Vapour of the free acid of the herbicide 2,4-D is toxic to tomato and lettuce plants

Tomato and lettuce plants were exposed to vapour of the free acid of 14C-phenyl] 2,4-D at concentrations in the range 1-600 pg litre(-1) for periods of 6, 24 or 72 h. The rate of uptake of radiolabel by tomato was about twice that by lettuce at the same vapour concentration. Uptake rates were linearly related to external vapour concentration. The relationship between uptake and vapour concentration of 2,4-D for the two species was similar to published values for the butyl and iso-octyl esters. The distribution of herbicide residue in the plant immediately after exposure indicated that the apical leaves of lettuce are particularly active in assimilating vapour, whereas for tomato, leaf position had no influence. Forty days after exposure, both species showed symptoms of toxicity and reduction in shoot dry weight typical of similar doses of 2,4-D esters. It is concluded that the vapour of 2,4-D represents a potential hazard to susceptible plants, and that further work is needed to determine the conditions likely to lead to the production of vapour of the free acids of phenoxyalkanoic herbicides following spraying.