ON THE NASTIC AND TRAUMATIC RESPONSES IN THE PEA TEST

Abstract

THE PEA TEST of Went (1934), a test method for the plant growth hormone auxin, depends on the curvature of longitudinally slit, growing stems (see sketch, fig. 1). Because of released tissue tensions and compressions, the slit halves of the stems curve strongly outward, a new equilibrium position being reached after a short time. In auxin solutions the preliminary outward curvature is soon followed by an inward curvature, i.e., a curvature toward the cut surface. This inward curvature is a result of unequal growth of the inside and outside layers of tissue and is a function of the auxin concentration. There has been a series of investigations on the physiological mechanism of this response (Went, 1934; Jost and Reiss, 1936; van Overbeek and Went, 1937; Tbimann and Schneider, 1938, 1939; Jost, 1938; Went, 1939). The explanations offered cover most of the conceivable possibilities and include: 1. Growth of the layers of cells near the longitudinallv cut surface such that these inner cells become more nearlv isodiametric; i.e., they decrease in length although they increase in volume. Thus the inside becomes shorter and causes an inward curvature. 2. Auxin can enter only through the intact surface (not through the cut surface), and establishes, therefore, an auxin gradient such that the 'outer layers are stimulated to grow more than the inner. 3. All layers have about equal access to the supplied auxin, but the central and peripheral layers inherently accumulate auxins to different degrees or respond to them to different degrees-i.e., the response is nastic. 4. The injury resulting from slitting reduces the potential growth of cells near the cut-i.e., the response is traumatic. The first two explanations have already been abandoned. The nastic and traumatic explanations remain; their evaluation in relation to each other is the subject of this paper. In making this evaluation, allowances must be made for the variable condition of the material used. For example, the finding that a given fraction of the curvature of highly responsive halved stems is the result of trauma or of nasty need not necessarily hold exactly in another case where only slightly responsive halved stems are encountered. In this connection it is of interest that with pea stems the reactivity varies to such a degree as to give the large curvatures reported by Went (1934, 1939) or no inward curvatures at all as reported by Jost and Reiss (1936); similarly with dandelion (Taraxacum) flower stalks, either inward pea test curvatures or only extreme outward curvatures can be obtained, in this case, de-