Growth substances

Some classic accounts of growth substance in onions are those of Thomas and Isenberg (1972) and Isenberg et al. (1987). They detailed the movement of inhibitors from the leaves into the bulb tissues at maturity, the gradual change to lower inhibitor- and higher growth-substance levels during overwinter storage at 5-8°C in the UK, in the order: gibberellins (with a first peak in December), cytokinins and auxins. A second gibberellin peak accompanied sprouting in March. Isenberg et al. (1987) concluded that UK storage onions have a rest period from harvest until midwinter, with several weeks' cessation of sprout growth even under favourable conditions. The peaks of activity of growth substances in sequence, roughly 30 days apart, were thought to correspond to internal development of the growth apex, representing floral initiation under cold conditions (initial gibberellic acid (GA) peak), cell multiplication (cytokinins), the initiation of sprout growth (auxins) and the appearance of a visible floral initial; the second gibberellin peak accompanied actual sprouting at the end of the dormant period. Thus, the apparently inert dormant onion actually undergoes important internal changes, leading towards flower production (Kamenetsky and Rabinowitch, Chapter 2, this volume).

Growth inhibitors increased and gib-berellins decreased throughout drying in cv. 'Sochaczewska' (Kielak and Bielinska-Czarnecka, 1987). In Japan, abscisic acid (ABA) levels were high at the onset of dormancy, reaching a maximum 1 month after storage, gradually decreased during storage and increased again during sprouting (Matsubara and Kimura, 1991; see also Section 7 below).

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