Agriculture Reference
In-Depth Information
The control of bulbing by photoperiod and temperature
As indicated by Tables 4.1 to 4.3, the timing of bulb initiation in relation to leaf
canopy development is critical in determining yields in bulb-forming alliums.
An understanding of what controls bulb initiation is therefore important.
Garner and Allard (1920) first showed that onions develop bulbs in response to
long photoperiods, and later it was shown that onion varieties grown at
different latitudes could be distinguished by the minimum day-length needed to
induce them to bulb (see Chapter 1 'Onion Cultivars'). Further research showed
that, in a given day-length, bulbing was faster the higher the temperature.
These effects of photoperiod and temperature have since been confirmed by
many studies (Brewster, 1990a). Onion leaves must be exposed continuously to
bulb-inductive photoperiods in order to complete bulbing. If bulbing plants are
transferred to short, non-inductive photoperiods green leaf production can
resume, even in plants at an advanced stage of bulb development (see Fig. 4.28).
Photoperiod and temperature responses have been quantified for two
cultivars grown in the UK, 'Hyton' used for spring sowings and 'Keepwell' for
autumn sowings (see Figs 4.29a and 4.30). These data were obtained by
regularly sampling plants growing in constant photoperiods and temperatures
and determining when bulb scales first appeared. The interval between
transfer of plants to test photoperiods and when bulb scales were initiated
measured the time needed for bulbing, and the reciprocal of this time was taken
as the measure of 'rate of bulbing'.
Fig. 4.28. The reversal of bulbing in onion cv. 'Rocket' at an advanced stage of
bulbing, following transfer from 16 to 8 h photoperiods. Note the resumption of
foliage leaf production even though many bulb scales had formed (redrawn from
Wright and Sobeih, 1986).
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