Studies in the Physiological Action of 2,2-Dichloropropionic Acid

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Abstract
The factors determining the pattern of uptake by Lemna minor of 2,2-dichloropropionic acid (DCPA), containing chlorine-36, have been examined. Entry takes place via both the roots and frond and is largely in the ionic form. Initially, the net rates are high and are replaced by slower but steady rates. It is concluded that over the first 30 minutes, net uptake is dependent on the physico-chemical processes since (i) the rate is directly proportional to the external concentration, (ii) on transference to buffer containing non-radioactive DCPA up to 90 per cent of the radioactivity is exchangeable, (iii) the temperature coefficients for the rates of net influx and efflux range from 1.2 to 1.4. During the second phase, the net rates of uptake are curvilinearly related to concentration, the temperature quotients are higher and the rates of efflux markedly lower than in the first phase. Phenylmercuric nitrate and cupric sulphate at 5×10−6 M halve the net rate of uptake but higher concentrations are demanded for dinitrophenol, arsenite, aside, and iodoacetate. The depression induced by phenylmercuric nitrate can to some degree be reversed by the addition of either glutathione or cysteine. For pyruvic acid to halve the rate of uptake of DCPA, it requires more than a three hundredfold greater concentration. It is postulated that in the second phase, uptake is mediated by metabolic processes involving thiol groups. The ability of calcium pantothenate and β-alanine to reverse the inhibitory effects of DCPA on the growth of L. minor were investigated in multifactorial experiments. Over a narrow range of concentrations, calcium pantothenate partially offset the inhibitions of growth but the interactions for β-alanine were not significant. These results do not support the view that the primary action of DCPA is to interfere with the biosynthesis of Co-enzyme A. In a comparison of the maximal capacity of different species to accumulate DCPA in their roots, up to a six-fold difference was recorded in the eight species examined. On the basis of these findings, the mechanisms determining selective action and the inhibition of meristematic activity are discussed.