Studies in the Physiological Action of 2,2-Dichloropropionic Acid: III. FACTORS AFFECTING THE LEVEL OF ACCUMULATION AND MODE OF ACTION

The factors determining the pattern of uptake by Lemna minorof 2,2-dichloropropionic acid (DCPA), containing chlorine-36,have been examined. Entry takes place via both the roots andfrond and is largely in the ionic form. Initially, the net ratesare high and are replaced by slower but steady rates. It isconcluded that over the first 30 minutes, net uptake is dependenton the physico-chemical processes since (i) the rate is directlyproportional to the external concentration, (ii) on transferenceto buffer containing non-radioactive DCPA up to 90 per centof the radioactivity is exchangeable, (iii) the temperaturecoefficients for the rates of net influx and efflux range from1.2 to 1.4. During the second phase, the net rates of uptakeare curvilinearly related to concentration, the temperaturequotients are higher and the rates of efflux markedly lowerthan in the first phase. Phenylmercuric nitrate and cupric sulphateat 5x10^–6 M halve the net rate of uptake but higher concentrationsare demanded for dinitrophenol, arsenite, aside, and iodoacetate.The depression induced by phenylmercuric nitrate can to somedegree be reversed by the addition of either glutathione orcysteine. 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 mediatedby metabolic processes involving thiol groups. The ability ofcalcium pantothenate and ß-alanine to reverse theinhibitory effects of DCPA on the growth of L. minor were investigatedin multifactorial experiments. Over a narrow range of concentrations,calcium pantothenate partially offset the inhibitions of growthbut the interactions for ß-alanine were not significant.These results do not support the view that the primary actionof DCPA is to interfere with the biosynthesis of Co-enzyme A.In a comparison of the maximal capacity of different speciesto accumulate DCPA in their roots, up to a six-fold differencewas recorded in the eight species examined. On the basis ofthese findings, the mechanisms determining selective actionand the inhibition of meristematic activity are discussed.