The Effects of Indole-3-Acetic Acid and 2:4-Dichlorophenoxyacetic Acid on the Growth Rate and Endogenous Rhythm of IntactAvenaColeoptiles

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Abstract
The rates of elongation of the coleoptiles of Avena seedlings, subjected to intermittent immersion in solutions of IAA or 2:4-D for various total periods, were determined from measurements of photographs taken every hour by infra-red radiation. Immersion in 17·5 mg./l. IAA for 1–5 hours caused a large increase in the growth rate followed by a depression. When the seedlings were immersed in 8·75 mg./l. IAA for periods of 12 or 24 hours the depression was partially overcome so long as the treatment was continued. Absorption of additional IAA by the coleoptiles reduced their geotropic sensitivity. Penetration of 2:4-D (sodium salt) into the coleoptiles was slower than that of IAA and the resulting stimulation of the growth rate was less, particularly in unbuffered solutions. After the treatment the growth rate declined slowly to about the normal value. Results with coleoptiles were very similar to those previously obtained with rhizomes of Aegopodium and suggest that inhibition of growth following stimulation by IAA may be of general occurrence. Possible causes of the inhibition are discussed and a comparison is made between the results with intact coleoptiles and observations made by others on coleoptile sections. Temporary immersion of the seedlings in auxin solutions depressed the rate of elongation of the primary leaf while it increased that of the coleoptile. It caused little disturbance of the endogenous rhythm induced by change from light to darkness. The suggestion that such rhythms can be explained in terms of variation in concentration of IAA-oxidase is not supported.