Introduction

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 is 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 tea’s invasion of 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).

Cultural Practices

Propagation

Processing: An Overview

Fermentation

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.

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Figure 1. Basic chemical reaction steps that occurs during fermentation (oxidation) of black tea.

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).

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 US$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.

Environmental Impacts

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 extreme weather events, rising sea-levels, mega-wildfires, 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.

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).

In addition to 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. To make matters worse, decades of studies encouraged 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 tea liquor that can put human life in jeopardy. 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.

Biotechnology

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. In addition, 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).

Omics Approaches

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).

Conclusion

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 livelihoods. 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.