THE EFFECT OF AUXIN ON PROTOPLASMIC STREAMING IN ROOT HAIRS OF AVENA

Abstract

WHETHER OR not auxin is necessary for elongation of roots is at present not definitely known. All auxins which have been investigated inhibit the elongation of roots in concentrations which accelerate the growth of coleoptiles, so that, if these are growth factors in root cells, then only minute amounts are necessary as compared with stem and coleoptile. Thus it seemed of interest to determine whether protoplasmic streaming in roots is affected by auxin, and, if so, what concentrations are effective. For purpose of comparison, it was obviously desirable to do this on Avena, but the delicate nature of Avena roots prevented sectioning for the observation of streaming. However, since root hairs are essentially elongated protuberances from root epidermal cells, a study of the effect of auxin in root hairs of Avena seedlings was made. METHODs.-Seeds of Victory oats were husked and soaked two to three hours in tap water. The seeds were then grown on moist filter paper in the dark room at 24?C. and 85 per cent relative humidity. Since, in Avena, root hairs did not develop if the roots were immersed, the seedlings, when two days old from soaking, were transferred to moist filter paper in a covered petri dish. Root hairs then developed abundantly overnight. On the third day after soaking, the whole plant was gently transferred to a slide, a cover slip was placed over the root to be examined, and oxygenated water was added. Root hairs were not damaged by this treatment and streaming continued at a reasonably constant rate for several hours. Only toot hairs which had not reached their final length were used (these measured 0.3 to 0.45 mm. in length). The auxin, indole-3-acetic acid, was used in these experiments. Solutions to be tested, made up in oxygenated water unless otherwise stated, were added as in the study of the coleoptile (Thimann and Sweeney, 1937), and were renewed every few minutes. Streaming was measured and recorded as described by Sweeney and Thimann (1942). In some experiments the plants were deseeded and the coleoptile was cut away at the first internode when the plants were two days old. They were then placed on filter paper as above. Although the whole root elongated only a few millimeters under these conditions, abundant root hairs were formed up to the tip of the root. RESULTS.-Root hairs from intact plants.-As in the coleoptile, auxin brings about an acceleration of streaming in the root hair. When auxin is added in water, without sugar, the acceleration of streaming is temporary, the effect being over in 30 to 35 minutes, as in the coleoptile (fig. 1, curve 1). The rate 1 Received for publication October 5, 1943. The author wishes to express her thanks to Dr. K. V. Thimann for many very helpful suggestions during the course of this work. of streaming is increased to about the same extent in roots as in coleoptiles, i.e., about 25 per cent. The acceleration may be maintained by the addition of 1 per cent fructose to the auxin solution. However, the effective auxin concentrations in the case of