Pre‐harvest retting of flax: effects of water stress on uptake and efficacy of glyphosate

Abstract

SUMMARYThe effects of water stress on the uptake, translocation and efficacy of glyphosate in flax were investigated in relation to pre‐harvest retting.Glyphosate (applied at a rate equivalent to 1.44 kg a.e. ha‐1 at 0, 10, 20 or 30 days after the start of flowering) caused little desiccation of flax grown in pots under restricted watering. Glyphosate application to well‐watered plants caused the moisture content to decline from an initial value of 70 – 80% to approximately 40% at 3 wk after spraying.Glyphosate was applied 2 wk after the mid‐point of flowering to flax grown in soil with moisture contents of 35, 31, 26, 22, 16 or 12%. Soil moisture levels (16% and 12%) which restricted evapotranspiration also reduced the efficacy of glyphosate but did not affect uptake of 14C‐glyphosate. Translocation of 14C‐glyphosate out of treated leaves was reduced only in the most severely stressed plants (12% soil moisture). Experiments with young plants (4 wk old) confirmed that water stress slightly reduced downward translocation of glyphosate.When the herbicide was applied to young plants under conditions which minimised differences in translocation, 10.8 μg glyphosate was sufficient to desiccate unstressed plants but 108 μg had little effect on stressed plants. This indicates that, in addition to any reduction in translocation which occurs during drought, water stress may reduce the susceptibility of flax to glyphosate. Thus only relief of plant water stress by irrigation is likely to improve response of the flax crop to glyphosate.