Agriculture Reference
In-Depth Information
< 25°C (Solberg, 1997). Curing temperatures of above 25°C (e.g. 27°C) have
been recommended where dark-coloured skins are desired (see 'The Curing of
Bulb Skins', above), and there might be some conflict of requirement here.
Dormancy has been studied in both Japanese (Takagi, 1990) and Californian
(Mann and Lewis, 1956) strains of garlic. The clones studied from both these
temperate regions show many similarities to onion bulbs as regards the
physiology of dormancy. Clones differ considerably in their innate duration of
dormancy, as measured by the time to sprouting, either in storage or after
planting.
As with onion, the garlic bulb passes through successive phases, each with
a different response to temperature. While the bulb is still immature and
growing and soon after maturity, the depth of dormancy, as measured by days
to sprouting after planting, is decreased by temperatures of 35°C, as with
onion. When the bulbs are mature, dormancy is most rapidly depleted by
storage at 5-10°C and prolonged by both lower and higher temperatures.
Between 15 and 30°C, time to sprouting increases progressively with tempera-
ture. Respiration rates of bulbs, measured soon after maturity, are more rapid
at 5, 10, and 15 than at 0 or 20°C. So, as with onion, there is a prolongation of
dormancy, and suppression of metabolic activity by moderately warm storage.
The optimum temperatures for sprouting in garlic (5-10°C) are about 5°C
lower than with onion.
In the Japanese cv. 'Yamagata', which is not innately a long-storing strain,
leaf initiation ceases as the bulb matures. It does not resume, even in cloves
planted in ideal conditions, for 3 weeks or so. So, around the time of foliage
collapse, there is a period of 'rest' during which growth at the shoot apex ceases,
as with onion. At this stage, the time from clove planting to visible sprouting is
maximal. This interval decreases the longer the bulbs are stored. Leaf initiation
resumes within stored bulbs, and respiration rates increase slowly, so there is a
gradual emergence from the state of innate rest and dormancy. Ultimately, the
response of sprouting to storage temperature changes, so that in cloves about to
sprout, temperatures of 20-30°C cease to suppress sprouting.
Root appearance, following planting cloves on moist sand, takes only a few
days, typically just a quarter of the time for visible sprouts to emerge. So, as
with onion, rooting precedes sprouting. As with sprouting, rooting is fastest
after storage at 5-10°C. Immersing garlic cloves in benzyl adenine solutions
before planting greatly accelerates sprouting, indicating a role of cytokinins in
breaking dormancy, another feature in common with onion bulbs.
In practice, garlic bulbs are normally stored at ambient temperatures.
Experiments have shown that the optimum temperature for prolonged storage
is -1 to -3°C. Bulbs sprout most rapidly in storage at 5-10°C. They store
