Agriculture Reference
In-Depth Information
Fig. 4.33.
A schema showing the main factors that control bulbing in onions and
how they interrelate.
raised from 40 to 120 g/l (Kahane
et al.
, 1992). However, bulb scales did not form
and there was no reduction in cell division or development of dormancy at the
centre of the 'bulb'. A similar increase in bulbing ratio, along with an
accumulation of soluble carbohydrates, is induced by nitrogen deficiency in
onions in non-inductive photoperiods (Brewster and Butler, 1989).
Attempts to isolate a 'hormone' responsible for bulbing have not been
successful, but auxin, cytokinin, gibberellins and ethylene have all been implicated
in bulbing. Externally applied ethylene (released from sprays of 'ethephon')
induces a swelling of leaf sheaths and some increase in bulbing ratio, but the
elongation of sheaths that normally accompanies bulb swelling does not occur.
Consequently, the appearance of ethylene-treated plants is not typical of normal
bulbing. Furthermore, although leaf bases swell as a result of ethephon treatment,
the bladeless bulb scales typical of normal bulbing do not develop (Sobeih and
