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Botany
and taxonomy of the onion
The
fresh marktet onion (Allium cepa L.) has been classified to the
Liliaceae and belongs to the genus Allium, section "Schoenoprasum".
It is biennial, i. e. the plants blossom only in the second year.
The
onion bulb is formed in the first year by swelling of the leaf wall
at the base of the leaves. Classifying the onion by its use in the
vegetative stage, we can assign it to the long-day plants, but botanically
this is not correct. Only when the required day length (which depends
on the variety) has been reached, the swelling of the leaf base
and thus, the formation of the bulb begins. In so-called short-day
varieties, which nowadays are used as overwintering onions, the
swelling of the bases begins already at a day length of 11 hours,
in the traditional long-day varieties only at a day length of 15
- 16 hours. In the period before the formation of the bulb, foliage,
i. e. assimilation surface, is produced, which is the precondition
for a high yield potential.
For
the cultivation, this means that both, autumn sown and spring sown
onions, need sufficient time, before the critical day length is
reached. If this period is too long, the onions tend to form thick
necks, which can be observed in some late maturing autumn sown onions.
The
bulb serves as a storage organ for cold and dry periods, containing
mainly water and only in second place reserve nutrients. This means
that after the maturing process the onion enters a so-called sprouting
resistance. The duration of this stage depends on the individual
variety. This period can be used for the storage of the onion. To
maintain the vital functions during the germ rest, the onion scales
are emptied one after another beginning from the outside so that
always new loose skins are produced, whereas the bulb itself remains
solid.
Only
when the onion begins to sprout again, water and nutrients are extracted
from all skins at the same time, so that the bulb becomes soft and
the storage period ends. Low temperatures can extend the sprouting
resistance, whereas physical stimuli as motion, temperature changes,
changes in atmospheric humidity, light, etc. disturb the sprouting
resistance. For the practice this means that, especially at the
end of the sprouting resistance, all disturbing influences on the
warehouse climate should be avoided.
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Onions
