Effects of Illuminance and Time on Accumulation of Glyphosate in Johnsongrass (Sorghum halepense)

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Abstract
Translocation of 14C-glyphosate [N-(phosphonomethyl) glycine] in johnsongrass [Sorghum halepense (L.) Pers.] was examined in relation to time and illuminance. Plants were treated with 14 C-glyphosate and harvested 3 days and 6 days after treatment. Translocation increased significantly between the 3 day and 6 day harvest. The greatest accumulation of 14 C occurred in areas of meristematic growth. The concentration of 14C recovered in rhizomes nearly doubled between the 3 day and 6 day harvest. Plants harvested after 3 days of treatment in the dark translocated 1.3% of the applied 14C, while plants exposed to full light translocated 4.7%. Plants harvested after 6 days of treatment in the dark translocated 2.1% of the applied 14C, while plants exposed to full light translocated 5.8%. Translocation data indicated that movement of glyphosate in johnsongrass was via the phloem. Accumulation of 14C in roots and rhizomes was significantly greater in plants exposed to full light compared with plants exposed to either darkness or shade.