The Uptake of Growth Substances: XI. VARIATIONS IN THE ACCUMULATION OF SUBSTITUTED PHENOXYACETIC ACIDS OF DIFFERING PHYSIOLOGICAL ACTIVITY BY SEGMENTS OFAVENAMESOCOTYL

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Abstract
An examination has been made of the phase of continuous accumulation of phenoxyacetic acid (POA) and the 2-, 4-, 2,4-, 2,6-, and 2,4,5-chloro-derivatives, containing carbon-14 in the carboxyl group, by segments of the Avena mesocotyl. On the basis of previous findings to eliminate the initial transient components of uptake (Type I processes) the segments were pretreated for 13 to 18 h in buffer or buffer containing the respective non-radioactive compound. For five of the compounds the relationship between the rate of uptake and the external concentration takes the form of a rectangular hyperbola, but for the sixth, 2,4,5-T, this relationship does not hold. The data, except those for 2,4,5-T, have been evaluated as linear regressions of rate of uptake against the quotient of rate over concentration. From each regression equation two constants have been derived: the point ‘B’ where the line intersects the rate axis (the theoretically maximum rate) and the slope of the regression ‘K’, which can alternatively be expressed as the concentration at which the observed rate equals half the value of ‘B’. The calculated values of B and K for POA are approximately twice as great as the corresponding values for 2-CPA, and about 25 times greater than for 4-CPA. The values for 2,4-D are close to those for 4-CPA, and 2,6-D is intermediate between 2-CPA and 4-CPA. Although the constants for 2,4,5-T could not be calculated precisely, the rates of accumulation are about one-fourth of those measured for 2,4-D at equivalent concentrations. The uptake of radioactive 2,4-D is slightly depressed in the presence of nonradioactive POA. Greater reductions are caused by 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 competition for common sites of uptake, while POA appears not to be competitive. In corresponding experiments with POA, the presence of the other regulators only caused small inhibitions and the order was different. Earlier work showed that in Avena accumulation is accompanied by the conversion to a varying degree of the individual substituted phenoxyacetic acids to conjugated derivatives. It is suggested that the variation between compounds in their rates of accumulation is in part due to differences in the stability of the conjugated derivatives, and that the facility of conversion is a factor in determining physiological activity.

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