Current applications

Node culture is of value for propagating species that produce elongated shoots in culture (e.g. potato and Alstroemeria), especially if stimulation of lateral bud break is difficult to bring about with available cytokinins. Nowadays the technique becomes more and more popular in commercial micropropagation. The main reason is that it gives more guarantee for clonal stability. Indeed, although the rate of multiplication is generally less than that which can be brought about through shoot culture, there is less likelihood of associated callus development and the formation of adventitious shoots, so that Stage II subculture carries very little risk of induced genetic irregularity. For this reason, node culture has been increasingly recommended by research workers as the micropropagation method least likely to induces somaclonal variation.. Some of the plants for which node culture has been described are listed in Table 2.2.

3.1.4. Multiple shoots from seeds (MSS)

During the early 1980's it was discovered that it was possible to initiate multiple shoot cultures directly from seeds. Seeds are sterilised and then placed onto a basal medium containing a cytokinin. As germination occurs, clusters of axillary and/or adventitious shoots ('multiple shoots') grow out, and may be split up and serially subcultured on the same medium. High rates of shoot multiplication are possible. For instance, Hisajima (1982a) estimated that 10 million shoots of almond could be derived theoretically from one seed in a year.

It is likely that multiple shoots can be initiated from the seeds of many species, particularly dicotyledons. The technique is effective in both herbaceous and woody species: soybean (Cheng et al., 1980; Hisajima, 1981; Hisajima and Church, 1981): sugar beet (Powling and Hussey, 1981): almond (Hisajima, 1981; 1982a,b,c): walnut

3.1.5. Shoots from floral meristems

Meristems that would normally produce flowers or floral parts can sometimes be induced to give vegetative shoots in vitro. Success depends on the use of young inflorescences where the determination of individual flower meristems is not canalized. Meristems in older inflorescences are likely to give rise to floral structures. Culture of immature inflorescence segments has, for example, resulted in shoot formation in:

• Broccoli - Anderson and Carstens (1977)

• Cauliflower - Pow (1969), Margara (1969a,b,c; 1977a), Crisp and Walkey (1974), Grout and Crisp (1977), Trimboli et al. (1977)

(Rodriguez, 1982): pumpkin and melon (Hisajima, 1981): cucumber and pumpkin (Hisajima, 1981, 1982c): pea, peanut, mung bean, radish, Zea mays and rice (Hisajima, 1982c). This technique does only make sense when elite seed is used or to gain preliminary information on the behaviour of a plant species under in vitro conditions.

• Dendranthema - Shu O Wang and Su Shien Ma (1978)

• Gerbera - Topoonyanont and Dillen (1988)

The exact origin of the shoots produced has not always been determined. In cauliflower and coconut they were thought to originate from actual flower meristems, but in sugar beet, from floral axillary buds. Some shoots formed from onion flower heads arose from various parts of the flower buds, but they were accompanied by other shoots which arose adventitiously over the entire receptacle surface. Shoots formed from young flower buds may therefore

Table 2.2 Examples of the use of node culture in micropropagation

Monocotyledons

Alstroemeria Cymbopogon spp.

Hussey et al. (19SG)

Jagadish Chandra and Sreenath (19S2)

Solanum spp.

Woody dicotyledons

Carpinus betulus

Haberlach et al. (1985), Levy (1988), Sihachakr et al. (1988)

Chalupa (1981a)

Poa pratensis

Pieper and Smith (19SS)

Castanea sativa

Vieitez and Vieitez (1980b) Yang et

Asparagus officinalis

Yang and Clore (1973, 1974a)

Castanea mollissima

al. (1986)

Dioscorea spp.

Ammirato (1976, 19S2), Chaturvedi

Eucalyptus grandis

Cresswell and Nitsch (1975)

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