Uptake and Distribution of N-Phosphonomethylglycine in Sugar Beet Plants

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Abstract
Glyphosate (N-phosphonomethylglycine) was readily transported in sugar beet plants (B. vulgaris L., Klein E type, monogerm). Concentrations in sink leaves reached 2.5-13.7 .mu.M in 10 h from a 15 mM solution supplied to 1 mature leaf. Distribution of glyphosate followed that of [3H]sucrose used as a marker for materials transported by the phloem, indicating that this is the primary means for distribution of glyphosate. Possible mechanisms of entry into the sieve tubes were evaluated using isolated leaf discs. Concentration dependence of uptake and kinetics of exodiffusion from tissue indicate a passive, nonfacilitated mechanism. Uptake was not affected by pH, eliminating the passive, weak acid mechanism. Permeability of the plasmalemma to glyphosate was calculated as 1.7 .times. 10-10 m/s. This characteristic would allow slow entry and exit from the phloem, and together with other physiological parameters of the plant, is postulated to allow accumulation and transport in the phloem.