Summinary. Six dichloro-, 3 trichloro-, 2 triiodo-, and 3 heterosubstituted benzoic acids (amiben, dinoben, dicamba), and N-1-naphthylphthalamic acid have been tested for effects on growth and on polar auxin transport. Growth activity with and without kinetin was measured by effects on fresh and dry weights of 30-day cultures of fresh tobacco pith. Transport inhibition was measured by following uptake and output of IAA-2-14C through 10 mm bean epicotyl sections. The distribution of callus growth on vascularized tobacco stem segments was also observed. Avena first internode extension assays established the relative activities: dicamba> amiben> dinoben suggested by pith growth results. Growth effects of active compounds were similar with and without kinetin, except that amiben was less active with kinetin, while 2, 3, 6-trichlorobenzoic acid was more active with kinctin than alone. The weak auxin activity of NPA was confirmed. Transport experiments showed that NPA was the most inhibitory compound tested, followed by TIBA. Other compounds tested were at least 300 times less inhibitory to IAA transport. The best growth promoters were the least inhibitory to transport, and the most effective transport inhibitors were at best poor auxins. It is suggested that the weak auxin and auxin synergistic activity of TIBA (and perhaps 2, 3-dichlorobenzoic acid) in extension growth tests arises from its inhibition of transport of endogenous or added auxin out of the sections, rather than from its intrinsic auxin activity. Chemically induced apolar callus growth on vascularized tobacco stem explants can arise from inhibition of native auxin transport, apolar growth stimulation by auxinic action of the test compound, or both.Prior studies (10,11,20,22,23,24,25,32) of the effect of chemicals on polar auxin transport have shown that those which A) interfere with ATP formation ( 3DNP), B) are sulfhydryl binders (TIBA, iodoacetate, PCMB) or C) are weak (NPA, 2,5-dichlorobenzoic acid) or strong (2,4-D) auxins can all decrease the amount of IAA reaching a receptor block from the basal end of a section of stem tissue to whose apical end it has been applied. A limited survey of 10 phenoxyacetic acids (24) showed that transport inhibition increased with the number of substituted chlorine atoms up to 3, and that the active molecules tended to have higher This studN' was aided by funds from the National Science Foundation, Grants C 14545 and GB 153.2 Present address: Department of Botany, Duke University, Durham, North Carolina. 3 The following abbreviations and trivial names are used throughout: DNP = 2,4-dinitrophenol, NPA = N-1-naphthylphthalamic acid, PCMB = p-chlorophenylmercuribenzoate, 'rIBA -2,3,5-triiodobenzoic acid, amiben = 3-amino-2,4-dichlorobenzoic acid, dicamba = 2,5-dichloro-6-methoxv benzoic acid, dinobein = 2,5-dichloro-3-nitrobenzoic acid.
