Agriculture Reference
In-Depth Information
By contrast, in bulbs there is a wide range of size or weight over which the
rate of inflorescence initiation increases with size. For example, in the Japanese
cv. 'Sapporiki', 50 g (fresh weight) bulbs could be induced to 50% flowering by
20-30 days exposure to 9°C but 5 g bulbs need more than 80 days. As a
consequence, larger bulbs complete the thermo and competition phases of
inflorescence development before smaller bulbs. This is why small bulbs (sets)
are used for vegetative propagation where flowering is not wanted.
Similarly to seedlings, spring-sown cultivars can be induced to flower as
smaller bulbs than the more bolting-resistant, autumn-sown cultivars. The
spring-sown cv. 'Sapporiki' initiates inflorescences at half its maximal rate in bulbs
of 14 g fresh weight, whereas the autumn-sown cv. 'Senshuki' has to be about 28
g. In cv. 'Sapporiki', inflorescence initiation reaches its maximum rate in bulbs of
50 g weight and in cv. 'Senshuki' in bulbs of 100-150 g. Studies on the British
spring-sown cv. 'Ailsa Craig' indicate an absolute minimum bulb weight for
inflorescence development of about 4 g; this would represent a dry weight of
0.40-0.50 g, considerably heavier than the 0.06 g minimum dry weight needed
for seedlings of cv. 'Rijnsburger', indicating that the minimum critical dry weights
of bulbs (sets) are seven or eight times higher than for seedlings (Brewster, 1987).
Environmental control of inflorescence initiation and development
Temperatures needed for inflorescence initiation have been studied in a
number of temperate region cultivars and, from the combined results, the
relationship shown in Fig. 4.37 was derived. Optimum temperatures in this
thermo phase are 8-12°C, and vernalization is definitely slower at 6°C or less.
The optimum temperature is higher than the optimum of about 5°C for
vernalization in most temperate vegetables (Wiebe, 1990).
The optimum temperature for onion vernalization varies with cultivar; the
West African cv. 'Bawku' is optimally induced at 15-21°C, whereas strains
from northern Russia have an optimum of 3 to 4°C. The time required for
vernalization may also vary with cultivar; for example, seedlings of the Japanese
spring-sown cvs 'Sapporiki' and 'Imai-wase' can be 50% induced by just 20 days
at 9°C, whereas the autumn-sown 'Senshuki' requires 30-40 days.
Temperatures of 28-31°C applied to bulbs previously induced by low
temperatures can reverse inflorescence induction, and this is termed
'devernalization'. The duration of storage at 28-31°C needed to prevent
flowering increases the more advanced the stage inflorescence initiation. This
is illustrated by the results of Aura (1963) on Finnish cultivars. In accord with
the results discussed above, he found that inflorescence initiation was more
rapid in bulbs stored at 9-13°C than at 3-5°C and that it was more rapid in
large bulbs than in small ones. Reflecting these differences, in a total storage
period of 8 months a terminating duration of 5 months at 28°C was needed to
suppress flowering in bulbs stored at 9-13°C, but only 2 months in those stored
at 3-5°C. Also, the duration of 28°C treatment needed to suppress flowering in
15 g bulbs was less than needed for 108 g bulbs.
