Studies in the Physiology of the Onion Plant: V. AN INVESTIGATION INTO THE GROWTH SUBSTANCE CONTENT OF BULBING ONIONS

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Abstract
Extracts have been prepared from induced and non-induced onion plants. These extracts have been fractionated and the ether-soluble, acidic components investigated using the wheat coleoptile straight-growth assay, colorimetric determination of 3-indolylacetic acid (IAA), and a new bioassay which has been introduced and modified to give reproducible results. This last is an assay of substances which will cause ‘bulbing’ (swelling of the leaf bases) of onion plants, and it has been used to demonstrate a number of factors which influence the initiation of bulbing in the onion. It has been shown that substances are present in extracts of onion tissue which will cause an increase in the ‘bulbing ratio’ of test onion seedling sections. Results indicate that there is an increase in the IAA content to a very high level during the first week following induction—before there is any visible sign of swelling of the leaf bases—but that this falls off rapidly after 5–7 days and eventually falls below the level found in non-induced plants.