Coffee belongs to the botanical family Rubiaceae, which has some 500 genera and over 6,000 species. Most are tropical trees and shrubs which grow in the lower storey of forests. Other members of the family include the gardenias and plants which yield quinine and other useful substances, but Coffea is by far the most important member of the family economically.

Since Coffea was first correctly described, by Linnaeus in the mid 18th century, botanists have failed to agree on a precise classification system. There are probably at least 25 major species, all indigenous to tropical Africa and certain islands in the Indian Ocean, notably Madagascar. Difficulties in classification and even in designation of a plant as a true member of the Coffea genus arise because of the great variation in the plants and seeds. All species of Coffea are woody, but they range from small shrubs to large trees over 10 metres tall; the leaves can be yellowish, dark green, bronze or tinged with purple.

The two most important species of coffee economically are Coffea arabica (Arabica coffee) - which accounts for over 70% of world production - and Coffea canephora (Robusta coffee). Two other species which are grown on a much smaller scale are Coffea liberica (Liberica coffee) and Coffea dewevrei (Excelsa coffee).

Some differences between Arabica and Robusta coffee

Coffea arabica - Arabica coffee

Coffea arabica was first described by Linnaeus in 1753. The best known varieties are 'Typica' and 'Bourbon' but from these many different strains and cultivars have been developed, such as caturra (Brazil, Colombia), Mundo Novo (Brazil), Tico (Central America), the dwarf San Ramon and the Jamaican Blue Mountain. The average arabica plant is a large bush with dark-green oval leaves. It is genetically different from other coffee species, having four sets of chromosomes rather than two. The fruits are oval and mature in 7 to 9 months; they usually contain two flat seeds (the coffee beans) - when only one bean develops it is called a peaberry. Arabica coffee is often susceptible to attack by pests and diseases, therefore resistance is a major goal of plant breeding programmes. Arabica coffee is grown throughout Latin America, in Central and East Africa, in India and to some extent in Indonesia.

Coffea canephora - Robusta coffee

The term 'robusta' is actually the name of a widely grown variety of this species. It is a robust shrub or small tree growing up to 10 metres in height, but with a shallow root system. The fruits are rounded and take up to 11 months to mature; the seeds are oval in shape and smaller than those of C. arabica. Robusta coffee is grown in West and Central Africa, throughout South-East Asia and to some extent in Brazil, where it is known as Conillon.

Coffea liberica - Liberica coffee

Liberica coffee grows as a large strong tree, up to 18 metres in height, with large leathery leaves. The fruits and seeds (beans) are also large. Liberica coffee is grown in Malaysia and in West Africa, but only very small quantities are traded as demand for its flavour characteristics is low.

Standard references

Clifford M.N. and Willson K.C. (Editors) - Coffee; botany, biochemistry and production of beans and beverage. London, Croom Helm, 1985

Wrigley G. - Coffee. London, Longman, 1988

PLANT BREEDING

Coffea arabica

C. arabica is a tetraploid (44 chromosomes) and is self-pollinating. There are two distinct botanical varieties: arabica (typica) and bourbon. Historically, typica was cultivated in Latin America and Asia, whereas bourbon arrived in South America and, later, East Africa via the French colony of Bourbon (Reunion). Because C. arabica is self-pollinating, these varieties tended to remain genetically stable. However, spontaneous mutations showing desirable characteristics have been cultivated in their own right, as well as being exploited for cross-breeding purposes. Some of these mutants and cultivars are described below.

Mutants: Caturra - a compact form of bourbon Maragogipe - a mutant typica with large beans San Ramon - a dwarf typica Purpurascens - purple leaved forms

Cultivars have been developed to give the maximum economic return under specific regional conditions such as climate, soil, methods of cultivation and the prevalence of pests and diseases. Some of the better known cultivars are:

Blue Mountain - grown in Jamaica and Kenya

Mundo Novo - a cross between typica and bourbon, originally grown in Brazil

Kent - originally developed in India, showing some disease resistance

Catuai - developed as a hybrid of Mundo Novo and Caturra, characterized by either yellow or red cherries: Catuai-amarelo and Catuai-vermelho respectively.

Coffea canephora

C. canephora is diploid and self-sterile, producing many different forms and varieties in the wild. The identification of cultivars is confused, but two main forms are recognised:

'Robusta' - upright forms

'Nganda' - spreading forms

Arabica / robusta hybrids

Coffee has been selectively bred to improve characteristics of: growth and flowering, yield, bean size and shape, cup quality, caffeine content, disease resistance, drought resistance.

Crosses between arabica and robusta aim to improve arabica by confering disease resistance and vigour or to improve on the cup quality of robusta.

Hibrido de Timor is a natural hybrid of arabica x robusta which resembles arabica coffee and has 44 chromosomes.

Catimor is a cross between Caturra and Hibrido de Timor and is resistant to coffee leaf rust (Hemileia vastatrix).

A new dwarf hybrid called Ruiru Eleven, developed at the Coffee Research Station at Ruiru in Kenya, was launched in 1985. Ruiru 11 is resistant to coffee berry disease and to coffee leaf rust. It is also high yielding and suitable for planting at twice the normal density.

Icatu hybrids are the result of repeated backcrossing of interspecific arabica x robusta hybrids to arabica cultivars Mundo Novo and Caturra.

Arabusta hybrids are fertile interspecific Fl hybrids from crosses between arabica and induced auto-tetraploid robusta coffee.

Techniques used in coffee breeding

1. Controlled pollination and multiplication by seed

2. Vegetative (clonal) propagation

• Traditional methods: grafting, taking cuttings
• New methods (tissue culture): micropropagation, somatic embryogenesis

In recent years the potential of genetic manipulation of Coffea using recombinant DNA technology and tissue culture techniques has been investigated. By introducing new genes for characteristics such as resistance to pests or to herbicides, or genes coding for desirable cup quality attributes, it may be possible to produce plants with any combination of features required.