Agriculture Reference
In-Depth Information
Because of the importance of the stage of bulb development at which MH is
applied, the best results occur when all plants in the crop are at the same stage
of development. Such a crop requires plants of a uniform cultivar, a small
spread in emergence date, and an even plant density and uniform growth that
have not been checked differentially by limitations of nutrients and water or by
pest, disease or herbicide damage.
IRRADIATION
A disruption of cell division at the shoot apex and inhibition of
sprout growth similar to that caused by MH can be achieved by treating dormant
bulbs with
(gamma) irradiation (Komochi, 1990; Gubb and MacTavish, 2002).
Doses of 20-150 Gy are sufficient to prevent sprout growth, and Benkeblia
et al.
(2000) showed that doses of 150 Gy (0.15 kGy) cause only a small initial
increase in respiration rate (see Fig. 7.10). Doses in this range cause no detectable
differences in the taste or nutritional composition of the bulbs. Much higher
doses can reduce the content of flavour-determining compounds.
To be effective, the bulbs must be irradiated soon after harvest while fully
dormant. Delay in irradiation decreases effectiveness as sprouts develop
internally. How long it can be delayed depends on both the duration of dormancy
of the cultivar and the temperature of storage. The long-storing cv. 'Senshuki'
was strongly inhibited from sprouting when treated 2-3 months after harvest,
but the short-dormancy cv. 'Sapporoki' had to be irradiated within 1 month of
harvest (Komochi, 1990). If internal sprout development is slowed by storage at
low temperatures (0-5°C), then the time available for effective irradiation is
increased.
A darkening of tissue near the growing point has frequently been observed
to develop a few weeks after irradiation. This can be prevented by low-
temperature (0-5°C) storage of treated bulbs, but darkening develops several
weeks after bulbs are returned to normal ambient temperatures. Ultimately,
cell death and decay occur in irradiated bulbs, after 25-30 weeks in the case of
cv. 'Rouge d'Amposta' (Benkeblia
et al.
, 2000).
No harmful effects have been observed in numerous toxicological tests
using irradiated onions. Because there are no toxic effects or serious detri-
mental effects on eating quality, the sale of irradiated onions is permitted in
many countries, and in many of these the irradiation of shallot and garlic bulbs
for sprouting inhibition is also permitted (Komochi, 1990).
Onion storage systems
Some of the physiological information discussed above has been applied in the
development of appropriate techniques of onion storage. In particular, since
temperature, humidity and gas atmosphere requirements for long-term storage
are known, air conditioning and ventilation techniques have been developed
that allow these needs to be met in large-scale, bulk stores. The onion bulb is a
natural food store for the plant, but it is a living system undergoing a process of
development towards sprouting, and subject to decay by various disease-
