Physiological and biochemical characteristics associated with tolerance to water stress in tea

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Origin of tea and global spread of tea cultivation

The tea plant (Camellia sinensis (L) O. Kuntze) has been cultivated and used in one form or another in many parts of the world for about 2000 – 5000 years (Kaundun & Matsumoto, 2002; Wright, et al., 2002). However, its origin and earliest home as a wild plant has been a matter of some speculation and debate. The Va-Ye or Bohea mountain of Fu-Kein province of China is considered by some authors to be the place of origin where tea was first discovered (Saha & Gazi, 1994). However, other reports indicate that the cultivated tea originated between China and the Indian sub-continent, on the India/Myanmar border region (Chen et al., 2005; Carr, 2010).
Tea cultivation gradually started to spread to other parts of world following the discovery of an Assam type of tea (Bezbaruah, 1975). At present, tea is grown in 52 countries across the world, mainly in South-East Asia and Eastern Africa (De Costa et al., 2007). Most tea producing areas experience Mediterranean or hot, humid climates in the tropics (Carr, 2010).
The tea plant therefore thrives in these areas because they receive adequate amounts of precipitation (≥1000 mm per year) and favourable temperatures (from ≥12.5 to 30°C) for optimal growth and productivity. These conditions are similar to the warm, wet summer and cool, dry winter conditions experienced in areas where tea had originated (De Costa et al., 2007), notwithstanding the micro-climatic variations within and among regions (Kingdon- Ward, 1950; Netto, et al., 2010). In Africa, tea was first introduced in Morocco in 1854 by the British. However, the first commercial cultivation of tea in Africa started in Malawi in 1878 (Anonymous, 2008), after which cultivation later spread to Kenya, Uganda and northern Tanzania in the 1930s (Palmer, 1985). The plantations in Malawi were established from seed imported from India and China (Carr, 2010).

Tea classification and taxonomy

The cultivated tea belongs to the family Theaceae, and genus Camellia (Chen et al., 2005; Sabhapondit et al., 2012). The genus Camellia comprises over 80 taxa but only C. sinensis (L) O. Kuntze is commercially used to produce beverage tea (Latip et al., 2010). Several ecotypes have evolved over time. Tea classification has been revised several times by various researchers. Wight (1962) revised the genus Camellia, based on differences in reproductive structures and assigned specific status to var. sinensis, var. Assamica, and the Southern or Cambod form of Camellia assamica (Masters) Wight, which was classified as Camellia assamica ssp. lasiocalyx (Planchon ex. Watt) (Kaundun et al., 2000; Premkumar et al., 2008).
All commercially-grown teas are thought to be hybrids of the Assam-type (also known as ‘Assam-jat’) and China-type (China-jat) (Wight, 1959; Anesini et al., 2008). Despite several taxonomic revisions of the genus Camellia, all teas are generally grouped under Camellia sinensis, regardless of their taxonomic differences. However, some tea varieties are still described as of Chinary (Camellia sinensis), Assam (Camellia assamica) or Cambod (Camellia assamica ssp. Lasiocalyx) species, based on various morphological and biochemical parameters (Kaundun et al., 2000; Kaundun & Park, 2002).
The Chinary-type (var. sinensis) is characterized by small (3-6 cm long), relatively erect, dark-green leaves and it only grows into a shrub of up to two metres high and one metre in diameter (Hadfield, 1975; De Costa et al., 2007). On the other hand, the Assam-type (var. assamica) has larger (15-20 cm long) and light-green leaves with a glossy surface and may naturally grow into a tree of up to 10 m high and 6 m in diameter (Hadfield, 1975; Takeda, 1994; De Costa et al., 2007). The Assam-type is believed to have originated under the shade of humid, tropical forests, whereas the Chinary-type is thought to have originated under open conditions in the cool, humid tropics (Carr & Stephens, 1992). The performance of these ecotypes in some environments has been related to the conditions that prevailed in areas of their origin.

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Economic importance of tea

Tea plays a very important role in the economies of many producing countries in Africa and Asia (Kaundun & Park, 2002; Zhao et al., 2008). This is because tea exports are a major source of foreign exchange for most tea producing countries. In addition, tea cultivation creates employment opportunities for the population living around the tea growing areas (Damayanthi et al., 2010). Tea has gained further popularity because of the potential health benefits associated with drinking tea (Mondal et al., 2004; Bharadwaz & Bhattacharjee, 2012). The relative economic importance of tea varies between producing countries, depending on the size of the tea industry, as well as other drivers of the economies of individual countries.
In Malawi, tea is the third most important export cash crop that contributes about 9%to the country’s foreign exchange earnings (Anonymous, 2008). The Malawi tea industry covers about 20,000 ha planted to both seedling and improved vegetatively propagated (VP) tea cultivars. The industry is dominated by the commercial estate sector, with smallholder farms constituting only about 14% of the total tea area (Van der Wal, 2008). On a global scale, about 2.7 million ha of cultivatable land is planted to tea (Mondal et al., 2004).

CHAPTER 1  
GENERAL INTRODUCTION  
1.1 PROBLEM STATEMENT
1.2 AIM OF THE STUDY
CHAPTER 2  
LITERATURE REVIEW  
2.1 ORIGIN OF TEA AND GLOBAL SPREAD OF TEA CULTIVATION
2.2 TEA CLASSIFICATION AND TAXONOMY
2.3 ECONOMIC IMPORTANCE OF TEA
2.4 FACTORS AFFECTING TEA PRODUCTION IN CENTRAL AFRICA (MALAWI AND ZIMBABWE)
2.5 TEA BREEDING AND SELECTION AT THE TRFCA
2.6 TRAITS OF ECONOMIC IMPORTANCE FOR TEAS PRODUCED IN CENTRAL AFRICA
2.7 CONVENTIONAL SELECTION METHODS FOR DESIRABLE TRAITS
2.8 MARKER ASSISTED SELECTION AND BREEDING
2.9 SUMMARY
CHAPTER 3  
IDENTIFICATION OF RAPD MARKERS ASSOCIATED WITH BLACK TEA QUALITY, DROUGHT AND LOW TEMPERATURE TOLERANCE  
3.1 INTRODUCTION
3.2 MATERIALS AND METHODS
3.3 RESULTS
3.4 DISCUSSION
3.5 CONCLUSIONS
REFERENCES
CHAPTER 4  
PHYSIOLOGICAL AND BIOCHEMICAL CHARACTERISTICS ASSOCIATED WITH TOLERANCE TO WATER STRESS IN TEA  
4.1 INTRODUCTION
4.2 MATERIALS AND METHODS
4.3 RESULTS
4.4 DISCUSSION
4.5 CONCLUSIONS
CHAPTER 5  
PHYSIOLOGICAL AND BIOCHEMICAL CHARACTERISTICS ASSOCIATED WITH LOW TEMPERATURE TOLERANCE IN TEA  
5.1 INTRODUCTION
5.2 Materials and methods
5.3 RESULTS
5.4 DISCUSSION
5.5 CONCLUSIONS
CHAPTER 6  
CONCLUDING DISCUSSION AND RECOMMENDATIONS  
6.1 CONCLUDING DISCUSSION
6.2 RECOMMENDATIONS

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