The Correlation of Different Aspects of Auxin Action

Abstract

Three aspects of auxin activity, namely, stem elongation, increase in fresh and dry weight and initiation of adventitious roots, have been studied. Stem elongation took place only when the auxin was applied to the apical end of the apical 5 mm of the hypocotyl. Maximum elongation occurred at a concentration of from 0.1 to 0.01 [mu]g/ml indole-3-acetic acid (IAA). Higher concentrations inhibited elongation. Fragments from the base of the hypocotyl failed to elongate on any concentration of auxin. Elongation was reduced but not eliminated by light. Maximum increase in fresh weight of the fragments took place at between 1 and 10 [mu]g/ml IAA in the apical fragments, and this increase was greater when the IAA was applied at the basal rather than the apical end of the fragment. Fragments from the base of the hypocotyl responded with maximum weight increase to a concentration of 0.1 to 1.0 [mu]g/ml IAA, Initiation of roots was greatest at a concentration of 0.1 [mu]g/ml IAA and attained a maximum value when the IAA was applied to the apical end of the basal hypocotyl fragments. It is concluded that the strictly growth-promoting activity of IAA, as measured by weight increase, reaches a peak at between 1 and 10 [mu]g/ml. Its auxin activity reaches a peak between 0.01 and 0.1 [mu]g/ml. The existence of two such widely separated peaks of activity must be taken into account in any comprehensive theory of auxin action.