The Efect of Indole Acetic Acid and other Growth Promoting Substances on the Endogenous Respiration of the Avena Coleoptile

1. By means of the Warburg manometric technique some details were studied of the respiration of starved Avena coleoptile tissue. 2. For this purpose the oxygen uptake was measured on 3 mm sections cut from different levels on the coleoptile (5-7, 8-10 and 11-13th mm from the tip). Instead of being thoroughly mixed — a procedure followed by other authors — the sections from a given position on the coleoptile were studied separately. 3. The amount of oxygen taken up per section per hour showed a slight increase in the basal direction of the coleoptile. 4. Addition of glucose to starved sections caused an increase in the oxygen uptake of an average of one hundred percent. This is considered an indication that the respiration of such sections is limited by the substrate concentration. 5. The intensity of the endogenous respiration showed diurnal fluctuations. Arguments were moved to connect this with diurnal fluctuations in the rate of starch mobilization. 6. Addition of indole acetic acid (1 mg/1) to sections, starved over a period of 24 hours, generally caused an increase in the oxygen uptake. 7. The respiration of sections of different positions on the coleoptile showed quantitative differences in the response to indole acetic acid addition. The zone of maximal sensitivity, though generally situated between the 5-10 mm from the tip of the coleoptile, in some cases was found at the level of the lowest sections. 8. In experiments with freshly cut sections, the indole acetic acid being added within three hours after sectioning, without any exception, considerable stimulations of the oxygen uptake were found. The stimulations were preceded by a period of inhibition, the duration of which varied according to the position of the sections in the original coleoptile. 9. The stimulative effect of indole acetic acid on the oxygen uptake of freshly cut sections lasted nearly 20 hours on an average. The total extra oxygen uptake in the treated sections amounted to 100-150 mm³, when indole acetic acid was present in the concentration of 1 mg per liter. 10. Addition of new indole acetic acid to the sections at the end of the period of stimulation was not followed by another increase in the oxygen uptake. 11. Addition of ethanol in a concentration as usually applied in dissolving growth substances (40 mg/1) increased the oxygen uptake of freshly cut sections by 8-23 percent. The presence of ethanol influenced the stimulation caused by indole acetic acid. 12. The action of other growth promoting substances was compared with that of indole acetic acid. Whereas the response of the endogenous oxygen uptake to alpha naphthyl acetic acid appeared to equal that of indole acetic acid, beta naphthyl acetic acid had no influence at all. The activity of gamma phenyl butyric acid being about half that of indole acetic acid, even exceeded that of indole butyric acid. 13. The degree of the stimulation in non-growing sections (cut from very long coleoptiles) was not less than in growing sections. The increase in oxygen uptake, therefore, was not the result of increased growth of the sections. 14. In accordance with data from the literature no effect of indole acetic acid was observed on the hydrolysis of starch by amylases in vitro. The unsuitability of the iodine staining technique for such investigations was demonstrated, as indole acetic acid combines with the iodine. 15. It is suggested that the auxin action, underlying the increased endogenous oxygen uptake, is exercised on inner protoplasmic mem- branes (lipophilic interfaces) thus making the reserve food more accessible to the metabolic processes.