The naturally occuring auxin indole-3-acetic acid (hereinafter IAA) was shown shortly after its discovery to stimulate adventitious root formation in cuttings of the easy-to-root plants, also called "good rooters" (see Went et al. 1937. Phytohormones. Mac Millan Co., N.Y. 294p). Synthetic auxins such as indole-3-butyric acid (IBA) and Naphthalene-acetic acid (NAA) have more effectively induced rooting in cuttings, particularly cuttings of difficult-to-root species also call "poor rooters" (see Audus, 1959, Plant growth substances, 2nd Ed. Interscience Pub, Inc. N.Y. 553p; also see Hartmann et al. 1975).
However, cuttings from many species of plants remain difficult to root because they do not respond to known formulations of auxins, of which many have been tested (Audus, supra). Mature trees in general, particularly of certain commercially important types, are an important class of plants that remain difficult or impossible to propagate economically.
Simple phenolics when administered with an auxin sometimes cause a synergistic response in adventitious root initiation (Haissig. 1974. Influrences of auxins and auxin synergists on adventitious root primidium initiation and development ment. N.Z.J. For. Sci. 4: 311-323). Several theories have been proposed to explain the synergism, one theory being that auxin molecules become bonded to the phenolic moiety, and that the conjugate induces root initiation more effectively than either chemical separately (Haissig,supra). Methyl or ethyl esters of auxins have sometimes been shown to enhance adventitious root initiation better than free acids (Veldstra. 1944. Researches on plant growth substances, Enzymologia II, 97-163). The naturally occuring auxin IAA sometimes appears naturally esterified to other compounds (Schantz. 1966. Chemistry of naturally occuring growth regulating substances. Ann, Rev. Plant Physiol. 17: 409-438), and poorly defined auxin-phenolic conjugates have been reported.
Among the known auxins, IAA does not appear to work as a rooting hormone on woody, as opposed to herbaceous, cuttings at all. All commercially available rooting hormone preparations comprise IBA, naphthylene acidic acid (NAA), amides of naphthylene acidic acid, mixtures of the foregoing, or mixtures of the foregoing with IAA. These materials have all been available for many years, but they are generally unsatisfactory for "poor rooters" and "non-rooters" and nothing better has appeared over the years, in spite of the need for effective rooting hormones for such poor rooters and non-rooters. Thus, Dr. Hudson T. Hartmann, Head of the Pomology Department at the University of California (Davis) has stated: "No growth regulators useful in stimulating production of adventitous roots on cuttings that are better than the long standard indolebutyric acid and napthaleneacetic acid are appearing." (New Vistas in Plant Propagation. International Plant Propagators Society Combined Proceedings. 27:106-113 (1977).
It has been reported [Haissig. 1978. Influence of phenyl indole-3-acetate on adventitious root premordium initiation and development. Plant Physiology (Supplement) 61(4):65. (Abstract of paper presented at the annual Meeting of the Am. Soc. of Plant Physiologists, Va.-Poly. Inst. and State Univ., Blacksburg, Va., June 25, 1978)] that P-IAA produced 2-4 times as many root primordia per leafy bean cutting (Top Crop) as did IAA. But as beans are herbaceous, this work suggests nothing regarding the effectiveness of P-IAA on woody cuttings, particularly on poor rooters, and it suggests nothing regarding the effectiveness of aryl esters of other auxins as rooting hormones.