Agriculture Reference
In-Depth Information
high level of bolting when autumn-planted in the field; 34% flowering in the
long-type 'Longor', which rarely flowers in the field; but no flowers in cv.
'Jermor', another long type that does not bolt naturally.
Taken together, these investigations indicate a range in the flowering
responses of shallots. Some tropical types flower easily when exposed to cool
temperatures, while some temperate types can be induced to flower given
sufficient time under cool conditions when they are large enough. However,
some strains fail to flower when exposed to long periods at temperatures normally
favourable for flower initiation.
Growth and bulbing
The diploid hybrid between
A. fistulosum
and shallot type
A. cepa
,
A.
wakegi
is grown in eastern Asia, from Korea to Indonesia, for its readily branching
green shoots which are harvested in the spring (Inden and Asahira, 1990). In
the late spring and early summer it forms bulbs which remain dormant until
late summer or autumn planting in some cvs. It is vegetatively propagated only.
Fujieda
et al.
(1980) compared 202 clones from Korea, Japan and Taiwan and
grouped them into two broad 'ecotypes', the 'Southern' types and the
'Japanese' types. The 'Japanese' types grew slowly in the winter but luxuriantly
in the spring and formed bulbs which stored well over the summer. The
'Southern' types continued to grow through the winter and bulb earlier in the
spring, but the bulbs did not become fully dormant. Within these two broad
ecotypes, 22 subgroups characterized by leaf types were described.
Physiological studies on bulb formation in
A.
wakegi
show striking
parallels in the environmental control of bulb formation with onion (see Fig.
4.33). Bulbing is promoted by long photoperiods, and 'Southern' types bulb
more strongly than 'Japanese' types in a given photoperiod, and also bulbing is
accentuated in dry soil conditions (Okubo
et al.
, 1981). Growing temperatures
of 20°C are optimal for bulbing of both Japanese and Indonesian cvs (Okubo
et
al.
, 1999). Yamazaki
et al.
(2003) investigated the effects of the temperature at
which mother bulbs were stored before planting on both growth and bulbing
after planting using the cv. 'Kiharabansei no.1'. Bulbs kept at 5 or 15°C for 35
days before planting were then grown at 20°C under photoperiods ranging from
11 to 14 h. Bulbs did not form under 11 or 12 h photoperiods but the cold-
stored plants bulbed strongly under 13 h photoperiods, with all shoots forming
bladeless bulb scales (see Fig. 4.26) after 46 days, whereas the warmer-stored
plants did not bulb. In 14 h photoperiods the plants from both storage
temperatures bulbed but the cool-stored ones had a higher bulbing ratio.
Therefore, low-temperature bulb storage did not induce bulb formation in the
shortest photoperiods but it did reduce the critical photoperiod from 14 to 13 h.
