Agriculture Reference
In-Depth Information
Fig. 7.5.
Sections of long-storing, spring-sown onion bulbs 2 months (left) and
12 months (right) after entering storage, showing sprouting, internal root growth
and the associated changes in bulb shape: a, skin; b, storage scale from bladed leaf;
c, bladeless storage bulb scale; d, sprout leaf with blade; e, sprouting leaves; f, old
stem plate; g, new stem plate; h, outer rooting; i, inner rooting; j, spathe; k,
developing inflorescence (from Tanaka
et al.
, 1985b. Courtesy of
Research
Bulletins, Hokkaido National Agricultural Experiment Station
).
particularly in bulbs that are approaching sprouting (Tanaka
et al.
, 1985c;
Tanaka, 1991). All treatments that delay or suppress sprouting, including pre-
harvest MH (see below), will help to maintain skin integrity and therefore
minimize water loss from the bulbs.
Temperature effects
Abdalla and Mann (1963) stored bulbs of two cultivars - 'Excel', with a short
storage life and 'Australian Brown', with a long storage life - at constant
temperatures of 0, 5, 10, 15, 20, 30 and 40°C. After 0, 2, 4, 8 and 16 weeks of
storage, samples of 30 bulbs were planted on moist peat at 15°C, and the time
for 50% of the bulbs to sprout visibly was recorded. For both cultivars the rate
of sprouting was fastest in bulbs stored at 10-15°C and was slower at both
lower and higher temperatures (see Fig. 7.6).
The rate of elongation of sprouts within the bulb, and the rate of leaf
initiation, were much faster at 15 than at 0 or 30°C (see Fig. 7.7). Therefore, the
rate of sprout development in onion bulbs, unlike most physiological processes,
does not increase in rate progressively as temperature increases. More recently,
