Agriculture Reference
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kujo' of the Warm Region Group, cv. 'Kincho' of the Intermediate Region Group
and cv. 'Cho-etsu', which is later bolting than the other two and which requires
longer at cool temperatures for flower induction. Comparing night
temperatures of 3, 7, 11 or 15°C established that 7°C was the optimum for
vernalization, with a 20°C day temperature for 8 h per day. This was indicated
by the highest percentage of plants developing an inflorescence, the lowest
number of leaves to the inflorescence initial, the most advanced stage of the
inflorescence initials after 30 or 60 days of cool treatment and the most rapid
bolting after transfer to 20°C minimum. Some flowering occurred with a night
temperature as high as 15°C in the warm and intermediate region cvs 'Asagi-
kujo' and 'Kincho', respectively, but this was too warm to vernalize cv. 'Cho-
etsu' (Yamasaki et al. , 2000a). Day temperatures of 26.5 rather than 20°C
with a night temperature of 7°C reduced flowering in cvs 'Asagi-kujo' and
'Kincho'. For cv. 'Cho-etsu' a day temperature of 20°C was less favourable for
flowering than 13.5°C. The duration of vernalization at 20/7°C d/n needed for
near 100% inflorescence induction was about 40 days for cv. 'Asagi-kujo',
50-60 days for cv. 'Kincho' and more than 75 days for cv. 'Cho-etsu'.
Short photoperiods (8 h) accelerated inflorescence induction compared
with long photoperiods (16 h) in plants given a temperature regime of 16 h at
7°C and 8 h at 20°C, and long photoperiods (16 h) before cool treatment
slowed induction (Yamasaki et al. , 2000b). As with onion, low nitrogen
promotes flower initiation and leads to swelling of the leaf sheaths (Yamasaki
and Tanaka, 2005). Once inflorescences had developed to the floret initiation
stage, long photoperiods accelerated scape elongation. Temperatures of 35°C
were devernalizing and suppressed inflorescence development. By growing
plants from mid-December to mid-April in plastic tunnels in which maximum
daytime temperatures of 35-40°C occurred, and by also extending the
photoperiod in the tunnels to 16 h using incandescent lamps, bolting was
prevented in cvs 'Asagi-kujo' and 'Kincho' (Yamasaki et al. , 2003). Using this
technique it should be possible to extend the season of marketability of these
two cvs until June in Japan.
In summary, the requirements for flowering in A. fistulosum are similar to
those of onion in that there is a juvenile phase, the size at the termination of
which increases from warm region to cool-region cultivars, an optimum
temperature for vernalization of 7°C or slightly higher (Yamasaki et al. , 2000a)
and a time requirement for induction under optimum vernalizing conditions
varying from 30 days to more than 75, the time needed being longer for cvs of
cool regions. Long photoperiods during vernalization slow inflorescence
development, in contrast to onion where they can accelerate the process (see
Fig. 4.40). Flower initiation can occur in temperatures as warm as 15°C in
warm and intermediate region cvs but high temperatures, starting at about
20°C in cool region cvs and 27°C in warm and intermediate region types, are
inhibitory to floral development.
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