Dissipation of Glyphosate Injected into the Lead Tree (Leucaena leucocephala) in Different Seasons in Taiwan

To determine the appropriate way to control the lead tree (Leucaena leucocephala (Lam) de. Wit), an invasive plant in Taiwan, 3 concentrations with an equal amount of glyphosate (18.8 mmole) were applied in spring and summer, respectively, and glyphosate residues in the plants and soils were monitored. A good control effect was observed with injecting a commercial product of glyphosate without dilution, especially in spring. Measurements of glyphosate residues in the upper (25 cm above the upper injection hole), middle (between the 2 injection holes), and lower (25 cm below the lower injection hole) parts of the lead tree, as well as in forest soils in spring and summer experiments showed that this herbicide began to sharply dissipate after treatment, which was accelerated by high temperatures and heavy rainfall from typhoons. In the spring experiment, it required ca. 1 yr for glyphosate residues in both the lead trees and soils to drop to a few parts per million, while this period was further shortened in the summer experiment. Thus, there is only a small possibility of glyphosate pollution of the soil environment of lead tree forests. Analysis of glyphosate in xylem and phloem from different parts of the plants revealed that after injection, glyphosate gradually diffused acropetally, basipetally, and laterally from the xylem tissue to the surrounding phloem tissue. Glyphosate appeared to preferentially diffuse to and accumulate in phloem near the injection hole for further long-distance translocation in phloem tissue.