Search

A rapid bioassay is described for the detection of growth substances. The bioassay is simple to perform under ordinary laboratory or classroom conditions and the sensitivity to auxin is comparable to other bioassays, including those using Avena first internode sections. The bioassay is performed on isolated first internode sections from partly etiolated bean plants (Phaseolus vulgaris L. cv. Bush Burpee Stringless Bush Bean). The test material is applied to one side and near the base of the section. Auxin treatments induce curvature and the distance of the lateral displacement of the apical portion of the segment is proportional to the concentration applied. Routinely, the indole 3-acetic acid response is measured 1 to 3 h after treatment. The sections also respond to gibberellic acid (GA3), which induces an increase in overall length, but unlike auxin, GA3 does not cause curvature. The test compounds are applied via a small filter paper disk that provides a convenient means for studying interactions of two or more compounds by co-applications. Auxin-induced growth can be monitored with a sensitive motion detector. Data are presented which confirm the usefulness of the assay.

The metabolism of IAA is correlated with tumor formation in the Nicotiana hybrid, N. glauca x langsdorffii. Higher IAA-oxidase activity was found in leaves of hybrid plants than in leaves of parental types. Furthermore, topographical distribution study revealed that young expanding leaves contained more IAA-oxidase activity than older leaves. In hybrid plants, there is a close correlation between IAA-oxidase activity and the ability of a leaf to produce tumors. Leaves which yield the greatest number of injury-induced tumors also yield the highest IAA-oxidase activity. IAA incites callus-like outgrowths in hybrid plants which do not differentiate into a multitude of apices as is the case with spontaneous tumors. The results are interpreted on the basis of a concept that the IAA-oxidase system in plants may function as part of an IAA activating mechanism rather than in the basis of an IAA destroying system.