DEPENDENCE OF THE AVENA COLEOPTILE GROWTH‐RATE ON THE PREVIOUS AUXIN SUPPLY

As is well known, the curvature in the Avena test (Went, 1928) is dependent on concentration of the applied auxin only up to a certain limiting angle. The value of this angle, the so-called "maximum angle," was shown by Schneider and Went (1938) to depend on the IDP (the Interval between the first Decapitation and the Putting on of the auxinagar block). When the IDP is short the maximum angle may be of the order of 450 but, as the IDP approaches two hours, this maximum angle passes through a minimum value of about 10-15 ?. Coleoptiles such as would give this 150 maximum angle are the basic experimental objects of this paper. The auxin content of coleoptiles used in maximum angle curvatures is probably dependent on the IDP. Thus for short IDP the content is high as a result of auxin production by the tip before the first decapitation, for IDP about three hours long the content is moderately high as a result of auxin production by the "regenerated physiological tip," and at intermediate intervals the auxin content passes t-hrough a minimum value corresponding to the lack of auxin production before regeneration of the physiological tip. The variation of the size of the maximum angle with IDP parallels this probable auxin content of the coleoptiles so closely as to lead to the suggestion that the size of the maximum angle is directlv related to the auxin content of the coleoptile at the beginning of the period of curvature. The investigations reported here test this suggestion that the maximum angle curvature varies with the auxin content of the coleoptile. The content of the coleoptiles is varied by artificially supplying auxin before the maximum angle auxin-agar blocks are applied. Schneider and Went (1938) have performed comparable experiments in which the auxin content was artificially increased to verv high levels. The experiments reported here deal only with concentrations comparable to those normally in the coleoptiles. The coleoptiles most used were those whose auxin content was lowered by removing the normal source of auxin, i.e., by decapitation. The auxin content was then brought up to anv desired level by artificially supplying auxin before performing the curvature tests, i.e., by auxin pre-treatments. Many other auxin pre-treatments have been reported in the literature. For the rooting of cuttings, Cooper (1938) and Went (1939) performed pretreatments leading them to the conclusion that auxin 1 Received for publication March 22, 1940. The author wishes to express his appreciation to Professor K. V. Thimann for stimulating advice and criticism in carrying out this investigation. is instrumental in the translocation of some second factor essential for root initiation. For the pea test, Went (1939a) developed a pre-treatment such that the sensitivity of the pea test was increased. For straight growth of the Avena coleoptile, Went (1 939b) performed pre-treatments from which he concludes that one of the activities of auxin is to cause the mobilization of a second factor (food factor) which is necessary for growth (by cell elongation). The results of the present paper are consistent with Went's conclusion of a mobilization of a "food factor" by auxin, although other explanations are not excluded. The pre-treatments reported here are unlike those of earlier investigations in three different ways. The assay of pre-treatment effects depends primarily on maximal growth responses. The pre-treatments cause no increase of sensitivitv to threshold concentrations. Physiological concentrations of highly active auxins are the most efficacious for the pre-treatments; that is, the pre-treatment is effected with just the kind and quantities of auxins that greatly accelerate cell elongation. VARIABILITY.-For each of the types of measurements quoted herein there has been some vari-ation in magnitude of response even for duplicate sets of experiments. Perhaps these could be best considered after the experimental evidence, but they are mentioned here in order that the reader may consider them along with the experiments. Thus for curvatures, the maximum angle response shown in figure 5 is ten degrees lower than that of figures 1 and 2. For straight growth, the response shown in figure 6 is considerably less than that shown in figures 3 and 4. These variations are comparable to those commonly encountered in the Avena test (see, e.g., Went and Thimann, 1937). A most striking variation occurs in the comparison of the effectiveness of different auxins for pre-treatment. For example, in experiments that were the duplicates of the one in figure 7, the positions of the curves for 3-indole-propionic and phenvl-acetic acids, were sometimes reversed. But perhaps this is not too surprising for similar variations of sensitivity of coleoptiles to these auxins are reported by Thimann and Schneider (1939). Curiously, the response of plain agar pre-treated controls was also variable (fig. 1, 5, 7). Efforts to find variation in the agar itself produced only negative results. It will be noticed that in the experiment of figure 1, the agar controls gave the same response as the plain controls, i.e., the agar pre-treatment was without effect, but in the experiments of figures 5 and 7 the agar pre-treatment had a marked effect. To a limited extent, this agar pre-treatment effect