The Role of Growth Substances in the Regulation of Onion Bulb Dormancy

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Abstract
The correlation between sprouting and changes in endogenous growth substances was investigated in stored onion bulbs (Allium cepa c.v. Rijnsburger and Lancastrian). In the main experiments bulbs were removed from store at approximately fortnightly intervals, samples were assessed for percentage sprouting and non-sprouting bulbs were either extracted for hormone assay or treated with growth substances in an attempt to induce sprouting. In other experiments the hormone content of bulbs at different stages of sprouting was assessed. Growth-inhibitor and gibberellin activity decreased before sprouting, but there was an increase in gibberellin and auxin activity as sprouting commenced. Gibberellin activity was highest in bulbs with well-developed sprouts whereas auxin activity occurred mainly in bulbs in which early sprout development was visible only on their being cut open. There was no conclusive evidence that bulb dormancy could be broken by application of the gibberellins GA3 and GA4/7, or the auxin 1-naphthylacetic acid (NAA). Maleic hydrazide (MH) completely inhibited root and sprout development but the growth retardant (2-chloroethyl)trimethylammonium chloride (CCC) was mainly effective in reducing root development and sprouting was only slightly inhibited.

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