Effect of Bud vs Rosette Growth Stage on Translocation of14C-Glyphosate in Canada Thistle (Cirsium arvense)

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Abstract
The effect of photoperiod and growth stage on translocation of14C-glyphosate was compared in Canada thistle plants at the bud and rosette stage of growth. Canada thistle plants grown under controlled environment conditions with a 10 h photoperiod remained as low growing rosettes and developed a mature root system. When the photoperiod for half of these plants was increased to 16 h, stem elongation occurred and flowering was initiated. Growth stage at the time of application affected the distribution of14C-glyphosate within the elongated shoot and between the shoot and root. The shoot tissue of the bud stage plants contained 25.9% of the14C-glyphosate recovered, while the rosette plants contained only 3.6%; a seven-fold difference.14C-glyphosate was applied to leaves 19 and 20, which corresponded to the mid-point of the elongated stem of the bud-stage plants. In the bud-stage plants,14C-glyphosate moved preferentially into the apical portion of the stem, with three to four times as much in the apical portion of the elongated stem as in the basal portion. In the roots, the effect of growth stage on distribution of14C-glyphosate was reversed, application at the rosette stage resulted in a four-fold increase in the amount of14C-glyphosate in the root. When applied in the rosette stage, 19.1% of the14C-glyphosate detected was in the root compared to only 4.9% when applied at the bud stage. Although the root of the rosette plants was larger than in bud-stage plants, the concentration of14C-glyphosate in the root tissue of the rosette plants was three times greater. Photoperiod indirectly affected the distribution of14C-glyphosate in Canada thistle by its effect on growth.