Agriculture Reference
In-Depth Information
SHALLOTS
Flowering
Shallots have traditionally been propagated vegetatively (Messiaen et al. ,
1993), but more recently seed companies from both The Netherlands and
Israel have introduced both temperate and tropically adapted cvs grown from
seed (Rabinowitch and Kamenetsky, 2002). Experiments to induce flowering in
traditional vegetatively propagated types have shown that some flower readily
whereas others do not flower (Krontal et al. , 2000; Esnault et al. , 2005).
Flowering is undesirable in the vegetatively propagated crop and, as with
garlic, it has probably been selected against for many generations.
Studies on a short-day clone derived from Thailand that flowers and seeds
readily in Israel have established that the requirements for flower induction are
similar to those of onion (Krontal et al. , 2000). The optimal temperature for floral
induction is 5-10°C; inflorescences can be induced in growing plants or within
stored bulbs, and warm temperatures (29/21°C d/n) after an inductive cold
treatment, prevent flowering. Inflorescence initials formed after the production of
only six leaves, including primordia, following a mid-October sowing in Israel
(Krontal et al. , 1998). The more bolting-susceptible cvs of onion become sensitive
to vernalizing temperatures after seven leaves are initiated (see Fig. 4.36), but at
least one and usually two or three further leaf primordia are initiated before the
shoot apex becomes floral, thereby giving a minimum of about nine leaf initials
before the floral apex. Therefore, juvenility ends at an earlier physiological age in
this tropical shallot than in any bulb onion so far investigated.
From a study of two tropical and one temperate shallot cvs grown from seed,
the data of Tabor et al. (2005) show that the tropical cvs needed, respectively, 3.50
or 4.75 visible leaves (dry weight about 0.06g) - probably equivalent to 6.50
or 7.75 leaves including primordia - to be 50% responsive to vernalizing
temperatures. The temperate cv needed seven visible leaves (d.w. about 0.26g),
equivalent to ten leaves with primordia. These authors emphasized that the plants
had to be considerably larger than this to achieve near 100% flowering. These sizes
are similar to those found for bolting-susceptible and bolting-resistant cvs of bulb
onion, respectively (see Fig. 4.36).
Esnault et al. (2005) investigated the conditions needed to induce flowering
in the traditional Jersey long and half-long shallot types, which are vegetatively
propagated in France. They started with stored bulbs harvested in summer and
found that 1 month of high-temperature storage with periods at 35 and 40°C
during September or October improved sprouting and growth after an October
planting, and increased flowering in the most bolting-susceptible type. The
highest level of flowering was obtained by planting bulbs in a greenhouse at
16.5/13.5°C d/n for October and/or November so that they sprouted and grew
two leaves, followed by 3 months in a phytotron at 10/6°C d/n with a 12 h
photoperiod. This gave 94% flowering in the half-long cv. 'Mikor', which has a
 
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