Agriculture Reference
In-Depth Information
the plants cannot respond to these factors unless they have been previously
exposed to low temperatures, either as stored cloves or after planting in the
field. Exposure to temperatures of 15°C or below with an optimum of 2-4°C is
necessary to induce bulbing in Japanese cultivars. Cool storage of bulbs also
induces flower initiation after planting, but flower induction is optimized by
storage at rather colder temperatures (-2 to 2°C) than the 2-4°C optimal for
bulb induction. The longer the bulbs are stored at inductive temperatures, the
older the axils in which cloves may form, culminating in the potential to form
an apical clove (single-clove bulb). Therefore, the developmental effect of cold
storage depends not just on temperature but on its duration. For the pro-
duction of normal bulbs, at least in temperate bolting varieties, the degree and
duration of cold should be sufficient to induce inflorescences and break apical
dominance, but should not be so prolonged as to promote bulbing at the shoot
apex after planting, or to promote laterals in the outermost leaf axils, thereby
favouring 'rough' bulbs.
Normally, the cold requirement will be met following an autumn planting,
but if garlic is stored at 20°C or above and then planted in the late spring when
mean temperatures are already warm, then the plants fail to bulb and mature and
make continued leaf growth into the autumn. There is an interesting parallel with
onion sets here, where warm (27-30°C) storage delays bulbing compared with
cool storage, although it does not actually prevent bulbing in the case of onion. To
produce measurable effects on subsequent bulbing, storage at controlled
temperatures for 1 month or more is required. The effects of temperature on stored
bulbs are reversible. Cloves from cv. 'California Late' stored for 3 or 4.5 months at
5°C matured by mid-August, whereas cloves stored at 20°C for 1.5 months
following 3 months at the lower temperature were delayed in bulbing and failed to
mature. Following such a reversal of bulb induction by high temperatures, bulbing
can by re-induced by a further period of cool storage. Again, a parallel reversibility
has been observed in onion sets.
In addition to influencing bulbing, the storage temperature of bulbs affects
the rapidity of sprouting following planting, the vigour of growth and the shape
of the plant after sprouting. In cv. 'California Late', cloves stored at warm
temperatures (20°C or above) are delayed in sprouting and make slow growth
with narrow leaves after sprouting. Cloves stored at 5-10°C sprout rapidly and
produce plants with broad leaves and necks. Clove storage at 0°C delays
sprouting but, when plants do sprout, leaf growth is vigorous and slender leaves
are produced, which outgrow in height those from cloves stored at warmer
temperatures. All these effects of storage temperature are more pronounced the
longer the period of storage before planting. For cloves planted in the autumn,
cool temperatures in the field after planting are likely to override any effects of
pre-planting storage temperature, although this will vary with both season and
location. In the mild winters of the Imperial Valley of southern California, just 8
weeks pre-planting storage at 0-10°C advanced bulbing and maturity even
from plantings as early as mid-October (Mann and Minges, 1958).
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