Agriculture Reference
In-Depth Information
below). However, at 20°C, unlike green onion, K
m
was higher, and this equation
did not fit the data very well; it yielded an approximate 'apparent' K
m
of 6.3 kPa
(equivalent to 6.2% oxygen in the atmosphere).
It is likely that the long diffusion path of oxygen into bulbs (see below) is
partially rate limiting for respiration. This makes the Michaelis-Menten equation
inappropriate for describing respiration rate kinetics in a bulky organ like the
onion bulb, whereas it is appropriate for the leafy tissue of green salad onions,
which has a much shorter diffusion path from the free atmosphere to respiring
cells.
If the dry, outer skins of onions are removed, the respiration rate of bulbs
increases nearly twofold and the rate of water loss also increases (Apeland, 1971).
Bulbs with the skin removed also sprout more rapidly than those with intact skins
(Tanaka
et al
., 1985a). Intact dry, outer skins may act as a strong barrier to gas
diffusion, resulting in the decreased O
2
:CO
2
ratio found within stored bulbs (see
Fig. 7.8). Increased CO
2
and decreased O
2
prolongs the time to bulb sprouting,
compared with storage in air (see 'Controlled atmosphere storage', below). It has
been suggested that intact outer bulb skins maintain an internal atmosphere
within the bulb, which slows both respiration rates and sprouting growth
(Ladeinde and Hicks, 1988). Furthermore, wounding, which promotes sprouting,
may, by puncturing the skin, change the internal atmosphere of the bulb, par-
ticularly since sealing cuts with wax nullifies this effect on sprouting (Boswell,
1924). However, Yoo
et al.
(1997) found that sprout growth was not inhibited by
internal carbon dioxide concentrations raised to 7.9% by sealing bulb necks.
Starting at the end of bulbing and before harvest, fructan (see Chapter 8)
concentrations decline and fructose levels increase in storage scales, most
rapidly in the outermost scales. Fructan concentrations increase in the bulb
base plate, and it appears that this acts as an intermediate store for carbo-
hydrates to supply the developing sprout. The decrease in fructan and increase
in fructose with time is not prevented by storing bulbs in ethylene concen-
trations that inhibit sprout leaf growth (Bufler, in press, b). Dry weights of
storage scales decline and base plate and sprout weights increase during
storage, reflecting this translocation of carbohydrates. The sprout always
contains enough carbohydrate for growth (50-60 mg/g dry weight, of which
30% is fructan), and therefore growth is not limited by the source of supply of
carbohydrates. As sprouts grow, sucrose synthase enzyme activity increases,
indicating an increasing sink strength for carbohydrates (Pak,
et al.
, 1995;
Yasin and Bufler, 2007).
From a large number of individual bulb measurements on two cultivars with
a clear dormant period, as shown by the absence of sprout leaf elongation (see
'Cultivar Effects', above), it was found that ATP content and sucrose synthase
enzyme activity increased within both the sprout leaves and enclosed apical
tissues when initial sprout leaf growth occurred (Yasin and Bufler, 2007). These
increases were concomitant with a small increase in total bulb respiration rate.
Increases in water-soluble carbohydrates and fructans were detectable in the
