Auxin Content of Extracts of Certain Tolerant and Susceptible Host Plants of Toxoptera graminum, Macrosiphum pisi, and Therioaphis maculata and Relation to Host Plant Resistance1

Abstract

The free auxins of certain tolerant and susceptible varieties were determined by the Avena coleoptile bioassay method used in conjunction with one-dimensional paper chromatography. The effect of feeding on the auxins present in tolerant and susceptible varieties by the green bug Toxoptera graminum (Rondani) and pea aphid Macrosiphum pisi (Harris) was measured. The following auxins were detected in significant concentrations in the various tolerant and susceptible varieties and were identified on the basis of Rf values of known growth compounds: (1) susceptible Reno barley-3-indoleacetic acid, indolepyruvic acid, 3-indolcbutyric acid, ethyl-3- indoleacetate, plus two unknown auxins; (2) tolerant Dicktoo barley-indolepyruvic acid, 3-indolebutyric acid plus two unknown auxins; (3) susceptible Pawnee wheat-3-indoleacetie acid; (4) tolerant Dickinson wheat-no known auxins in significant concentrations, one unknown auxin in significant concentration; (5) susceptible alfalfa clone (50–1266)–3-indoleacetic acid, indolepyruvic acid, 3-indolebutyric acid, 3-indoleacetonitrile; (6) resistant alfalfa clone (C84)-3-indoleacetic acid and 3-indolebutyric acid; and (7) broad bean, Vicia faba L.-3-indoleacetic acid, indolepyruvic acid and 3-indolebutyric acid. All uninfected susceptible varieties, except Pawnee wheat, differed significantly from the tolerant varieties by having one or more free neutral auxins present and also usually greater concentrations of free acid auxins. Short aphid infestation periods significantly reduced the free acid auxin content of all varieties except tolerant Dickinson wheat. Infestation of susceptible Pawnee wheat caused a significant increase in neutral auxin content. Significantly less acid auxins were destroyed or removed from tolerant varieties than susceptible varieties even though in some eases significant differences of inherent acid auxins did not exist. A relationship between tolerance and auxin content was found to exist in reference to absence of neutral auxins in tolerant plants. An investigation of bound plus free auxins in tolerant plants must be conducted before it can be positively confirmed that tolerant plants have less total auxins present than susceptible plants. Auxins appear to be concerned primarily with the tolerance component of resistance and not preference or antibiosis unless auxins act as an attractive substance or as a source of a necessary nutrient. This study presents the first indication of the basis of the tolerance component of resistance.