Agriculture Reference
In-Depth Information
the beginning of the leaf blade, is always less than the blade length. So a ratio
(blade length:sheath length), termed the 'leaf ratio', of less than unity for any
developing leaf is characteristic of bulbing (Heath and Hollies, 1965) (see Fig.
4.26). Bulbing can also be characterized by increases in the ratio (maximum bulb
diameter:minimum pseudostem diameter), which is termed the 'bulbing ratio'.
Although more easily measured than leaf ratio, initial increases in bulbing ratio
depend on the extent of swelling in the sheaths of bladed leaves, and this increases
with light intensity and nitrogen deficiency. Therefore, decreases in leaf ratio are
a more reliable measure of bulb initiation than increases in the bulbing ratio.
As the bulb ripens, the outermost one to three sheaths develop into thin,
dry protective skins. Since leaf blades cease to form on the inner bulb scales, the
pseudostem becomes hollow. Also, the sheath tissues in the middle region of
the neck soften and lose turgidity, ultimately causing the foliage to fall (see Fig.
2.11). This foliar fall-over is a useful indicator of bulb maturity. A typical onion
Fig. 2.11.
Scanning electron micrograph of the apical region of a cold-treated garlic
plant 6 weeks after planting. The leaf bases have been removed and concentric
groups of cloves are differentiating in the axils between the leaf bases. C, clove; CP,
clove primordium; PL, protective leaf; AM, apical meristem. The scale bar is 500
m
(0.5mm) (from Rahim and Fordham, 1988. Courtesy of
Scientia Horticulturae
).
