Modified Bio-assay for Growth Substances in Bulbing Onions

Abstract

THE onion seedling section assay developed by Clark and Heath^1,2 estimated the effectiveness of known and unknown growth substances extracted from onion plants, both before and after photoinduction, and separated by paper partition chromatography; it depended on bulb development in isolated 13-mm. sections cut from onion seedlings at the second leaf (6-wk.) stage. The special significance of this bio-assay was that it tested the ability of these substances to produce the effect under investigation, namely, bulbing, in standard onion tissue maintained under otherwise non-inductive conditions. Clark and Heath showed that indolyl-3-acetic acid (IAA) supplied externally to the sections, as a 1 × 10⁻⁴ M solution in the presence of 1 per cent sucrose, caused a significant increase in bulbing ratio (that is, ratio of maximum diameter of base to minimum diameter of neck) after periods of exposure to the test solutions for as long as nine days. No significant changes were found in sucrose alone. The variety used by them, Unwin's Reliance, proved to give rather variable and small responses, and we decided that a more uniform test material was required.