Growth Stimulation by High Indole-3-Acietic Acid Concentrations

IT is well established that the curve depicting the extension growth of coleoptile sections as a function of indole-3-acetic acid concentration in the surrounding medium (the concentration curve) assumes the shape of an optimum curve. In the case of wheat coleoptile sections the curve ascends at a constant rate until the optimum is reached at approximately 10-5 M indole-3-acetic acid. The descent of the curve above the optimum is conditioned by the inhibiting effect of higher indole-3-acetic acid concentrations. This clear and distinct picture is found at least when an experimental period lasts 24 hr., while for shorter periods the course of the concentration curve is not so well known.