Second positive phototropism in the Avena coleoptile.

A method has been developed whereby the second positive phototropism can be observed separately from the first positive and negative phototropic responses which also occur in oat coleoptiles. Although the second positive phototropic response has often been referred to as the base response, photoreception for it is shown to occur mainly in the apical 3 mm of the coleoptile. The Bunsen-Roscoe reciprocity law, so typical of first positive phototropism, does not apply to the second positive responses, and the amount of curvature increases linearly with the duration of the stimulus. However, although this linear proportionality between stimulus duration and response is the major factor determining response at all intensities tested, the intensity of the stimulus does influence the response somewhat. The action spectrum for the response shows no activity above 510 nm and has peaks at 375 and 450 nm. In all but one particular it closely resembles the action spectrum for the first positive phototropism, and it is concluded that the same, or similar, pigments may well be the photoreceptors for both types of response. The identity of this blue light absorbing pigment is not known.