Agriculture Reference
In-Depth Information
whole bulb. Leathery scales can be seen in the field late in the growing season
and weak symptoms may disappear during warm-air curing. A slight rise in pH
to 6.4 from the normal 5.9 in fleshy scales is first observed, a feature associated
with excess carbon dioxide toxicity in plants. Later, the pH declines to 4.1 and
ethanol levels increase. The electrical conductivity of the tissues increases as
the disorder develops, a characteristic of tissues with membrane leakage and
breakdown.
Oxygen permeabilities of dry scales and the epidermis of fleshy scales were
10 6 cm/s/atm., respectively, lower than the
permeability needed to supply respiring fleshy scales, but the permeability of
the neck and base (10.8 and 1.9
10 6
11
and 6.8
10 4 cm/s/atm., respectively) were much
higher, indicating that the neck was the important path for respiratory gas
exchange and that there could be negligible radial gas exchange through onion
skin and epidermal layers into the bulbs. Blocking the neck by clamping or
wetting reduced its oxygen permeability by a factor of four.
Mechanical treatments likely to simulate events during harvesting and
curing could cause internal carbon dioxide to reach damaging concentrations
within bulbs. Dropping onions on to a hard surface caused a 4% rise in internal
carbon dioxide, through increased respiration rate and therefore carbon
dioxide production (see Fig. 7.12). Pressure on necks by tying restricted oxygen
permeability, leading to a 4% rise in carbon dioxide. A combination of these two
treatments led to internal carbon dioxide levels likely to cause damage.
Pressure on the necks of onions in a bin from overlying bulbs was observed in
about 5% of onions. This could restrict gas exchange in such bulbs and may
explain why some show symptoms and others do not.
Field experiments showed that late harvesting, high drying temperatures
and long duration of drying give high internal carbon dioxide levels and the
high occurrence of translucent scales. Yoo et al. (1997) reported higher internal
carbon dioxide levels in stored bulbs and lower internal gas volumes with
increasing temperature (see Fig. 7.8). They speculated that the lower gas
volumes in the neck at warm temperatures could restrict gas exchange and
partly explain the greater internal carbon dioxide levels they observed at high
temperatures in situations where respiration was not increasing with
temperature (see Figs 7.8b, c ). In uncured onions straight after harvest a high
drying temperature is likely to be associated with a high respiration rate, leading
to high internal carbon dioxide levels. Late harvesting is likely to be associated
with more moisture on the necks and more collapsed necks, both of which will
lower gas permeability compared with conditions at an earlier harvest. Survey
data showed that the occurrence of these disorders was associated with years of
high rainfall and humidity at harvesting and curing time.
Recommendations for avoidance of these disorders included: (i) early
harvesting (not later than 50-80% leaf-fall); (ii) gentle handling of the onions;
(iii) avoidance of wet harvesting conditions; (iv) drying in boxes rather than in
deep piles to avoid pressure on the onion neck; and (v) drying temperatures
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