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

Abstract

Translocation of14C-glyphosate [N-(phosphonomethyl) glycine] in tall morningglory [Ipomoea purpurea(L.) Roth., ‘Heavenly Blue’] was compared with movement of14C-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 of14C distribution patterns indicated minor differences between glyphosate and assimilate sink partitioning when14C-glyphosate or14CO2were applied to a single leaf. Labeled glyphosate moved upward through stem girdles in greater proportions than14C from photosynthate, suggesting a greater capacity for glyphosate to transfer from symplast to apoplast. When14C-glyphosate was applied to the stem,14C 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.