Redroot pigweed (Amaranthus retroflexus L.) and robust foxtail [Setaria viridis (L.) Beauv. var. robusta-alba Schreiber or Setaria viridis var. robusta-purpurea Schreiber] were investigated to measure the accumulation of nonstructural carbohydrates throughout two separate growing seasons. Pigweed tended to accumulate more total nonstructural carbohydrates (TNC) than foxtail, particularly in the roots. In both weeds, peak levels of TNC in leaves, stems, and roots occurred during early flowering. The primary nonstructural carbohydrate in the leaves of both weeds was starch. Reducing sugars were the predominant nonstructural carbohydrate in the stems. Starch accumulated more in the leaves of the upper third of the plant and reducing sugars occurred in the greatest quantity in the middle strata of the stem. Accumulation levels of TNC in the plant organs suggested that the stem was a major sink during the vegetative growth phases of both pigweed and foxtail.

Experiments were conducted in growth chambers to evaluate the effect of soil temperature and soil moisture on the distribution of 14C-photoassimilates and 14C-glyphosate [N-(phosphonomethyl) glycine] in quackgrass [Agropyron repens (L.) Beauv. ♯3 AGRRE]. When 14C-glyphosate was applied to leaves, the radioactivity was less in the rhizome buds of plants exposed to 7-C soil temperature than in plants exposed to 12- and 18-C soil temperatures after 2 days. In plants with leaves exposed to 14CO2, the radioactivity from 14C-photoassimilates was greatest in rhizomes and rhizome buds of plants at the 12-C soil temperature. As soil moisture levels were decreased, uptake of C-glyphosate into leaves declined, and transport to the daughter shoots, rhizomes, and rhizome buds was reduced. The concentration of 14C-photoassimilates in the rhizome system of water-stressed quackgrass plants was similar to that in nonstressed plants. This study shows that the patterns of glyphosate distribution differ from those of photoassimilate distribution in quackgrass plants exposed to water stress.

Glyphosate transport was studied using the 14C-labeled herbicide. Because glyphosate appears to undergo little metabolism in plants, detected 14C was assumed to indicate the presence of glyphosate. Leaf-girdling experiments demonstrated that 14C-glyphosate [N-(phosphonomethyl)glycine] transport in treated quackgrass [Agropyron repens (L.) Beauv. ♯3 AGRRE] leaves occurred in both leaf symplast and apoplast. Symplastic transport was toward the leaf tip and the leaf base. Apoplastic transport was toward the leaf tip only. Rhizome-girdling experiments showed that translocation of glyphosate from a treated shoot to another shoot on the same rhizome occurs primarily in the rhizome symplast. Girdling the base of quackgrass culms showed that acropetal transport of glyphosate from the rhizome into quackgrass shoots is in the apoplast and the symplast of the culms of quackgrass shoots. Excision of rhizome buds and rhizome apices indicated that the rhizome is a sink for glyphosate independent of rhizome buds or rhizome apices. The concentration of 14C-glyphosate in rhizome buds was less in plants with rhizome apices removed than in those with apices intact. The concentration of C-glyphosate in rhizome apices did not differ between plants with buds removed and those with buds intact. 14C-glyphosate applied to a girdled rhizome apex remained primarily in the apex. Some transport of glyphosate past the girdle occurred, indicating apoplastic transport of glyphosate in the rhizome. Apoplastic transport of 14C-glyphosate in the rhizome was greatest in water-stressed plants.