Citrus Germplasm 1 страница

The genus Citrus is one of 33 genera in the subfamily Aurantioideae (or Citroideae) of the family Rutaceae (Table 4.1). Most Aurantioideae genera are native to and have their centre of diversity in north-eastern India, southern China, the Indochinese peninsula and nearby archipelagos, although some related Aurantioideae genera are native to Asia, Africa and Australia.

Much of the descriptive information on these genera, as well as the establishment of one of the most widely accepted taxonomic systems used therefore, was published in various places by Walter T. Swingle of the United States Department of Agriculture (USDA) in the early decades of the 20th century. These publications, as well as ear- lier accounts from various flora, plant exploration reports, etc. were summarized in Swingle (1943) and its minor revision as Swingle and Reece (1967). This is still the most complete single source of information in this area and will form the basis for much of the following information. Except in the rare cases where new information is added in Swingle and Reece (1967), refer- ence will be made to Swingle (1943) with the understanding that the same material is available in Swingle and Reece (1967).

Although some recent information has been generated on the Aurantioideae, much of the information on these genera is quite old and relies heavily on the work of Swingle, which in turn often refers to even older materials. In some cases, a species description by Swingle (1943) is based upon a single herbarium specimen or previous description. It is quite probable that not all of the species described by Swingle (1943) con- stitute valid taxa, due both to assumptions made about the materials observed by the authors and possible loss of species or gene pools since the original descriptions were published or herbarium specimens collected. On the other hand, as will be seen below, some genera that have had attention paid to them recently have actually had increases in the number of species catalogued.

Table 4.1. The Aurantioideae (orange subfamily) of the plant family Rutaceae.

Review of the Aurantioideae

The Rutaceae are a family of approximately 160 genera and 1650 species of mostly trees and shrubs. The leaves are compound, with glands and without stipules, often thorny; the fl owers are 4- or 5-merous, regular and perfect, usually with a superior ovary and 3–5 locules; fruit varies, usually a capsule or berry. The subfamily Aurantioideae is further characterized by leaves and bark with oil glands, and most characteristically a fruit which is a hesperidium, i.e. a berry whose fl eshy parts are divided into seg- ments surrounded by a rind or hard shell,

the seeds of which are without endosperm and sometimes with two or more nucellar asexual embryos.

The Aurantioideae is divided into tribes. Although Engler (1931) recognized only a single tribe and Tanaka (1932) eight, the most commonly used division is that of Swingle (1938), which recog- nizes two tribes (the Clauseneae and Citreae) as shown in Table 4.1 (see also Swingle, 1943). These two tribes are further divided into six subtribes, 33 genera, and 203 species as per Swingle (1943). For a complete but not up-to-date enumeration of published species, the

reader is referred to Carpenter and Reece (1969).

The Clauseneae are considered to be the most primitive members of the Aurantioideae. This tribe is distinguished from the Citreae by never having axillary spines, having leaves alternately attached to a non-articulated rachis, and having a rachis that does not break into segments when the leaves fall. The Citreae are the opposite of the Clauseneae: axils have single or paired spines, leaves are oppo- sitely attached to an articulated rachis, and the rachis breaks into segments when the leaves fall. This is the larger and more eco- nomically important of the two tribes, and includes Citrus and its closes relatives. The Clauseneae and Citreae are further divided into subtribes, and the subtribes of the Citreae (but not the Clauseneae) are further divided into subtribal groups (Table 4.1). The reader is referred to Swingle (1943) for more information on the distinguishing characteristics of these subdivisions.

Recent molecular phylogeny studies have suggested slightly different relation- ships within the Aurantioideae and Rutaceae. Herrero et al. (1996b) suggest that Swinglea is more closely related to Murraya than to Afraegle and that Aeglopsis is more closely related to Fortunella, Microcitrus and Citrus than to the other hard shell species, while Chase et al. (1999) suggest that Luvunga is more closely related to other subfamilies (Rutoideae and Toddaliodae) than to other members of the Aurantioideae. Recent work in molecular systematics (Samuel et al., 2001) and karysystematics (Guerra et al., 2000) found that the traditional division into tribes and subtribes did not refl ect phylogenetic rela- tionships. Because these arrangements are less intuitively obvious than the classical taxonomy of Swingle (1943), the latter will be utilized for this brief discussion of Aurantioideae genera. This is not a taxo- nomic treatment (see Nicolosi, Chapter 3, for that) but it is convenient for presenta- tion and discussion.

For the current chapter, the genera of the Aurantioideae will be briefl y discussed

in a somewhat reverse order, starting with the most important genus, Citrus, and pro- ceeding to the most distantly related genera at the end.

Citrus

The taxonomy of the genus Citrus is not precisely established. There have tradition- ally been two major systems of Citrus tax- onomy utilized: Swingle and Tanaka. The Swingle system (Swingle, 1943) recognizes

16 species in two subgenera (Citrus and Papeda). Modifi cations of Swingle recog- nize 17 species (Bhattacharya and Dutta, 1956; Stone, 1994a), 36 species (Hodgson, 1961) or 31 species (Singh and Nath, 1969). The other widely utilized taxonomic system for Citrus and related genera is that of Tyô zaburô Tanaka of the University of Osaka. The Tanaka version of Citrus taxon- omy was developed more or less concur- rently with the Swingle system (Tanaka, 1954, 1966, 1969a, b, 1977). The Tanaka taxonomy recognizes up to 162 species within 13 primary elements in its most highly developed form (Tanaka, 1977). This lack of agreement with Swingle refl ects dif- ferences of opinion as to what degree of dif- ference justifi es species status and whether or not supposed hybrids among naturally occurring forms should be assigned species status. Although Tanaka’s differences with Swingle primarily involved the genus Citrus itself, Tanaka also catalogued and described many related Aurantioideae

genera.

There is no defi nitive work on Citrus taxonomy, and the two major systems are both currently in use. In practice, some germplasm banks (such as France, Spain and the USA) use a sort of hybrid system that is in many ways closer to Tanaka than to Swingle. In addition, international scien- tific citriculture societies, such as the International Society of Citriculture (ISC) and the International Organization of Citrus Virologists (IOCV), utilize both systems.

Recently it has been suggested that only citron (C. medica), mandarin (C. reticulata)

and pummelo (C. maxima) constitute valid species (within the subgenus Citrus) and that other important types (orange, grape- fruit, lemon and lime) originated from one or more generations of hybridization between these ancestral genera (Scora, 1975, 1989; Barrett and Rhodes, 1976; Mabberley, 1997). Interestingly, the earliest workers also believed that there were only three or four valid species of citrus (Linnaeus, 1753; Hooker, 1875), and recent molecular studies have supported this concept (Así ns et al., 1996; Federici et al., 1998; Herrero et al., 1996b; Nicolosi et al., 2000; Gulsen and Roose, 2001; Moore, 2001).

None of the systems is ideal, but the Tanaka system is better adapted to the hor- ticultural traits of different groups. The Swingle system is poorly adapted to these traits, particularly in regard to the man- darin group. A system with only three species may be scientifi cally more rigorous, but is even less cognizant of horticultural traits than is that of Swingle. The Swingle system will be mainly followed here in order to discuss the various types of Citrus. A technical discussion of Citrus taxonomy is found in Nicolosi (Chapter 3).

The three ancestral species only repro- duce sexually since they are not apomictic. Consequently, some mandarins, pummelos and citrons have higher levels of genetic diversity since many of the cultivars have arisen through sexual hybridization. On the other hand, most of the cultivars of orange, grapefruit, lemon and some mandarin groups such as satsumas and clementines originated from nucellar seedlings or bud- sports. Consequently, the amount of genetic diversity within these groups is relatively low, in spite of there being many named varieties with important differences in hor- ticultural traits (Herrero et al., 1996a). Table 4.2 summarizes the current under- standing of the origin, mode of reproduc- tion and level of genetic diversity within certain commercially important species of the genus Citrus.

Citrus originated and has as its centre of diversity in the south-eastern region of Asia, including India, southern China, the Indochinese peninsula and nearby archi- pelagos. Tanaka (1954) proposed a theoreti- cal dividing line (the Tanaka line) that runs south-eastwardly from the north-west border of India, above Burma, through the

Probable

Yunnan province of China, to south of the island of Hainan. Citron, lemon, lime, sweet and sour oranges, and pummelo orig- inated south of this line, while mandarins, kumquats and trifoliates originated north of the line. The mandarins apparently devel- oped along a line north-east of the Tanaka line, along the east China coast, through Formosa and to Japan, while the trifoliates and kumquats are found in a line crossing south-central China in an east–west direc- tion. Yunnan, China, through which the Tanaka line runs, has recently been pro- posed as a major centre of origin for citrus (Gmitter and Hu, 1989, 1990).

The genus Citrus is by far the most eco- nomically important in the Aurantioideae and probably the entire Rutaceae. It includes the cultivated sweet and sour oranges, mandarins, lemons, limes, pum- melos, grapefruits, etc., as well as other types not commonly consumed. It also includes the papedas (subgenus Papeda). The fruits of the latter subgenus are not

edible due to their pulp vesicles having dense aggregations of acrid oil that give the juice a bitter, unpleasant fl avor. The fl owers are smaller than those of the subgenus Citrus, and the leaves have elongated peti- oles with broad wings.

Since most readers will be well acquainted with the cultivated types of Citrus and they are covered in more detail in a previous chapter, only a few comments will be made concerning them. For descrip- tions of some important commercial culti- vars, the best concise sources are Hodgson (1967) and Saunt (2000). The use of Citrus spp. as germplasm resources is, of course, concerned with more than fruit quality and characteristics. Such traits as disease resist- ance and tolerance, adaptation to different soil and environmental conditions, resist- ance to insects, tolerance of cold condi- tions, etc. are important characteristics of genotypes maintained in germplasm collec- tions and utilized by scientists. Table 4.3 presents examples of some characteristics

Table 4.3. Some reported attributes of Aurantiodeae genera.

Table 4.3. Continued

Table 4.3. Continued