Inhibitory and Stimulatory Effects of Indoleacetic Acid on Development of the Bean Hypocotyl

Abstract

Study was made of histological changes preceding and concomitant with observed gross inhibitions and stimulations induced in the Red Kidney bean seedling (Phaseolus vulgaris) following unilateral and band applications of 3% indoleacetic acid-lanolin paste to various loci of the hypocotyl in various stages of straightening and elongation. 2. Within 24-48 hours after application of IAA to the arched seedling gross changes include discoloration of tissues, temporary inhibition of straightening and elongation of the hypocotyl, and excessive increase in its diameter at, below, and, to a slight extent, above the treated area. Following application the cells of the concave side of the seedling arch at first stretch along radial and tangential, but not along the longitudinal, axes. This change in the normal pattern of orientation of the axes is responsible for delay in straightening and for deficient hypocotyl elongation and, at this stage, is the main factor in abnormal increases in diameter and in localized tumor formation. Eventual straightening of the treated hypocotyl and its finally deficient elongation are due to lengthening along the longitudinal axis which is less than in untreated hypocotyls but eventually equals the depressed elongation of cells on the convex side of the arch. Later both sides lengthen in the final phase of elongation. The abnormal increase in the hypocotyl volume at, below, and, to a slight extent, above the locus of application is due to increases in the cell volume and to enlarged intercellular spaces, formation of cortical schizogenous cavities, and to root meristems arising in the tumor opposite one or more rays. 3. When IAA paste was applied unilaterally to various loci on the straightened hypocotyl 24-48 hours after the arch stage, negative bending of the hypocotyl was induced within 3-6 hours after application. Bends always occurred in the hypocotyl zone which was in process of elongating to mature length at the time of application. Curvatures were at, below, or above the treated area depending on the relative positions of the zone of elongation and of locus of treatment, provided the former was not too far above the latter. 4. Unilateral applications of 3% IAA to various loci on the straightened hypocotyl 72 hours after the arch stage did not induce hypocotyl bending but did lead to tumor development at each locus. If the treated area was directly below the cotyledonary node on either the convex or the concave side, negative bending occurred in the zone of elongation of the first internode. 5 There is some evidence that IAA, or some substance activated by it, is capable of moving against the normal polarity gradient and under proper conditions can affect tissues above the site of application. 6. Histological studies of the bends indicated that they probably were entirely due to elongation which temporarily is excessive in rate and finally slightly deficient in amount in the cells on the treated side, and initially is deficient in rate and ultimately more deficient in amount in the cells on the opposite side. Changes in the normal pattern of cell enlargement along the three axes probably are functions of the IAA concentration in, and of the developmental stage of, the affected cell. Tumors formed at the locus of application in straightened hypocotyls treated with 3% IAA paste 24, 48, and 72 hours after the arch stage have their origin in excessive cellular stretching mainly along radial and less extensively along tangential axes. Later, cell divisions set in, and tumors develop as in treated arched hypocotyls.