Calendarizaci´ on

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

within themselves and form the ovules, two of which occur in each carpel. The style arises at the apex of the three fused carpels, and is still elongating when the flower opens (see Fig. 2.17b).

The development of the inflorescence, florets and top-sets in garlic is shown in Fig. 2.18. Development to anthesis in garlic is rare and, when it can occur, requires a specific sequence of conditions (see Chapter 4). Usually top-sets (sometimes termed bulbils) form in the inflorescence and swell to suppress the florets (see Fig. 2.18E–G). The same can happen in onion, leek and other species under certain conditions (see Figs 4.35 and 4.41).

The inflorescences of these crops are umbels (see Plate 1). When ripe, the leafy spathe that encloses the umbel during development splits and the inflorescence opens. The flower colour and the pattern of opening of individual flowers within the umbel vary with species (see Table 1.2). Individual flowers are carried on short stalks (pedicels). In onion, flowers are continuously carried to the outer boundary of the inflorescence by elongation of the pedicels before the tepals open. This leads to successive cohorts of flowers opening over the whole surface of the umbel. In onion there are commonly 200 to 600 flowers per umbel, depending on cultivar, growing conditions and whether the umbel is formed from the main growing point or an axillary shoot. Similar umbels containing large numbers of flowers are produced by leeks and Japanese bunching onions. Chives typically have about 30 flowers per umbel, rakkyo six to 30 flowers and Chinese chives have approximately 40 white, star-shaped, fragrant flowers in a flat-topped umbel.

Fig. 2.17. The development of individual florets. (a) Top view of a young flower in which primordia of all perianth segments (Iabelled I to VI) and stamens have differentiated: ipl, inner perianth lobe; ost, outer stamen; ist, inner stamen (
60) (from Jones and Emsweller, 1936. Courtesy of Hilgardia). (b) Open flower with the inner whorl of anthers shedding pollen. (c) Late in flowering when the stigma is still sticky and receptive but the tepals and anthers are withering (b and c approx.
2) (from Currah and Ockenden, 1978. Courtesy of New Phytologist).

In each flower nectar accumulates in three cups formed between the lower ovary walls, where the nectaries are located under a small flap, and the broad bases of the filaments of the inner whorl of stamens. The stages of flower opening in onion are: (i) opening of the tepals and the beginning of nectar secretion; (ii) dehiscence and pollen release by the stamens (see Fig. 2.17b);

then (iii) the stigma becomes sticky and receptive when the anthers have ceased to shed pollen (see Fig. 2.17c).

SEEDS

The development of onion seeds following fertilization is described by Rabinowitch (1990b) (see also Figs 6.17 and 6.18). Seed size and therefore the number of seeds per g varies with species; approximate figures are: onion and Japanese bunching onion, 300; leek, 350; kurrat, 440; chives, 1150. The Fig. 2.18. Scanning electron photomicrographs of garlic apical and floral

development. (a) Vegetative meristem (vm) with differentiated leaf primordium (lp);

bar = 0.1 mm. (b) Floral transition; the reproductive meristem (rm) is swollen and hemispherical; the spathe (sp) was removed; bar = 0.1 mm. (c) Differentiation of flower primordial in a reproductive meristem with spathe removed; first flower primordia (fp) are initiated in an apex of diameter 0.8 mm; bar = 0.3 mm. (d) Differentiation within the floral primordial; floral parts are seen in the oldest floral primordial (fp) while the younger ones still appear as undifferentiated meristematic domes; bar = 0.4 mm. (e, f and g) Newly developed meristems appear near leaf-like bracts at the base of the inflorescence; they quickly differentiate and grow to form small inflorescence bulbs termed top-sets (t); bar = 0.8 mm in E, 1 mm in F and 0.1 mm in G. (a–f from Kamenetsky et al., 2004. Courtesy of Journal of the American Society for Horticultural Science; g from Kamenetsky and Rabinowitch, 2001. Courtesy of Sexual Plant Reproduction.)

internal structure of the onion seed is illustrated in Fig. 2.19. The embryo is curled within the seed and consists of a short root below the shoot apex, which is located along with the primordium of the first leaf at the base of a slit at the lower end of the cotyledon. Following germination, the first leaf will emerge from this slit. The cotyledon forms the bulk of the embryo and consists mostly of small cells densely packed with reserves of globular fat, protein and sugar phosphate (phytin) (de Mason, 1990). The beginning of conducting tissue is seen as a central procambial strand. At the tip of the cotyledon, embedding into the surrounding endosperm, there is a swelling termed the haustorium. During germination this absorbs nutrients from the endosperm reserves and transfers them to the growing cotyledon. The cells of the thick-walled endosperm are also packed with reserve globules of protein and lipids.

During germination the lower portion of the cotyledon elongates first and the root emerges. Cell divisions occur throughout the cotyledon as it initially elongates. The primary root grows downwards and the slit containing the

Fig. 2.19. Longitudinal section through a mature onion seed with embryo. The embryo consists of a root, a hypocotyl with shoot apex and a long cotyledon. The procambium extends the entire length of the embryo. The cotyledon ends in a haustorium, which absorbs nutrients from a thick-walled endosperm during germination. A small amount of nucellus lies under the seedcoat at one end (from Esau, 1977. Courtesy of John Wiley and Sons Inc., New York).

apical meristem remains at seed-sowing depth, while a sharp, inverted U-shaped bend develops in the cotyledon, midway between the apical slit and the haustorium, which stays embedded in the seed. The bend forms a sharp ‘knee’, which is pushed upwards through the soil surface by the elongation of the cotyledon on either side of it. As a result, the cotyledon emerges from the soil as a loop, before straightening and finally pulling the tip clear of the soil (see Fig.

2.1C). The same pattern of germination and emergence occurs in the other seed-grown vegetable alliums.

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© J.L. Brewster 2008. Onions and other Vegetable Alliums, 51 2nd Edition (J.L. Brewster)