| Abstract: | An examination has been made of the phase of continuous accumulationof phenoxyacetic acid (POA) and the 2-, 4-, 2,4-, 2,6-, and2,4,5-chloro-derivatives, containing carbon-14 in the carboxylgroup, by segments of the Avena mesocotyl. On the basis of previousfindings to eliminate the initial transient components of uptake(Type I processes) the segments were pretreated for 13 to 18h in buffer or buffer containing the respective non-radioactivecompound. For five of the compounds the relationship between the rateof uptake and the external concentration takes the form of arectangular hyperbola, but for the sixth, 2,4,5-T, this relationshipdoes not hold. The data, except those for 2,4,5-T, have beenevaluated as linear regressions of rate of uptake against thequotient of rate over concentration. From each regression equationtwo constants have been derived: the point ‘B’ wherethe line intersects the rate axis (the theoretically maximumrate) and the slope of the regression ‘K’, whichcan alternatively be expressed as the concentration at whichthe observed rate equals half the value of ‘B’. The calculated values of B and K for POA are approximately twiceas great as the corresponding values for 2-CPA, and about 25times greater than for 4-CPA. The values for 2,4-D are closeto those for 4-CPA, and 2,6-D is intermediate between 2-CPAand 4-CPA. Although the constants for 2,4,5-T could not be calculatedprecisely, the rates of accumulation are about one-fourth ofthose measured for 2,4-D at equivalent concentrations. The uptake of radioactive 2,4-D is slightly depressed in thepresence of nonradioactive POA. Greater reductions are causedby 2-CPA or 2,6-D, and 2,4-D or 2,4,5-T are even more inhibitory.The pattern of inhibition caused by 2,4,5-T indicates competitionfor common sites of uptake, while POA appears not to be competitive.In corresponding experiments with POA, the presence of the otherregulators only caused small inhibitions and the order was different. Earlier work showed that in Avena accumulation is accompaniedby the conversion to a varying degree of the individual substitutedphenoxyacetic acids to conjugated derivatives. It is suggestedthat the variation between compounds in their rates of accumulationis in part due to differences in the stability of the conjugatedderivatives, and that the facility of conversion is a factorin determining physiological activity. |
