A quantitative method used to correlate the degree of curvature obtained in a plant part, after the application of an external stimulus, with the amount of endogenous growth substance (* auxin) required to bring it about. The tips of Avena (oat) coleoptiles are used as a standard. These are removed and placed on an agar block and auxin allowed to diffuse out for a predetermined time. The auxin-impregnated agar is then used to measure the curvature produced when pieces are placed acen-trally on decapitated coleoptiles. The response is plotted against the auxin concentration in the block. Over a certain range, increases in auxin concentration result in a regular increase in the degree of bending in the coleoptile, but the linear response is lost at higher levels. Only the straight line section of the graph is useful for assays. By noting the angle of curvature produced by a given stimulus under otherwise controlled standard conditions, one can find the quantity of auxin involved in producing the response. See also tropism. |
