Agriculture Reference
In-Depth Information
Fig. 2.1.
Key stages in the growth of bulb onions from seed (from Rey
et al
., 1974.
Reproduced by courtesy of
Revue Suisse de Viticulture, Arboriculture et Horticulture
).
(a) Seed in soil after sowing. (b) The 'loop' stage. After germination underground the
cotyledon appears as a loop or hook above the surface. (c) First leaf 'crook' or 'whip'
stage. The first true leaf appears while the cotyledon is still sharply bent to form a
shape like a shepherd's crook or a whip. (d) Cotyledon senescence. After the
appearance of the second and third true leaves the cotyledon desiccates, withers and
falls. (e) Fourth leaf 'leek' stage. The fourth leaf appears and the neck of the plant
starts to thicken while the first leaf shrivels. (f) Fall of the first leaf. The first leaf falls
and the second leaf becomes detached at the sheath and begins to senesce from the
tip while leaves five, six and seven appear. (g) Start of bulbing. The bulb begins to
form; the second and third leaves desiccate while leaves eight to 13 appear; the plant
reaches maximum height. (h) Bulb swelling. The bulb swells rapidly alongside the
progressive desiccation of leaves four to six plus tips of the younger leaves. The leaves
may bend or fold sharply under their own weight. One or two more short leaf blades
may appear. The dry outer bulb skin begins to form. (i) 'Fall-down' or 'soft-neck'. The
neck or pseudostem becomes hollow as new leaf blades cease to grow within it, and
the neck tissues lose turgidity and soften so that the foliage collapses under its own
weight. The bulb reaches its final size. (j) Bulb ripening. The outer skins dry, cure and
set whilst the foliage senesces completely and desiccates.
