Agriculture Reference
In-Depth Information
inflorescence-inducing cold treatment of 5-6 months. As described above, high-
temperature bulb storage (28-31°C) can cause growing points at a critical stage
of inflorescence initiation to revert to the vegetative state and to form leaf
initials. If the inflorescence has already developed to a certain extent at the start
of such a heat treatment the meristem may be diverted to bulbil development.
Such a meristem in the early stages of inflorescence differentiation will probably
produce one bulbil on a short scape whereas, if the heat treatment occurs late in
inflorescence development, a mixture of bulbils and normal florets may develop.
Therefore, the stage of inflorescence development reached by the apex at the
start of heat treatment probably determines the outcome.
The 'top' or 'tree' onion (see Chapter 1, 'Cultivated Hybrids of
Allium cepa
'),
represents an extreme condition in which large bulbils are always produced in
inflorescence-like structures, and normal flowers do not occur. Bulbils can be
induced to form in normal inflorescences by clipping off the developing florets.
Such bulbil production can be increased by spraying the clipped heads with
water, or better, benzyl adenine solution.
Another deviation from normal inflorescence development has been
observed when induced plants have been allowed to develop inflorescences in
abnormally short photoperiods, for example 8-10 h. Under these conditions
inflorescence appearance is slow, scapes are short and spathes may become
long, green, curved and leaf-like (Scully
et al.
, 1945), or several leafy vegetative
shoots may occur in the inflorescence (van Kampen, 1970).
The completion phase, flower opening and pollination
Once a spathe is visible the plant enters the 'completion phase' of the flowering
process. Here, higher temperatures become optimal. About 400°C days > 10°C
are required to progress from spathe appearance to peak flowering, and 900°C
days > 10°C to reach a stage where ripe seeds begin to shed from seed capsules
(Brewster, 1982). Scapes normally elongate to a length of 1-2 m. Individual
bulbs may produce between one and 20 inflorescences depending on genotype,
bulb size and environmental conditions; three to six inflorescences is common.
The spathe opens to produce a roughly spherical umbel containing between 50
and 2000 individual flowers, although 200-600 is the normal range. There is
no very regular sequence of flower opening on an umbel. The most strongly
insolated parts of the umbel tend to produce open florets first, and there is a
general tendency for upper florets on the umbel to precede the lower ones in
opening. The pedicels of later-opening florets elongate to carry them clear of
earlier-opening, maturing florets.
The sequence of individual flower development consists of petal opening
(anthesis) coinciding with the start of nectar secretion, the dehiscence of the
inner whorl of anthers, and then the outer whorl. Meanwhile, the style elongates
from 1-2 to 5-6 mm and develops a sticky stigma knob after the anthers have
dehisced. Then nectar secretion ceases and petals, stamens and style start to
wither. The whole process takes 10 days at 18°C or 5 days at 30°C. The greatest
