Active Absorption — Three Herbicide Examples
Carrier-mediated transport of the 2,4-D anion has been proposed in addition to the simple, nonionic diffusion of the 2,4-D acid discussed earlier. Absorption of 2,4-D was saturable at increasing external concentrations and competed with IAA absorption, suggesting carrier-mediated absorption. Additional evidence suggested 2,4-D absorption may also involve facilitated diffusion, an increased rate of diffusion down an electrochemical gradient through a protein in the cell membrane (Figure 11). It is thought that 2,4-D is taken up by the cell using a protein carrier that normally recognizes the plant hormone, auxin.
Glyphosate absorption was nonlinear over time with an initial rapid phase of absorption followed by a slower, steady-state phase. The saturable component of glyphosate transport was competitively inhibited by phosphate, supporting the involvement of a phosphate carrier recognizing the phosphate group on the glyphosate molecule (Figure 12). It is thought that glyphosate is taken up by the cell using a protein carrier that normally recognizes the mineral, phosphate.
Absorption of the divalent cation paraquat also involved active transport and was dependent on concentration. Absorption kinetics of putrescine, a divalent polyamine with a charge distribution similar to paraquat, were similar to the kinetics of paraquat absorption. In addition, putrescine competitively inhibited the saturable component of paraquat absorption (Figure 13). It is thought that paraquat is taken up by the cell using a protein carrier that normally recognizes the polyamine, putrescine.