Translocation of¹⁴C-Glyphosate and¹⁴CO₂-Labeled Photoassimilates in Canada Thistle (<i>Cirsium arvense</i>)

Abstract

Translocation of 14 CO 2 -labeled photoassimilates was compared to movement of 14 C-glyphosate [ N -(phosphonomethyl)glycine] in Canada thistle [ Cirsium arvense (L.) Scop. ♯ CIRAR] under field and controlled-environment conditions. Field-grown Canada thistle plants were treated on mature upper leaves at the midflower stage in mid-June and harvested 8 days later. No differences were found in glyphosate and assimilate distribution, and movement was primarily basipetal. Of the glyphosate and photoassimilates translocated from the treated leaves, 25 and 31%, respectively, were recovered from the roots, while 59 and 58%, respectively, were found in the shoot tissue below the treated leaf. Concentration of labeled glyphosate in the roots as much as 95 cm from the treated shoot was as high as at the base of the treated shoot. Photoperiods that induce flowering (15 h) or maintain vegetative growth (13 h) did not differentially affect the distribution of photoassimilates or glyphosate in 7-week-old Canada thistle plants in the growth chamber. In growth chamber-grown plants equivalent amounts of glyphosate and assimilates were translocated out of treated leaves; however, relatively more glyphosate than photoassimilates accumulated in the roots.