F.N. Wachira1,3, S. Kamunya1, S. Karori2, R. Chalo1 and T. Maritim1
1Tea Research Foundation of Kenya, Kericho, Kenya
2Department of Biochemistry, Egerton University, Egerton, Kenya
3ASARECA, P.O. Box 765, Entebbe, Uganda
Classification in Camellia
The tea plant (Camellia sinensis) from which the beverage tea is processed, is placed in the genus Camellia. The genus has over 200 species and is largely indigenous to the highlands of Tibet, north eastern India and southern China (Sealy, 1958). Sealy (1958) classified the genus into 12 subgeneric sections, one of which (Thea) contains species of cultivated tea. However, in his monograph Sealy recognized a group of 24 inadequately known species which he called ‘Dubiae’ (Dubious). In their work, Chang and Bartholomew (1984) not only translated the 1981 monograph of the genus Camellia by H.T. Chang but also included publication of new taxa and moved many species treated by Sealy to different sections. They divided the genus into four subgenera (sub groups), i.e. Protocamellia, Camellia, Thea and Metacamellia, and twenty sections (Figure 1.1).
FIGURE 1.1 Summarized Schematic Diagram Showing Species Relationships within Genus Camellia.
Taxonomy of the genus Camellia has been complicated by the free hybridization between species, which has led to the formation of many species hybrids (Chuangxing, 1988). Similarly, most species are unavailable to scientists for study. Genetic relationships and taxonomy has therefore remained controversial and recent interest has seen the discovery of many new species and a revision of taxonomic relationships (Chuangxing, 1988; Lu and Yang, 1987; Tien-Lu, 1992). Tea is, however, the most important of all Camellia spp. both commercially and taxonomically. Though the other non-tea Camellia’s are not widely used to produce the brew that goes into the cup that cheers, several species, e.g. C. taliensis, C. grandibractiata, C. kwangsiensis, C. gymnogyna, C. crassicolumna, C. tachangensis, C. ptilophylia, are used as sources of tea-like beverages in parts of China, which indicates that the economic potential for beverage production from additional underutilized species is very great (Tien-Lu 1992; Chang and Bartholomew, 1984). Seed oil from several species including C. fraterna, C. japonica and even C. sinensis are important sources of cooking oil in China. In addition, many Camellia species are of great ornamental value.
At the species level, tea taxonomy failed to attract much attention and interest once the species of economic importance were identified. It continues to be a low-priority area in most tea research programs. The array of hybrids available which might suggest unrestricted introgression of many species of Camellia and tea compound the taxonomic jigsaw. Several minor taxa have been treated as conspecific with major taxa, although more recently accumulated evidence has shown that these minor taxa have no natural distribution and are derived from hybridization events involving different species (Parks et al., 1967; Uemoto et al., 1980). The taxonomic affinities of most interspecific and intraspecific hybrids are unknown, but could provide clues to the evolutionary organization of the tea gene pool. Information on taxonomic characteristics, genetic diversity and biogeography of Camellia in living collections are scantily documented, though vital in identifying sources of desirable genes (Banerjee, 1992).
Tea was initially classified as Thea sinensis by Linnaeus (Linnaeus, 1753). Following the discovery of its economic importance, and the subsequent extensive collection of indigenous teas from the forests contiguous to the upper Assam–Burma–Tibet borders, two distinct taxa were identified and classified by Masters (1844) as Thea sinensis, (the small-leaved China plant) and Thea assamica (the large-leaved Assam plant). For a long time, Thea and Camellia were considered as separate genera (Fujita et al., 1973) and some authors even considered Camellia to be a ‘section’ under the genus Thea (Roberts et al., 1958; Barua and Wight, 1958).
Another group of authors (Sealy, 1958; Barua, 1965) considered that CameIlia and Thea were so much alike in morphological, anatomical and biochemical features that the classification schemes proposed above were unrealistic. According to them, the apparent difference in leaf pose, patina and pigmentation was a part of the total variation in leaf features. Wight (1962) considered Thea to be synonymous with Camellia and the name Camellia prevailed. Thus, today tea is botanically referred to as Camellia sinensis (L.) O. Kuntze, irrespective of species-specific differences. Camellia sinensis is classified under section Thea along with 18 other species (Figure 1.1).
At the species level, several intergrades resulting from unrestricted intercrossing between disparate parents have been documented, but have not been assigned the status of separate species (Sealy, 1958). However, three distinct tea varieties have been identified on the basis of leaf features like size, pose and growth habit. These are the China variety, Camellia sinensis, var. sinensis (L.); the Assam variety, Camellia sinensis var. assamica (Masters) Kitamura; and the southern form also known as the Cambod race, C. assamica ssp. Lasiocalyx (Panchon ex Watt). The three main taxa can be differentiaed by foliar, floral and growth features (Tables 1.3 and 1.4) and by biochemical affinities (Sanderson, 1964; Robert et al., 1958; Hazarika and Mahanta, 1984; Ozawa et al., 1969; Fujita; et al., 1973; Owuor et al., 1987). It is common to find the three different varieties (China, Assam and Cambod) referred to as separate species, namely, Camellia sinensis, C. assamica and C. assamica ssp. Lasiocalyx, respectively (Bezbaruah, 1976). Research has shown that cultivated tea is an out-crosser with an active late-acting pre-zygotic gametophytic self incompatibility (LSI) system (Wachira and Kamunya, 2005a; Muoki et al., 2007). Because of its out-breeding nature and, therefore, high heterogeneity, most cultivated teas exhibit a cline extending from extreme China-like plants to those of Assam origin. Intergrades and putative hybrids between C. assamica and C. sinensis can themselves be arranged in a cline of specificity (Wight, 1962). Indeed because of the extreme hybridizations between the three tea taxa, it is debatable whether archetype (original) C. sinensis, C. assamica or C. assamica ssp. lasiocalyx still exist (Visser, 1969). However, the numerous tea hybrids currently available are still referred to as Assam, Cambod or China depending on their morphological proximity to the main taxa (Banerjee, 1992).
TABLE 1.3 Criteria Used for Differentiating Two Major Tea Varieties and Sub-Varieties of Camellia sinensis
TABLE 1.4 Types of Tea Differentiated on the Basis of Foliar Characteristics
1 = Extreme China
2 = Typical between Assam and China
3 = Typical ...