AbstractTomato plants (5–7 weeks old)were exposed to a range of concentrations of vapour of 2,4‐D‐butyl (0.12–2.4 ng litre−;1), for periods of 6 or 24 h, using an air‐flow system. Net carbon dioxide exchange of the whole plant was measured for up to 2 days after exposure and was found to decrease in treated plants approximately in proportion to vapour concentration. Six weeks after exposure, shoot dry‐weights of plants exposed to herbicide vapour were less than control plants in all cases, by about 20 or more. All characteristics measured were affected more by a 24‐h exposure period than 6 h, at the same vapour concentration. The amount of herbicide vapour, expressed as the product of exposure period and vapour concentration, gave a similar effect at a low range of vapour concentrations (≤1.0 ng litre−;1)and air‐flow rates of about 200 litre min−;1, but not at higher concentrations at a flow rate of about 120 litre min−;1. Better agreement between the experiments carried out at the two concentration ranges was obtained by expressing the herbicide vapour as the total amount to which the plants were exposed (i.e. the product of air‐flow rate, vapour concentration and duration of exposure)ins
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