Agriculture Reference
In-Depth Information
regimes on storage life but the causes of loss, whether by rotting due to disease
or sprouting, were not always distinguished. Reviews on these topics were
published by Komochi (1990), Gubb and MacTavish (2002) and Chope (2006),
and these authors provide extensive bibliographies of the research literature.
The physiology of dormancy in onions
Bulb development and structure in relation to dormancy
Many fundamental aspects of what is known about onion dormancy were
discovered or elegantly demonstrated by Abdalla and Mann (1963). They followed
bulb development, rest and sprouting in cv. 'Excel', a cultivar with innately short
dormancy, grown in the field at Davis, California, USA. During growth, leaves were
initiated at the shoot apex at a rate of about one per week (see Fig. 7.1).
Leaves initially differentiated into bladed leaves, and later, as bulbing
commenced, into three or four bladeless storage leaves (bulb-scales) (see Fig.
4.26). Within these were formed five or six non-elongated leaf initials that
again had blades, and which later formed the shoots that sprouted from the
bulb; these are termed the 'sprout leaf ' initials. Leaf initiation at the shoot apex
ceased about 20 days before bulb harvest (see Fig. 7.1), while the leaves were
still green and erect. Root initiation ceased at about the same time. Leaf
initiation resumed within stored bulbs about 2 weeks after harvest.
Cell division at the shoot apex continued right up to harvest and then
declined after curing, to remain at a low level during storage. Sprout elongation
within the bulb during storage was due to the elongation of pre-existing cells in
the 'sprout leaf ' initials that were formed late in bulb growth and was not,
Fig. 7.1.
Leaf initiation on the main axis (solid line) and first axillary bud (broken
line) of onion cv. 'Excel' sown on 31 December at Davis, California, USA. The first
eight to nine leaves were bladed, the next three to four were bladeless bulb scales
and the last five to six were bladed sprout leaf primordia (see Chapter 2) (from
Abdalla and Mann, 1963. Courtesy of
Hilgardia
).
