Camellia sinensis (PROSEA)

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Plant Resources of South-East Asia
Introduction
List of species


Camellia sinensis (L.) Kuntze


Protologue: Um die Erde ("chinensis"): 500 (1881) et in Acta hort. petrop. 10: 195 (1887).
Family: Camelliaceae (Theaceae s.l.)
Chromosome number: 2n= 30

Synonyms

  • Thea sinensis L. (1753),
  • Camellia thea Link (1822),
  • C. theifera Griff. (1854).

Vernacular names

  • Tea (En).
  • Théier (plant), thé (product) (Fr).
  • Indonesia and Malaysia: teh
  • Philippines: tsa (Tagalog).
  • Burma (Myanmar): leppet
  • Cambodia: taè
  • Laos: s'a:, hmièngx
  • Thailand: cha (central), miang (northern)
  • Vietnam: chè, trà.

Origin and geographic distribution

The natural habitat of C. sinensis is the lower montane forest on mainland Asia from south-western China (Sichuan) to north-eastern India (Assam). The primary centre of origin is presumed to be near the source of the Irrawadi (Ayeyawadi) river in northern Burma (Myanmar), but early human interest in the stimulating properties of tea may have been instrumental in its wider dispersal in Asia. The tea plant was already known to the Chinese peoples more than 4000 years ago. Written records dating from the 5th Century AD confirm its widespread cultivation and general use as a refreshing beverage in several Chinese provinces. Tea cultivation in Japan was started in the 9th Century with seed introduced from China. Tea became an important export commodity for China, first through the Mongols by old overland trading routes in central Asia to Turkey and Russia (mainly as brick tea), and then from the early years of the 17th Century also to Europe by sea, through the Dutch and English East India Companies (green, and later black tea). For more than 300 years all the tea drunk in the Western world came from China (100 000 t in 1850), but this monopoly on the international tea market gradually came to an end with the development of tea plantations in India (1840), Sri Lanka (1870) and Indonesia (1880). By 1925 very little of the 300 000 t of tea imported into Europe came from China. Tea exports from China were resumed in quantity in the 1960s.

The tea grown in China and Japan is all C. sinensis var. sinensis ("China tea"), which has smaller leaves and more cold tolerance but grows less vigorously than C. sinensis var. assamica (Mast.) Kitamura ("Assam tea") discovered in the forests of north-eastern India in 1823. Assam tea and subsequently hybrids between the two varieties ("Indian hybrid tea") became the basis for the tea industries of South, South-East and West Asia, as well as for those established in Africa and South America. In South-East Asia, tea cultivation is most important in Indonesia, Vietnam, Papua New Guinea, Malaysia and Thailand.

Uses

Tea, the beverage obtained by infusing the leaves in hot water, is used worldwide. Called "tay" or "cha" by the Chinese, the beverage derives its stimulating and refreshing properties from the high concentration of the alkaloid caffeine, specific polyphenolic compounds, and the aroma complex present in the leaves of young shoots. In China, Japan, Vietnam and to some extent in other South-East Asian countries people drink green (unfermented) tea, which is prepared from pan-fired or steamed and dried young leaves plucked from China tea plants. This produces a pale and mildly flavoured liquor. More than 78% of the present world tea production is consumed as black tea, which involves withering, rolling or crushing, fermenting and drying the young leaf shoots. Black tea prepared from China tea produces a light brown and delicately flavoured liquor. Black teas made from Assam or Indian hybrid tea shrubs generally give a darker, stronger tasting liquor, hence these are often drunk with milk. "Oolong" or "Bohea" is a semi-fermented China tea originally from the Fujian province and Taiwan. In Burma (Myanmar) tea is also consumed as pickled food called "leppet".

In addition to regular black tea, several specialty teas are offered to the consumers in the industrialized world based on origin (e.g. Darjeeling), blend (e.g. English Breakfast) or added flavour (e.g. bergamot, orange, jasmine). Herbal teas have nothing to do with real tea. The conventional method of retailing loose tea in small packets (50-250 g) has been largely replaced by tea bags (250-400 bags to 1 kg fine grade tea), which many Western consumers find more convenient. Real instant teas are inferior in liquor quality, but have a significant market share in the United States as bottled or canned cold drinks. Bottled and cartoned tea compete with soft drinks on the Indonesian market. There is also a small market in the United Kingdom and the United States for decaffeinated tea.

Production and international trade

World production of tea during 1995-1998 averaged 2.6 million t/year (22% green tea) from a total area of 2.5 million ha in 30 countries. China has the largest area under tea (1.1 million ha), but with 580 000 t (71% green tea) is the second largest producer after India, which produces 755 000 t from 425 000 ha. The third largest tea producer is Sri Lanka (250 000 t) followed by Kenya (240 000 t), Indonesia (140 000 t) and Turkey (120 000 t). South Asia (India, Sri Lanka, Bangladesh) produces about 40% of the world tea, East Asia (China, Japan, Taiwan) 27%, Africa (10 countries) 14%, West Asia (Turkey, Iran, Georgia, Azerbaijan) 10%, South-East Asia 7% and South America (Argentina, Brazil) 2%.

About 50% of all black and 78% of green teas are consumed domestically, leaving about 1.1 million t (93% black tea) for the international tea market. Sri Lanka and most African countries export more than 90% of their tea, Indonesia 60%, China 27% (black and green tea) and India 20%. The largest tea importers are the United Kingdom plus Ireland (155 000 t/year), the Russian Federation (150 000 t), Pakistan (110 000 t), the United States (85 000 t) and Egypt (70 000 t). Annual tea consumption per head varies from 0.1-3.1 kg: e.g. Italy 0.1, United States and Indonesia 0.3, China and Russian Federation 0.5, India 0.6, Japan and Egypt 1.1, Turkey 1.9, United Kingdom 2.5, Ireland 3.1.

The international tea trade is based on regular public auctions in producing countries (e.g. Calcutta, Colombo, Jakarta, Mombasa), but partly also on direct sales from large plantations or national tea selling organisations (e.g. China) to overseas buyers. Tea prices have fluctuated considerably over the years, but there has been a general decline over the last decade. There are also price differentials of 50-200% between plain and very high quality teas. At mean auction prices of US$ 1.20-1.90/kg, the total value of the international tea trade for 1996 (1.1 million t) can be estimated at US$ 1.3-2.1 billion.

In South-East Asia, Indonesia is the most important tea producer with 80 000 ha plantations (110 000 t black tea) and 50 000 ha smallholdings (30 000 t green tea). Vietnam produces some 40 000 t green tea on 70 000 ha (mostly smallholdings) all for domestic consumption. Malaysia produces 6000 t annually from 4000 ha (plantations), but also imports another 7000 t black tea; Papua New Guinea produces 7000 t black tea from 5000 ha (plantations); Thailand 5000 t from 10 000 ha; Laos 1500 t from 2000 ha.

Properties

Young green tea shoots (bud and two leaves) of Assam tea have the following approximate chemical composition per 100 g dry weight: polyphenolic compounds (mostly six catechin flavanols) 30-35 g, polysaccharides and carbohydrates 22 g, protein 15 g, caffeine 3-4 g, amino acids (including theanine) 4 g, inorganics 5 g, organic (mainly ascorbic) acids 0.5 g and volatile substances 0.01 g. In addition to these (hot) water soluble components, there are also non-soluble cellulose 7 g, lignin 6 g and lipids 3 g. Compared with Assam teas, the flavonol content of China teas is half and Indian hybrid teas about three-quarters. The polyphenol content decreases with leaf age, being highest in the bud and first leaf and low beyond the third leaf.

In the unfermented green teas most of the components remain unchanged and these determine the colour, taste and aroma of the liquor. During the manufacturing process of black tea, complex biochemical reactions take place, as a result of cell disruption and the mixing of cytoplasmic polyphenol oxidase with the contents of the cell vacuoles. Part of the polyphenols are oxidized and polymerized to theaflavins and thearubigins that give the orange-brown colour, strength and taste typical of black tea liquors. At the same time several hundreds of secondary volatile compounds are formed, mainly derived from carotenes, amino acids, lipids and terpene glycosides. Together with the primary volatile substances already present in fresh tea leaves they form the aroma complex of the black tea liquor. So far, some 650 of such aroma compounds have been detected in the liquor of black tea, compared with 250 in green tea brews. Tea quality is determined not only by the briskness, strength and colour of its liquor but even more so by the composition and concentration of the aroma complex. Research has made considerable progress in distinguishing highly desirable aroma (group I) compounds from those deleterious (group II) to tea quality, but using the ratio between the two as a quantitative method for determining tea quality has limited value. Tea quality is still largely assessed organoleptically by experienced tasters.

The genetic background of the cultivar planted, the climatic conditions (e.g. altitude), the age of the bush, the period since pruning and the agronomic practices, all affect the quality. However, potentially high quality tea can easily be destroyed by poor methods of plucking, handling and processing.

The 1000-seed weight is 450-500 g.

Description

  • Evergreen, multi-stemmed shrub up to 3 m tall (var. sinensis), or tree up to 10-15 m tall with one main stem (var. assamica); in cultivation pruned to 1-1.5 m and trained as a low profusely branching and spreading bush. Strong taproot and many lateral roots giving rise to a dense mat of feeding roots in the top 50-75 cm of the soil; some lateral roots grow 3-4 m deep; feeding roots in association with endotrophic mycorrhiza, lacking root hairs. Branchlets finely pubescent at apex.
  • Leaves alternate, with short petiole; blade obovate-lanceolate, 4-30 cm × 1.5-10 cm, serrate, obtuse with broad cusp to acuminate, slightly pubescent on lower surface when young, stomata on lower surface only, characteristic sclereids (stone cells) in mesophyll and calcium oxalate crystals in phloem of petiole; in var. sinensis leathery and usually stiff-erect, narrow and usually less than 10 cm long, dark green with dull, flat surface and indistinct marginal veins; in var. assamica thinly leathery and horizontal or pendant, wider and longer (15-20 cm long), lighter green with glossy, bullate upper surface and distinct marginal veins.
  • Flowers axillary, single or in clusters of 2-4, 2.5-4 cm in diameter, very fragrant; pedicel short; sepals 5-7, persistent; petals 5-7, obovate, concave, white or light pink, slightly coherent at base; stamens 100-300, with 2-celled yellow anthers, outer filaments fused at base and coherent with petals, inner ones free; pistil with superior 3-5-loculed ovary, 4-6 ovules per carpel, and 3-5 free (var. sinensis) or partly fused (var. assamica ) styles with stigmatic lobes.
  • Fruit a subglobose capsule, 1.5-2 cm in diameter, thick-walled and woody, brownish-green, usually 3-lobed and 3-loculed, 1-2 seeds per locule.
  • Seed globose or flattened on one side, 1-1.5 cm in diameter, testa (shell) light brown and hard, integument thin papery; endosperm absent; cotyledons thick and rich in oil, embryo straight.
  • Seedling with epigeal germination.

Growth and development

There is no seed dormancy. Viability of freshly harvested seed diminishes quickly at ambient temperatures, but can be maintained at more than 60% for 6-10 months by storage at 0-4°C at 100% relative humidity. Seeds germinate readily upon removal of the shell. Seedlings have one vertical stem with lateral branches from buds in the leaf axils; cotyledons are shed after 5-6 months. Flowering starts when tea plants are about 4 years old. Root development, whether the initial taproot of seed-grown tea or the adventitious roots of cuttings, is important in tea cultivation. Generally, when roots reach a diameter of 1-2 mm starch reserves already begin to be laid down. This stored carbohydrate plays an important role in the regrowth of shoots following pruning.

Endogenously determined growth cycles in unpruned tea cause a series of flushes of shoot growth alternating with periods of inactivity. These so-called "banjhi" cycles last 10-14 weeks each; shoots with dormant apical buds are called banjhi shoots. However, the yield cycles observed in cropped tea are mainly the result of synchronization of shoot development.

Pruning and harvesting cause a large number of branched twigs with leaves to develop in the top 20-40 cm of the closely planted bushes. The leaf area index in a mature tea planting is 4-10, and is usually larger in China than in Assam types. Favourable weather conditions induce a synchronized growth flush of apical shoots, which produces the first peak in crop production. When a bud comes out of dormancy 2 scale leaves are formed, one of which usually drops off soon after, followed by a small, non-serrated "fish" leaf and a flush of normal leaves. Removal of these apically dominant shoots by harvesting encourages a second generation of shoots to grow, but some of these shoots stop growing after only 2 or 3 leaves and end in a dormant (banjhi) bud. A dip in yield occurs until this second flush of shoots has grown to harvestable size and produces the next peak in crop production. The number and length of the shoot replacement cycles (SRC) - defined as the duration between the start of bud expansion and the moment that the new shoot is harvestable - is determined by climatic conditions. Fast-growing flushes (SRC of 30-40 days) in warm and wet weather will reach harvestable shoot size almost simultaneously and so lead to distinct peaks in crop production. Under uniformly cool and humid conditions (SRC of 100 days or more) each shoot flush will reach harvestable size spread out over a longer period, resulting in a more even distribution of the crop over the season.

In the tropics flowering in non-plucked tea occurs year-round. Flowers are pollinated mainly by insects (e.g. bees). Only cross-pollination gives good fruit set and seed, especially in var. assamica , which appears to have a system of self-incompatibility. Fruits take 10-12 months to mature; ripe fruits dehisce the seeds by splitting open from the apex into 3 valves.

Other botanical information

In the classification of true teas (no hybrids) a third taxon has often been distinguished called "Cambod" or southern tea, C. sinensis (L.) Kuntze var. lasiocalyx (Watt) W. Wight, with characteristics intermediate between China and Assam teas. The outbreeding nature of C. sinensis and the extensive intercrossing between the taxa has resulted in a wide spectrum from pure China to Assam plant types and all intergrades in presently existing tea populations. There have been suggestions that C. irrawadiensis P.K. Barua and C. taliensis (W.W. Smith) Melchior have also contributed to the gene pool of tea, although these species as such do not produce a tea of acceptable quality.

The term "jat" is often used to indicate seedlings from a certain district, but also to distinguish a group of plants that appear to belong to a distinct type on the basis of leaf and floral characteristics.

To classify cultivars it is more appropriate to distinguish cultivar groups, e.g. the cv. groups China Tea, Assam Tea, Cambod Tea and Indian Hybrid Tea, each with their respective cultivars.

Ecology

Tea originated from an area of monsoon climates (warm, wet summers and cool, dry winters) but is presently grown in a range varying from Mediterranean to tropical climates, between 42N (Georgia) and 27S (Argentina) latitudes, as well as from sea-level to 2300 m altitude.

Mean annual rainfall varies from 1500 mm (Uganda) to 3500 mm (West Java). About 1700 mm annual rainfall is the minimum requirement for economic tea production. Additional irrigation is only effective under sufficiently high air humidity. On the other hand, rainfall of 5000 mm has no adverse effect on tea growth. Rainfall should not fall below 50 mm per month for any prolonged period. Hail can be a serious hazard to tea, causing yield losses of 10-30% in some areas (e.g. in Kenya above 2000 m altitude).

Generally, optimum temperatures for shoot growth are between 18-30°C. The base temperature (Tb), below which shoot growth stops, is about 12.5°C, but this can vary between genotypes from 8-15°C. The thermal time - i.e. the product of number of days and effective temperatures (T - Tb) - for the SRC in tea is on average 475 day°C, in the absence of water stress. It has been shown to be a very useful parameter to estimate seasonal and geographical effects of temperature on the length of the SRC and consequent yield patterns. At average daily temperatures of 22.5°C the SRC would thus be 48 days against 79 days at 18.5°C. The thermal time parameter is not applicable at temperatures above 30°C, as the co-occurring high vapour pressure deficits of the air (> 23 mbar) depress shoot growth. Tea is not killed by the night frost that occurs in important tea-growing areas at higher latitudes. China-type teas are more tolerant of colder climates. Daylength does not have a large influence on seasonal variation in growth (yield) or flowering.

At high altitudes in tropical areas the photosynthesis of whole canopies of the tea crop becomes saturated at 75% of full sunlight. Tea is generally more productive without shade, but shade trees may be necessary to reduce air temperatures during hot periods, e.g. in Assam and Bangladesh. Shelter belts of trees planted between fields are beneficial in protecting tea against prevailing strong winds.

Tea is grown successfully in a wide range of soil types developed from diverse parent rock material under high rainfall conditions. Soils suitable for tea cultivation should be free-draining, have a depth of 2 m, a pH between 4.5 and 5.6, a texture of sandy loam to clay and good water-holding capacity.

Climatic conditions have a great influence on the quality of the tea, especially on the flavour. Fast shoot growth - for instance at low altitudes, during the best part of the growing season or shortly after the bushes have been pruned back - is detrimental to the quality of tea, particularly the flavour, but induces high production. Hence, both in respect of plucking method and inputs to encourage growth (e.g. heavy manuring) the grower has to choose between high yield and good quality. Nevertheless, high yields and excellent quality tea can be obtained in tropical countries on fertile soils, especially at elevations of 1200-1800 m above sea-level. At still higher elevations, the tea will have a well-developed flavour but it will lack strength and yields will be lower. Likewise, the retarded shoot growth during a dry period and the proliferation of growing points with an attendant reduction in shoot vigour shortly before the next pruning round result in better flavour but low yields.

Propagation and planting

Both seedlings and cuttings are used as planting material for tea. Seeds are produced in a special orchard, or "bari", with free-growing, widely spaced (5 m × 5 m) trees from a selected jat, or a restricted number of selected clones (biclonal and polyclonal seed). Seeds with a diameter of 12.5 mm or more are considered to have sufficient food reserves. They are immersed in water for up to half an hour to select the sinkers which show better germination and subsequent vigour. To germinate, seeds are usually placed between wet gunny or hessian cloth and inspected twice weekly. Those that have an emerging radicle are transferred to nursery beds. After 1.5-2.5 years, the stems are cut back to a height of 15 cm, the plants dug up and transplanted to the field.

The techniques of vegetative propagation were mastered commercially during the 1960s in India, Sri Lanka and Kenya, in the 1970s in Indonesia. Since then, rooted cuttings have been used almost everywhere. Multiplication plots of selected clones are being established; the shoots are left to grow up to 15 nodes before being cut. Green, semi-hardwood cuttings with a full leaf, taken from primary shoots, are the best. At the nursery, up to 8 single-node leaf cuttings are made with a sharp knife from the middle part of each shoot. Each cutting is then placed in a small polythene sleeve (10 cm wide, 30 cm long) with the leaf and bud just above soil level. The rooting medium should be acid and low in organic matter. The bags are then watered and placed in small airtight polythene tunnels under shade. The tunnels are periodically opened for watering, while the shade is gradually removed to harden the plants off before planting out at an age of 6-9 months.

A recently developed technique of propagation is the composite tea plant, which is produced by chip-budding on unrooted cuttings. It offers the opportunity of increasing yield without loss of quality, by combining improved vigour of selected rootstocks with scions yielding high-quality tea. Micropropagation by tissue culture appears possible, but the tea industry is currently showing little interest in this. Tea is planted at densities of 11 000 to 14 000 bushes per ha, depending on climatic and edaphic conditions, as well as on varietal vigour. On slopes, tea is planted in contour rows. Although trials in various regions have shown that there is no particular optimum spacing, the need for soil conservation has led to closer planting (60 cm) in the rows, with sufficient space (120 cm) between the rows to allow pluckers to walk and work. To further check erosion and provide some shade for the young plants, Tephrosia candida (Roxb.) DC., Crotalaria micans Link or C. trichotoma Bojer are often sown between the rows of tea. Laying cut or mown matter from these leguminous plants or from Guatemala grass ( Tripsacum andersonii J.R. Gray) alongside the tea plants provides a mulch to conserve moisture and to control erosion and weed growth. The use of permanent shade trees (most important are Paraserianthes falcataria (L.) Nielsen, Leucaena leucocephala (Lamk) de Wit and Erythrina subumbrans (Hassk.) Merrill) is restricted to low elevations.

Husbandry

Regular weeding of tea is needed only during the first few years after planting, until shade from the continuous plucking table and the mulch provided by prunings prevents the further growth of most weeds. Surviving weeds can be controlled by spot application of herbicides. Pruning in tea has the following main objectives:

  • Frame formation of young plants: this involves pruning at various levels to induce lateral growth leading to a permanent and wide frame for a continuous gap-free plucking table. Bending and pegging down of branches avoids some of the pruning and accelerates the formation of a good and lower frame. Pegged plants come into bearing earlier, producing a first small crop in the second year after planting.
  • Final shaping of the plucking table: 3-5 rounds of "tipping" (breaking back) of upright growing shoots is carried out to level the plucking surface at a height of 50-60 cm for the first crop. This produces the required 20-25 cm of maintenance foliage and a high density of points from which the flush will grow.
  • Maintenance pruning: at 2-5 year intervals (depending on regional differences in climate) all the stems and leaves above the basic frame are removed to lower and rejuvenate the plucking table, which may have reached a height of 120-150 cm and become less productive. Maintenance prunings may vary from very heavy to light (skiffings), the best time being usually at the onset of a cool or dry period, when the starch reserves in the roots are high and regrowth will therefore be fast. Maintenance pruning is followed by rounds of tipping to prepare the plucking table for the new crop.
  • Collar or down pruning: cutting below the lowest level of normal pruning, close to ground level, to rejuvenate very old tea bushes; it may take 5-6 years to regain full production after collar pruning.

Tea requires regular fertilizer application to sustain satisfactory yield levels, but types and rates will vary with local conditions of soils, climate, agronomic practices, absence or presence of shade, age of the bushes and type of variety planted. Fertilizer recommendations are based on field experiments, soil and leaf analyses. Foliar analysis (the third leaf below the bud is sampled) provides useful information on the nutrient status of the tea plant. Nutrients removed by one t of processed tea are: 45 kg N, 5 kg P205, 20 kg K20, 8 kg CaO and 3 kg MgO. A considerable part of the nutrient uptake is returned to the soil by the prunings, which are left between the bushes and also improve the organic matter content of the soil. Nitrogen is the most important nutrient in tea; clear yield responses have been recorded to rates of up to 500 kg N/ha in mature tea, but 150-200 kg N/ha is the most economic level. Excessive N application can adversely affect the tea quality. The rates of application of P and K often depend on the type of compound fertilizers used, e.g. NPK 25-5-5 (Kenya), 6-1-2 (Indonesia). Other major (Ca, Mg, S) and minor elements (Mn, Cu, Zn) are applied separately when required, minor elements also as foliar applications.

Diseases and pests

Blister blight (Exobasidium vexans), which attacks young leaves and shoots, is of major economic importance in all tea-growing areas of Asia, but has not yet occurred in Africa or South America. It spread from Assam (first reported in 1868) to all tea areas in India, to Japan and Taiwan (1920), Sri Lanka (1946) and reached Indonesia in 1950. It can be controlled by regular sprays of copper and also by systemic fungicides. Some clones are less susceptible to blister blight than others, but so far no true host resistance has been found. Anthracnose (Colletotrichum theae-sinensis) and net blister blight (Exobasidium reticulum) are of importance mainly in Japan and Taiwan. Grey blight (Pestalotia theae) and brown blight (Colletotrichum camilliae) are weak parasites of mature leaves. They can become a problem in mechanically harvested tea. There are several stem cankers in tea (e.g. Macrophoma theicola and Phomopsis theae), but these can be controlled by careful pruning, protecting pruning cuts by fungicidal paints and by removing affected branches. A number of important root diseases are extremely difficult to control in tea. They include charcoal root disease (Ustulina deusta), red root disease (Poria hypolateritia), brown root disease (Fomes noxius), root splitting disease (Armillaria mellea) and Ganoderma pseudoferreum. Initial infection is often from mycelial strands that spread from old stumps and roots of previously cleared forest or shade trees. Control measures include uprooting affected bushes and some apparently healthy ones surrounding them, applying soil fumigants like methyl bromide and observing a 2-year fallow period with grass (e.g. Guatemala grass) before replanting.

More than 300 species of insects and other pests are known to infest tea. The most important tea pests occurring in Asia and accounting for 6-14% of annual crop losses include:

  • foliage feeders: mites (Oligonychus coffeae, Tetranychus spp., Brevipalpus spp.), thrips (e.g. Scirtothrips dorsalis), mosquito bugs (e.g. Helopeltis theivora), scale insects and aphids (Aphis spp., Toxoptera aurantii), leaf-feeding (e.g. Homona spp.) and leaf-rolling caterpillars (e.g. Caloptilia theivora) and flushworm (Cydia leucostoma). Apart from direct crop loss, these foliage feeders also cause loss of quality in the processed tea.
  • pests damaging stems: red coffee borer (Zeuzera coffeae), scolytid shot-hole borer (Euwallacea fornicatus) and termites.
  • pests of the root system: in particular root knot (Meloidogyne spp.), root lesion (Pratylenchus spp.) and root burrowing (Radopholus similis) nematodes.

Pest management by chemical control in tea has to be limited to narrow spectrum pesticides with a half life shorter than 8 days to avoid pesticide residues in processed tea exceeding internationally accepted FAO/WHO limits. With few exceptions, methods of biological control and IPM (Integrated Pest Management) are not yet very advanced in tea.

Harvesting

Most tea is still harvested manually by plucking the fresh shoot tips as they appear above the plucking table. The best quality of processed tea (flavour and strength) is achieved by light or "fine" plucking, which includes only active shoots with 2 young leaves and the bud or pecco. Hard or "coarse" plucking of 3 young leaves and the pecco increases yield at the expense of quality. Banjhi shoots (with dormant buds) appearing above the plucking table are harvested only when the top leaf is still young (one leaf and the bud), otherwise they are broken off and discarded. The harvested shoots are transferred to a bag or basket on the plucker's back, taking care not to compress or damage the leaves. Full baskets are taken to shaded collecting points for weighing and checking of quality. Plucked tea must subsequently reach the factory as soon as possible and undamaged to prevent early deterioration of quality. The interval between plucking rounds may vary from 4-7 days during peak flushes to 14 days or longer in adverse seasons.

There should be a balance between light and hard plucking in order to prevent the plucking table from rising too fast and at the same time to retain an adequate layer of maintenance foliage on the bushes for continued vigour and yield.

The best teas are produced by skilled manual harvesting, but it is very labour-intensive and accounts for about 60% of total production costs. Mechanical harvesters - from hand-operated devices to tractor-mounted and self-propelled machines - are increasingly being used in countries where labour costs are relatively high, or where the quality aspects are less stringent (e.g. bulk green tea production in Japan).

Yield

As a rule, Assam tea and Indian hybrids have a higher yield potential than Chinese teas. Estates, which generally have better management, yield more than smallholdings, whilst coarse plucking produces more than fine plucking. Climatic factors (such as drought, night frost and hail), volcanic activity and outbreaks of diseases and pests influence yields. Constraints in the supply of agricultural inputs (e.g. fertilizers and pesticides) and insufficient labour can affect the yield in a particular period. Price declines since the 1950s have accelerated the uprooting of old plantings and their replacement by higher producing jats or clones.

World average yield for China-type teas is estimated at about 900 kg of processed tea per ha per year, ranging from less than 500 kg in China to 1600 kg in Japan. Mixed plantings of China and Assam hybrid teas in Sri Lanka yield about 900 kg/ha annually. The predominant Assam and Indian hybrid teas produce per year 2100 kg/ha in Kenya, 1800 kg/ha in Malawi and 1700 kg/ha in India. In South-East Asia national average yields range from 600 kg/ha in Vietnam, 1100 kg/ha in Indonesia, and 2000 kg/ha in Malaysia, to 2100 kg/ha in Papua New Guinea (the latter two wholly from estates). In Indonesia annual yields are on average 550 kg/ha for smallholders, about 900 kg/ha for private estates and 2050 kg/ha for state plantations.

Handling after harvest

Black tea is manufactured in tea factories in a process lasting a little over 24 hours and consisting of the following stages:

  • withering: by partial removal of the moisture from the leaves over a period of 12-16 hours (down to 70% moisture content) in open or enclosed withering troughs;
  • leaf disintegration: by orthodox roller, rotorvane or CTC (crush,tear and curl) machines (about 30 minutes); the CTC machine produces smaller leaf particles, which is of advantage to the modern tea-bag market;
  • fermentation (oxidation): in thin-layer floor, or deep-layer and fan-assisted fermentation systems with temperature control (1-3 hours);
  • drying: by multi-band or fluid-bed dryers with heated air to reduce the moisture content to a final 2.5-3.5%;
  • sorting and fibre removal: to remove stalks and fibre and subsequent sorting into different sizes of tea particles; grades applied in the tea industry include whole-leaf grades, brokens, fannings and dusts;
  • packing: in India, Sri Lanka and Indonesia the traditional plywood chest to hold 40-45 kg processed tea is still being used; the United States and United Kingdom encourage alternative packaging in laminated paper sacks (e.g. in Kenya and Malawi), which can protect the tea quality equally well, are cheaper (US$ 1.50 against US$ 4.50 for a tea chest) and lessen demands on dwindling timber reserves.

Green teas are always prepared from C. sinensis var. sinensis. Chinese tea (Kamaira cha) is produced by firing fresh leaves in a pan for 10-15 minutes with frequent agitation to avoid burning of the leaves, followed by rolling and drying. In the case of Japanese Sen-cha, freshly plucked leaves are steamed for one minute, then subjected to rolling and drying with heating in three different stages until a moisture content of 6% is achieved and a needle-like tea. The pan-firing or steaming destroys the enzymes and thus prevents any fermentation (oxidation) as occurs during the process of black tea manufacturing.

Genetic resources

Most tea research institutes in Asia and Africa have living collections of tea germplasm of various origin. The seedling populations in cultivated tea of var. sinensis, var. assamica and the hybrids between these taxa also provide a rich pool of genetic variation whose potential has not yet been fully exploited. On the other hand, useful host resistance to important diseases and pests is hardly available in tea. For these and other reasons there is ample justification for collecting "wild" germplasm of tea and related species in the main areas of origin, for preservation and future use. Recent surveys in the Yunnan province of China have identified several interesting genotypes of var. sinensis and new tea species.

Breeding

Much of the tea in the world is established from open-pollinated seed collected in seed orchards, which at first consisted of unselected families or clones but which nowadays contain mass-selected and even progeny-tested families or clones. Tea is a highly heterogeneous outbreeder and large between-plant variation continues to exist in the most advanced seed cultivars. The development of efficient methods of vegetative propagation by single-node cuttings in India, Sri Lanka and Kenya in the 1960s set the stage for large-scale introduction of clonal cultivars in several tea-producing countries. Clonal selection offers an opportunity for uniform tea plantations and instant fixation of superior genotypes. However, the probability of finding these by simple mass selection within existing tea populations has been shown to be extremely small; better results are obtained when clonal selection is preceded by recombination crosses between selected tea genotypes (mother bushes or existing clones). In tea, as in many other crops, crosses between plants from genetically diverse subpopulations often show considerable hybrid vigour for yield. Molecular marker techniques can assist in measuring genetic diversity of initially selected parents and so increase breeding efficiency.

Improvement of tea cultivars has so far focused on plant type, yield and quality. Satisfactory levels of host resistance to diseases and pests have not been found within existing C. sinensis populations. Introgression of resistance from related species has been unsuccessful, not because of crossing barriers but due to inability to recover the quality of the recurrent parent. The product may look like tea, but the taste is commercially unacceptable.

Another handicap in tea breeding is the lack of information (from proper genetic studies) on the inheritance of yield and quality, beyond general observations and assumptions that all components of yield and quality appear to be inherited quantitatively.

Response to selection on components of yield is higher for number of shoots and weight of plucked shoot than rate of shoot growth. The quality of processed tea is positively correlated with the degree of pubescence on the underside of young leaves and with the "greenness" of mature leaves (dark and pale green leaves give poor quality).

Separate selection of clones to serve as vigorous rootstock (with deep and extensive root system) and as high-quality scions is an effective novel approach in tea breeding first developed in Malawi and Kenya. The resulting composite clonal cultivars are higher yielding (mainly because they produce more harvestable shoots) without loss of quality in processed tea.

Prospects

The world surplus of tea - due to steadily increasing production (2-3% per year) against stagnant consumption - will continue to put pressure on tea prices, which have been declining in real terms in recent years. An international tea agreement could stabilize tea prices to acceptable levels, but it appears to be entirely up to the main tea-producing countries to take effective steps in that direction. A strong association of tea-producing countries should also invest in promoting tea as an all-purpose and healthy beverage, in order to stem the present strong competition tea faces from other beverages such as herbal teas and soft drinks.

The economics of tea production can be improved considerably by increasing the inherent yield potential of cultivars, as well as by mechanizing of harvesting operations.

Much of the tea is still produced from old seed cultivars. Replacing these by high-yielding clonal cultivars, particularly those composite clones, would considerably increase economic returns. It also provides an opportunity of concentrating national tea production targets in smaller areas, thus releasing fertile land for alternative crop production. Molecular biology and genetic transformation could become an alternative approach for developing cultivars with host resistance to important diseases and pests, if such traits cannot be found in natural tea germplasm. Disease and pest management by chemical means will become increasingly difficult under the stringent rules for pesticide residues in processed tea.

Where tea is mainly a plantation crop and cost of labour is rapidly increasing, tea production will become completely uneconomic unless plucking is mechanized. Tractor-mounted and self-propelled machines capable of plucking tea without serious damage to leaves and stems in clonal plantations with uniform and strictly managed plucking tables have been developed for green-tea production and will soon be applied in black-tea production too, wherever quantity has priority over prime quality.

Literature

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  • Paul, S., Wachira, F.N., Powell, W. & Waugh, R., 1997. Diversity and genetic differentiation among populations of Indian and Kenyan tea (Camellia sinensis (L.) O. Kuntze) revealed by AFLP markers. Theoretical and Applied Genetics 94: 255-263.
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Authors

A.F. Schoorel & H.A.M. van der Vossen