Citrus
L.
Shrubs or small trees; young branchlets angular with axillary, single spines. Leaves alternate, unifoliolate or simple (as in C. medica); petioles usually Winged. Inflorescences axillary, racemose, or cymose or of a solitary flower. Flowers bisexual or functionally unisexual. Calyx cupular or urceolate with 4 or 5 sepals. Petals 4 - 8, imbricate, fleshy, white or tinged purplish or reddish. Stamens numerous, 4 times or more as many as the petals (20 - 40), free or polyadelphous. Disk annular. Ovary depressed-subglobose, 8 - 18-locular; locule with 4 - 8 ovules in 2 collateral rows; style cylindric, caducous; stigma globose or capitate, glandular-sticky. Fruits multilocular, many-seeded. Berries specialized known as hesperidia, of variable size and shape; pericarp leathery; outer layer densely glandular, often glossy, yellowish or orange coloured when mature; inner layer white; endocarp spongy; interior of locules filled with numerous stalked, soft or firm, purple vesicles containing sweet, sour or acidic juice; seeds angular-obovoid, mono-or polyembryonic; cotyledons white or green.
A native of S.E. Asia, particularly Indo-Malesia and China, and now under cultivation in the tropks and subtropics throughout the world; ca 17 species with innumerable cultivars and hybrids; 10 in India.
Notes. Citrus includes some of the principal fruit crops of the world like the citrons, lemons, limes, grape fruits, oranges and pomelos. The genus is very diverse in nature and the classification of its innumerable cultivated forms has ever been a difficult problem. An untraceable wild ancestry of many of the presently known cultivated species coupled with the peculiarities in their mode of reproduction makes the taxonomic delimitation in this genus and related genera a complex and rather perennial problem. Most of the species of Citrus not only breed true but also form hybrids with species of its related genera like Fortunella and Poncirus. This leads to intrageneric or inter-generic hybrids (simple or complex) which by means of adventive nucellar polyembryony multiply true to their maternal genotype and exist as stable natural populations. The spontaneous formation of autotetraploids and budsports is also not uncommon.
Two principal systems of Citrus taxonomy in current use are that of Swingle (1943) revised by Reece in 1967 and of Tanaka (1954, 1961). The former is basically a biological approach and the latter a horticultural one. Swingle's system recognized two subgenera: subgenus Citrus with 10 species, and subgenus Papeda with 6 species. Tanaka (1954) classified Citrus consisting 2 subgenera, 8 sections, 13 subsections, 8 groups, 2 subgroups, 2 microgroups and a total of 145 species. Later, Tanaka (1961) added 2 new subsections, one new group and 12 new species, thus recognising a total of 157 species (for acomparison of the above 2 classifications, refer to Swingle and Reece 1967: 364 - 367).
Swingle's and Tanaka's systems differed chiefly in their basic concepts. Swingle recognized only 16 valid species and did not accord species status to 'budsports' or natural hybrids. But, Tanaka accepted 157 species that mostly represented variants and hybrids. However, some of the basic species commonly called citron, lemon, sourlime, grape fruit, oranges (mandarin, sour orange, sweet orange) and pomelos were accepted in both the systems. Subsequent classifications on the genus proposed by various other botanists and citrologists are mere modifications of either Swingle's or Tanaka's System (e.g. Bhattacharya & Dutta, 1956; Hodgson, 1961; Hodgson, Singh & Singh, 1963a,b,c;Hodgson, 1965; Singh, 1967; Singh & Nath, 1969).
India enjoys a remarkable position in the 'Citrus belt of the world' due to her rich wealth of Citrus genetic resources, both wild and cultivated. The introduction, cultivation and trade of Citrus fruits in India have a very remarkable past (Bonavia, 1890; Lushington, 1910). Apart from the most important and widely known cultivated species, certain wild and indigenous species of Citrus like C. ichangensis (Ichang papeda), C. nindica (Indian wild orange), C. latipes (Khasi papeda) and C. macroptera var. annamensis (Melanesian papeda) also occur on the Khasi hills and adjoining areas in N.E. India. Of these, C. indica is considered the most primitive and progenitor of all Citrus species. The natural diversity of Citrus indica was observed on the Garo hills of Meghalaya (Singh, 1984). A 'Citrus gene sanctuary' for preserving the indigenous Citrus germplasm, particularly the Indian wild orange, has been set up in the 'Nakrek Biosphere Reserve' along the Tura ridges of the Garo hills in Meghalaya.
Literature. BHATTACHARYA, S.C. & S. DUTTA (1956). Classification of Citrus fruits of Assam. ICAR Sci. Monogr. 20: 1 - 110. BONAVIA, E.(1888 - 90). The cultivated oranges and lemons etc. of lndia and Ceylon. pp 384 & Atlas of 259 plates. CHEEMA, C.S. & S.S. BHAT (1934). A study of the Citrus varieties of the Bombay Presidency. Curr. Sci. 2: 298 - 304. HODGSON, R.W. (1961). Taxonomy and nomenclature in Citru. fruits. in: Prince, W.C. (ed.), Proc. 2nd Conf. lnternatl. Organ. Citrus Virol. pp. 1 - 7. HODGSON (1965). Taxonomy and nomenclature in Citrus fruits. in: Advances in Agricultural Science and their application. Madras Agric. j. 317 - 33. HODGSON, R.W., R. SINGH & D. SINGH. (19638). Some little known Indian Citrus species. Calif. Citrog. 48: 188. HODGSON, R.W., R. SINGH & D. SINGH (l963b). Some little known Indian Citrus species. Calif. Citrog. 48: 288. HODGSON, R.W., R.SINGH & D. SINGH (1963c). Some little known Indian Citrus species. Calif. Citrog. 48: 357. LUSHINGTON, A.W. 1910. The genus Citrus. Indian Forester 36: 323 - 353. SINGH, B. (1984) Conservation of genetic resources of Eastern Himalayan region with special reference to Citrus. in: Tripathi, R.S. (ed.), Resource potentials of north-east India. Vol. 2.17 - 21. SINGH, R. (1967). A key to the Citrus fruits. Indian J. Hort. 24: 71 - 83. SINGH, R. & N. NATH (1969). Practical approach to the classification of Citrus in:Chapman, H.D. (ed.), Proc. Intenatl. Citrus Symp. 1: 435 - 440. TANAKA. T. (1954). Species problem in Citrus..... (Revisio Aurantiacearum XI). Citrologia, semi-centennial commemoration papers on Citrus studies.
A native of S.E. Asia, particularly Indo-Malesia and China, and now under cultivation in the tropks and subtropics throughout the world; ca 17 species with innumerable cultivars and hybrids; 10 in India.
Notes. Citrus includes some of the principal fruit crops of the world like the citrons, lemons, limes, grape fruits, oranges and pomelos. The genus is very diverse in nature and the classification of its innumerable cultivated forms has ever been a difficult problem. An untraceable wild ancestry of many of the presently known cultivated species coupled with the peculiarities in their mode of reproduction makes the taxonomic delimitation in this genus and related genera a complex and rather perennial problem. Most of the species of Citrus not only breed true but also form hybrids with species of its related genera like Fortunella and Poncirus. This leads to intrageneric or inter-generic hybrids (simple or complex) which by means of adventive nucellar polyembryony multiply true to their maternal genotype and exist as stable natural populations. The spontaneous formation of autotetraploids and budsports is also not uncommon.
Two principal systems of Citrus taxonomy in current use are that of Swingle (1943) revised by Reece in 1967 and of Tanaka (1954, 1961). The former is basically a biological approach and the latter a horticultural one. Swingle's system recognized two subgenera: subgenus Citrus with 10 species, and subgenus Papeda with 6 species. Tanaka (1954) classified Citrus consisting 2 subgenera, 8 sections, 13 subsections, 8 groups, 2 subgroups, 2 microgroups and a total of 145 species. Later, Tanaka (1961) added 2 new subsections, one new group and 12 new species, thus recognising a total of 157 species (for acomparison of the above 2 classifications, refer to Swingle and Reece 1967: 364 - 367).
Swingle's and Tanaka's systems differed chiefly in their basic concepts. Swingle recognized only 16 valid species and did not accord species status to 'budsports' or natural hybrids. But, Tanaka accepted 157 species that mostly represented variants and hybrids. However, some of the basic species commonly called citron, lemon, sourlime, grape fruit, oranges (mandarin, sour orange, sweet orange) and pomelos were accepted in both the systems. Subsequent classifications on the genus proposed by various other botanists and citrologists are mere modifications of either Swingle's or Tanaka's System (e.g. Bhattacharya & Dutta, 1956; Hodgson, 1961; Hodgson, Singh & Singh, 1963a,b,c;Hodgson, 1965; Singh, 1967; Singh & Nath, 1969).
India enjoys a remarkable position in the 'Citrus belt of the world' due to her rich wealth of Citrus genetic resources, both wild and cultivated. The introduction, cultivation and trade of Citrus fruits in India have a very remarkable past (Bonavia, 1890; Lushington, 1910). Apart from the most important and widely known cultivated species, certain wild and indigenous species of Citrus like C. ichangensis (Ichang papeda), C. nindica (Indian wild orange), C. latipes (Khasi papeda) and C. macroptera var. annamensis (Melanesian papeda) also occur on the Khasi hills and adjoining areas in N.E. India. Of these, C. indica is considered the most primitive and progenitor of all Citrus species. The natural diversity of Citrus indica was observed on the Garo hills of Meghalaya (Singh, 1984). A 'Citrus gene sanctuary' for preserving the indigenous Citrus germplasm, particularly the Indian wild orange, has been set up in the 'Nakrek Biosphere Reserve' along the Tura ridges of the Garo hills in Meghalaya.
Literature. BHATTACHARYA, S.C. & S. DUTTA (1956). Classification of Citrus fruits of Assam. ICAR Sci. Monogr. 20: 1 - 110. BONAVIA, E.(1888 - 90). The cultivated oranges and lemons etc. of lndia and Ceylon. pp 384 & Atlas of 259 plates. CHEEMA, C.S. & S.S. BHAT (1934). A study of the Citrus varieties of the Bombay Presidency. Curr. Sci. 2: 298 - 304. HODGSON, R.W. (1961). Taxonomy and nomenclature in Citru. fruits. in: Prince, W.C. (ed.), Proc. 2nd Conf. lnternatl. Organ. Citrus Virol. pp. 1 - 7. HODGSON (1965). Taxonomy and nomenclature in Citrus fruits. in: Advances in Agricultural Science and their application. Madras Agric. j. 317 - 33. HODGSON, R.W., R. SINGH & D. SINGH. (19638). Some little known Indian Citrus species. Calif. Citrog. 48: 188. HODGSON, R.W., R. SINGH & D. SINGH (l963b). Some little known Indian Citrus species. Calif. Citrog. 48: 288. HODGSON, R.W., R.SINGH & D. SINGH (1963c). Some little known Indian Citrus species. Calif. Citrog. 48: 357. LUSHINGTON, A.W. 1910. The genus Citrus. Indian Forester 36: 323 - 353. SINGH, B. (1984) Conservation of genetic resources of Eastern Himalayan region with special reference to Citrus. in: Tripathi, R.S. (ed.), Resource potentials of north-east India. Vol. 2.17 - 21. SINGH, R. (1967). A key to the Citrus fruits. Indian J. Hort. 24: 71 - 83. SINGH, R. & N. NATH (1969). Practical approach to the classification of Citrus in:Chapman, H.D. (ed.), Proc. Intenatl. Citrus Symp. 1: 435 - 440. TANAKA. T. (1954). Species problem in Citrus..... (Revisio Aurantiacearum XI). Citrologia, semi-centennial commemoration papers on Citrus studies.
KEY TO THE SUBGENERA
| 1a. Pulp-vesicles of fruits free of acrid oil droplets; petioles wingless, marginate or narrow to broadly winged; wings never exceeding more than 3/4 of the breadth of blade; stamens usually polyadelphous | 1. Citrus |
| b. Pulp-vesicles of fruits contain acrid oil droplets; petioles broadly winged; wings as broad as blade; stamens usually free | 2. Papeda |
