The influence of formulation, buffering, pH and divalent cations on the activity of endothall on hydrilla

Abstract

Endothall has been used as an aquatic herbicide for more than 40 years and provides very effective weed control of many weeds. Early research regarding the mechanism-of-action of endothall contradicts the symptomology normally associated with the product. Recent studies suggest endothall is a respiratory toxin but the mechanism-of-action remains unknown. To further elucidate the activity of endothall, several endothall formulations were evaluated for their effects on ion leakage, oxygen consumption and photosynthetic oxygen evolution from hydrilla shoot tips. The influence of pH, buffering and divalent cations was also evaluated. Based on ion leakage, the LD50 of the alkylamine formulation was substantially lower under both light and dark conditions than the acid or dipotassium salt formulations. Lowering the pH increased ion leakage, and therefore activity, of the dipotassium salt, while buffering had the opposite effect on the acid formulation. Neither pH nor buffer concentration had an effect on the alkylamine salt, however addition of divalent cations increased ion leakage by all formulations. The rate of oxygen consumption was initially increased followed by a sharp decrease for both the dipotassium and alkylamine salt formulations. All formulations also caused a marked reduction in photosynthetic oxygen evolution within 60 minutes of treatment. Based on these data, endothall appears to inhibit respiration and photosynthesis, possibly due to a similar mechanism. There also appears to be fundamental differences between the alkylamine salt and the other formulations that may help to explain the mechanism-of-action of endothall in hydrilla and other plants.