Interactions in the Physiological Effects of Growth Substances on Plant Development

Abstract

The interrelated effects of 3-indolylacetic acid and sodium 2 : 4-dichlorophenoxyacetate on the growth and development of Lemna minor and Helianthus annuus have been investigated by means of multifactorial experiments, involving three to five levels of each compound. A clonal population of L. minor was maintained under constant conditions of light (550 foot-candles) and temperature (25° C.) and the growth regulators were added to a phosphate-buffered culture solution (pH 5·1), which was changed every 2 days. In the H. annuus experiments the compounds, in aqueous solution, were placed by means of a micro-pipette on the leaves of young plants grown in a greenhouse or in the open. Indolylacetic acid, at concentrations up to 25 p.p.m., increases the relative growth rate of L. minor on either a dry-weight or frond-area basis. Sodium dichlorophenoxyacetate at 0·025 p.p.m. has little effect on growth, but at and above concentrations of 0·2 p.p.m. growth is depressed. The ratio of frond area to total dry weight is increased by indolylacetic acid but is depressed by the phenoxyacetate. In the presence of indolylacetic acid, more particularly at the higher levels, the relative inhibition of growth caused by sodium dichlorophenoxyacetate is reduced. When sunflowers are treated daily with indolylacetic acid (0, 6, 30, 150 μg./ plant) or initially with sodium dichlorophenoxyacetate (0, 20, 100 μ.g./plant) in all possible combinations, then the relative growth rates of the treated leaves, the leaves above the treated leaves, the first internode, and the roots are depressed when each compound is applied alone. On the other hand, the growth rate of the hypocotyl is increased by both growth regulators, while for the second internode indolylacetic acid decreases the growth rate but the phenoxyacetate at the lower level accelerates it. The combined action of the two compounds on the different parts tends to be cumulative, save for the hypocotyl, where the effect of indolylacetic acid is dependent on the level of the phenoxyacetate. It is concluded that these results cannot be explained on the concept of wholly additive effects. There is evidence that the compounds can mutually interfere in their actions and it would seem that more than one mechanism is involved. Many of the results can be satisfactorily interpreted on the basis that the two compounds act on a common system and that they compete with one another, but the results are not compatible with the postulate that the dichlorophenoxyacetic acid directly inactivates indolylacetic acid.