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

Translocation of14C-glyphosate [N-(phosphonomethyl) glycine] in johnsongrass [Sorghum halepense(L.) Pers.] was examined in relation to time and illuminance. Plants were treated with14C-glyphosate and harvested 3 days and 6 days after treatment. Translocation increased significantly between the 3 day and 6 day harvest. The greatest accumulation of14C occurred in areas of meristematic growth. The concentration of14C 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 applied14C, while plants exposed to full light translocated 4.7%. Plants harvested after 6 days of treatment in the dark translocated 2.1% of the applied14C, while plants exposed to full light translocated 5.8%. Translocation data indicated that movement of glyphosate in johnsongrass was via the phloem. Accumulation of14C in roots and rhizomes was significantly greater in plants exposed to full light compared with plants exposed to either darkness or shade.