Cultivation, Improvement, and Environmental Impacts of Tea
Summary and Keywords
Tea, the globally admired, non-alcoholic, caffeine-containing beverage, is manufactured from the tender leaves of the tea [Camellia sinensis (L.)] plant. It is basically a woody, perennial crop with a lifespan of more than 100 years. Cultivated tea plants are natural hybrids of the three major taxa or species, China, Assam (Indian), or Cambod (southern) hybrids based on the morphological characters (principally leaf size). Planting materials are either seedlings (10–18 months old) developed from either hybrid, polyclonal, or biclonal seeds, or clonal cuttings developed from single-leaf nodal cuttings of elite genotypes. Plants are forced to remain in the vegetative stage as bushes by following cultural practices like centering, pruning, and plucking, and they are harvested generally from the second year onward at regular intervals of 7–10 days in the tropics and subtropics, with up to 60 years as the economic lifespan. Originally, the Chinese were the first to use tea as a medicinal beverage, around 2000 years ago, and today, around half of the world’s population drink tea. It is primarily consumed as black tea (fermented tea), although green tea (non-fermented) and oolong tea (semifermented) are also consumed in many countries. Tea is also used as vegetables such as “leppet tea” in Burma and “meing tea” in Thailand.
Green tea has extraordinary antioxidant properties, and black tea plays a positive role in treating cardiovascular ailments. Tea in general has considerable therapeutic value and can cure many diseases. Global tea production (black, green, and instant) has increased significantly during the past few years. China, as the world’s largest tea producer, accounts for more than 38% of the total global production of made tea [i.e. ready to drink tea] annually, while production in India, the second-largest producer. India recorded total production of 1233.14 million kg made tea during 2015–2016, which is the highest ever production so far.
Since it is an intensive monoculture, tea cultivation has environmental impacts. Application of weedicides, pesticides, and inorganic fertilizers creates environmental hazards. Meanwhile, insecticides often eliminate the fauna of a vast tract of land. Soil degradation is an additional concern because the incessant use of fertilizers and herbicides compound soil erosion. Apart from those issues, chemical runoff into bodies of water can also create problems. Finally, during tea manufacturing, fossil fuel is used to dry the processed leaves, which also increases environmental pollution.
Tea is the second most extensively consumed beverage in the world (Mukhopadyay et al., 2012), following water, and has a slightly cooling, astringent flavor (Mondal et al., 2004). The drink is prepared from the evergreen perennial shrub called Camellia sinensis (L.) (Mukhopadhyay et al., 2013a, 2013b). The tender leaves are processed to make a drink that gives people the crucial pep and stimulus necessary for doing mental and physical work.
China was the first country to use tea as a medicine and drink (Chen & Sheng, 1981), and plants that are more than 1,500 years old are still blooming in the Yunnan province of southwestern China (Hara et al., 1995). Under normal conditions, tea plants can grow as high as 20–30 m, but for ease of cultivation, they are maintained as evergreen shrubs by pruning. In the tropics, harvest of the apical bud and the young leaves continued throughout the year, but in temperate environments, plucking is done seasonally.
There are diverse kinds and qualities of products generated from different cultivation practices, growing conditions, and processing methods (Bhatia & Ullah, 1962; Hara et al., 1995). It is principally consumed as fermented tea or black tea. In addition to use as a beverage, tea leaves are consumed as vegetables, such as in the Burmese “leppet tea” and “meing tea” of Thailand which are consumed as semi-fermented or pickled tea. Also, tea has considerable therapeutic value and can cure many diseases, including cancer. While green tea is known for its antioxidant properties, black tea is valued for its positive role in cardiovascular ailments (Sen & Bera, 2013). Black tea, green tea, yellow tea, oolong tea, white tea, and pu-erh teas are diverse varieties that all originate from the plucked leaves of C. sinensis, but disparities in processing determine which one is produced. After a tea is processed into any one of the five basic types, it can also be blended, flavored, or scented.
Origin and Spread of Tea
Tea was used as a medicinal beverage for the first time in China (Eden, 1958). According to the Ben Chao, the Chinese medical book, the role of tea in Chinese history can be dated to the year 2737 bc, when the emperor, Shen Nong, discovered it (Hara et al., 1995; Harbowy & Balentine, 1997). The people of Southwest China used teas for paying tribute to the Chinese emperors. A Chinese landlord, Wang Bao, wrote an essay called “Tong Yue” in 59 bc, in which the making and sale of tea was mentioned (Chen & Sheng, 1981). In addition, excavated tombs dating to 200 bc revealed that tea used to be included with the burial objects (Hara et al., 1995). “Cha Jing,” written by Lu Yu in 780 ad, highlighted the fact that, tea was a staple commodity in China. The book highlighted the origin, characteristics, names, and qualities of tea, along with details about tea plantations, plucking, processing, making, drinking, and storage (Chow & Kramer, 1990). Southeast China, near the source of the Irrawaddy River, is considered to be the probable center of origin of tea (Taylor & McDowell, 1993). Later, it was taken to South parts of China and beyond, to some parts of Southeast Asia. From there, by 221 bc, tea spread to a number of tropical and subtropical countries (Hara et al., 1995) by migration of peoples in war time, including Vietnam, Myanmar, Laos, and Thailand.
Hence, techniques of tea processing in certain mountainous regions of these countries are somewhat similar to those employed in ancient China (Zhuang, 1988). During the 5th century, China established the tea trade with Turkey, which further extended to Iran, Rome, Arabian peninsula, Afghanistan, Pakistan, and Korea, creating the Silk Road (Lu, 1987). Zen Buddhist missionaries introduced tea to Japan in 805 AD as a medicine due to its meditation-enhancing properties. There are two schools of thoughts regarding the invasion of tea to Europe. It had been suggested that a Portuguese Jesuit father first introduced tea to Europe in 1560 (Lu, 1987). However, other studies suggested that the first tea reached Europe around 1610, carried by Dutch ships from Java (Chow & Kramer, 1990). The first cultivations introduced in various countries were in Indonesia in 1684, Russia in 1833, Sri Lanka in 1839, Malawi in 1875, Iran in 1900, Kenya in 1903, and Argentina in 1924 (Chen, 1987).
Botany of Major Tea Varieties
Tea plants belong to the family Theaceae (Mukhopadhyay & Mondal, 2014). The cultivated varieties are hybrids of C. sinensis and Camellia assamica. Leaf size, flowers, and branching are considered the major taxonomic criteria for tea plants. For instance, Assam types are characterized by large leaves, whereas China types are characterized by very small leaves and Cambod leaves are between the two (Mondal, 2007). The C. sinensis variety (the China type) can withstand low moisture, stress, and frost. These plants grow in big shrubs, with thick, hard, leathery leaves. The young leaves are erect and purple (Mondal et al., 2004). On the other hand, C. assamica (Masters), the Assam type, is a small tree with robust branches and thin, glossy leaves (Mondal et al., 2004). The tropical variety of C. assamica is sensitive to drought and cold (Das, Mukhopadhyay, Sarkar, Saha, & Mondal, 2015). Cambodiensis tea, Camellia assamica sub sp. lasiocalyx (Planch. MS), is a small, fastigiated tree with many upright branches and erect, glossy, light green leaves whose petioles turn pinkish red upon maturity (Mondal et al., 2004).
Ecophysiology of Tea
Tea plants grow in a wide range of latitudes, from South Africa (30°S), Russia (45°N), and New Guinea (150°E) to Argentina (60°W) (Caffin, D’Arcy, Yao, & Rintoul, 2004). The adequate annual rainfall for successful cultivation of tea is about 1,200 mm. The average optimum relative humidity of the ambience ought to be more than 70%–75%, and it is essential throughout the vegetative phase because lower relative humidity of the atmosphere has a depressing influence on shoot development (Lemmesa, 1996). Alternatively, the critical value of atmospheric pressure below which shoot growth is subdued is 2.3 kPa [i.e., relative humidity of 28% at 25°C and 45% at 30°C (Willson, 1999)]. High-quality tea plants, mainly required for the orthodox type of made tea, i.e., the final product, are cultivated at higher elevations (4,900 ft), whereas in the equatorial regions, tea plants grow profusely at between 1.000 and 3,000 m altitude. The optimum altitude drops as the distance from the equator increases and tea plants, growing closer to sea level, are also a reality (Willson, 1999).
Growth of tea is temperature-dependent, and the bushes do not grow when temperatures are either too low or too high, regardless of other climatic factors (Lemmesa, 1996). Tea plants can survive a wide range of temperatures; starting from zone 8 (the hardiness zone, as defined by the U.S. Department of Agriculture), where temperature dips as low as ‒12°C, to warmer areas of the subtropics, the bushes grow vigorously under sufficient rainfall and preferable soil pH (Jim & Cave, 2003). Solar radiation is another significant factor. The amount of radiation reaching a tea bush at high altitudes in equatorial regions can be up to 600 Wm–2. The Assam type of tea benefits more from the availability of shade than does the China type, which is attributable to their differences in leaf angle (Willson, 1999). Excessive wind has a negative influence on tea plants. Wind breaks, therefore, are essential to prevent the high evapotranspiration and water stress, which can occur in unprotected tea plants (Lemmesa, 1996).
Tea plants thrive best on well-drained, permeable, and fertile soils at a minimum of 2 m in depth (Lemmesa, 1996). Tea soils should have a high capacity of water retention, but water should not remain stagnant (Willson, 1999). Soil temperature is also related to growth, and the most favorable range is 19°C to 22°C but beyond that range, a linear relationship is maintained. Tea roots have been noted growing up to 15 m deep under ideal soil (Willson, 1999). Lack of soil moisture decreases the growth of the branches, and as a result, the leaves become very hard and tend to produce a number of sterile buds, which reduces the yield and quality (Lemmesa, 1996).
The prerequisite for higher productivity is being planted correctly. Correctly planted tea plants establish themselves in the field swiftly and grow vigorously. Otherwise, improper planting can cause a high degree of mortality or growth retardation. Prior to planting, they should be hardened in the nursery by stepwise exposure to full daylight. Planting should be carried out with sufficient irrigation. Only 9- to 12-month-old healthy plants attaining 40–60 cm height with a minimum of 12 physiologically mature leaves of 0.5-cm thickness should be selected.
Pit and trench are the two major types of planting. In pit planting, individual pits of 45 cm deep and 45 cm wide are excavated. The excavated soil is then fortified with 150–200g of well-decomposed oil cakes or 4–5 kg of well-rotten cattle manure, along with 30 g of rock phosphate and 30g single super phosphate. In trench planting, trenches of 30 cm wide and 45 cm deep are dug along the rows. The excavated soil is then treated in the same way as in pit planting. About 14,000–16,000 (up to 17,000 in hilly areas) plants per hectare comprise the ideal bush population, with spacing of 105–110 cm between rows and 60–75 cm between plants. The planting can be done either as single or double hedges.
Shade is an absolute requirement of tea plantation. The reduction of heat and excessive light radiation is the main benefit of shading. Above 30°C–32°C of ambient temperature during active growing periods demands moderate shade for tea cultivation, which is provided by planting leguminous trees. This medium level of shade has been found beneficial for productivity of tea bushes. Bushes with broad leaves benefit more from shade than thin-leafed bushes. In addition, shade trees added organic matter to the soil by shading their leaves, preserving soil moisture during the dry winter months, and reducing red spider infection. Principally, shade trees are of two types, permanent and temporary. Permanent shade trees are planted for longer duration (more than 40 years). At the initial stage of plantation, both temporary (Gliricidia, Indigofera, etc.) and permanent shade trees are installed to protect the tea plants from direct light. As soon as the permanent shade trees become established, the temporary shade trees are removed.
Another advantage of shade trees is the continuous addition of organic matter to the soil. In East Africa, the total annual leaf fall from Grevillea robusta shade trees and tea bushes is 6.5 tons of dry matter per acre (Goodchild & Foster-Bartham, 1958), as it is well known that plants in the Leguminosae family fix atmospheric nitrogen. Nevertheless, the nitrogen-fixing capacity of the shade trees is not convincingly established. However, in the presence of large quantities of supplementary nitrogenous fertilizers in tea gardens, nitrogen fixation by shade trees is greatly diminished (Visser, 1961).
In order to prevent damage from epidemic pests and diseases, it is desirable to use at least four species in combination. The ideal mixture is called intimate, where no two trees of a single species are adjacent to each other. Some important permanent shade trees are Albizzia, Derris, Dalbergia, Acacia, Gravillea, and Leucaena (Deka et al., 2006).
Since tea is a perennial crop, regular replenishment of soil nutrients is of the utmost importance. Fertilizers are applied only when the bushes are ready to exploit them. Normally, the best time for fertilizer application is after the first spell of rain, which moistens the soil up to a depth of 45 cm, and if there is some new growth in unpruned plants. In case of pruned and skiffed teas, fertilizers should be applied after the emergence of two new leaves. In mature teas, after complete coverage of the ground, fertilizers are applied uniformly as broadcast. Hence, weed-free, clean ground is desirable at the time of manuring. Nitrogen and potassium fertilizers are recommended in two splits if the dose exceeds 100 kg/ha. This will ensure adequate supply of nutrients throughout the growing period and higher utilization efficiency. The first split consists of 60% of the total nitrogen and potassium being applied, and the remaining 40% is applied in the second split. The entire quantity of phosphate fertilizer may be applied in the beginning of the season, along with the first split of nitrogen and potassium. In very light-textured soil, the number of splits may be increased. Soil application of sulfur at 20–45 kg/ha improves the yield and quality of tea if the soil test indicates values lower than the critical limit (40 ppm of available sulfur). It can be applied to well-drained tea fields as broadcast along with the nitrogen, phosphorus and potassium, fertilizers in spring (Deka et al., 2006).
Among micronutrients, zinc offers a consistent response and is suggested for unpruned or skiffed bushes. Foliar application of ZnSO4 (zinc sulfate) (1%–2% concentrations; w/v) improve yield by an average of 10%. Indiscriminate ZnSO4 application may lead to high residues in made tea. Foliar nutrition is beneficial under stress conditions and in low-potash soil; NPK mixture (2-1-2 or 2-1-3) at 0.5%–1% can be sprayed. Foliar application of manganese, boron, and molybdenum has shown varied responses. Foliar sprays of potash at 1% Muriate of potash (MOP, w/v) during moisture stress period can enhance the water use efficiency. MOP @ (1%) and MgSO4 (magnesium sulfate; 1%) application alleviate the harmful effects of moisture stress (Deka et al., 2006). The high-yielding clonal varieties require a generous supply of fertilizer. The nutrient uptake from the soil of 4 tons of harvested green leaves is up to 45–60 kg of nitrogen, 4–7 kg of phosphorus, 20–30 kg of potassium, and 4 kg of calcium per hectare.
Irrigation is compulsory in areas where the distribution of rainfall is uneven. Throughout the world, there are areas where rainfall is scant during some times of year; the quantity may vary from 5% to 10% of the annual rainfall. In these phases, average evaporation exceeds average precipitation, and as a result, conservation and supplementation of soil moisture by irrigation becomes necessary to mitigate moisture stress. For tea, estate sprinklers and drip irrigation systems are widely used.
Tea plants are vulnerable to stagnant water and cannot thrive in areas where waterlogging is an unending attribute. This can be rectified with the development of sufficient drainage systems. Removal of excess moisture near the root zone within a precise interval is the primary objective of drainage, so the field capacity of the soil is retained without any impairment to the bushes. Swift removal of water without any soil erosion from the catchments forms the foundation of a sound drainage plan. Improvement of suitable outlets can enhance the rate of outflow, and in so doing decrease water logging. Drainage planning cannot succeed without proper outlets, and therefore an efficient drainage layout requires development of outlets for lowering the water table below the rhizosphere. A contour drainage system is the prime requirement for hilly land, but for flat surfaces, a rectangular drainage system is recommended. On sites where major drainage work and leveling is required, ground work should begin one year before the anticipated period of transplant. For certain areas under tea cultivation, rainfall may surpass the probable evapotranspiration for some time, and successful conservation of moisture is essential to prevail over the recurrent drought stress periods during the growing season. Importantly, it has been found that proper drainage systems can increase the yield around 30%–35% (Deka et al., 2006).
Pests and Disease
The tea-growing environment is amenable to a large number of pests and diseases, but adequate and timely measures reduce crop loss from this. The perception of pest control by pesticide application has undergone comprehensive changes over time, with ever-growing alarm over pesticide residues (which may render tea inappropriate for consumption) and the rising costs of the new-generation pesticides. Therefore, the concept of integrated pest management (IPM) has prevailed, which includes monitoring pests for early detection, introduction of predators, manual control, use of biological pesticides, and discretion on the choice of pesticides to be used on tea bushes.
The major tea pests are listed in Table 1.
Table 1. Major Tea Pests
Bunch caterpillar (Andraca bipunctata), looper caterpillar (Buzura suppresseria), red slug caterpillar (Eterusia magnifica), flush worm (Lespeyrasia leucostoma)
Helopeltis theivora (tea mosquito bug), jassid (Empoasca flavescence), thrips (Scirtothrips dorsalis)
Purple mite (Calacarus carinatus), pink mite (Acaphylla theae), scarlet mite(Brevipalpus phoenicis),red spider mite(Oligonychus coffeae)
Tea bushes often suffer from various types of diseases as well. These diseases often result in huge crop losses throughout the world. One of the major menaces is blister blight, which is caused by the Exobasidium vexans pathogen. This fungal disease occurs during rainy season and disseminates by airborne spores. Generally, succulent leaves recovering from pruning are susceptible to blister blight infection. When the environment is foggy and moist, the fungus attacks young succulent leaves on all tea plants. Within 10–21 days after infection, blister blight makes its appearance.
Another disease that the tea plant is vulnerable to is red rust, caused by the Cephaleuros parasiticus algae, Red rust causes severe damage, especially to the young stem and ultimately to the leaves. Tissues of the stem, in patches, are killed and dieback is frequently observed. Poor drainage, improper shade, and low potash content in soil are other prime causes of this disease.
In addition, black rot is an important disease caused by two fungi, Corticiumtheae and Corticiuminvisum. The maintenance leaves are infested, which causes continuous decline in the health of bushes and subsequent crop loss. During the winter, fungi hide in sclerotial form in the cracks and crevices of stems, endure during this adverse weather, and upon the return of favorable conditions, they germinate and reinfect. Improvement of drainage, shorter pruning cycles, and alkaline wash are the cultural remedies. Fungicides are also very effective against black rot, as well as application of Bacillus subtilis.
Young buds are affected by anthracnose, which is caused by Colletotrichumtheae-sinensis. Trunk and branch cankers appear when the bushes are infected by Pseudomonas. Armillaria, Ganoderma, and Hypoxylon are the causal organisms behind root rot.
Tea bushes suffer from root diseases too; the most widespread primary root diseases are charcoal stump rot, brown root rot, and red root rot. These diseases spread through direct contact, from pre-existing diseased plant parts, or from airborne fungal spores. Since the roots of tea plants remain in close proximity, whenever a dead plant is noticed, proper cultural practices are adopted to keep its neighbours disease free. Violet root rot, which is caused by Spherostilbe repens due to attacks of Diplodia sp. on weak plants, is the important secondary root disease of tea (Deka et al., 2006).
Different varieties of annual and perennial weeds infest tea plantations. Tropical climates with plentiful sunshine, heat, and moisture pave the way for flourishing growth of weeds, which is again compounded by manure broadcast on the ground. Afterward, these weeds compete with plants for space, water, and nutrients, and their vigorous growth shades the tea plants, especially when tea plants are young and thus comparatively smaller in height. The dominant weed flora that affects optimum growth of young teas are Mimosa pudica, Borreriahispida, Cynodondactylon, Sidaacuta, and Paspalum sp. In mature tea, Mikaniamicrantha, Paspalum, Axonopus, Lantana, and Mimosa, among others, dominate (Deka & Barua, 2015). Weeds can increase the humidity around the bushes, creating conditions favorable for diseases; and they also can hinder the tea harvest (Hasselo & Sandanam, 1965).
Different practices are implemented for weed management, which includes manual, chemical, cultural, and biological methods. Integrated weed management is important, which means the utilization of available expertise on weeds and their management to accomplish successful and environment-friendly weed control.
After plucking, pruning is one of the most fundamental operations that play a vital role in the productivity of tea bushes. Although it incurs some percentage of crop loss, it is carried out cyclically to provide stimuli for vegetative growth, to rectify bush architecture, and to maintain the perfect frame height of the bushes for efficient plucking. Alternatively, if pruning is delayed, the size and weight of growing shoots on the plucking surface are reduced. In addition, there is a predominance of pests and diseases, bush hygiene deteriorates, sterile shoots on the plucking surface become prevalent, and buds fail to grow due to loss of vitality of the growing points. Hence, pruning is necessary to stimulate the production of growing shoots from stored energy and to maintain vegetative growth. The time period from one light-pruning year to another is called a pruning cycle. In such a cycle, all types of pruning and skiffings are carried out serially for renewal of wood, which ultimately produces high-quality crops. In order to keep well-, balanced yield throughout the year, scientifically programmed, extended pruning cycles are adopted consisting of 33% pruned, 33% skiffed (light cut), and 33% untouched or leveled-off skiffed areas.
The choice of pruning cycles depends on several factors, such as yield, capacity of the bushes, availability of labor, desired liquor quality, aroma and flavor of the made tea and market requirements, pests and diseases, climate and soil, bush height, age, and the vigor and kind of tea. In the plains under normal growing conditions, 3–4-year pruning cycles are implemented, and at higher altitudes, 4–5-year pruning cycles are followed (Deka et al., 2006). Based on the magnitude of reduction of height, there are a few types of pruning, discussed next.
Tea bushes are usually pruned 4–5 cm above the last pruning mark every 3 or 4 years, which is called light pruning (LP). It helps in wood renewal, crop distribution, pests and diseases reduction and maintenance of ideal frame height.
Height Reduction Pruning, Medium Pruning, and Heavy Pruning
As tea bushes grow in height, plucking becomes difficult, so then they are trimmed to an optimal height. If the pruning is done 60–70 cm above the ground, it is called height reduction pruning (HRP), and 45–60 cm above ground level, it is known as medium pruning (MP). Both types of pruning help in rejuvenating the tea bushes and rectifying the frame. Heavy pruning (HP) refers to cutting 15–45 cm above ground level for complete frame renewal. In another variation, collar pruning, the entire above ground parts of the tea bushes are removed, while considering the strength of the root system to survive the shock and commence regrowth. However, it is rarely done, as it can cause heavy mortality.
Between two consecutive LP years, tea bushes undergo lighter cutting, which is known as skiffing. Skiffing comes in diverse types such as deep skiffing (DS), medium skiffing (MS), light skiffing (LS), level of skiffing (LOS), and untouched (UT) skiffing. In DS, tea bushes are trimmed 12–15 cm above the last LP mark, which helps to reduce the ill effects of stresses, excessive creep, and the height of the plucking table because if the plucking table goes up it may become unmanageable to maintain the required productivity. In MS, the last DS cut mark is usually taken as the benchmark and then a cut is made5 cm above that, and thus sterile shoot formation is reduced. If a 1-cm cut is made above the previous tipping height, that is LS, and a leveling of the plucking table (4–6 cm above the tipping mark) is LOS.
Universally, pruning is carried out at a time when the bushes are dormant. For LP, if there is inadequate foliage, the tea bushes are not plucked three weeks before pruning, whereas bushes meant for MP should be rested 5–8 weeks prior to pruning. For MP and HP, additional doses of potash and phosphate are applied on the pruning year. After pruning, bushes are drenched with an aqueous solution of copper fungicide to reduce infestation of Poria, Aglaospora, and other pests. Occasionally, de-mossing and lime-caustic washing are also required. After pruning, knife cleaning is started to remove knots of dead, diseased, and unproductive wood, and this is completed before bud-break. In MP, HRP, or HP, the bushes are covered with the pruning litters immediately after the prune to minimize sunburn (Deka et al., 2006).
Young shoots, consisting of two leaves and a terminal bud, are the harvestable part of the tea plant, although in certain cases, more mature leaves are plucked. Plucking stimulates the growth of the immediate dormant apical buds, which start emitting shoots consisting of normal leaves and another type of appendage known as fish leaves. Appendages carry axil buds, which start growing as vigorous shoots. The time between two successive harvests is known as the plucking round, which may extend from 4 to 14 days, but to keep a balance between yield, quality, and bush health, usually a plucking round of 6–8 days is considered optimum for the growth rate and the desired quality.
Until the middle of the 19th century, tea was propagated from seeds from various sources. The seed-bearing plants were selected for yield, quality, and other desirable properties. However, seedling plantations, in general, had variable yields and quality. This bottleneck led to the utilization of selected elite plants and vegetative multiplication by single-node cutting, the products of which were released as clones, and after prolonged life cycles, they had lower productivity due to susceptibility to diseases.
Conventionally, tea is propagated through seeds that are collected from seed orchards. Tea seeds are recalcitrant, with short storage viability. Entirely mature healthy seeds, either from the mother plants or ground (recently dehisced), are collected. The seeds are soaked in water, and the healthy undamaged sinker ones are selected for propagation. Floater seeds are disposed of, as they have dried cotyledons due to punctures made by Poecilocoris latus (the tea seed bug), and hence fail to germinate. Seed propagation is primarily carried out in nursery beds (Mondal, 2014).
Wherever possible, the nursery site should be on virgin soil consisting of sandy loam with manure. But, in dearth of sufficient land, seedlings are raised in polyethylene sleeves with soil properly tested for acidity and eelworms. Germination of the seeds is done in a sand bed. Pregerminated healthy seeds are then planted in poly-sleeves that are kept in a permanent nursery in the shade. After 5–6 months, vigorous seedlings can be planted in the field in April and May.
The first attempt for vegetative propagation of tea was done in Indonesia by budding as well as grafting. However, due to sluggishness, neither of them could serve the purpose. Faster propagation by single-leaf internode cutting was developed simultaneously in India, Sri Lanka, and Indonesia (Mondal, 2011). This was further modified to meet the commercial demand. Cuttings from semi-hard wood are usually taken from current-year growth. Upon fungicide application and rooting-hormone treatment, the cuttings are kept in the nursery for 45–60 days, depending on geographical location and the planting material. The successfully rooted cuttings are then transferred to polyethylene sleeves filled with good virgin soil (pH around 4.5), with adequate water-holding capacity, and kept for another 8–12 months in the nursery, when they become ready for field transfer. Cuttings with a single leaf and internode are made from the primary shoot with dormant apical buds. The unbranched portions of the primaries, where lateral shoots are produced from the lower portion of the primaries quite early, are preferably taken. An ideal cutting should have a fully mature undamaged healthy leaf with a dormant or just-swelling bud on a hardwood green. A slanting basal cut parallel to the slope of the leaf is made for easy handling and quick propagation of cuttings.
Processing: An Overview
The quality of made tea varies due to climate, cultivar, and the level of oxidation due to varying degree of leaf massacre during processing. Processing of tea is complex, with multiple steps to each phase that finally determine the type of tea.
Tea manufacturing, in general, introduces disruption of the cell walls of tea shoots meant for mixing of substrates, polyphenols, and enzymes. This initiates a cascade of biochemical reactions in the presence of ambient oxygen, which ultimately formed pigmented compounds. The various steps are (a) withering (basically reduction of water content of plucked leaves), (b) rolling (physical breakage of leaves), (c) fermentation (biochemical reactions in the presence of a moist air supply), (d) drying (reduction of moisture content to a stipulated level), (e) sorting (fiber removal and size-wise grading).
Withering is physical and biochemical changes made for the synthesis of quality-related compounds by reducing the turgor, weight, and volume of the leaves. It is done either by thin spreading of the leaves on the floor or by sparing the leaves in troughs for 8–18 hours, depending upon the condition of the leaves. It increases cell membrane permeability, which affects the mixing of substrates and enzymes during fermentation. This process activates oxidative, hydrolytic, and proteolytic enzymes, causing increased levels of soluble protein, free amino acids, caffeine, and simple sugars. It also enhances the level of organic acids, followed by pH reduction and improvement of polyphenol oxidase (PPO) activity that, in turn, improve theaflavin (TF) formation during catechin oxidation. This process acts on volatile compounds that impart aromas. Volatile compounds belong to two groups: (a) compounds that are deleterious to quality and (b) compounds that impart a sweet flowery aroma. The ratio between the latter and the former is known as the flavor index, a parameter that normally represents better aromas (Mauskar, 2007).
The aim of the rolling process is to macerate the tea leaf so that the enzymes and their substrates are thoroughly mixed, leading to the exposure of crushed surfaces to air. Rotorvane and crushing, tearing, and curling (CTC) machines achieve this mechanically. The rolling operation breaks the cellwall to release the enzymes. The rotorvane machine consists of a central rotor with a number of knives that bruise and cut the leaves.
The disrupted leaves next enter the CTC rollers. The rolling is done either by orthodox rollers or by a combination of roller and CTC or by the CTC machines. The surface of each roller is made of a number of stainless steel segments. The withered leaves pass through a battery of three to five such rollers, which cause them to be crushed, torn, and curled.
Fermentation refers to the oxidation of cellular substrates into compound biochemical products by the activity of endogenous enzymes. The process initiates right from the inception of cell maceration and continues until the enzyme proteins are denatured by elevated temperature in the dryer. The subsequent chemical reactions are displayed in Figure 1.
Made tea usually contains 0.3%–2.0% of TF (dry weight), which is orange red in color and has great involvement in astringency, briskness, brightness, and the color of liquor, whereas TR consists of 9%–19% of made tea (dry weight), is red brown in color and impart the color and strength of liquor. Throughout the process of fermentation, TF content of oxidized leaf keeps increasing and after reaching a peak, it starts declining, whereas TR content continually increases right through oxidation. The visual indication of the end of fermentation is the development of a coppery color and a pleasant aroma. There are some limiting factors upon which the rate of oxidation chiefly depends, such as concentration of substrates, accessibility of crushed leaf to oxygen, enzyme activity, and the pH and temperature of the bed. As it is not a uniform system, intimate contact between the enzyme and substrate is important. During fermentation, polyphenols form an insoluble complex with the enzyme by feedback reaction, and consequently, the temperature of the leaf increases. TF production depends on the concentration of oxygen and ambient temperature. However, fermentation at lower temperatures with longer durations is advantageous because the highest TF production can be sustained over a period of time (Mauskar, 2007).
The fermentation process persists at a certain rate until the temperature of the leaves is reduced to around 55°C, and thus unoxidized catechin continues to be converted to TF until PPO is totally denatured. Drying stops the biochemical reactions by denaturing the enzymes involved, decreases the percentage of volatile compounds, reduces the moisture content from 45%–50% to 3%, and boosts some biochemical reactions related to black tea aroma and liquor quality. The desired black color is brought about by the conversion of chlorophyll to pheophytin. Some of the flavor attributes of made tea are evolved as the fermented leaf is heated during drying. Preferably, drying is carried out in a fluid bed dryer at 125°C for 20 minutes.
Sorting is a practice where dried and cooled bulk tea is divided into various grades by their size. This separation can be accomplished by means of different sorting equipment, or sometimes it is done manually. The size of the grades is not directly related to liquor quality as such, but it does affect the color and strength of the final product. As a consensus, “leaf” grades are the largest; “broken” grades are sequentially smaller, and the smallest pieces are known as “dust.”
Economic Importance of Tea
Since tea is one of the most important plantation crops, with a financially viable lifespan of not less than 60 years, the Indian tea industry has a market of around $40.7 billion. Interestingly, the majority of tea produced in India is consumed domestically. For example, of the 1.2 million tons produced during 2013, around 80% was consumed domestically, indicating the great domestic demand for this product. Nevertheless, it also is a major foreign currency earning commodity for most tea-producing countries. That gives the tea industry a noteworthy place in the country’s economy as an earner of foreign currency.
In addition, tea planting provides lucrative employment to a large number of both poorly educated people and well-educated ones. It is labor intensive and offers employment opportunities to both female and male workers. Tea plantation offers work to underprivileged people living in the undeveloped regions and creates employment opportunity per household to these people. Apart from that, the tea industry supports the growth of several ancillary industries. It is one of the major stakeholders in the jute and plywood industries, as tea is generally packed in either jute bags or plywood chests for transportation. A huge amount of coal or gas is required for manufacturing made tea, which helps the coal and petroleum industries. Additionally, it is a major user of fertilizer, pesticides, and weedicides.
Ecotourism is another important aspect of preserving and sustaining the diversity of the natural and cultural environments and it has ushered in a novel additional method of income for tea estate inhabitants. Visitors to the estates are put up in a way that is minimally intrusive and sustains the native cultures and local products, and thus tea growers can expose their products to a large audience. Moreover, the revenue generated by ecotourism encourages local bodies to fund developmental projects to preserve the flora, fauna, and cultural heritage of their area.
The Earth is currently facing some critical problems, such as unsustainable use of resources, rapid decline in freshwater reserves, hasty exhaustion of conventional energy sources, carbon emission beyond the planet’s absorption capacity, and degradation of soil beyond reclamation. All of this ultimately has led to global warming, manifested by tsunamis, decreases in groundwater levels, melting of icebergs, and other negative effects. Conversely, in accordance with persistent population growth, demand of tea is constantly mounting. Thus, the urgent need for the present is to promote agriculture and food supply in a manner that upholds and improves the fertility of the soil, protects the quality and availability of water, conserves biodiversity, and is produced and consumed in a way that is socially, environmentally and economically sustainable (Munasinghe, 2012).
Tea is produced in almost all of the continents on Earth, and the total area under cultivation is still growing. Hence, in order to make enough land available for tea cultivation, vast areas are being annexed throughout the world, and that obviously entails deforestation, which has numerous negative effects on ecosystems. The most hazardous environmental impact of tea production is the alteration of habitat (Clay, 2003). According to a number of reports, forests are still being uprooted in East Africa to facilitate new plantations (McLennan, 2011). Nevertheless, tea cultivation is not the solitary cause of deforestation. Around 70% of the terrestrial plants and animals live in the forest ecosystem, so massive alteration of natural habitats forces the flora and the fauna of that area to struggle to live, and this can culminate in loss of biodiversity if the species do not survive. Simultaneously, trees also play a key role in stopping global warming, as they absorb greenhouse gases. Thus, the felling of trees en masse for tea planting has environmental implications. Furthermore, with the elimination of trees, accumulation of leaf litter on the soil surface shrinks, resulting in the concurrent reduction of organic matter content in the soil and the subsequent hampering of the soil’s water-holding capacity. During heavy downpours, soil is subjected to erosion, followed by siltation of riverbeds and adjacent drains, which ultimately threatens irrigation schemes.
Alternatively, in order to achieve higher productivity, estates are kept weed free by weeding and applying scrapers. Manual weeding using scrapers resulted in severe soil loss of about 30 cm of topsoil per hectare by erosion in Sri Lanka (Ekanayake, 1994). This loss translates into average soil erosion of 40 metric tonnes/ha/yr (Krishnarajah, 1985).
Thus, apart from soil erosion, the water table is lowered, with consequent harmful effects on the moisture status of the surface layers of soil, compelling the tea plants to literally withstand lingering periods of drought. Alternatively, decades of studies formulated the application of inorganic fertilizers for gearing up productivity. However, due to the poor organic status of the soil and deterioration of water-holding capacity, infiltration is retarded, with consequential runoff problems. The problem has been aggravated by the continual broadcast of ammonium sulfate, which led to a lowering of pH. The recurring use of nitrogenous fertilizers causes the release of other elements from bound sites and leaching out into rivers and other water bodies, rendering them absolutely irreconcilable with biodiversity.
Decreasing soil pH, continual use of ammonium sulfate, and frequent application of weedicides have taken a toll on earthworms, which keep the soil in fine tilth. Hence, the tilth and fertility conditions of tea estate soil have deteriorated over the years due to the disappearance of these beneficial annelids.
In some tea-growing countries, profuse and erroneous application of insecticides has disturbed the environment. As tea plantations are basically a monoculture, they offer the perfect conditions for a number of pests. In order to keep productivity to the desired level, a number of lethal, detrimental pesticides are extensively used. However, erroneous application of these substances obliterates beneficial predators and destroys biodiversity as well. Insecticides are dispersed by fossil fuel–driven power sprayers, which produces mist that is harmful to the local environment, especially human beings. Further, if not applied well ahead of plucking, pesticides with higher half-lives may produce residual effects on liquor that can put human life in jeopardy. Alternatively, though broad-spectrum insecticides provide many benefits by means of good control, higher yields, and handsome profits, they also have raised grave concerns, such as growth of resistance against pesticides, the reappearance of pests, an epidemic of secondary pests, detrimental effects to the environment, and unwanted residual effects in the liquor (Gurusubramanian et al., 2008).
After fermentation, the partially processed leaves are allowed to dry under controlled conditions until a particular level of moisture is attained. This step is carried out in dryers run by gas or diesel fuel. However, in some cases, firewood is also used for heating. Bear in mind that fossil fuel is a source of greenhouse gas, and the aftermath can be dreadful.
Another aspect of the tea industry that is deleterious to the environment is its waste. Most tea estates in the developing countries do not possess effluent treatment plants, so after every manufacture of tea, the contaminated water is thrown out. The effluents are rich in biological oxygen demand, chemical oxygen demand, lignin, and theaflavins, among other waste, and all of them are hazardous to the environment.
Escalation in tea production to meet the ever-rising need to develop high-yielding clones with better quality and enhanced stress tolerance has occurred; hence, the use of deoxyribonucleic acid (DNA)–based techniques for molecular characterization of clones is valuable. Conventional tea breeding, though well established for the varietal improvement of tea, still has some bottlenecks, such as long gestation periods, self-incompatibility, nonavailability of mutants, low success of hand pollination, and long intervals for seed maturation, as well as complex life cycles and outbreeding nature (Mondal, 2009). Yet, constant impoverishment of the tea gene pool is a significant problem, and thus biotechnology is necessary to develop better-quality clones and to divulge the unexplored genetic variations because the categorization of molecular markers coupled to major quantitative trait loci (QTLs) for the desired traits could be exploited for standardization of elite clones (Mondal et al., 2005; Mukhopadhyay et al., 2016).
Agrobacterium-mediated gene transformation has been widely used for the production of transgenic plants (Mondal et al., 2001a, 2001b, 2001c), as it is inexpensive and easier than other gene transfer processes. Simultaneously, it permits the modest reorganization of transgenes and well-organized integration into the genome (Mukhopadhyay et al., 2016). Successful transformation reflects the nonappearance of bactericidal effects on leaf polyphenols, proper expression of Agrobacterium virgenes, and persistence of growth potential of the transformed plants for regeneration. However, frequently used antioxidants or polyphenol adsorbents could not efficiently ameliorate the bactericidal effect of tea polyphenols and the inhibited regeneration process (Mukhopadhyay et al., 2013c). Even so, despite several attempts, production of the transgenic tea plant remains elusive.
The narrow genetic base of tea cultivars is a hindrance to improving productivity due to rapid vulnerability of genetically uniform cultivars. Hence, studying the genetic diversity of the newly improved clones under exploitation is necessary to avoid narrowing the genetic pool (Borthakur et al., 1995; Leonida et al., 2013). Based on detection, molecular markers are fundamentally categorized as hybridization-based, DNA sequence–based, and polymerase chain reaction (PCR)–based. Furthermore, germplasm characterization will assist in varietal improvement of deserved agronomical significance, avoidance of any unwarranted entry in the gene pool, proficient selection, and taxonomic classification of tea based on molecular markers.
In tea, randomly amplified polymorphic DNA (RAPD) was the initial molecular marker that was widely used by several researchers (Mondal, 2000; Mukhopadhyay et al., 2016). RAPD was found in diverse varieties of China, Assam, and Cambod tea and revealed that Korean tea was genetically diversified following introduction from China (Mondal, 2007). Nevertheless, Chinese and Indian teas were found to have closer relationships with Japanese tea. PPO activity was investigated in 15 accessions, and total PPO activity was divided into three groups: high, moderate, and low (Ramkumar et al., 2011). Many other studies have been carried out by employing RAPD analysis. However, due to limitations such as lower extent of polymorphism, interest was diverted to other advance markers, such as Inter Simple Sequence Repeats (ISSR) (Mondal, 2002) and restriction fragment length polymorphism (RFLP) for genetic characterization. ISSR markers demonstrate superior repeatability and constancy of map position, apart from comparing strongly related individuals. It also provides a competent technique for germplasm discrimination (Mukhopadhyay et al., 2013c). ISSR analysis exposes higher genetic polymorphism between tea plants, apart from projecting a sensible and efficient way to distinguish tea germplasm at the interspecific level (Mukhopadhyay et al., 2013c).
Similarly, simple sequence repeats (SSRs), also called microsatellites, are tandem repeat motifs that are recognized for hypervariability, PCR scoring simplicity, and codominance, vis-à-vis high reproducibility. SSR has been found to be a prevailing genetic marker for tea genetic linkage studies, along with diversity analysis (Mukhopadhyay et al., 2016) and marker-assisted selection. Alternatively, amplified fragment length polymorphism (AFLP), a persistent DNA marker, detects polymorphisms better than RFLPs or RAPDs and offers meticulous genetic studies in closely interrelated populations (Mondal, 2014). AFLP, in a single analysis, produces multilocus fingerprints, appreciably decreasing the expenditure and increasing the opportunity of discovering polymorphisms (Vos et al., 1995). Sequence-tagged microsatellite site (STMS) markers can recognize genetic variability between Camellia genotypes (Mukhopadhyay et al., 2016). Single nucleotide polymorphism (SNP) of the coding region of PPOs from various genotypes of tea has been identified, which ushers in a remarkable scope for applying SNP markers pertaining to beverage quality (Mukhopadhyay et al., 2016).
Tea has a diploid genome size of around 4.0 Gb, with a basic chromosome number of n = 15 (Mukhopadhyay et al., 2013c). Due to the swift increase in DNA sequence information, tea research started focusing on functional genomics that classify the function and expression of genes in spatial, sequential, cell-dependent, and regulatory mechanisms. Upon completion of genome sequencing, putative functions of identified genes can be inferred by comparing the protein sequence of unknown genes with known genes. Expressed sequence tag analysis is a novel technique to understand expressed sequences in the genome; thus, it facilitates the identification of several genes that are accountable for many important traits. Additionally, express sequence tag are excellent resource for gene expression analysis using microarrays (Mukhopadhyay et al., 2016) and are potential candidates for gene tagging. In tea, enormous quantities of EST data have been gathered (Das et al., 2015) and stored in databases.
Based on the revelations of SSRs from EST sequences, functional annotation has been undertaken to identify the putative functions of the generated ESTs and to realize the vital functional domain markers (FDMs) that are related to the SSR-ESTs for subsequent study of gene ontology. In C. sinensis, fundamentally, a couple of attempts have been made, such as individual gene cloning associated with particular attributes and differential gene expression that ultimately paves the way to identify genes linked to a particular trait (Mondal, 2014). Several biosynthesis-related genes, such as PAL, chalcone synthase, flavanone 3-hydroxylase, and flavonoid 3p, have been cloned. Caffeine synthesis pathway related genes, such as caffeine synthase and 7-N-methyltransferase, have revealed an approach to the development of caffeine-free tea using RNA interference (RNAi) technology (Mukhopadhyay et al., 2016).
Theanine is one of the most important amino acids found in tea, which greatly influences the taste of the liquor. As a result, theanine biosynthesis manipulation would be valuable for improvement of green tea liquor quality. Cloning of glutamine synthase genes, belonging to the theanine biosynthesis pathway, has been successfully achieved (Mukhopadhyay et al., 2013c). The transcriptome from the poly(A)+ RNA of tea has been investigated by applying the high-throughput Illumina RNA-seq technique, followed by sequence similarity analyses against public databases. Available public databases such as Uniprot, NR, and COGs at NCBI, Pfam, InterPro, and KEGG identified unigenes, which were annotated.
Precise searches applying the annotations recognized plenty of genes that are coupled to several pathways associated with primary metabolism and natural products significant to the quality of tea, such as flavonoid, theanine and caffeine biosynthesis. New candidate genes belonging to the secondary pathways were also discovered (Mukhopadhyay et al., 2016). In order to nullify loss of flavor, insecticides are usually avoided against the tea geometrid (Ectropis oblique), whose larvae cause severe damage to leaves. Nevertheless, metabolic pathway analysis indicated that secondary metabolites of tea plants and the signaling pathways probably play a vital defensive role against this chewing pest (Wang et al., 2016).
A comprehensive survey of transcriptome profiles of tea plants against chilling temperatures revealed the molecular mechanisms of C. sinensis, which is activated while the cold acclimation process is in effect (Mukhopadhyay et al., 2016). The EST markers vis-a-vis EST-SSRs will, in all probability, improve the investigation of key traits and the study of molecular genetics of tea plants (Taniguchi, Fukuoka, & Tanaka, 2012). Proteomics studies, in general, relate the probable modifications of proteins to certain phenotypes by techniques like high-performance liquid chromatography (HPLC), mass spectrometry (MS), sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis, two-dimensional gel electrophoresis, in silico protein modeling, and matrix-assisted laser desorption/ionization (MALDI).
Proteomic analysis of tea pollen revealed proteins related to cold stress. Similarly, a detailed study of albino leaf expressed proteins that critically participate in albinism (Mukhopadhyay et al., 2016). ABA (Abscisic acid) action on tea plants under drought stress is essential, and proteome analysis is a major tool used for this. Proteomic studies identified certain proteins, which upon up-regulation participate in glycolysis, photosystem II, response to drought stress, carbohydrate as well as nitrogen metabolism, counteract reactive oxygen species (ROS), defense, signaling, and metabolism of nucleic acid (Zhou et al., 2014).
Tea is a perennial, nonalcoholic beverage crop that is vital to the world. It is an employment-generating, labor-intensive, ecofriendly, and sustainable agricultural product that brings in income from export for tea-producing countries. It also supports several ancillary industries that further improve people’s livehoods. It also has several medicinal benefits, and it is taken as a morning drink by nearly half of the world’s people. In addition to black tea, several other forms of tea are available today due to its product diversification. Manufacturing processes, though, varied depending upon the type of tea, and this point has been well studied. Horticultural practices such as pruning, plucking, agronomical practices such as manuring are well established.
Although conventional breeding is the backbone for varietal improvement of tea, it is extremely slow. Biotechnological practices area potential alternative, but successes are limited due to several factors, such as poor response in tissue culture and long life cycle. However, the tea industry has a high impact on the environment. While being monoculture, it is hub a particular type of fauna, it is ecologically friendly, that is, not harmful to the environment, absorbs greenhouse gases, and supports tea-tourism. However, it also raises certain concerns, including the huge use of pesticides and irrational use of chemical fertilizers.
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