PHYSIOLOGY OF CROP GROWTH, DEVELOPMENT AND YIELD - Onions and other Vegetable Alliums - page 166

Dragland (1972) also observed more and earlier bolting at cooler
temperatures within the range 12-18°C. He also noted that flower stalks
elongated faster the longer the photoperiod over the range 9-24 h, indicating
that once inflorescences are initiated their rate of growth increases with
photoperiod, as is the case with onion provided bulbing does not suppress the
scape. Using a statistical analysis of temperatures and flowering from leeks
growing from a range of planting dates at two sites, with and without protective
crop covers, Wurr et al. (1999) concluded that 7°C was optimal for inducing
flowers in cv. 'Prelina'. The strongest influence on flowering was the duration of
temperature in the 4-8°C range during the 42 days subsequent to the plants
reaching the three visible leaf stage, suggesting an end of juvenility at an earlier
stage than four to five leaves of Wiebe (1994).
In summary, the control of flowering in leek is similar to that in onion, with
a juvenile stage before which vernalizing temperatures are ineffective, an
optimum temperature for vernalization around 7°C falling off to zero flowering
when plants are exposed to temperatures of 18-20°C, and devernalization
possible from temperatures of 22°C and above. Elongation of scapes is promoted
by both long photoperiods and warmer temperatures (Wurr et al. , 1999).
JAPANESE BUNCHING ONION
Inden and Asahira (1990) grouped A. fistulosum cvs into three ecological
groups. Those from regions with cold winters ('Cool Region Group') like Korea,
northern China and northern Japan are dormant in winter, grow quite slowly
in mean temperatures around 5°C and grow well at mean temperatures of
25°C and above. The 'Warm Region Group' from southern Japan, Taiwan and
southern China do not have winter dormancy, grow well at mean tempera-
tures around 5°C but not well at summer mean temperatures of 25°C and
above. The 'Intermediate Region Group' do not have winter dormancy and
grow well at both the cool and warm temperatures specified above. The
optimum temperature range for growth, germination and photosynthesis is
15-20°C.
The plants are grown for either green leaves or the succulent pseudostems,
which may be elongated and blanched by earthing-up. Flowering (bolting) is
undesirable because the seedstalks are hard and unpalatable (Yamasaki et al. ,
2003). Flowering is induced by exposure to cool temperatures when the plants
are larger than a certain critical size. The cv. 'Kaga' (Cool Region Group) must
have initiated 11 or 12 leaves corresponding to a pseudostem diameter of
5-7 mm before it can respond to flower-inducing conditions. Plants of the
Warm Region Group cv. 'Pei Chang' respond to vernalizing temperatures when
the pseudostem is just 4.5 mm in diameter (Inden and Asahira, 1990).
Yamasaki and colleagues (Yamasaki and Miura, 1995; Yamasaki et al. ,
2000a,b,c, 2003) have investigated vernalization and bolting in cv. 'Asagi-