Species that naturally produce bulbs can be induced to form small bulbs (bulbils or bulblets) in culture. Bulbils can be produced from axillary buds, but frequently they are formed from adventitious buds developed on pieces of leaf, on inflorescence stalks, or on ovaries, and particularly on detached pieces of bulb scale.
Both axillary and adventitious shoots and bulbils are formed on bulb scale pieces in vitro. Strong dominance of the main shoot apex often prevents the formation of axillary buds at the bases of bulb scales in vivo, but buds capable of giving rise to bulblets (or to shoots upon which bulbs will be formed later) are freely produced when bulb scales or bulb sections are cultured. In some species it is important to include part of the basal plate in the explant. Depending on the kind of bulb being cultured, explants for continued Stage II propagation may consist of scales taken from bulblets, swollen shoot bases, or bulblets which have been trimmed and split. Propagules for transfer from Stage III to the external environment can be plantlets, plantlets with a bulblet at the base, or dormant bulblets.
Instead of producing storage organs composed of swollen leaf bases (bulbs), some monocotyledons store food reserves in swollen stem bases (corms). Small corms (cormlets) of Gladiolus may be formed directly on explanted tissue or on callus in culture (Ziv et al., 1970; Ziv and Halevy, 1972), or they are produced on rooted plantlets grown in culture jars until the leaves senesce. Cormlets formed in vitro can be planted in soil or used to start new in vitro cultures (Hussey, 1978a,b).
The production of plantlets from genera producing bulbs and corms is discussed more fully in Volume 2. Miniature tubers
Under appropriate environmental conditions, plants that naturally produce tubers can be induced to produce miniature versions of these storage organs, in a medium containing high cytokinin levels. Tubers normally formed on underground stolons are produced in vitro in axillary positions along in vitro shoots (Fig 2.6). Two crops where miniature tubers have been utilised for propagation are potato and yams. Methods for inducing the in vitro tuberization of potatoes were first described by Lin et al. (1978, in Wang and Hu, 1980) and Hussey and Stacey
(1981a,b), and since by several other workers. Potato tubers form best in darkness, but those of Dioscorea mainly appear at the base of stem node cuttings in the light (Ng, 1988).
Miniature tubers have the great advantage that they can be readily removed from culture flasks in a dormant condition and stored ex vitro without precautions against sepsis. When planted in soil they behave as normal tubers and produce plants from axillary shoots. If they are produced in vitro from virus-tested shoots, miniature tubers provide an ideal method of propagating and distributing virus-tested stock to growers.
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