Pre-harvest retting of flax with glyphosate: effect of growth stage at application on uptake, translocation and efficacy of glyphosate

Flax cv. Hera was grown in pots at a density of 2000 plants/m². At various stages of maturity plants were either sprayed with glyphosate (Roundup, Monsanto plc) at a rate equivalent to 1.4 kg a.e./ha or the uppermost leaves of the main stem were treated with ¹⁴C-glyphosate. Uniform desiccation to greater than 80% DM within 5 wk was obtained with sprays applied up to and at 2 wk after the mid-point of flowering (MP) and also at 10 wk after MP, when all seed had been shed. Sprays at 4–6 wk after MP caused little or no desiccation of the lower stem. Autoradiography demonstrated that ¹⁴C-glyphosate applied to the 10 uppermost leaves of the main stem at MP, or to the upper stem at 10 wk after MP, was distributed throughout the plant within 48 h. However, no downward translocation was detected within 48 h of applications during seed filling, i.e. at 2 or 5 wk after MP. Quantitative determinations of distribution of ¹⁴C-glyphosate were made at 48 h after application either to the five uppermost leaves on the main stem or, after leaf drop, to the base of the inflorescence. Translocation out of the treated zone was 58–67% of applied ¹⁴C-glyphosate with applications at 0, 2 or 6 wk after MP but only 13% with application to senescing leaves at 4 wk after MP. Little downward translocation occurred with any of these applications, ¹⁴C-glyphosate accumulating mainly in the upper stem and inflorescence. Thus failure of glyphosate to desiccate the lower stem may be related to poor downward translocation during seed development and to reduced uptake into, and export out of, senescing leaves.