Water stress and triclopyr on clopyralid efficacy in honey mesquite

Water stress may affect herbicide efficacy in herbaceous and woody plants. Chamber studies were conducted to evaluate the influence of water stress (-1.3 to -2.8 MPa) and triclopyr on the absorption and translocation of clopyralid in greenhouse-grown honey mesquite (Prosopus glandulosa Torr.). Xylem water potential was determined in honey mesquite at time of herbicide application. Absorption and translocation of clopyralid was determined at low (-1.3 MPa), medium (-2.2 MPa), and high (-2.8 MPa) water stress at 4 h after application for 1.5-mo-old plants, while only translocation was determined at either a low (-1.4 or -1.6 NIPa) or a high (-2.4 MPa) water stress treatment at hours after herbicide application for 3-mo-old plants. Water stress did not affect (P < 0.05) absorption or translocation of clopyralid alone in either study. With 1.5-mo-old plants, the addition of triclopyr to clopyralid increased (P < 0.05) clopyralid absorption in leaves at low (63 mug) and medium (54 mug) water stress compared to high water stress (33 mug) but did not affect (P > 0.05) translocation at 4 hours after application. On 3-mo-old plants, triclopyr decreased (P < 0.01) clopyralid translocation 24 hours after treatment at high water stress. The reasons for reduced uptake and 24 hours post-treatment translocation of clopyralid when applied with triclopyr at high water stress are unclear, but have implications for field applications.