From Plant Resources of South-East Asia No 16: Stimulants, PROSEA Foundation
by A. F. Schoorel and H. A. M. van der Vossen
Taxon
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 Numbers
2n = 30
Synonyms
Thea sinensis L. (1753), Camellia thea Link (1822), Camellia 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: ch[ef], tr[af].
Origin and Geographic Distribution
The natural habitat of Camellia 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 Camellia sinensis var. sinensis ('China tea'), which has smaller leaves and more cold tolerance but grows less vigorously than Camellia 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.
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, Camellia sinensis (L.) Kuntze var. lasiocalyx (Watt) W. Wight, with characteristics intermediate between China and Assam teas. The outbreeding nature of Camellia 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 Camellia irrawadiensis P.K. Barua and Camellia 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 x 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 Camellia 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 Camellia 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 Camellia 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
Eden, T., 1976. Tea. 3rd edition. Longmans, London, United Kingdom. 205 pp.
Ellis, R.T., 1995. Tea, Camellia sinensis
(Camelliaceae). In: Smartt, J. & Simmonds, N.W. (Editors):
Evolution of crop plants. 2nd edition. Longman Scientific &
Technical, Harlow, United Kingdom. pp. 22—27.
Guinard, A., 1953. La
culture du thé en Indochine [The cultivation of tea in Indo-China].
Archives des Recherches Agronomiques et Pastorales au Viêt-nam No 20.
179 pp.
Harler, C.R., 1956. The culture and marketing of tea. 2nd edition. Oxford University Press, London, United Kingdom. 263 pp.
International Tea Committee, 1997. Annual Bulletin of Statistics. 133 pp.
Kayange, C.W., Scarborough, I.P. & Nyierenda, H.E., 1981. Rootstock influence on yield and quality of tea (Camellia sinensis L.). Journal of Horticultural Science 56: 117—120.
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.
Purseglove, J.W., 1968. Tropical crops. Dicotyledons 2. Longmans, London, United Kingdom. pp. 599—612.
Tanton, T.W., 1981. Growth and yield of the tea bush. Experimental Agriculture 17: 323—331.
Visser, T., 1969. Tea, Camellia sinensis.
In: Ferwerda, F.P. & Wit, F. (Editors): Outlines of perennial crop
breeding in the tropics. Miscellaneous Papers No 4. Landbouwhogeschool
Wageningen, the Netherlands. pp. 459—493.
Willson, K.C. &
Clifford, M.N. (Editors), 1992. Tea: cultivation to consumption.
Chapman & Hall, London, United Kingdom. 769 pp.
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