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Citrus Germplasm 4 страница







 

1960a, b).

Shannon et al. (1960a, b) collected material from suckers of trifoliate root- stocks used in commercial production in several areas of California and divided them into small- and large-fl owered vari- eties. These selections, along with others from Florida and other areas of the southern USA, are still the basis for most collections of trifoliates in the USA and, to a lesser extent, in other countries. Seedlings of large- and small-flowered varieties pro- duced similar biomass, but large-fl owered varieties had an upright habit of growth with a single trunk whereas small-fl owered varieties produced multiple trunks result- ing in a bushy growth habit. Suckering of the multiple shoots of the small-fl owered types reduced the weight of the small-fl ow- ered types as compared with the large-fl ow- ered types. Fruits from the small-fl owered varieties matured earlier than those from the large-fl owered varieties.

Shannon et al. (1960a, b) reported that ‘Washington’ navel trees on the large-fl ow- ered ‘Christian’ trifoliate were larger than those on the small-fl owered ‘Rubidoux’ tri- foliate. Similarly, Bitters et al. (1973a) com- pared 24 selections of trifoliates and found that trees were larger on almost all large- fl owered selections than on small-fl owered selections. However, yields and fruit qual- ity were similar from trees on both large- and small-fl owered varieties. The small- flowered types produced slightly larger fruit and were more effi cient on a canopy volume basis.

This system of dividing the trifoliates into small- and large-fl owered varieties is used in the USA and many other countries. Japan, which has a far longer history of cul- tivation of P. trifoliata, has a somewhat dif- ferent system (Okamoto, 1935; Iwasaki, 1943; Iwasa and Shiraishi, 1957; Iwasaki and Nishiura, 1963; Tanaka, 1969). In the Japanese system, four different strains are recognized based upon leaf size: diploid large leaf; tetraploid large leaf; small leaf; and normal (medium leaf). Iwasa and Shiraishi (1957) report other strains in addition to these, including Hiryû (see


 


below), ‘multi-leaf strain’, ‘ P. trifoliata var. microcarpa ’ (which is described as a ‘use- less fruit – details not known’), a ‘spotted’ strain (i.e. variegated), a purple-fl owered strain and a bent-thorn strain.

Iwasaki and Nishiura (1963) note that the common (Chû ba-kei) strain is superior to either the large-leaf (Ô ba-kei) or small- leaf (Koba) strains as a nursery tree and rootstock. Chû ba-kei developed a larger root system with abundant fi brous roots, whereas Ô ba-kei had a root system with few branches and thick roots, and Koba-kei had well-branched, slender roots. Chû ba-kei produced a buddable diameter trunk sooner than the other strains. Tree growth was greatest and most uniform on Chû ba-kei. Ô ba-kei and Koba were considered variants not suitable for use as rootstocks.

It may be tempting to correlate the large-fl ower and large-leaf strains, and the small-fl ower and small-leaf strains. Iwasaki (1943) does correlate fl ower size and leaf size in the Japanese strains studied, and notes differences in growth habit similar to those reported by Shannon et al. (1960a, b). However, it is not entirely certain how these Japanese strains relate to the US strains studied by Shannon et al. (1960a, b). There is not enough information on grada- tions between small- and large-fl owered types in the USA to determine if there could be an intermediate group. If this could be done, it might be possible to des- ignate a third, medium-fl owered group and see where its horticultural characteristics fi t in with the small- and large-flowered groups. However, the fact that the most common strain in Japan is medium leafed and that the strains with small and large leaves are not suitable for rootstock use speaks against this. Although there were differences in performance as rootstocks of the varieties tested by Shannon et al. (1960a, b) and Bitters et al. (1973a), the dif- ferences were small and none of the vari- eties was apparently unfit for use as rootstocks. This might be related to disease status of the scion varieties. The early Japanese work may well have utilized scion varieties that were infected with Exocortis


or other viroids (to which trifoliates are sus- ceptible), which were common in Japan in those times. The scion varieties utilized by Shannon et al. (1960a, b) and Bitters et al. (1973a) were not likely to have been infected with viroids. These differences in pathogen status could have infl uenced hor- ticultural performance and account for some of the differences noted.

More probably, the differences between the USA and Japanese classifi cations can be traced to the fact that there were few intro- ductions of trifoliates into the USA, with a resultant lack of genetic diversity. Shannon et al. (1960a, b) took cuttings from rootstock suckers and developed these into selec- tions. The original providence of the root- stocks from which the buds were taken is unknown, but probably represented very few introductions of P. trifoliata. This is supported by recent molecular work (Fang et al., 1997). Inter-simple sequence repeat (ISSR) markers revealed that trifoliates in the UCR/USDA Citrus Variety Collection clustered into four groups. All small-fl ow- ered accessions clustered into group 3 and all large-fl owered accessions clustered into group 4. Group 2 had only two accessions, which were apparently derived from zygotic seedlings of individuals similar to the common genotype. On the other hand, three trifoliate accessions introduced from China more recently had relatively different fingerprints and probably represented a more divergent genetic make-up. This would appear to support the idea that the genetic base of the US-derived accessions is relatively narrow. That there is more genetic variability in Poncirus than previ- ously thought is demonstrated by the range of adaptations exhibited by Poncirus germplasm recently introduced into Australia from China (Broadbent et al., 2003).

One variety of P. trifoliata of particular interest is ‘Flying Dragon’ (P. trifoliata var. monstrosa), also known as ‘Hiryû ’ (Swingle, 1943). This is a dwarf variety, with very slender leaves and crooked, tor- tuously curved branches and spines. Although all trifoliates produce a small tree


 


 

when used as a rootstock, ‘Flying Dragon’ is the only variety that is a true dwarfi ng root- stock. Trees budded on ‘Flying Dragon’ seldom exceed 2.5 m in height. Swingle (1943) stated that he had introduced it to the USA in 1915 but that it had never fl ow- ered or fruited and that if it could be induced to do so it would be interesting to ‘study the chromosomes … to see if perhaps they are abnormal’. More recently, Cheng and Roose (1995) demonstrated that the dwarfi ng characteristics were inherited as if they were a single dominant gene for which ‘Flying Dragon’ is heterozygous. The curved thorns and trunk appeared to be pleiotropic effects of the dwarfi ng gene. The data also suggested that ‘Flying Dragon’ originated as a mutant of a non-dwarfi ng genotype and has not undergone sexual recombination since. The phenotypic effects may be tied to plant growth hormone metabolism, since ‘Flying Dragon’ had sig- nifi cantly higher abscisic acid (ABA) in leaves and roots and signifi cantly lower indoleacetic acid (IAA) in shoots than normal types of trifoliates (Xiao et al., 2001).

The above statements were written based upon what is normally called ‘Flying Dragon’ in the USA. This may actually be different from the ‘Hiryû ’ of Japan. According to some workers, Swingle’s (1943) description of ‘Flying Dragon’ may be closer to the Japanese variety ‘Unryû ’ (letter from M. Iwamassa to W. P. Bitters,

~1970). On the other hand, Iwasa and Shiraishi (1957) describe Hiryû as a ‘deco- rative bonsai plant which has flexible stems’, a description which seems consis- tent with that of Swingle (1943). The acces- sions present in germplasm collections in the USA (and others which obtained their material from the USA, particularly from Riverside) are morphologically similar to one another and to Swingle’s (1943) description. That does not mean that the name was not misapplied. A separate acces- sion arriving in the USA as ‘Unryû ’ did not establish so it is not possible to compare it with the existing ‘Hiryû ’ accessions. However, the imported ‘Unryû ’ was from


 

the same source as one ‘Hiryû ’ accession, so several types of confusion are possible.

Poncirus was for many decades a monotypic genus. Recently, a putative new species, P. polyandra, was published (Ding et al., 1984). This was discovered in 1978 in Yunnan Province (Duan, 1990). This differs from P. trifoliata in having larger leaves, some fl oral differences and, most strikingly, in being evergreen (Duan, 1990). Perhaps this last characteristic is related to its prov- idence in Yunnan, the southernmost Province of China. Poncirrus polyandra is little known in the west (and perhaps in China). Duan (1990) suggests that it is dwarfi ng as a rootstock. Its other character- istics are not known to the writer other than possibly being susceptible or tolerant (rather than resistant) to CTV. Fang (1993) considered it as deserving of species status based on isozyme data, while others see it as a possible hybrid (F. Gmitter, personal communication, 1998). Recent ISSR data suggest that P. polyandra is genetically dis- tinct from the other groups of Poncirus, falling outside the four groups of Fang et al. (1997) (Krueger and Roose, 2003).

 

 

Atalantia

The genus Atalantia, along with Citropsis, constitute the ‘near citrus fruit trees’ of Swingle (1943). As with many genera of the Aurantioideae, the taxonomic history of Atalantia is somewhat confusing; it was formerly included in Limonia. The back- ground for the current status of Atalantia can be found in Swingle (1943). Swingle (1914–1917) enumerated ten species of Atalantia; this was expanded in 1940 (Swingle, 1940a, b). Some species of Atalantia are considered rare (Jones, 1990). Members of Atalantia are small, attrac-

tive trees with fragrant white fl owers and small, greenish-yellow fruits. The juice vesicles differ from those of Citrus in being sessile rather than stalked. These juice vesi- cles are conical and are arranged radially in the locules, attaching to the dorsal wall of the locules and imbedded in the inner layer


 


of the rind. Atalantia ceylanica, A. rotundi- folia, A. guillaumini and A. hainanensis have few or no vesicles, and large seeds that nearly fi ll the locules. These species consti- tute the subgenus Rissoa. The leaves of Atalantia are simple or unifoliolate like those of Citrus, but differ in having more prominent and numerous lateral veins.

Atalantia is graft compatible with Citrus. It has been reported to be resistant to the burrowing nematode (Table 4.3) but is not as well characterized as some other genera. Since the trees are attractive, there has been minor use of Atalantia in orna- mental plantings. The wood is also very attractive.

 

 

Citropsis

Citropsis was formerly included as a sec- tion of Limonia until separated by Swingle (1914a). Additional species were added several decades later (Swingle, 1940b). Swingle (1943) recognizes 11 species and one variety. The members of this genus are native to the tropical forests of Africa rather than to South-east Asia.

Citropsis has fruits similar to those of Atalantia but has pinnate, trifoliolate or fi ve-foliolate leaves. Citropsis is graft com- patible with Citrus and has been experi- mented with as a rootstock. It has been tested for nematode and disease reaction, as well as for several other properties (Table 4.3). Although its fruits resemble those of Atalantia, Citropsis is not as attractive as a tree. The leaves of most species are fairly large and coarse, and the trunk and branches are quite spiny. The fruits, while reasonably attractive, have a strong, fetid odour.

 

 

Hesperethusa

Hesperethusa, Limnocitrus, Burkillanthus, Pleiospermium and Severinia have primi- tive pulp vesicles, a fact that was not recog- nized until the middle part of the 20th century. Swingle (1938, 1943) used this, at


that time new, fact to separate these fi ve genera into the new group, the ‘primitive citrus fruit trees’.

Hesperethusa is a monotypic genus (H. crenulata) native to the Indian subconti- nent and the Indochinese peninsula. There is some controversy as to whether Hesperethusa or Naringi is the correct genus name. Naringi has been put forth by Panigrahi (1975) and is accepted by Wiersma and Leó n (1999).

Hesperethusa has leaves that are simi- lar to those of Citropsis in having odd pin- nate leaves with broad petioles. Unlike those of Citropsis, the leaves of Hesperethusa have prominent, slightly raised glands. The small black fruits have a few primitive pulp vesicles in each locule which are attached to the dorsal wall and also to the base of the locule. The trees are medium sized and attractive. Hesperethusa is described as ‘subdeciduous’ (Swingle. 1943) and, indeed, those in ex situ collec- tions sometimes lose some or all of their foliage, apparently with no obvious envi- ronmental cue.

Hesperethusa is graft compatible with Citrus. Often an interstock improves com- patibility. It is used in its native area as a condiment, but the fruits are not eaten as they are ‘bitter (not acid)’ (Swingle, 1943).

 

 

Pleiospermium

Pleiospermium was separated from Limonia by Swingle (1916b). The existence of the primitive vesicles mentioned above was not known in some of the other genera of the ‘primitive citrus fruit trees’ at that time, and consequently the relationships between Pleiospermium and other Aurantioideae genera stated by Swingle (1916b) were not correct. It was not until the 1930s that the relationship of Pleiospermium to the other genera in this group was published (Swingle, 1938, 1943). The original paper by Swingle (1916b) published the trifoliate P. alatum and the polymorphic (1- to 3-foliate) P. dubium. Swingle (1939) later added three unifoliate


 


 

species: P. longisepalum, P. sumatranum and P. latialatum. The leaves of all fi ve species have long, articulate, winged peti- oles. The small fruits have small, ovoid or cylindrical pulp vesicles with bluntly rounded or acute tips arising from the inner wall of the ovary. A more or less well-dif- ferentiated outer cortical layer surrounds a central portion that disintegrates as the fruits mature into an oily mass. There are various anatomical differences in the fruits between the different species (Swingle, 1943).

Swingle (1943) considers Pleiosper- mium to be a signifi cant genus from the evolutionary standpoint, with primitive pulp vesicles linking the ‘near citrus fruit trees’ and ‘true citrus fruit trees’ of the Citrinae with the genera from the other, more primitive tribes of Triphasiinae and Balsamocitrinae. The fact that some genera exhibit the more primitive trifoliolate leaves and others the more evolved unifoli- olate leaves is cited to support this con- tention.

Pleiospermium is graft compatible with Citrus. It has been shown to be susceptible to CTV but is otherwise not well character- ized (Table 4.3).

 

 

Burkillanthus

Burkillanthus, represented by only one species (B. malaccensis), is similar to Pleiospermium in its leaf characteristics but is quite different in its fruit. The pulp vesi- cles of Burkillanthus somewhat resemble those of Pleiospermium, but are much larger and less obvious corticate, and the central core does not disintegrate. The ovary of Burkillanthus has 22–26 ovules in each of the fi ve locules; this is the largest number of ovaries of any plant in the Aurantioideae. The fruits are much larger than those of Pleiospermium and have a thick, leathery rind. This species is not well represented in ex situ collections and there has apparently been no characterization work on it. It is considered rare and threat- ened by Jones (1990).


 

Limnocitrus

The monotypic genus Limnocitrus (L. lit- toralis) was formerly included in several different genera until separated out by Swingle (1940a). It is similar to Pleiospermium but has simple leaves that are thick and sparingly veiny, with slightly crenulate margins and straight rigid thorns in the axils. Limnocitrus also has slender, fusiform pulp vesicles with acute tips and slightly narrowed bases. These are different from those of Pleiospermium and Atalantia. The fruits resemble small oranges exter- nally.

Limnocitrus is native to the tidal marshes of the South-east Asian archipela- gos and mainland. Because of this habitat, it probably possesses a high tolerance to soil moisture and salinity. Although it was said by Swingle (1943) to be widely distributed, it is not well represented in ex situ collec- tions. It is graft compatible with Citrus. More information on the properties of this species is needed.

 

 

Severinia

Severinia is probably the best known of the ‘primitive citrus fruit trees’ but in actuality it is only the type species, S. buxifolia, that is well known. Severinia was generally included in Atalantia, which it superfi - cially resembles, until Swingle (1916c) re- applied the 19th century generic name Severinia. Severinia differs from Atalantia in having a dark, usually nearly blackberry- like fruit at maturity. The pulp vesicles are quite primitive, in contrast to the relatively well developed ones found in Atalantia, and two or three oil glands are found in the mesocarp of the young ovary. The vesicles do not break down into a gelatinous mass as in Pleiospermium. The leaves of Severinia are somewhat similar to those of Atalantia, but have non-articulated petioles.

Severinia buxifolia is a relatively common plant in the southern part of China and adjacent areas. It is a handsome shrub, and is sometimes used for hedges. The


 


leaves are (or were) used in China for making yeast cakes. According to Swingle (1943), there are different horticultural vari- eties in Asia that differ in height, growth habit, etc., but have not been recognized as subspecies or varieties. Even in ex situ col- lections there are variant forms and multi- ple accessions. A common variant is the ‘brachytic form’ with shortened internodes and dense foliage.

Severinia buxifolia has received more horticultural attention than the majority of citrus relatives (Table 4.3). It is graft com- patible with Citrus and the resultant plants are generally long lived. The dwarfi ng effect of S. buxifolia as a rootstock is well known. It has been tested for reaction to diseases and nematodes more than most other citrus relatives. It is known to be tolerant of cold, boron and salinity. Probably the main reason for its more extensive characteriza- tion than other related genera is simply the fact that it is commonly found in ex situ collections and is easy to grow.

The other species of Severinia include

S. disticha, S. linearis, S. paniculata, S. retusa, S. trimera and S. lauterbachii. These were named or re-named by Swingle (1938) from other species (mostly Atalantia). Of these, S. disticha is the one most similar to

S. buxifolia and the other most common Severinia accession in ex situ collections. Some species of Severinia are considered rare by Jones (1990).

 

 

Luvunga

Luvunga, Paramignya, Pamburus, Triphasia, Merope, Monanthocitrus and Wenzelia are the ‘minor citroid fruit trees’. These genera do not have pulp vesicles, but the fruit often contains mucilaginous gum.

Luvunga, along with the very similar Paramignya, comprises the Luvunga group of the ‘minor citroid fruit trees’. Luvunga is a genus of 12 rather non-diverse species, which are not well known or characterized. Unlike many other genera within the Aurantioideae, it has retained the same


generic name since the 1830s (Swingle, 1943). Luvunga is known mostly from the work of Swingle (1943), which relies mainly on old descriptions from herbaria specimens. A new species, L. minutifl ora, was proposed by Stone (1985c). Luvunga is undoubtedly one taxon that, when defi ni- tively studied, will end up being different from the present description.

All species of Luvunga are woody vines of the south and South-east Asian continent and archipelagos. These vines cling to the forest trees by recurved thorns borne in the leaf axils. The leaves are usually trifoliolate with long, wingless clusters. Occasionally Luvunga bears unifoliolate leaves, which have shorter petioles than usual and thus resemble normal Paramignya leaves. The smallish fruits are yellowish and fi lled with mucilage. The rough skin has small oil glands on the peel and there are larger ones between the peel and the locules.

 

 

Paramignya

Paramignya is closely related to Luvunga and resembles it in general appearance. Surprisingly, most species were formerly included in Atalantia despite such pro- nounced differences as the lack of pulp vesicles in Paramignya. It is native to the same geographical area as Luvunga and shares its vinous habit. The main vegetative difference is that Paramignya has unifolio- late leaves borne on short petioles. The leaves have a well-defi ned pulvinus so that they can maximize interception of sunlight. The fruits of Paramignya and Luvunga are very similar.

Also like Luvunga, Paramignya is not a well characterized genus. The 15 species are apparently not very diverse and most are known mostly from herbari- um specimens. Several species were added to Paramignya by Swingle (1938, 1940a). Like Luvunga, Paramignya will undoubt- edly be very different from what is cur- rently thought when it is definitively studied.


 


 

Triphasia

Triphasia and Pamburus comprise the Triphasia group of the ‘minor citroid fruit trees’. In the work of Swingle (1943), this group differs from the Luvunga group pri- marily in leaf and spine characteristics. The Triphasia group are small trees or shrubs rather than vines, and do not have recurved thorns. The petioles are not articulated and there are no pulvini. They are native to the south-eastern Asian archipelagos.

Triphasia has small reddish or pink fruits with 3–5 locules of one or two ovules apiece in the ovaries. They are sometimes used for marmalade. The best-known species, T. trifolia, was for many years the only species in the genus. It is an attractive small tree or shrub with trifoliate leaves, and is grown as an ornamental in some tropical and subtropical regions. It is some- what sensitive to cold, but has been said to show some salt tolerance. It is also the only species usually found in ex situ collections. Despite this relative commonality, Triphasia is not well characterized (Table 4.3). It is graft compatible with Citrus. Small (1933) and Brizicky (1962a) state that it has become naturalized in some areas of south Florida; however, current Floridians are not acquainted with this (K. D. Bowman, personal communication, ~2001). Observations in Riverside suggest that Triphasia is fairly tender. A 4 n form, T. tri- folia forma tetraploidea, was described by Swingle (1940c).

Other species are less well known. They include T. grandifolia, which appar- ently differ mainly in having large, simple leaves. Triphasia brassii was renamed by Swingle (1938) from the Paramignya brassii of White (1926).

 

 

Pamburus

Pamburus is a monotypic genus (P. missio- nis). It differs from Paramignya in the leaf characteristics and growth habit cited above, and from Triphasia in having single, rather than paired, spines; longer anthers;


 

and lacking an infl ation at the base of the fi laments. Pamburus missionis is a robust, thorny tree with dark green foliage. It is native to southern India and Sri Lanka, where its wood is sometimes used for fur- niture. Swingle (1943) stated that it was almost as cold tolerant as commonly culti- vated citrus, but it sometimes has been dif- fi cult to establish in the fi eld in subtropical climates. Although it is not uncommon in ex situ collections, little more has been published on Pamburus since the old Swingle treatments (1916a, 1943).

 

 

Wenzelia

The Wenzelia group of the Triphasiinae consists of the genera Wenzelia, Monanthocitrus, Oxanthera and Merope. They have 4–8 ovules (most commonly six) in each locule, and simple or unifoliolate leaves. All four genera are native to the South-east Asian archipelagos; only Merope has been reported from the mainland (Myanmar/Burma). These genera constitute some of the least known of the Aurantioideae. They are not well represented in ex situ collections and little is known of their horticultural properties.

Wenzelia has large, simple leaves, unique in the ‘minor citroid fruit trees’ except for Monanthocitrus. The fruits are ellipsoid with 6–8 seeds per locule immersed in a mucilaginous substance. All species of Wenzelia were described and published in the 20th century, but some- times from little material. Swingle (1943) recognized nine species divided into two subgenera (Wenzelia and Papualimo), which are perhaps somewhat dubious as valid subspecies (Stone, 1985b). Stone (1985b) reviewed the paucity of material with which Swingle was able to work and indicated a need for a modern revision based upon additional specimens as well as fi eld collections. Unfortunately, Stone died before being able to accomplish this, so the genus remains somewhat confused. The ‘clearly distinct’ (Swingle, 1943) W. palu-



dosa was transferred to Monanthocitrus paludosa by Stone (1985b).

Wenzelia species are shrubs. As stated, little is known of their horticultural proper- ties. However, their graft compatibility with Citrus is somewhat problematic.

 

 

Monanthocitrus

Monanthocitrus is very similar to Wenzelia, with the main difference being the spotted seeds of the former (Swingle, 1943). This seems a somewhat questionable character- istic upon which to separate genera, partic- ularly since Swingle had little material to work with and not all specimens examined had seeds. Stone (1985b) states that ‘The distinctness of Wenzelia and Monan- thocitrus remains somewhat problematical’ and ‘needs reconsideration’. Monan- thocitrus was construed as a monotypic genus (M. corniculata) by Swingle (1943). Monanthocitrus grandifl ora was transferred to Wenzelia by Swingle (1938, 1943). Stone (1985b) believed that it belonged better to Monanthocitrus but did not publish that combination. Swingle also assigned W. grandiflora and W. dolichophylla to Wenzelia rather than Monanthocitrus even though seeds were not available for inspec- tion. More recently, two new species have been published: M. bispinosa (Stone, 1985b) and M. oblanceolata (Stone and Jones, 1988). This genus is little known nor collected.

 

 

Oxanthera

Oxanthera is a distinctive genus of 4–5 species. The leaves of these shrubs or small trees are glabrous and glaucous, usually rather blunt at the tip and cuneate at the base, and borne on thornless branches. The leaves of three of the four species are thick and coriaceous, but O. undulata has thin leaves. The fl owers are large and fragrant. The fruits are distinctive in appearance, being elongate and longitudinally ribbed. In at least one species (O. neo-caledonica), the






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