Studies on Plant Growth Hormones

Abstract

Two neutral plant hormones, one isolated recently from plants (3-indolylacetonitrile) and the other (3-indolylacetaldehyde) reported to be present in plants, are avaible as pure synthetic compounds for investigation of their biological activities. This paper is mainly concerned with their effects on cellelongation in the Avena coleoptile 3-Indolylacetaldehyde is considerably less active than 3-indolylacetic acid in the Avena straight-growth test; for example, a 1.0 mg./l. solution of the aldehyde shows an activity equivalent to that of a 0.1 mg./l. solution of the acid An acidic substance is produced in solutions of the aldehyde during the period of assay. In some experiments it accounts for all of the activity shown by the aldehyde solutions, on the assumption that it is 3-indoylacetic acid, and in other experiments it shows a greater activity than that of the aldehyde solutions from which it was obtained. Therefore, it is concluded that 3-indolylacetaldehyde, itself is either inactive or inhibitory. Acid production in aldehyde solutions in vitro is much lower, a fact which suggests that there is enzymatic oxidation of aldehyde to acid in the presence of coleoptiles. The activities of 3-indolylacetaldehyde in the pea test and in root-inhibition and of 3-indolylacetone in the straight-growth test are briefly reported. 3-Indolylacetonitrile is considerably more active than 3-indolylaceatic acid in the Avena straight-growth test; for erample, a 0.1 mg./l. solution of the nitrile shows an activity equivalent to a 1.0 mg./l. solution of the acid. The inhibitory effect at concentrations above 1.0–10.0 mg./l. is less with the nitrile than with the acid. There is negligible production of acid in solutions of the nitrile both in vitro and in the presence of Avena coleoptiles at temperatures ranging from −18° to 25° C. for varying lengths of time. The possibility of enzymatic convesion of nitrile to acid inside the cells of the coleoptile is discussed The activities of 3-indolylacetamide and of 2:4-dichlorophenoxyacetic acid and the corresponding nitrile are considered in this connexion The nitrile is destroyed by treatment with alkali but not by acid. In the light of these results, it is desirable to re-examine previous work on identification of auxins in plants by their acid and alkali sensitivity. Evidence for the existence of the nitrile in a number of other plants is presented.