Agriculture Reference
In-Depth Information
Wright, 1987). In addition, ethylene antagonists do not prevent bulbing in
inductive photoperiods. The increases in bulbing ratio caused by ethylene or by
high sugar levels do not result in dormant bulbs with bulb scales, and hence this
should be regarded as 'pseudobulbing' rather than true bulbing.
Several studies implicate gibberellins in onion bulbing. An inhibitor of
gibberellin biosynthesis, coded S-3307, promotes bulbing in non-inductive
photoperiods. It also causes microtubules in the cells of onion leaf sheaths to
orientate longitudinally, whereas gibberellin application causes transverse
orientation (Mita and Shibaoka, 1984). The orientation of microtubules deter-
mines the orientation of the cellulose microfibrils that reinforce cell walls. Bulb-
inductive photoperiods cause microfibrils to lose the transverse orientation
found in non-inductive conditions. Therefore, changes in the architecture of the
cellulose reinforcement of sheath cell wells are associated with bulbing, and
these changes can be mimicked by the gibberellin antagonistic S-3307. Shallot
plants growing in the
in vitro
system mentioned above could be prevented from
bulbing in inductive photoperiods by inclusion of 10
M GA
3
in the culture
medium. Inclusion of the anti-gibberellin ancymidol in the medium increased
bulb swelling but did not induce normal, complete bulbing in non-inductive
photoperiods (Le Guen-Le Saos
et al.
, 2002).
Various approaches have been taken in developing models to predict onion
growth and/or development. Lancaster
et al.
(1996) used field data on the
growth and bulb development of cvs 'Pukekohe Long Keeper' and 'Early Long
Keeper' grown from a wide range of sowing dates in two regions in New
Zealand to develop an empirical model to predict the photoperiods and
temperatures needed for bulbing and the date and bulb size at maturity. For
bulbing to start, a minimum photoperiod of 13.75 h and a thermal time (T
b
=
5°C) since sowing > 600°C days were required. The bulb size at maturity could
be predicted from the bulb diameter at the start of bulbing, d
B,
and the number
of leaves appearing after bulbing, n
L
: d
B
depended on the thermal time from
sowing to bulbing and n
L
was a function of the date bulbing started. A similar
model could be useful for other cvs in other localities, but it would need to be
developed using local field data.
Scaife
et al.
(1996) fitted a range of equations containing biologically
meaningful parameters to the growth in dry weight of a bulb onion crop in
central England. They found that the best fit to the data with no indication of
systematic deviations was given by an 'expolinear' equation using accumulated
'effective day-degrees (EDD)' (Eqn 4.19) rather than time or day-degrees as the
independent variable. This equation models the growth of mutually shading
plants in a crop as they pass from the exponential growth phase of spaced
seedlings to linear growth in crop dry weight when the leaf canopy is closed.
