The bulbs of rakkyo, onion and shallot and garlic show a gradation of increas-ing complexity (see Fig. 2.8). Bulbs in the true sense do not form in Japanese bunching onion, chives or Chinese chives, although the thickened basal sheaths may act as food reserves during winter dormancy in the former two species. In Allium ampeloprasum, great-headed garlic, pearl onions and most wild types form bulbs but, with leek and kurrat, bulbs form only in some specimens after bolting or in unnaturally long photoperiods (see Chapter 4).
Rakkyo bulbs form as a result of swelling of the foliage leaf sheaths, and these appear as concentric swollen rings in transverse section. There is some growth of leaf blades as the sheaths thicken, resulting in oval-shaped bulbs.
The leaf blades decay at the end of bulbing. Many lateral buds form in the axils between sheaths, and these develop into the following season’s shoots. The apical buds of the bulb-forming shoots differentiate into inflorescences during bulb formation and dormancy.
The early stages of bulb formation in onion and shallot are similar to those in rakkyo, involving the swelling of leaf sheaths. The thickening of the sheaths is frequently preceded by their sudden elongation. Thickening occurs as a result of lateral expansion of cells in the lower third of the sheaths and does not involve cell division. During bulbing the young developing leaves cease to form blades but develop into swollen, bladeless ‘bulb scales’. In all but these leaves the length from the base of the sheath to the pore where the next leaf emerges, and which marks
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).
bulb at maturity has two dry skins enclosing four swollen sheaths from bladed leaves. These in turn enclose three or four swollen, bladeless bulb scales, and finally at the centre are found five leaf initials with blades (see Fig. 2.7). These inner bladed leaf initials emerge when the bulb sprouts (see Fig. 7.5), and hence are termed the ‘sprout leaves’. Usually, one or more lateral shoots occur towards the centre of the bulb, consisting of a few swollen, bladeless bulb scales enclosing several sprout leaves or, nearer the centre, just sprout leaves.
The reserve tissue in garlic bulbs consists solely of bladeless storage leaves, which constitute about 75% of the weight of trimmed bulbs and form the bulk of the familiar garlic clove. They arise as buds within the axils of the sheaths of foliage leaves, on the side of the axis below the leaf blade. Each clove primordium may divide as the axil in which it develops expands, so that up to six or seven cloves can be found in the axil of a single foliage leaf (see Fig. 2.11).
In a typical mature bulb, the number of cloves per leaf axil increases from one or two in the innermost leaf axil to six or seven in the fourth youngest axil and then decreases again in the axils of older leaves. Ultimately, the apical bud may also develop into a clove. In plants grown from very small cloves, the terminal clove may constitute the only storage leaf; such single-cloved bulbs are termed ‘rounds’. The sheaths of the foliage leaves remain non-swollen, but dry to form a papery protective envelope surrounding the cloves within. When the bulb is fully ripened, the original stem and leaves are dead, and merely act as dry container for the dormant cloves. As bulbs ripen, parenchymal cells degenerate and collapse on the inner side of the leaf sheaths that form the pseudostem. This leaves alive just the outer epidermis, the conducting vessels and some surrounding cells. As a result the neck of the bulb softens and the foliage collapses, as in onion (Mann, 1952).
Individual cloves consist of a tough, outer, dry protective leaf, a swollen storage leaf that constitutes the bulk of the weight and which contains 30–40% dry matter, a sprout leaf and three or four foliage leaf primordia surrounding the apical meristem (see Fig. 2.3A). The protective, storage and sprout leaves have only vestigial leaf blades, but the foliage leaf primordia have blades. As with onion bulbs, two or so additional foliage leaf primordia differentiate at the clove apical meristem during storage. During sprouting, the sprout leaf elongates and emerges from the pore at the tip of the storage leaf, and then the first foliage leaf pushes through the pore near the tip of the sprout leaf (see Fig. 2.3B). In development, the protective leaf forms first and this ensheaths the developing storage leaf (see Fig. 2.11).
Ideally, ripe garlic bulbs should be near circular in transverse section and covered by several outer skins; such bulbs are termed ‘smooth’. In some conditions (see Fig. 4.47), lateral buds may initiate in the axils of the outer leaf blades and give rise to green side shoots, which themselves go on to develop cloves, resulting in bulbs made up of several sub-clusters of cloves. Such bulbs have an irregular outer surface and are termed ‘rough’. This is a common defect in garlic production (see Fig. 2.12).
The bulb structure of great-headed garlic is similar to that of garlic. It frequently forms a single terminal clove or ‘round’. In the following season a round usually develops an inflorescence surrounded by multiple axillary cloves, like normal garlic except that the clove size is usually larger (van der Meer and Hanelt, 1990). Frequently, additional smaller cloves or bulblets form in the outer leaf axils of the bulbs, a feature that is occasionally observed in leeks.
The structure of rakkyo, onion and garlic bulbs is diagrammatically summarized in Fig. 2.8.