Morphology and Distribution
Ginger (Zingiber officinale Roscoe) is a perennial plant with fleshy and bent finger-like rhizomes. The rhizomes branch usually up to the 4th order and show negative geotropism, and also possess oil cells.
Zingiber officinale is believed to be native to tropical Asia. However, this plant is widely cultivated for the aromatic and pungent rhizomes in the tropical, subtropical and temperate regions, such as India, Ceylon, Indonesia, Nigeria, Sierra Leone, Jamaica and West Indies, Florida (USA), Queensland (Australia), China, and Japan.
Zingiber officinale rhizomes are roughly classified into three categories, such as the fresh rhizome (Shinshoga in Japanese), the propagation rhizome (Taneshoga) and the 1-year-old rhizome (Oyashoga). These rhizomes are widely utilized not only as food, but also as medicine.
Pharmacological studies show that ginger rhizomes are effective for intestinal disorders and salivary secretion, for stimulating the vasomotor and respiratory centres, for relaxing the tracheal and ileal smooth muscles, and for lowering serum and hepatic cholesterol levels. Recent investigations also indicate that pungent principles of ginger rhizomes, such as gingerols and shogaols, have pharmacological action. In addition, antioxygenic effect of the rhizomes on food has been found.
The dried rhizomes of this plant are used as one of the most important spices in cooking, particularly in curries. In addition, the essential oil and oleoresin from the rhizomes are utilized as flavour additives in foods and beverages. The raw rhizomes are often used in ginger-growing countries as vegetables, pickles and preserves, and as condiments in fish and meat dishes in addition to curries. The young fresh rhizome is most relished as garnishing because of the pleasant aroma.
Conventional Practices for Propagation
Zingiber officinale usually propagates itself by rhizomes stored for several months at 15 °C under high humidity. The stored rhizomes, after kept in the sunlight for a few days to promote germination, are planted as propagation rhizomes in spring, and cultivated under the sufficient irrigation and supplement of potash and nitrogenous manure. Then, newly propagated fresh ginger rhizomes in various stages of maturity are harvested extending from early summer to early winter, and these fresh ginger rhizomes and also their mother rhizomes (1-year-old rhizomes) are used for various purposes. For propagation in the next year, mature fresh ginger rhizomes are stored again. Thus, the cultivation of Z. officinale is repeated. The repeated cultivation of this plant on the same ground, however, should be avoided because of prevention from a poor crop caused by poor soil and pathogen attack.
Earlier investigations on the volatile constituents of ginger rhizomes were carried out on the commercial essential oil and the essential oils from the rhizomes in a less fresh condition and from the dried rhizomes. According to these investigations, major constituents in the essential oil of the ginger rhizomes are sesquiterpenes. In contrast with these ginger rhizomes, main constituents in the essential oil of the various kinds of the Japanese ginger rhizomes, including the young shoots, are monoterpenes, as described below in detail. Recent studies indicate that the essential oil of the green ginger from Fiji has a high content of monoterpene aldehydes, such as neral and geranial and that monoterpene content decreases with an increase in the ses-quiterpene content on drying of the rhizomes.
Cultivation of the Stored Rhizomes
The rhizomes stored for 5 months were planted as the propagation rhizome and then cultivated under natural environment. After cultivation for 7 months, newly propagated mature fresh rhizomes and their mother rhizomes were collected to analyze volatile constituents of these rhizomes. The result of the analysis indicates that through cultivation of the stored rhizomes, the content of neral and geranial decreases to a small extent (41%), whereas a large amount of geraniol (12%) and geranyl acetate (6%) occurs in the newly propagated fresh rhizomes. Thus, the conclusion from these results is that the essential oil of Z. officinale rhizomes of various ages is predominantly composed of the acyclic oxygenated monoterpenes, such as neral, geraniol, geranial, and geranyl acetate, of which the contents change greatly with ageing of the rhizomes. A small amount of geranyl acetate in addition to a large amount of neral, geraniol and geranial is present in other parts, such as blades and sheaths, of the ginger plant cultivated, and geranyl acetate seems to be a characteristic compound of the fresh rhizome.
Zingiber officinale: Conclusions and Prospects
Zingiber officinale rhizomes of various ages are commercially available in Japan and the raw rhizomes are often used for food. Young fresh rhizomes are most relished, having a characteristic pleasant aroma, so the essential oils of rhizomes at various stages of the life cycle were first investigated. The data indicate that the oils of rhizomes at the various stages are predominantly composed of four acyclic oxygenated monoterpenes including geranyl acetate, and that the content of the four compounds changes markedly with advancing stage.
Zingiber officinale often suffers from diseases. Therefore, the production of a pathogen-free plant has been needed. For this purpose, the propagation by shoot tip culture seems to be most suitable. The new shoot tip culture technique described in this chapter may realize clonal propagation of pathogen-free stock of Zingiber officinale by further raising the rate of shoot multiplication, because this culture technique enables the production of various types (B1-B3) of precocious branches growing at an excellent rate. Further, this new procedure based on culturing in vitro would be able to control the volatile oil composition of the Zingiber officinale plantlet rhizomes produced.
For direct use of in vitro cultures with aroma characteristic of the original plant, for instance, the modified B5 medium without NAA, in addition to a low level of BAP, may be recommended, because such a medium enables induction of the plantlet. For clonal propagation of this plant, modified MS medium supplemented with a high level of BAP may be suitable, because such a medium enables production of multiple shoots, followed by induction of multiple plantlets by transplanting them into the media suitable for the production of the plantlet at any time.
Selections from the book: “Medicinal and Aromatic Plants II”, 1989.