Agriculture Reference
In-Depth Information
Fig. 4.37.
The relative rate of vernalization
versus
temperature for European and
Japanese cvs of onion. Different symbols indicate results from different series of
experiments (from Brewster, 1987).
When an inflorescence initial has developed within a bulb to stage 4 of Fig.
2.6, its subsequent rate of elongation depends on temperature and, after the
bulb has sprouted and started to grow, on day-length. Long photoperiods
coupled with fairly cool temperatures (10-15°C) are optimal. A vegetative shoot
apex normally develops axillary to the inflorescence (see Fig. 2.6), and this can
develop more rapidly than the inflorescence under a combination of warm
temperatures (20°C or higher) and long photoperiods (e.g. 16 h, although this
probably depends on cultivar). This results in the swelling of the axillary bud to
form bulb scales and the shrivelling and degeneration of the young scape (see
Fig. 4.38), hence the term 'competition phase' coined by van Kampen (1970) to
describe the apparent competition between axillary bud and inflorescence.
When onions are planted out in bulb-inducing conditions - for example, in a
warm spring or in a glasshouse in spring - such inflorescence abortion can be
common, even from bulbs with advanced inflorescence initials.
As discussed under bulbing, bulb or set storage at 28-30°C also delays
bulb initiation. Because bulb development suppresses inflorescence elongation
during the competition phase, a short period (1-2 months) of storage at such a
high temperature at the end of a long period of cool storage can, by delaying
bulbing, actually result in increased bolting (see Fig. 4.39), although longer
periods at high temperature do suppress flower initiation and bolting.
