Agriculture Reference
In-Depth Information
Table 7.2.
Respiration rates (CO
2
output, mg/kg/h) at constant temperatures in air
of onion bulbs, leeks, garlic and a selection of root, shoot and leafy vegetables
(from Robinson
et al.
, 1975, Table 1; data for garlic from Kader, 1992).
Temperature (°C)
Vegetable
0
5
10
15
20
Onion bulbs, 'Bedfordshire Champion'
3
5
7
7
8
Garlic bulbs
-
5-10
-
-
-
Leeks, 'Musselburgh'
20
28
50
75
110
Potato, 'King Edward'
6
3
4
5
6
Cabbage, 'Decema' (storage type)
3
7
8
13
20
Carrots
13
17
19
24
33
Lettuce, 'Kloek'
16
24
31
50
80
Spinach
50
70
80
120
150
respiration rates increase quite sharply, possibly because these temperatures are
injurious to bulbs.
The Q
10
for respiration rate in fully dormant bulbs over the temperature
range 10-30°C is about 1.3. If bulbs are wounded, their rate of respiration
increases and reaches a maximum after about 12 h. In this condition the Q
10
for bulb respiration rate is about 2.3, more normal for a physiological process
(Miedema, 1989). As dormancy declines and internal sprout growth begins,
bulb respiration rates increase more rapidly with increases in temperature, and
Q
10
values are similar to most plant tissues. The respiration rate of damaged
bulbs remains elevated for many weeks after the initial damage, though in less
severe cases it can eventually return close to the level of undamaged bulbs.
Irradiation with 150 or 300 Gy of gamma radiation increased respiration
rates by 35 and 100%, respectively, 24 h after treatment in onion bulbs cv. 'Rouge
d'Amposta' stored at 20°C. Within 2 weeks of treatment the respiration rate of
irradiated bulbs was down to the level of untreated controls (Benkeblia
et al.
,
2000). Ionizing radiation probably causes cellular damage and stimulates repair
metabolism as does mechanical damage, and hence increases respiration rates.
In the longer term the respiration rate of irradiated bulbs continued to decline
slowly, while that of untreated bulbs began to increase (see Fig. 7.10). The Q
10
of
respiration rate in untreated bulbs increased from 1.8 to 2.9 during 2 months of
storage at 4°C but the Q
10
of irradiated bulbs did not increase over this interval.
The relationship between respiration rates of stored produce in reduced-
oxygen atmospheres has frequently been described by the Michaelis-Menten
equation, where respiration rate is plotted against oxygen concentration. Accord-
ing to this model, bulb onions had a K
m
value (oxygen concentration at which
respiration rate was half of maximal) of 1.6 kPa (equivalent to 1.58% oxygen
partial pressure in a standard atmosphere) at 4°C (Benkeblia
et al.
, 2000), similar
to the value reported for green
Allium fistulosum
(see 'Green Onion Storage',
