Importance, Distribution, and Morphology
Tea is the oldest caffeine-containing beverage, and has been used for two or three thousand years in south-east China). It is also the most widely consumed hot beverage, as being the cheapest. To date, the habit of tea drinking has become well established for more than half of the world’s population. The most common type of tea is black or fermented tea. The unfermented type is green tea, which is produced in China and Japan, and gives a distinctive taste different from black tea. A further semi-fermented tea is oolong, consumed in China and Japan. In some few cases tea leaves are used as vegetables, like leppet tea in Burma and meing tea in Thailand. Based on FAO records (1984), 2.7 million hectares of land are under tea cultivation and 2.2 million tons are produced every year in the world. Over 80% of the world’s tea exports come from India and Sri Lanka. Exports from China rank next to these two countries. The United Kingdom is the largest importer of the world, importing over 20% of the production. The second largest importer is the United States, but there only about half a pound per person each year is drunk.
The centre of origin of the tea plant is considered to be near the source of the Irrawady River and further north. It is variously stated that tea is found wild in Assam and Upper Burma and south Yunnan in upper Indo-China, but its real origin is still the subject of various theories. In China, cultivation of tea has long been established, while in Japan tea was introduced during the 12th century from China. However, in other countries tea cultivation was introduced as a commercial production since the discovery of the black tea variety assamica by R. Bruce in 1823. Introduction into East Africa occurred at the beginning of the 20th century. According to plantation areas, various botanical varieties are planted. Tea, Camellia sinensis (L.) O. Kuntze belongs to the Thea section of the Camellia genus in Theaceae. The tea plant is a shrub or tree, 9-15 m in height under natural conditions and about 1.5 m under cultivation. Leaves are alternate, elliptical on short stalks, leathery and with toothed margins. The cultivated varieties separate into two main groups on the basis of foliar and growth characteristics. China teas, Camellia sinensis var. sinenesis, are slow-growing, dwarf trees, with small, erect, comparatively narrow, dark green leaves and are resistant to cold. In contrast, Assam teas, C. sinensis var. assamica, are quick-growing taller tree with large, drooping leaves and resistant to cold, while adapting well to tropical conditions. Tea contains 30 chromosomes (2n = 30). Large-leafed triploid (2n = 45) and tetraploid (2n = 60) varieties have been discovered. Various hybrids between China and Assam types are planted according to easy intercrosses. Hybrids are characterized by the intermediate characteristics of leaves and growth of trees when compared between the two types.
Camellia sinensis: Diseases
Diseases of tea plants differ within the types of plant and planting areas. In areas where Assam teas (var. assamica) are grown, as in India, Sri Lanka and many other countries, blister blight caused by Exobasidium vexans Massee remains the most dangerous disease. Air-borne basidiospores of Exobasidium vexans spread and form white blisters on young leaves and young stems. Buds can be attacked and whole shoots may die. Anthracnose caused by Gloeosporium theae-sinensis Miyake is the most serious disease in Japan and China. It spreads over the area where the China tea (var. sinensis) is produced. It has been found very recently that fungus invades the plant only through the tricomes of three young leaves from the top of the growing shoots (Hamaya 1981, 1982). The fungus, however, does not attack the stem. Variety assamica and its hybrids are highly resistant to this disease. Grey blight caused by Pestalotia theae Sawada, Pestalotia longiseta Spegazzini and brown blight or copper blight by Glomerella cingulata (Stomen) Spaulding et Schrenk, white scab by Elsinoe leuospila Bitancourt et Jenkins (Sphaceloma theae Kurosawa) infect the leaves or growing shoots. Root disease, red root disease caused by Poria hypolateritia Berk, is very serious in Sri Lanka, India and Indonesia, but not observed in Japan. In its mode of development it resembles Armillaria, by causing the sudden death of the tree. Armillaria mella (Vahl: Fr.) Kummer is common in Africa but remains rare in Sri Lanka, India and Japan. Charcoal stump rot caused by Ustulina deusta (Fr.) Petrak and black root disease by Rosellinia arcuata Petch are serious diseases in India, Sri Lanka and Indonesia, while charcoal stump rot is sporadically found in Africa. In Japanese pythium cuttings, root rot by Pythium spp. is becoming a prevalent disease with the extention of vegetative propagation. The affected roots become watery, whitish brown then turn a brown colour and often flatten without inner substance. White root rot caused by Rosellia necatrix (Hartig) Berlese infests the plant and is also a very common disease in ornamental fruit trees. Stem disease, brunch canker caused by Phomopsis theae Hara, Nacrophoma theicola Petch, Nectria spp. and Poria hypobrunnea Petch are common in Sri Lanka and India. It is considered that a viral disease causes necrosis of phloem which can be present in the root, stem or leaf bud. However, the vector has not been discovered and the diagnostic symptoms are spreading in tea-producing areas of Sri Lanka and India. According to variety, tea plants differ in tolerance to some diseases such as anthracnose. It is suggested that there is scope for breeding of resistant varieties. Since some diseases are common in one area or country but not in others, the prevention of epidemics in international trade still represents a very important goal.
Methods of Propagation and Processing
Tea is propagated through seeds or cuttings. The plants are obtained from seeds because the flower has high cross-pollinating ability. Biclonal seeds, i.e. trees planted in two clones in the seed garden, are used for propagation. Cross-fertilized seed bushes are selected for yield and quality or other characteristics. Seeds are sown in special seed gardens and seedlings show high heterogeneity. Moreover, since seeds become recalcitrant and unable to retain their viability through long-term storage, seeds need to be sown immediately. Vegetative propagation by using cuttings has become one of the most frequently used methods. Usually, single internode cuttings taken immediately above a leaf and axillary bud are planted into the rooting bed. New growing shoots with five to six leaves are also used for cutting materials. The cuttings from selected trees are planted in shaded propagating beds and given moderate amounts of water. Rooting ability varies among cultivars. Methods of propagation with seeds or cuttings require a longer period of time and space for production of nurslings.
Shaping and pruning are done to maintain a convenient height for plucking, to induce vigorous vegetative growth and ensure a continuous supply of flushes. China types, being dwarf and slow-growing, require relatively little pruning, whereas Assam types and hybrids have to be kept within bounds. Pruning should be done during a dormant period. Newly grown terminal buds with two or three leaves are plucked and brought to the factory. The processing of black tea comprises four main steps: withering or drying, rolling, fermentation and firing. During firing, the fermentation is stopped by destroying the enzyme with heat. This is done either by steaming or roasting. The process takes about 40min at 71 °C and 15min at 99 °C. The fermented tea leaf enters the heating room, where it immediately undergoes heating at a high temperature of 90 to 95 °C. The firing process is performed in special dryers. The hot air from the heater reduces tea moisture content to 3% to 4%. The firing stage is followed by a sorting process to make the half-finished product marketable.
Camellia sinensis: Summary and Conclusion
The results of tissue culture research in C. sinensis indicate that callus can be obtained very easily on many media from any vegetative or flowering part of the plant. The regeneration occurred from the cotyledon or stem callus on medium supplemented with a high concentration of cytokinin and low auxin. Wu et al. (1982) used coconut milk for which reason the concentration of cytokinin for the regeneration from cotyledon callus may have been higher in the medium. Root formation from callus was limited on the 2,4-D medium. It is considered that patterns of differentiation differ with the kind of auxin. Bud formation from the callus varied with the origin of the tissues of the explants. The different patterns of regeneration of three types of explants may be due to the occurrence of an antagonistic correlation between epidermal layers and vascular tissue. In pre-cultured explants, the stem segments induced softer callus and regeneration occurred more readily than on the intact explant. Moreover, when pre-cultured explants were separated to epidermal layers, stem segments and stripped segments, the same phenomenon of regeneration was observed. Further studies are required to determine the correlation between epidermal layers and vascular tissue under culture conditions or on the differences in callus characteristics. Since somatic embryos were produced without callus formation on the surface of the cotyledon and the hypocotyl, it was considered that embryos appeared to be genetically stable clones. Moreover, in C. sasanqua () and C. chrysantha (), adventitious embryos were formed in the same way on the cotyledon as in C. sinensis and C. japonica. Since the cotyledon is formed after completion of fertilization, it is heterogenetic in hybrid seed. It is considered that cotyledon culture may provide a method for rapid, disease-free, clonal propagation of hybrid materials or selected clones in Camellia (). Somatic embryogenesis was also reported in anther culture in C. sinensis (). These somatic embryos are useful for the production of homozygous plants and the genetic improvement of tea plants. Somatic embryogenesis has application not only in micropropagation but also in the preservation of recalcitrant seed.
In tea and other species of Camellia, investigations on callus or somatic embryogenesis seem to be increasing. As mentioned above, tea plants require pathological, genetical and propagational improvements. New progress of in vitro research in these plants opens perspectives for whole plant regeneration from a cell; cellular transformation by the introduction of foreign gene provides tolerance to pathogenes or resistance to cold or high quality and quantity of tea production, and understanding physiological development and biosynthesis of tea alkaloid as in coffee.
Selections from the book: “Medicinal and Aromatic Plants II”, 1989.