Botany and Commercial Importance
Chrysanthemum cinerariaefolium (Trev) Boccone is a member of the family Compositae. The genus Chrysanthemum includes over 200 species and is classified as a member of the Anthemidea tribe in the Tubiflorae subdivision of the family. Economically, the Compositae family is of considerable importance, containing members important as sources of food, ornamentals and the contact insecticide pyrethrum. The term “pyrethrum flower” often refers to the perennial Chrysanthemum cinerariaefolium (Trev) Bocc, and occasionally to C. coccineum Willd. (garden pyrethrum) and C. marshalli Aschers.
Chrysanthemum cinerariaefolium grows to about 1 m high, its leaves are alternate and pinnately lobed. The flower has alternate, double-layered white petals with a yellow flower head. The flower head is a collection of florets surrounded by a ring of ray florets, set on a slightly convex receptacle. Both disc and ray florets have ribbed achenes which are seated upon the receptacle.
The commercially cultivated crop originated from temperate climatic areas and C. cinerariaefolium is native to Dalmatia (a region of Yugoslavia). The world’s leading producers are Kenya, Tanzania, Ecuador, Rwanda, and Japan, which grow over 25000 metric tons annually. The first four of these countries export over 95% of their production, primarily in the form of extract, but Kenya also exports ground flowers for the world’s mosquito coil market. The insecticidal components of pyrethrum are effective, nonpersistent insecticides used commercially in many applications, particularly in pediculocides and in preparations used for insect control in industrial kitchens. Most of the pyrethrum comes from small plots averaging in size from 1/4 to 1 acre. The individual plot holders are usually organized into cooperative societies. There are very few large-scale farms still in existence. Pyrethrum can be cultivated from either seed or vegetative clones. The plants are selected for high insecticidal content as well as high yield of flowers. Micropropagation of C. cinerariaefolium from capitulum or shoot meristem explants has been reported. Thousands of plants from pyrethrum clones have been successfully transplanted into fields in Ecuador. The tissue culture micropropagation method is sometimes preferred because it is more rapid than field splitting, and insures nematode-free plants for a period of time. Vegetative propagation, on the other hand, often leads to plants with decreased insecticidal content and/or low flower head numbers. Aseptic shoots may be initiated on media containing benzyladenine (BA) from 0.2 mg/1 to 3 mg/1, and roots may be induced in media containing 0.1 mg/1 to 0.2 mg/1 naphthaleneacetic acid.
Chrysanthemum cinerariaefolium (Pyrethrum): Conclusions and Prospects
In vitro micropropagation of high pyrethrin content/high flower head count plants has been accomplished. Flowering of shoots and/or plantlets in vitro, though possible, has not been realized. Organized shoots and aseptic regenerated plantlets produce the pyrethrins, the latter in levels and ratio comparable to field-grown commercial plants. Organized roots and undifferentiated callus generally do not produce the pyrethrins. There have been some isolated reports to the contrary, however for the most part these tissue are usually young in regard to initiation date. On the other hand, undifferentiated callus does produce small quantities of the irregular monoterpene precursors to the pyrethrins which makes them ideal for studying their biosynthesis. Callus has been shown to produce geraniol, chrysanthemyl alcohol, chrysanthemic acid, and chrysanthemum dicarboxylic acid. A cell-free homogenate prepared from undifferentiated callus contains the enzymes that form these monoterpenes, although initial research has been only marginally successful in separating and isolating the enzymes involved.
Selections from the book: “Medicinal and Aromatic Plants VI”, 1994.