Selective phytotoxicity of hexazinone to Pinus resinosa and Pinus banksiana

Abstract

Hexazinone (3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4(1 H,3 H)-dione) is an s-triazine herbicide that is used widely to control weeds in forest plantations and on industrial rights-of-way. Under controlled environmental conditions, red pine ( Pinus resinosa Ait.) has been shown to have greater tolerance to hexazinone than jack pine ( Pinus banksiana Lamb.). In a whole-plant nutrient-culture bioassay, hexazinone concentrations up to 40 μ M did not effect red pine root or shoot growth in terms of fresh weight; however, at the same concentrations, hexazinone was toxic to jack pine. After five different exposure times (i.e., 24, 48, 72, 96, and 192 hr), jack pine foliage contained significantly ( P < 0.05) higher levels of radioactivity, applied to the roots as [ 14C]hexazinone, than did red pine foliage. The higher level of radioactivity in jack pine foliage was an apparent result of greater transpiration rates in jack pine than in red pine. In a separate metabolism experiment, after 192 hr of root exposure to [ 14C]hexazinone, approximately 27 and 2%, respectively, of the radioactivity recovered from the foliage was found to be unaltered hexazinone in jack pine and red pine. In the roots of both species, the amount of unaltered hexazinone did not differ significantly ( P < 0.05), with approximately 87% of the radioactivity recovered as hexazinone. Metabolism of hexazinone was greater in the stem tissue than in the roots, but less than in the foliage. Two main factors (i.e., uptake and metabolism) were found to contribute to hexazinone selectivity in red pine and jack pine.