Agriculture Reference
In-Depth Information
transfer to inductive photoperiods, showed the younger leaves to be more
effective in inducing bulbing (Sobeih and Wright, 1986).
The increased time required for bulbs to initiate as photoperiods decrease
has been interpreted as evidence for a progressively decreasing 'critical' photo-
period needed for bulbing as plants age (Wiles, 1989). This is an alternative
interpretation of data such as those in Fig. 4.30 in invoking a slower
rate
of bulb
initiation at shorter photoperiods, as implied by Eqns 4.20 and 4.21. It has yet
to be shown conclusively which of these interpretations is correct, or whether
they are both partly true.
Pre-planting storage temperature and the bulbing of sets
If onions are grown from sets, there are numerous reports (e.g. Aura, 1963;
Brewster, 1990a) that storage of sets at 28-30°C for several months prior to
planting results in bulbing and maturity up to 1 month later than in sets stored
at 20°C or below. The consequent longer growing season allows more time for
leaf growth and gives higher yields. In shallots and multiplier onions also,
storage at 28-30°C - or in some strains 20°C - before planting delays maturity,
prolongs the growing season and increases yields. Therefore, when growing
onions and shallots from bulbs rather than seeds, the timing of leaf growth and
bulb development is influenced by storage temperatures prior to planting, as
well as by temperatures and photoperiods while the crop is growing in the field.
Figure 4.33 summarizes the environmental control of bulbing in onion
and indicates how factors interrelate.
The biochemistry of onion bulbing
When onion plants are transferred from non- to bulb-inducing conditions, a
rise in reducing sugar, sucrose and fructan (see Chapter 8, 'Carbohydrate
Biochemistry') concentrations occur in the sheath (pseudostem) tissue within
5-10 days. Reducing sugar concentrations in the leaf blades also increase. At
the same time, a rapid decrease has been measured in the level of a soluble acid
invertase, an enzyme that catalyses the conversion of sucrose to the reducing
sugars glucose and fructose. These changes occur before visible bulb swelling
(Lercari, 1982a, b).
Bulb swelling may involve the hydrolysis of previously accumulated fructans
to fructose and glucose (Darbyshire and Steer, 1990). This could increase the
osmotically active solutes in the outer sheath cells, thereby drawing in water and
driving cell expansion. A swelling of leaf sheaths and the cessation of leaf blade
development, along with bulbing ratios of three or more, have been induced in
non-inductive photoperiods in onion plants growing
in vitro
on agar containing
tissue culture nutrients when the sucrose concentration in the medium was
