Studies on the Geotropism of Roots:II.THE EFFECTS OF THE AUXIN ANTAGONIST α-(I-NAPHTHYLMETHYLSULPHIDE)PROPIONIC ACID (NMSP) AND ITS INTERACTIONS WITH APPLIED AUXINS

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Abstract
Previous experiments on the effects of auxins on the geotropic responses of seedling pea roots (Audus and Brownbridge, 1957) have been extended using the ‘anti-auxin’ α-(I-naphthylmethylsulphide)propionic acid (NMSP) alone and in combination with indole-3-acetic acid (IAA) and 2:4-dichlorophenoxyacetic acid (2:4-D). NMSP action differs from that of the auxins in that it reduces the rate of curvature progressively as the concentration is increased, irrespective of whether the overall extension growth of the roots is being stimulated (10 and 30 p.p.m.) or inhibited (100 p.p.m.). Correspondingly the reaction time is lengthened by 25–50 per cent. in all concentrations. Studies of responses in mixtures of growth-stimulating concentrations of NMSP (30 p.p.m.) and growth-inhibiting concentrations of IAA (10 −8 ) and 2:4-D (3 x 10 −8 ) show that auxins and ‘antiauxins’ are mutually antagonistic in most, if not all, their actions on growth and curvature. The results suggest that the anti-auxin NMSP may stimulate root growth and inhibit curvature by interfering with the synthesis or distribution of a natural endogenous inhibitor, which is not IAA. NMSP inhibition of root growth in high concentrations must, however, be exerted independently of this natural inhibitor. The mutual antagonisms shown between the auxins and NMSP are best explained in terms of an interference with access to the growth centres; competitive action at the growth centres themselves seems not to be involved.

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