A Comparison between Glyphosate and Assimilate Translocation Patterns in Tall Morningglory (Ipomoea purpurea)

Translocation of 14 C-glyphosate [ N- (phosphonomethyl) glycine] in tall morningglory [ Ipomoea purpurea (L.) Roth., ‘Heavenly Blue’] was compared with movement of 14 C-photosynthate. The flow of photosynthate was manipulated by creating assimilate sinks in mature leaves and cotyledons using combinations of stem girdling, leaf shading, and localized cytokinin application. Comparisons of 14 C distribution patterns indicated minor differences between glyphosate and assimilate sink partitioning when 14 C-glyphosate or 14 CO 2 were applied to a single leaf. Labeled glyphosate moved upward through stem girdles in greater proportions than 14 C from photosynthate, suggesting a greater capacity for glyphosate to transfer from symplast to apoplast. When 14 C-glyphosate was applied to the stem, 14 C still moved symplastically to natural and artificial sinks, but apoplastic movement into all transpiring tissues above the site of application greatly increased. Results support the classification of glyphosate as an ambimobile rather than a phloem-mobile herbicide.