AbstractSimultaneous measurements were made of evaporation and foliar uptake of formulated 2‐14C2,4‐D‐butyl, applied as 10‐μl droplets to the leaves of three‐week‐old barley plants, at different temperatures and boundary layer conductances using an air‐flow system. Evaporation was measured both by trapping the vapour and by the difference between the amount applied and the amount recovered from the plant. Assimilation of vapour by the plants during the experimental periods of about 6 h was found to be about 0·4 of the total amount of herbicide applied initially to the leaves. Uptake and evaporation were described in terms of rates and velocity constants. Uptake was generally the more rapid, by about 2·5 times, of the two processes. Temperature affected evaporation strongly whereas uptake was largely unaffected. Kinetics of evaporation from glass slides and leaf surfaces were very similar if surface temperature was taken into account. Evaporation from leaves increased in darkness and in conditions of high boundary layer conductance caused by rapid air circulation. The importance of the boundary layer has been underestimated in previous studies, and some misleading results have been reported as a consequence. Most of the herbicide that penetrated the plant (95) remained in the leaf to which it was applied, but it affected physiological processes within the plant because transpiration and carbon dioxide exchange were reduced by about 8 and 18, respectively. Comparison with published values of herbicide uptake suggested that 2,4‐D‐butyl is relatively rapidly taken up by barley leaves. Results predicted that, at 27°C, about 30 of the 2,4‐D‐butyl sprayed onto barley leaves would evaporate, and 70
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