| Abstract: | The interrelated effects of 3-indolylacetic acid and sodium2 : 4-dichlorophenoxyacetate on the growth and development ofLemna minor and Helianthus annuus have been investigated bymeans of multifactorial experiments, involving three to fivelevels of each compound. A clonal population of L. minor wasmaintained under constant conditions of light (550 foot-candles)and temperature (25 C.) and the growth regulators were addedto a phosphate-buffered culture solution (pH 51), which waschanged every 2 days. In the H. annuus experiments the compounds,in aqueous solution, were placed by means of a micro-pipetteon the leaves of young plants grown in a greenhouse or in theopen. Indolylacetic acid, at concentrations up to 25 p.p.m., increasesthe relative growth rate of L. minor on either a dry-weightor frond-area basis. Sodium dichlorophenoxyacetate at 0025p.p.m. has little effect on growth, but at and above concentrationsof 02 p.p.m. growth is depressed. The ratio of frond area tototal dry weight is increased by indolylacetic acid but is depressedby the phenoxyacetate. In the presence of indolylacetic acid,more particularly at the higher levels, the relative inhibitionof 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 leavesabove the treated leaves, the first internode, and the rootsare depressed when each compound is applied alone. On the otherhand, the growth rate of the hypocotyl is increased by bothgrowth regulators, while for the second internode indolylaceticacid decreases the growth rate but the phenoxyacetate at thelower level accelerates it. The combined action of the two compoundson the different parts tends to be cumulative, save for thehypocotyl, where the effect of indolylacetic acid is dependenton the level of the phenoxyacetate. It is concluded that these results cannot be explained on theconcept of wholly additive effects. There is evidence that thecompounds can mutually interfere in their actions and it wouldseem that more than one mechanism is involved. Many of the resultscan be satisfactorily interpreted on the basis that the twocompounds act on a common system and that they compete withone another, but the results are not compatible with the postulatethat the dichlorophenoxyacetic acid directly inactivates indolylaceticacid. |
