation of these macromolecules.…”