Agriculture Reference
In-Depth Information
season progressed and their root weight increased. It may be that the rather
sparse root system is a reflection of these crops having evolved from wild
species in which mycorrhizal (VAM) enhancement of the root-absorbing
surface was usual (see Chapter 5).
INFLORESCENCE AND FLOWER STRUCTURE AND
DEVELOPMENT
Bolting, or inflorescence production, can occur in all the vegetable alliums and
the process is similar in all. Successive stages of inflorescence initiation in onion
are shown in Fig. 2.6. The inflorescence develops from the shoot apical meristem
under appropriate environmental conditions (see Chapter 4). The inflorescence
terminates the shoot, so further vegetative growth after flowering must occur
from the lateral bud that usually develops adjacent to an inflorescence.
The internode between the last leaf and the spathe is the only stem node to
elongate. In onion it ultimately forms an inflorescence stalk (termed a 'scape')
that is 1-2 m long. As the scape elongates, cell division becomes confined to
the more basal regions, and proceeding towards the tip the cells become
progressively more elongated and mature. Consequently, elongation occurs
predominantly in the lower parts of the scape. The mature onion scape is
hollow, with a prominent swelling about one-third of the way from its base. It is
green, covered in stomata and similar in internal anatomy to a foliage leaf (de
Mason, 1990).
The other edible alliums produce similar scapes, although these differ in
stature depending on the size of the plants. In rakkyo the scape is 40-60 cm
long, in chives up to 70 cm long. Not all species have hollow scapes (see Fig.
1.2), and the presence or absence of swellings on the scape varies with species,
Japanese bunching onion having a nearly parallel-sided scape. The scape
emerges from ensheathing foliage leaves except in rakkyo, which is autumn
flowering. Here, the scape emerges from the bulb after a period of summer
dormancy during which the surrounding leaves wither. The foliage that
elongates with the scape is from a bud in its axil, and hence the flower stalks are
off-centred from the leafy shoots.
The development of individual onion flowers has been described in detail
by Jones and Emsweller (1936). Underneath the spathe on the broad top of the
stem numerous membranous bracts develop, each covering several young
florets arising on kidney-shaped regions of dividing cells. Looking from the
outside towards the centre of each floret there develop three members in each
of five whorls of floral organs: the outer perianth, the inner perianth, outer
stamens, inner stamens and the carpels. These develop as globose projections,
with the outer whorls developing first (see Fig. 2.17a). The carpels develop as
three U-shaped upswellings on the surface within the inner stamens. These
upswellings grow towards the centre and their inturned edges meet, fold
 
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