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

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Abstract
Translocation of 14C-glyphosate [N-(phosphonomethyl) glycine] in tall morningglory [Ipomoea purpurea (L.) Roth., ‘Heavenly Blue’] was compared with movement of 14C-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 14C distribution patterns indicated minor differences between glyphosate and assimilate sink partitioning when 14C-glyphosate or 14CO2 were applied to a single leaf. Labeled glyphosate moved upward through stem girdles in greater proportions than 14C from photosynthate, suggesting a greater capacity for glyphosate to transfer from symplast to apoplast. When 14C-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.