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Conservation Status of Citrus Genetic Resources






Citrus is one of the most ancient crop species domesticated by humans. The long history of selection and vegetative propaga- tion has led to the perpetuation of ‘elite’ germplasm lines. This has often led to the neglect and disappearance of progenitor wild types. In addition, Citrus hybridizes readily, and in many cases possesses nucel- lar embryony. These factors can also reduce genetic diversity. Consequently, ‘wild’ citrus are relatively rare, mostly existing as scattered trees in remote areas rather than as pure stands. Where ‘natural’ populations are located, it is often diffi cult to determine whether they represent wild ancestors or are derived from naturalized forms of intro- duced or selected varieties.

Agricultural utilization of citrus has


involved a narrow range of genetic material, making this a genetically vulnerable crop. Genetic diversity in the centres of origin is also threatened or severely threatened by habitat loss caused by deforestation, popu- lation pressure, fi re, hydroelectric develop- ment, clearance for agriculture or other development, tourism, etc. (World Wide Fund for Nature and International Union for Conservation of Nature and Natural Resources, 1994–1997). These factors may be especially important in countries such as India and China, which have rapidly expanding populations coupled with rapid economic/industrial development. This can result in very rapid losses of habitat and genetic diversity. This situation makes ex situ conservation of genetic resources of citrus and related genera imperative. This statement is not meant to diminish the importance of in situ conservation and habitat preservation, but to put into per- spective the very real potential for loss of citrus genetic resources in situ.

Aurantioideae genera related to Citrus are utilized much less frequently and there- fore exist most often as ‘wild’ unselected types. These 32 genera are mostly tropical and of limited commercial importance. Therefore, there has been less attention focused upon them except by local inhabi- tants. These areas are also often in danger of habitat destruction, and therefore the threat of losing genetic diversity is also present for these genera, particularly since their repre- sentation on ex situ collections is very lim- ited.

Assessment of the genetic vulnerability of any crop requires knowledge of the extent and distribution of genetic diversity. This is acquired by systematic sampling and mapping of the fl ora of the geographical areas in which the species in question are found, as well as an assessment of ex situ collections. Unfortunately, information on natural and semi-natural Citrus and other Aurantioideae germplasm is limited on the international level. This is due to the remoteness of some of the material, a lack of resources devoted to assessing these areas, and in a few instances political unrest or


 


 

other unsettled conditions. In some cases, information may be available at the local or national level, but not to the international genetic resource conservation community.

The information that is available is often simply a catalogue (sometimes quite old) of plants present in an area, with little more than names and phenotypic descrip- tions. Often even information on the fre- quency of occurrence is lacking. More detailed characterization and evaluation data are needed to assess adequately the actual amount of genetic diversity present. These data should include both descriptive phenotypic data and molecular genetic analysis of germplasm existing both in situ and ex situ (Albrigo et al., 1997; Gmitter et al., 1999).

The status of citrus genetic resources and their conservation has been reviewed by Reuther (1977), the International Board


 

for Plant Genetic Resources (1982), Albrigo (1997, 2001) and Broadbent et al. (1999). A limited amount of information is found in Food and Agriculture Organization of the United Nations (1996). Rouse (1988) and Bettencourt et al. (1992) have summarized the world citrus collection situation, identi- fying major and minor citrus collections. Outside of the centres of origin/diversity, collections consist mostly of advanced lines and commercial varieties. Large ex situ citrus collections of this sort are found in Argentina, Australia, Brazil, Corsica (France), Morocco, New Zealand, South Africa, Spain, Turkey and the USA. Some of the larger collections contain many selec- tions of the same variety, and so the genetic diversity is less than might be expected from the number of accessions.

Specifi c information on the status of citrus genetic resources in many countries


 

 

 
 

Table 4.4. Information sources for citrus germplasm status in various countries and areas.

Country or geographic area References

South East Asia Mehra and Sastrapodja, 1988; Jones, 1990; Verheij and Coronel, 1991;

Coronel, 1995; Osman et al., 1995; Saamin and Ko, 1997a, b; Hor et al., 1999

Thailand Anupunt, 1999

Philippines Garces, 1999

Malaysia Santiago and Sarkawi, 1962; Allen, 1967; Jones, 1985, 1989, 1991; Jones and Ghani, 1987; Saamin and Ko, 1997a, b; Ko, 1999

Vietnam Ca, 1999; Le et al., 1999

China Yin-min, 1985; Hu, 1989; Gmitter and Hu, 1989, 1990; Zhaomin, 1989;

Zhang et al., 1992; Xueqin, 1995; Zhusheng, 1997, 1999; Zhusheng et al., 1996; Deng et al., 1997a, b; Weidong et al., 2000

India Bhattacharya and Dutta, 1956; Dutta, 1958; Singh, 1981; Singh, 1985; Dass, 1990; Singh and Chadha, 1993; Chadha, 1995; Singh and Uma, 1995; Chadha and Singh, 1996; Rai et al., 1997a, b; Ghosh, 1999

Nepal Chaudhary, 1999

Japan Nishida et al., 1981; Iwamasa, 1988; Omura, 1996, 1997; Nito et al.,

Australia Armstrong, 1975; Forsyth, 1988; Sykes, 1993, 1997, 1999; Mabberley,

Spain Ortiz et al., 1988

Turkey Tuzcu, 2001

Cyprus Georgiou, 2001

Morocco El-Otmani et al., 1990

Latin America Anderson, 2001

Brazil Machado, 1997

 
 

USA Cameron, 1974; Reuther, 1988


 


and geographical areas is available from various sources, as referenced in Table 4.4. There is undoubtedly more information available in governmental or non-govern- mental organization (NGO) offi ces in some countries, but this is not generally available to the public. Table 4.4 includes informa- tion on countries that have only ex situ col- lections, as well as countries having in situ resources. These ex situ collections may maintain elite lines, undeveloped material or both. In addition, they may contain breeding lines, synthetic hybrids and related genera. All these are sources of genetic material for the breeder or biotech- nologist. The following brief comments summarize some of the information from the sources in Table 4.4, with an emphasis on areas near the centre of origin.

Southern PR China is one of the centres of diversity for Citrus and related genera and a wide range of genetic diversity is apparently still present in situ. However, some (though not all) areas are threatened with habitat degradation or lack of proper management that could result in decreases in genetic diversity. In PR China, explo- ration and collection of indigenous citrus genetic resources began in the 1950s and 1960s, but was interrupted by the Cultural Revolution of 1967–1972. Governmental surveys resumed during the 1970s and 1980s and uncovered a number of new putative species, including Citrus hongheensis, C. mangshanensis, C. daoxia- nensis and Poncirus polyandra. These puta- tive new species are mostly incompletely known outside of PR China. Areas that have been explored include Guangxi district, Guangzi province; Shennon Jia, Hubei province; Sichuan, Gansu and Shanxi provinces; Hainan Island and Tibet. There are also a number of indigenous Aurantioideae in southern China. There is exploitation (use) of indigenous germplasm and some attempts at in situ preservation have been made. However, conservation of citrus genetic resources in PR China is mostly ex situ at present. Beginning in the early 1960s, a National Citrus Germplasm Repository was established at Beibei,


Chongqing, Sichuan province. This is a large collection with over 1000 accessions. Regional citrus germplasm repositories were also established in Huangyan, Zhejiang province; Guiling, Guangxi province; Zhangsa, Hunan province; Guangzhou, Guangdong province; Jiangjin, Sichuan province; Wuzhung province and Hubei province. The exact composition of these collections is mostly unknown out- side PR China, but a high percentage is indigenous germplasm and undoubtedly represents a substantial amount of diver- sity, although some of the germplasm, indigenous and otherwise, consists of advanced lines or selections. The acces- sions at the repositories have had a limited amount of characterization and evaluation done on them. There have apparently been some diffi culties in maintenance owing to such factors as lack of funds, disease and weather (freezes and fl oods). Recently the Chinese government has provided increased resources for the support of the Beibei collection and it is again in a stable condition. The Beibei collection currently has slightly over 1000 accessions, including

296 local selections and 116 wild selec- tions. Huazhong Agricultural University maintains collections of 280 accessions in vivo and 110 accessions in vitro (X.X. Deng, personal communication, 2007). Regional Citrus Research Institutes in Shantou, Guangdong; Ichang, Hubei; Thouyang, Hunan; Ganzhou, Jianxi; Yuchi, Yunnan and Wu, Jiangsu maintain small amounts of citrus germplasm, as do botanic gardens such as Xithanbanna and Guanzhon.

In India, the north-east region is the centre of origin/diversity. Unfortunately, this region has sometimes experienced civil unrest, making evaluation of genetic diver- sity and plant exploration diffi cult. There are apparently a few stands of ‘wild’ citrus in these areas, but many of the ‘wild’ popu- lations apparently consist of dooryard plantings. A long history of cultivation and selection has produced many genotypes/landraces, which are diffi cult to separate from ‘wild’ citrus. Still, a wide range of genetic diversity undoubtedly


 


 

exists in these areas. There is an in situ gene sanctuary for citrus in the Garo Hills of the north-east, which is a field gene bank reported to have over 600 accessions. Other regions of diversity include the central and northwest Himalayas, Maharashtra and the southern peninsula. Ex situ conservation of citrus germplasm began in the 1950s in India, and there are now collections at eight sites (Chetalli, Bangalore, Rahuri, Tirupati, Abohar, Bhatinda, Yercaud and New Delhi), with smaller collections at 14 addi- tional sites (Akola, Barapani, Birouli, Hessaraghatta, Katol, Ludhiana, Nurpur, Parbhani, Pantnagar, Pedong, Periyakulam, Sirmour, Srirampur and Tinsukia). Total accessions are probably in the neighbour- hood of 600 accessions. The ex situ collec- tions in India are mostly of rootstock varieties and a few local cultivars, repre- senting less genetic diversity than might be expected. Many of the indigenous types described in historical accounts such as Bonavia (1890) and later works such as those of Bhattacharya and Dutta (1956) and Dutta (1958) are apparently not in any of the collections. As in China, there are apparently problems with the maintenance of these collections. The intention of the Indian government is to concentrate acces- sions from the various collections at the National Research Centre for Citrus in Nagpur and/or at regional research centres at Bangalore Tirupati (south), Ludhiana/Abohar (north), Rahuri (central), and Shillong and Assam (north-east).

South-east Asia (including Malaysia) is rich in indigenous germplasm and, with chance seedlings, semi-wild and wild types. Most indigenous types of citrus are grown in the hot lowlands. One species (C. halimii) is still found wild in the highlands, while the majority of the others are culti- vated. Some introduced Aurantioideae species (e.g. Aegle marmelos and Limonia acidissima) have become naturalized. This genetic diversity is threatened by deforesta- tion, development and disease. In 1983–1988, IBPGR coordinated four collect- ing missions to Thailand, Malaysia, Indonesia and Brunei. The materials are


 

maintained in Japan, the organizer of the missions. There are four collections in Malaysia, the main one being the University of Malaya (Rimba Ilmu) Botanical Garden. This was established at the request of IBPGR in 1986. Other Malaysian collections at Jerangau Station, Trengganu, Kuala Kangsar and Cameron Highlands are maintained by the Malaysian Agricultural Research and Development Institute. There are also some in situ con- servation efforts, such as at the Taman Negara National Park in Pahang and the Danum Valley in Sabah. There are appar- ently three collections in Thailand with over 500 total accessions. The most impor- tant are Phichit Horticultural Research Center, which has a collection of mostly native pummelos; and Nan Horticultural Research Station, which has approximately 70 accessions of mandarins, sweet oranges and citrus relatives, of which approxi- mately 25% are native. In the Philippines, the main collection of citrus genetic resources is maintained by the national Plant Genetic Resources Laboratory of the Institute for Plant Breeding in Los Bañ os, and consists mostly of commercial and imported varieties; there are said to be two other collections with slightly over 100 accessions. There are also three collections in Indonesia (~500 accessions) and at least two in Vietnam (National Institute of Agricultural Science and Technology and Phu Ho Fruit Research Center), which con- tain materials collected by an IBPGR-spon- sored programme in 1992.

Asia’s largest collections, outside of the centres of origin discussed above, are in Japan. Citrus entered Japan in ancient times, compared with its appearance in countries farther away from the centres of origin, and some types became semi-naturalized. There is a limited amount of in situ preservation of these naturalized types but, as in other areas, development is a threat. The Fruit Tree Research Stations in Tsukuba, Okitsu and Kuchinotsu have large collections that have a number of citrus relatives. Total accessions were said to be over 1200 in 1996. Of interest are the large numbers of


 


mandarin types, especially satsumas. There are also three other collections of citrus germplasm in Nagasaki, Kagoshima and Okinawa. Japan has been active in collect- ing in South-east Asia (see above), Nepal (1983–1985) and Vietnam (1996) in IBPGR- coordinated cooperative programmes. Accessions collected from these ventures are maintained in Japan.

Australia has several ex situ collections maintained by State Government Departments of Primary Industries and the Commonwealth Scientifi c and Industrial Research Organization (CSIRO) that consist primarily of cultivated types. However, this island country is the centre of origin for several related genera (most notably Eremocitrus and Microcitrus), that are included in the collections, as well as in certain botanic gardens (e.g. Royal Botanic Garden, Sydney, and Brisbane Botanic Gardens) and arboreta (e.g. Waite Research Institute, University of Adelaide Arboretum). Also of interest are hybrids of these native types.

The situation with the related Aurantioideae genera is less well known, particularly from outside the southern/south-east Asian region. Although these genera are sometimes repre- sented in collections, there is little informa- tion available about their status in situ. However, as many of them originated in countries which are currently rapidly developing, experiencing population growth and pressure, or being bothered with civil unrest, it is probable that at least some native populations exist in habitats which may be threatened. These factors also make assessment of the situation diffi - cult.

Overall, the genetic diversity of Citrus and related Aurantioideae is vulnerable. Habitat loss is common in areas in which these plants are endemic, and eco-geo- graphic assessments of these areas are often lacking. Although some efforts are being made in the areas of in situ and ex situ con- servation, it is probable that there has been considerable genetic erosion for these species. Due to the lack of eco-geographic


information, as well as characterization and evaluation data from the ex situ collections, it is impossible to say to what extent this erosion has occurred. It is imperative that more resources be devoted to these areas in the future.

Due to these factors, it has recently become evident that more intensive interac- tions and coordination between the various entities dealing with citrus germplasm con- servation is necessary (Albrigo, 1999; Ramanatha Rao and Arora, 1999). A pro- posal to establish a global network on citrus genetic resources conservation and utiliza- tion was recommended during the meeting of the FAO Intergovernmental Group on Citrus in April 1996. Accordingly, this pro- posal was followed up and further elabo- rated during the Symposium on the Conservation of Genetic Resources of Citrus and its Relatives, held in South Africa in May 1996, where the major technical issues to be addressed by a global cooperative pro- gramme were analysed (Albrigo, 1997).

The Global Citrus Germplasm Network (GCGN) was formally constituted under the aegis of the FAO. The GCGN will function on a voluntary basis and will involve national institutions as well as existing regional and inter-regional networks deal- ing with citrus genetic resources conserva- tion and utilization (Global Citrus Germplasm Network, 1998). It will serve to link different initiatives in different parts of the world dealing with citrus genetic resources exploration, conservation and utilization. The GCGN will also play a role in harmonizing and strengthening ongoing networking initiatives that are dealing with citrus germplasm conservation and utiliza- tion, and in promoting new undertakings in different regions of the world. The existing regional and inter-regional citrus networks (IACNET (Americas), MECINET (Medi- terranean region)) and those under consti- tution (Asia-Pacific and sub-Saharan Africa) will participate in the GCGN. The Global Network will be guided by a Coordinating Board chaired by the General Coordinator of the Network, and will include the coordinators of the technical


 


 

working groups and representatives of the different regional and inter-regional citrus networks. More information is available in Broadbent et al. (1999), Global Citrus Germplasm Network (1998), Albrigo (2001), and on the Internet (< https://www.lal.ufl.edu/CONGRESS/Gcgn rept.html>). It is hoped that this sort of international cooperation will increase the effi ciency of citrus genetic resource conser- vation efforts.

 

 






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