The Interactions of Auxins and Anti-auxins in the Stimulation of Root Growth

Publisher Website
Google Scholar
Abstract
Experiments on the interactions of auxins and anti-auxins in roots have been extended to studies with concentrations giving growth stimulations using 2-mm. sections excised from the extension zone of roots of Pisum sativum. The curves relating growth responses to log10 concentration for β-indolylacetic acid (IAA) and three anti-auxins, α-(I-naphthylmethylsulphide) propionic acid (NMSP), I-naphthylmethylsulphide acetic acid (NMSA), and 4-chloro-3-nitrobenzoic acid (CNB), are of very similar shape. A fourth anti-auxin, p-chlorophenoxy-iso butyric acid (PCIB), shows negligible stimulation of growth in any concentration. In multifactorial experiments involving stimulatory concentrations of IAA (10−4 and 10−5 p.p.m.) with several stimulatory concentrations of the first three anti-auxins (NMSP, NMSA, and CNB), consistent mutual antagonisms were clearly demonstrated. PCIB in non-inhibitory concentrations markedly antagonized stimulation by IAA Similar mutual antagonisms were shown in various mixtures of two anti-auxins. Both the similarities of the concentration-response curves and the consistent mutual antagonisms suggest that both auxins and anti-auxins in stimulating root growth are exerting identical physiological actions in the same growth system. On the assumption that these substances are active when adsorbed at some enzyme or other protoplasmic surface, it has been demonstrated that these experimental results are more easily explained by a direct action at those growth centres than by a competitive antagonism of a natural endogenous growth inhibitor, as suggested in a previous paper. The bearing of these results on current theories relating auxin and anti-auxin activities to molecular structure is discussed.