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Citrus Germplasm 3 страница






 

Table 4.3. Continued

 

Genus Attribute References
Severinia Reaction to citrus tristeza virus Grant and Costa, 1949; Knorr, 1956;
    Yoshida, 1996
Severinia Reaction to Mal secco Russo, 1976/77
Severinia Reaction to miscellaneous fungal diseases Fulton, 1925; Olson and Godfrey, 1953;
    Winston et al., 1927
Severinia Reaction to miscellaneous mycoplasmas Hung et al., 2001
Severinia Reaction to miscellaneous virus or Iwanami et al., 1993
  viroid diseases  
Severinia Reaction to Phytophthora Carpenter and Furr, 1962; Grimm and
    Hutchison, 1972
Severinia Salt tolerance Embleton et al., 1973b; Traub and
    Robinson, 1937
Severinia Tree size control Bitters, 1950; Bitters et al., 1977; Castle
    and Phillips, 1977; Phillips, 1969;
    Phillips and Castle, 1977; Traub and
    Robinson, 1937; Xuannan and Dehui,
     
Swinglea Hair and scalp preparations Brown, 1954
Swinglea Leaf characteristics Hirano, 1931
Swinglea Reaction to canker Lee, 1918; Peltier, 1918; Peltier and
    Frederich, 1920, 1924
Swinglea Reaction to citrus tristeza virus Knorr, 1956; Mü ller and Garnsey, 1984;
    Yoshida, 1996
Swinglea Reaction to miscellaneous fungal diseases Fulton, 1925; Winston et al., 1927
Swinglea Reaction to miscellaneous virus or Iwanami et al., 1993
  viroid diseases  
Swinglea Reaction to Phytophthora Rí os-Castañ o and Tafut, 1972
Swinglea Medicinal use Brown, 1954
Triphasia Anti-insect properties Bowman et al., 2001
Triphasia Food use Burkill, 1935
Triphasia Gum Burkill, 1935
Triphasia Medicinal use Burkill, 1935
Triphasia Reaction to canker Lee, 1918; Peltier, 1918; Peltier and
    Frederich, 1920, 1924
Triphasia Reaction to citrus tristeza virus Knorr, 1956; Mü ller and Garnsey, 1984;
    Yoshida, 1996
Triphasia Reaction to miscellaneous fungal diseases Winston et al., 1927
Triphasia Wood quality Burkill, 1935

 


of Citrus and other members of the Aurantioideae that have been reported. It is by no means complete or exhaustive, but gives a general overview of some of the characteristics of the Aurantioideae that have been reported in the literature.

The citrons, C. medica, are one of the primal three species of citrus mentioned above. They are monoembryonic, and so all seedlings are zygotic. This group represents one of the greatest pools of genetic diversity


within the genus Citrus. The actual place of origin of the citrons is not defi nitively known. It is assumed to be northern India or southern China, but Swingle (1943) spec- ulates that citrons may have arisen in southern Arabia. In any case, the citron arrived in the Middle East in ancient times and became important to the ancient Hebrews; it is still used in the Jewish Feast of Tabernacles (Goor and Nurock, 1968). The citron is also believed to be the


 


fi rst type of citrus to arrive in Europe. Citrons are not a large crop in most areas, but are locally important in the Mediterranean area. The most important food use is of the candied peel in confec- tions. It is also used as for pickle, and essential oils are sometimes distilled. The citron is an attractive and fragrant dooryard tree; for this use, the famous ‘Buddha’s Hand’ (C. medica var. sarcodactylis) must be mentioned. There are also traditional medicinal uses of citron, which suggest that it may have medical benefi ts. Citrons are sensitive to cold and tend to make small, shrubby trees, especially when grown from cuttings.

The pummelos, C. maxima, are also a primal, monoembryonic species and reser- voir of genetic diversity. They are native to South-east Asia and the nearby archipela- gos, and it is in this area where they have their greatest use. Pummelo fruit are very variable in size, shape, and internal and external colour. They have in common a thick peel with non-adherent larger, pulp vesicles than other Citrus species.

The mandarins, C. reticulata, comprise both monoembryonic and polyembryonic (apomictic) types. This is a variable group and is sometimes divided into subgroups based upon secondary compounds (Scora, 1989). As used by Swingle, this name is applied to wild or primitive fruits as well as highly selected or hybridized fruits. The mandarins include sour types (C. reticulata var. austera), large-fruited elite selections and small-fruited types used as rootstocks. There are distinctive subtypes such as the satsumas of Japan (usually referred to as C. unshiu) and the clementines of the Mediterranean areas (usually referred to as

C. clementina). The mandarins are the most cold-hardy species of the subgenus Citrus but are variable in other characteristics. Some rootstock types are citrus tristeza virus (CTV)-tolerant but these vary in their tolerance to salinity, pH and other soil con- ditions, as well as other graft-transmissible diseases (Wutscher, 1979; Castle, 1987). Mandarin trees in general are relatively small and spindly compared with many


other types of citrus. Many natural and man-made hybrids of mandarins exist, the most notable being the tangelos (C. reticu- lata ´ C. paradisi, sometimes referred to as

C. ´ tangelo) and the tangors (C. reticulata ´

C. sinensis, sometimes referred to as C. nobilis).

The relatively recently described monoembryonic C. halimii (Stone et al., 1973), which has characteristics intermedi- ate between Citrus and Fortunella, is some- times considered a valid species (Scora and Kumamoto, 1983), but there is some evi- dence that this is not actually a Citrus species at all (Berhow et al., 2000). Recent molecular studies have supported C. hal- imii as a distinct species (Luro et al., 1992; Herrero et al., 1996b; Fang et al., 1998b), closely related to Fortunella (Herrero et al., 1996b). Swingle (1943) also accepts the mandarin-like C. indica and C. tachibana as valid species. Molecular evidence also sup- ports species status for C. indica (Fang et al., 1998a; Federici et al., 1998) and C. tachibana (Hirai et al., 1990; Herrero et al., 1996b). These three species are somewhat obscure and do not often fi gure in taxo- nomic schemes.

The other species of the subgenus Citrus are probably hybrids or successive hybrids of these primal types. The grape- fruit, C. paradisi, should probably not be considered a distinct species but is accorded species status by Swingle (1943). Its probably hybrid origin is recognized in the designation C. ´ paradisi (Bailey Hortatorium, 1976). It probably arose rela- tively recently as a spontaneous hybrid of pummelo and sweet orange (Scora et al., 1982). The origin of the grapefruit and its relationship to the ‘forbidden fruit’ of the Caribbean region is an interesting story (Gmitter, 1995). The history of the various grapefruit cultivars is well documented compared with most other types. All cur- rent cultivars arose from the original grape- fruit type, being selected for seedlessness or pigmented fl esh (Gmitter, 1995).

The lemon (C. limon) probably arose as a hybrid of citron (paternal) and sour orange (maternal) (Nicolosi, et al., 2000;


 


 

Gulsen and Roose, 2001), sour orange prob- ably being in turn a hybrid of mandarin and pummelo (Scora, 1975, 1989; Berhow et al., 2000; Gulsen and Roose, 2001; Moore, 2001). Different varieties of mandarin and pummelo probably gave rise to the sweet orange (Barrett and Rhodes, 1976; Berhow et al., 2000; Nicolosi et al., 2000). Limes probably originated as hybrids between citron and either a Microcitrus species (Barrett and Rhodes, 1976; Scora, 1989; Federici et al., 1998) or a papeda (Nicolosi et al., 2000). Probably there is little justifi - cation for according separate status to such groups as sweet lemon, sweet lime and limetta (Federici et al., 1998). The situation with other groups (rough lemons, rangpur limes) is less clear, but probably there are contributions to both groups from man- darin and citron (Scora, 1989; Federici et al., 1998).

The papedas constitute the subgenus Papeda. The characteristics of this distinct group are stated above. Swingle (1943) rec- ognized C. ichangensis, C. latipes, C. micrantha, C. celebica, C. macroptera and

C. hystrix, and various subspecies and hybrids. More recently, a new putative species, C. hongheensis, has been described (Yinmin et al., 1976). Although the original description of C. ichangensis by Swingle (1913a) was based on only a few specimens, there are local variants including variously petioled (Kiang and Zu-Zhao, 1984) and long- and short-fruited types (Ding et al., 1990; Gmitter and Hu, 1989, 1990).

 

 

Fortunella

The genus Fortunella contains the kumquats. Fortunella spp. closely resemble Citrus spp. and were included in Citrus until Swingle (1915c) separated them into a separate genus based on: (i) differences in the ovary, ovules and stigma of the fl ower;

(ii) abaxial leaf glands; (iii) small more or less angular fl ower buds; and (iv) a sweet, edible, more or less pulpy skin. It is proba- bly the last characteristic that most people consider the most distinctive of the


 

kumquats. Yamamoto et al. (1993) could not distinguish between Fortunella and Citrus using restriction fragment length polymorphism (RFLP), and Mabberley (1998) included Fortunella in his revision of Citrus. However, at this time, Fortunella is generally considered a separate genus.

Swingle (1915c) originally described four species of Fortunella: F. margarita, F. japonica, F. crassifolia and F. hindsii. In Swingle (1943), the ‘Meiwa’ and ‘Changshou’ kumquats were considered to be hybrids and were described but not accorded species status, removing F. crassi- folia (‘Meiwa’) as a species. Swingle (1943) also rejected Tanaka’s (1933) F. obovata (‘Changshou’) but accepted as a ‘species of doubtful validity’ the large-fruited F. polyandra from Malaysia (which is specu- lated to be a limequat). An updated nomen- clature that recognizes the hybrid nature of

F. crassifolia and F. obovata is presented in Fantz (1988).

One unique facet of Fortunella is the existence of a wild tetraploid form of F. hindsii in addition to the diploid forms of this species. Fortunella hindsii was for- merly considered to be in the genus Atalantia, but was placed in Fortunella in 1915 (Swingle, 1915c). Swingle (1940b) later added the var. chintou, which is a diploid version of the ‘normally’ (?) tetraploid species. The tetraploid and diploid forms have the differences expected in identical varieties with different ploidy levels. The existence of both diploid and tetraploid forms in ex situ collections has been confi rmed by root squash chromo- some counts and fl ow cytometry (J. Juá rez and L. Navarro, unpublished).

Fortunella trees are evergreen, small, and slow growing. Most of the intrageneric differences are in the fruit size and shape. In China, kumquats have been cultivated and eaten since ancient times. Here, different selections within these cultivars are recog- nized (Yin-min, 1985; Dashen and Fangcong, 1989). Kumquats are currently considered to exist only in cultivation, with the possible exception of F. hindsii. Their area of origin in northern China makes them


 


one of the most cold tolerant of the Aurantioideae, partially due to their charac- teristic winter dormancy. Fortunella spp. are also slightly precocious and resistant to canker and Phytophthora (Table 4.3). The kumquats are attractive plants and recently there have been introductions of variegated and seedless types suitable for backyard use. Kumquats, as with most Aurantioideae, hybridize readily with their closely related genera. This has led to their use in breeding programmes, particularly in programmes for breeding cold-tolerant citrus. This resulted in the creation of many different ‘-quats’ in the early part of the 20th century (see Swingle, 1943 for additional references). These were bred with the objective of creat- ing a ‘citrus-like’ fruit that was cold hardier than citrus. The ‘-quats’ have not taken this role in commercial production but now rep- resent a separate group in many ex situ col- lections. The procimequat ((C. aurantifolia ´

F. japonica) ´ F. hindsii), which is found in

ex situ collections, is a triploid resulting from the hybridization of the tetraploid F. hindsii and a diploid limequat.

Fortunella has some interesting rela- tionships with Citrus as regards rootstock/scion compatibilities. These two genera are closely related, but their graft compatibilities are somewhat less congen- ial than are those of Citrus and some of the other closely related Aurantioideae genera. Fortunella is best propagated on trifoliate rootstocks, and often develops incompati- bilities with Citrus and even with citranges. A study in Florida indicated that after 4.5 years, ‘Calamondin’ ‘Meiwa’, and ‘Nagami’ on ‘Benton’, ‘Cleopatra’, ‘Flying Dragon’, ‘Sun Chu Sha’ and ‘X-639’ were free of bud union problems, whereas these same vari- eties showed various bud union problems on ‘Carrizo’, 80-18 citrumelo, ‘Swingle’ cit- rumelo, sweet orange and sour orange (Youtsey, 1997). This seems to be truer of ‘Meiwa’, which is sometimes grown with a ‘Nagami’ interstock. Fortunella hindsii is sometimes used as a dwarfi ng rootstock in China. The recently described citrus leaf blotch virus (Galipienso et al., 2000, 2001; Vives et al., 2001), originally found in


‘Nagami’ (Navarro et al., 1984b) and later in other kumquat sources, produces incompat- ibility on trifoliate rootstocks and may be the cause of some incompatibilities of kumquats described in the past.

The calamondins represent a distinct group and are sometimes accorded species status. However, Swingle (1943) does not accept this as a valid species, stating that it is an apparent hybrid between a sour man- darin and a kumquat [ C. reticulata var. austera? ´ Fortunella spp.? ]. When the cala- mondins are accorded species status, they are usually referred to as C. mitis or C. microcarpa. Calamondins are sometimes referred to as C. madurensis, but this is incorrect as C. madurensis actually refers to

F. japonica (Swingle, 1943; Wijnands, 1984). Ingram and Moore (1975) and Wijnands (1984) recognize the hybrid nature of this taxon with the coinages

´ Citrofortunella mitis and ´ Citrofortunella

microcarpa, respectively. Calamondin is extensively reviewed by Mabesa (1990), with an emphasis on fruit quality and pro- cessing properties. Although the calam- ondin is used as a fresh fruit and a condiment and has some known medicinal uses, it is not as well characterized as many other types of Aurantioideae germplasm. It is graft compatible with Citrus and has been used as a rootstock, as has its apparent hybrid the ‘Philippine calamandarin’ (which does not appear to be a calamondin

´ mandarin hybrid despite its name).

 

 

Microcitrus

The genus Microcitrus is very closely related to Citrus and was originally included in that genus. Swingle (1915b) separated Microcitrus from Citrus and rec- ognized four species: M. australis, M. aus- tralasica, M. inodora and M. garrowayii. Swingle (1943) added M. maideniana and

M. warburgiana. Microcitrus papuana was published by Winters (1976). Microcitrus warburgiana and M. papuana are native to Papua New Guinea, while the remaining species are native to Australia. Fortunella



warburgiana may be similar to the ancestral type from which both Microcitrus and Citrus evolved.

Recently, an additional species has been published: ‘ Citrus gracilis ’ (Mabberley, 1998). Mabberley attempted a major taxonomic revision of Citrus (see above) and some closely related taxa in which Microcitrus as well as Eremocitrus are re-inserted into the genus Citrus. This system has not at this point been widely accepted and the species status of ‘ C. gra- cilis ’ may be somewhat in doubt.

Microcitrus differs from Citrus in its dimorphic foliage, very small juvenile leaves, the distinct shape and venation of adult leaves, very small fl owers having free stamens and a very short pistil, a few celled ovary with numerous ovules in each cell, and the subglobose stalked pulp vesicles. Microcitrus spp. are shrubs or small trees. The fruit of most species except M. aus- tralasica are round, with green rinds. In the right environmental conditions, Microcitrus trees are quite vigorous. This genus has been reported to be a source of precocity, drought tolerance, nematode resistance, Phytophthora resistance and adaptations to low fertility situations (Table 4.3). Microcitrus is graft compatible with Citrus. There has recently been some interest in the use of Microcitrus fruits in Australia, where they are among the ‘bush foods’ (Sykes, 2001), and in the USA, where they are sometimes used as a garnish.

Microcitrus hybridizes readily with Citrus, Fortunella and other closely related genera. Swingle (1943) describes trigeneric hybrids, such as the Faustrimedin (Microcitrus australasica ´ (Fortunella sp. ´ Citrus sp.)). Also of note are the ‘Sydney Hybrid’ (M. australis ´ M. australasica) and the red-fruited sanguinea variant of M. aus- tralasica, both of which are often found in ex situ collections.

 

 

Eremocitrus

Eremocitrus is a monospecifi c genus (E. glauca) that is very similar to Microcitrus


 

(Swingle, 1914b). However, the fl owers are smaller and the fruit has 3–5 locules with two ovules in each. It is native to central and northern New South Wales and south- eastern Queensland in Australia.

As a native of desert habitats, E. glauca is xerophytic and cold tolerant. Its xero- phytic qualities include small, thick, grey- ish leaves with thick cuticles and small substomatal chambers and an extensive root system. As with some other species adapted to hot, arid climates, the leaves may abscise during severe drought condi- tions. There is some confusion as to the cold hardiness of E. glauca due to some temperature conversion errors in Swingle (1943) and Swingle and Reece (1967). However, E. glauca can probably tolerate temperatures of about –5.5˚ C (approxi- mately +20˚ F) or less, consistent with the original paper (Swingle, 1914b) and more recent observations (Yelenosky et al., 1978). Eremocitrus is graft compatible with Citrus and hybridizes readily with that and other closely related genera. Eremocitrus is reported to be a source of tolerance to heat, boron, salt and Phytophthora root rot (Table 4.3). When grafted to Citrus as a rootstock, it has some unusual reactions. Two E. glauca trees were planted in the University of California Citrus Variety Collection in 1983. One, grafted on ‘Carrizo’ citrange, is a normal sized tree, while one grafted on C. macrophylla, not normally a dwarfi ng root-

stock, was only about 1 m tall in 2003.

 

 

Clymenia

The genus Clymenia is another that is closely related to Citrus and was originally included in that genus. Swingle (1939) sep- arated Clymenia from Citrus based upon the structure of the pulp vesicles, which are short, plump, blunt, oval or subglobose, sessile or very short stalked, and attached to the side walls of the 14–16 locules; this is not like any species in the genus Citrus. Furthermore, the leaves of Clymenia are unlike those of any Citrus species, and the fl owers have enlarged disks with 10–20


 


times as many stamens as petals. Recent chemotaxonomic work (Berhow et al., 2000) suggests that Clymenia is closely allied with Fortunella and may be a hybrid between Fortunella and Citrus. Clymenia is considered a primitive genus in the ‘true citrus fruit trees group’ and may be a link between that group and the ‘near citrus fruit trees’ group.

For many years, Clymenia was a mono- typic genus (C. polyandra). However, Stone (1985a) described a ‘problematical new species’, C. platypoda, which differed from

C. polyandra in its leaf form; it was specu- lated that P. platypoda may actually repre- sent a hybrid between Clymenia and Citrus. Clymenia is not a well characterized genus. The trees are described as being small and are of small size when cultivated in ex situ collections. The fruits resemble sweet limes and are eaten by the natives (who give it the name a-mulis) of its place of origin in the Bismarck Archipelago north-east of Papua New Guinea. Clymenia is considered rare in the wild (Jones, 1990). It is graft compatible and hybridizable with Citrus but has been little studied as far as its

disease reaction and other adaptations.

 

 

Poncirus

Swingle (1943) described the genus Poncirus as ‘remarkable’ and it is, indeed, the most remarkable genus of at least the ‘true citrus fruit trees’ and probably the entire Aurantioideae. In fact, it is diffi cult to know how properly to approach Poncirus.

Poncirus, like the other genera of the ‘true citrus fruit trees’, was for many years included in Citrus despite its ‘many strik- ingly aberrant characters’ (Swingle, 1943). These characteristics were well known at least by the early part of the 20th century (Swingle, 1909), but Poncirus was appar- ently not separated from Citrus and the name Poncirus reapplied until 1916 (Swingle, 1914–1917). Its early inclusion in Citrus led to the still used common name (or misnomer) of ‘trifoliate orange’.


Swingle (1909, 1914–1917) noted the differences between Poncirus and Citrus. The leaves are trifoliolate and deciduous, the tree being winter dormant. The fl ower buds are formed in the early summer, and overwinter protected by bud scales. In the spring, the fl owers are borne on old wood. They are nearly sessile, with the petals opening fl at, the stamens entirely free, and the ovary having 6–8 locules. The fruits are densely and fi nely pubescent. The pulp vesicles carry scattered hair-like organs that bear at their expanded tips thick-walled, fissured cells secreting a viscous fluid allowing the pulp vesicles to slip past one another. Transverse plates composed of thick-walled cells are found in the pith of the stem. Poncirus trees are relatively small and shrub-like. They are very thorny, with a relatively thick trunk and limbs.

These pronounced differences make it diffi cult to place Poncirus in an orderly course of evolutionary progress from some remote common ancestor of Citrus and its related genera, suggesting that there are many gaps therein and that Poncirus is the most ‘isolated and aberrant of the “true citrus fruit trees”’ (Swingle, 1943). Swingle’s (1943) view is that the putative remote ancestor of the ‘true citrus fruit trees’ originated in a tropical or subtropical climate. While the other genera of the ‘true citrus fruit trees’ remained in those cli- mates, Poncirus ‘migrated’ to the temperate climate of north-eastern Asia. In doing so, it developed the adaptations to winter cold noted in the previous paragraph. These adaptations permit Poncirus to survive in temperate climates that would kill all other Aurantioideae.

The trifoliate has apparently been cul- tivated since ancient times in China and was introduced into Japan several centuries before the end of the fi rst millennium AD. It may be the oldest rootstock to be used in citriculture. It is also used as an ornamental or hedge in that area. The introduction of the trifoliate to the Western world was apparently later than the introduction of edible types of citrus. In the USA, despite several suggestions that it was introduced


 


 

during Colonial times and cultivated at Monticello and other historic sites, its actual date of introduction was most proba- bly about 1869 (Swingle, 1909).

The useful characteristics as well as the limitations of the trifoliate became evident in the last part of the 19th century. These are summarized in Table 4.3, and include resistance to Phytophthora root rot and CTV; cold tolerance; adaptation to heavy soils; small size of budded trees; lack of suitability to calcareous soils; etc. Poncirus is graft compatible with and hybridizes with Citrus. While the use of Poncirus as a rootstock in Asia is very old and it is also used in current commercial production in many locales, perhaps its major importance in this area has been as a parent in inter- generic hybridization with Citrus. This has produced the very important citranges (P. trifoliata ´ C. sinensis) and citrumelos (P. trifoliata ´ C. paradisi), as well as citran- darins (P. trifoliata ´ C. reticulata), cit- remons (P. trifoliata ´ C. limon) and citradias (P. trifoliata ´ C. aurantium). These have been coined ´ Citroncirus species by Ingram and Moore (1975). While normally slow growing, when used as a rootstock Poncirus is stimulated to add girth more quickly than the scion variety, causing the characteristic ‘bell’ at the bud union.

The early observations of Poncirus by Swingle (1909, 1914–1917, 1943) indicate that the trifoliates were not considered a very variable group. This is somewhat mis- leading. Swingle (1909) indicated that most trifoliates in the USA had small fl owers due to a stunting of the petals near the base. By 1916 Swingle (1914–1917) had recognized the small fl owered forms as being distinct from the ‘normal’ large fl owers. By the 1930s, the ‘normal small-fl owered form and a large-fl owered form of seemingly greater vigor’ had been recognized (Traub and Robinson, 1937). These large- and small- fl owered forms were different from the ‘normal’ large-flowered and imperfectly developed small-fl owered forms described earlier, however, being based upon the size and structure of the petals (Shannon et al.,






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