The general name of plants with certain genetic material and valuable for citrus production and breeding. Most scholars define citrus plants as a group of plants with typical citrus fruits in Rutaceae, Aurantioideae, Citrea and Citr-inae. Also known as real Citrus fruit trees, there are ***6 genera, namely Fortunella, Er-emocitrus, Poncirus, Clymenia, Microcitrus and Citrus. Among them, the most economically important genus is Citrus, with its origin center in China, India and Southeast Asia, followed by Kumquat and Fructus Aurantii, with its origin center in China. The other three genera are distributed in Australia and its vicinity, and they do not occupy an important position in economy.
The characteristics and diversity of citrus germplasm resources
Southeast Asia and Oceania are the natural distribution areas of real citrus fruit groups, and there are many kinds, but most of them belong to narrow-spread species in geographical distribution, so the genetic genes among the lines are very different, and at the same time they are internally related. The wild original species of citrus are mostly distributed in inaccessible areas, such as C.ichangensis Swingle, which is distributed in the middle and upper reaches of the Yangtze River in China. C.hongheensis YLDL., only distributed in the southeast of Yunnan Province, China; Taiwan Province tangerine (C.tachibana Tanaka) is only distributed in Japan except Taiwan Province Province of China and nearby islands. C.indica Tanaka is only distributed in a narrow area of Assam, India. Citrus cultivation has a long history and a vast geographical distribution. Citrus cultivation resources in different places are self-contained and vary from place to place, forming genetic diversity. It is more obvious in the related plants of citrus, such as Poncirus trifo-liata Rafin., which has defoliation and can withstand the low temperature of -26℃, while other citrus can hardly survive under such conditions: under suitable conditions, the one-year-old seedlings of Citrus papuana can blossom and bear fruit, while the one-year-old seedlings of Citrus glauca Swing can bear fruit.
Fertility of distant hybridization
Interspecific hybridization of citrus plants is easy to succeed, and there are many examples of successful interspecific hybridization. The common intergeneric hybrids in production are shown in table.
Eremoradia, a hybrid between Australian lemon and lime, is more important. Eremora-nge, a hybrid with sweet orange; Faustrim, a hybrid between Australian orange lemon and kumquat, etc. The diversity of germplasm increases the selection opportunities when it is used, but it also brings difficulties in collection and preservation. The seeds of citrus plants are multi-embryo, and many nucellar embryos are one zygote embryo, and nucellar embryos are dominant, and zygote embryos are often aborted. Therefore, no matter whether it is a sexual hybrid or a somatic mutation, even through seed reproduction, most of them can be preserved.
The main citrus germplasm resources
According to Zeng Mian's opinion, the classification of citrus plants can be divided into the following categories:
Large-winged oranges
There are C.hongheensis, C.hystrix DC, C. Yichang Orange, etc. in China, and the winged leaves are very large. Honghe Orange and Yichang Orange are native to China and distributed in the wild. There are many natural wild types of citrus. Some of them have been used as rootstocks, such as C.junos(Sieb.)Tanaka and C.macrophylla Wester, which are rootstocks of lemons.
citron lemons
there are citron (C. medica L.), lemon (C. limonburm.f.), 7351 lemon (C.limonia Osbeck) and lemon (C.aurantifolia Swingle). Winged leaves are small or hardly winged, and the juice cells taste sour. Besides being cultivated as sour citrus, they are mostly used as rootstocks. A kind of rough lemon originated from hybrid, which has strong root system, is resistant to drought and other virus diseases, but susceptible to root rot and nematodes. It is widely used as the rootstock of sweet oranges, grapefruit and oranges in the United States, Argentina, Australia, India and South Africa. High yield, but poor fruit quality, not suitable for the production of fresh fruit rootstock. 7351 lemon is used as citrus rootstock in southern China. Laimeng has triploid type and seedless fruit, which is the main citrus for sour fruit in tropical areas. It can also be used as the rootstock of lime or grapefruit and orange grapefruit, and its performance is similar to that of rough lemon. A lemon from Mexico can be used as an indicator plant of citrus virus. Grafting the citrus branches to be identified on it can tell whether it is infected with diseases such as recession in a short period of time.
pomelo
There are C.grandis Osbeck and C. paladis ii macf. Pomelo is native to China, and some people think that grapefruit is a hybrid of pomelo and sweet orange. Pomelo is a potential germplasm resource with large fruit, single embryo and many varieties. Acid-free varieties include Shatian pomelo, Liangping pomelo and Dianjiang pomelo. Sweet and sour varieties are Wendan and Late White Pomelo. In addition, there are many seedless types. As citrus rootstocks, pomelo and grapefruit are not ideal.
Oranges
There are sweet oranges (C.sinensis Osbeck) and sour oranges (C. aurantium L.) in China. Sweet oranges are the main citrus species in the world, and many varieties and strains have been selected for long-term cultivation in various places. In addition to ordinary sweet oranges, there are navel oranges with seedless fruits and blood oranges with red flesh. There are many varieties (lines) of common sweet oranges in China, such as Xinhui orange and Liuorange, Jincheng orange and Xianfeng orange, and Fujian improved orange selected from orange chimera. As a rootstock, sweet orange is not widely used, and lime is widely used in the world. Lime is a deep-rooted plant, which is the rootstock of sweet orange, grapefruit and orange. The fruit quality is excellent, but the disadvantage is that it is not resistant to decline disease and is vulnerable to nematodes. Citrus aurantium cv. Goutou in Zhejiang, China is a promising rootstock resource against recession.
Wide-skinned citrus
The flowers are solitary, the skin can be peeled off, and the seeds are green, which is different from the above categories. Originated in China, it has many varieties and is a treasure house of wide-skinned citrus. C.reticulata Blanco is widely cultivated with China as the center, Iraqi in the west and Philippine in the east. C.tan-gerina Hort.ex Tanaka is the main cultivated variety in Sichuan and Fujian. C.kinokuni Hort.ex Tanaka is a main variety in Jiangxi and Zhejiang provinces with small fruit, sweet taste and few stones. There are many natural hybrids, such as C.unshiu Marc., with seedless fruit, high yield and good taste, which are widely cultivated in China and Japan. C.tankan Hayata is widely cultivated in Guangdong and southern Guangxi. C.clementina Hort. is the main citrus planted in Mediterranean area, and it is widely used as a hybrid parent. Wide-peel oranges are widely used as rootstocks in China. Red orange, native orange, fragrant orange and built orange are all good rootstocks of sweet orange in Sichuan. Cleoptra was used as the rootstock in America to graft oranges, tangerines and grapefruit, which performed well in Florida. In Brazil, sour orange is a good rootstock for sweet orange.
related plants of citrus
mainly refer to five genera of plants in the real citrus fruit tree group of citrus subfamily and citrus subfamily of Rutaceae except citrus. Among these related plants, except for the genus Thymus, the ones that have been utilized and evaluated are:
Kumquat
China, with about 6 species and 1 variety. There are single embryo species and multi-embryo species with seeds, and natural tetraploid resources are found. Small fruit, edible skin, flowering many times a year, cold tolerance, shorter childhood than citrus, seedling is more sensitive to copper deficiency in soil. It is a germplasm with obvious dormancy period and pericarp oil characteristics.
The genus Hovenia
originated in China, is a deciduous tree, which is extremely cold-resistant, resistant to -26℃ low temperature, citrus nematodiasis, gummosis and decline disease, but not resistant to peeling disease and sensitive to soil with high chlorine and excessive lime. It is one of the important rootstocks in the world (see figure). Many named rootstock clones have been selected. More than 3 clones of Fructus Aurantii have been bred in Japan, which can be divided into two types: large flower type and small flower type, and there is no great difference among the clones. The variation types and natural hybrid resources of China Fructus Aurantii are very rich. Orange rootstock has replaced lime rootstock in many areas, and many selection lines have been named, which have different performances on different scion varieties in different areas. In addition to orange, there are many hybrids between orange and other citrus. Hybrid with orange (Citrandarin), hybrid with lime, hybrid with lemon (Citremons), hybrid with sweet orange (Citranges), hybrid with kumquat (Citrangemelons) and so on. The selected lines of them have been tried as rootstocks of citrus, and only Citrumelos, which is a hybrid with grapefruit, can be used for economic production, and the others are not as promising as Citrus.
The genus
is native to Migan County, eastern Australia, and its childhood lasts for more than 1 years. This genus is a germplasm resource with cold resistance, extremely early maturity and obvious dormancy period. It is easy to cross with citrus plants, but its grafting affinity with citrus plants is poor. The hybrid between them and citrus can survive by grafting, which is hopeful to be used as rootstock resources.
Australian orange lemon
is native to the rain forest in eastern Australia and New Guinea. It's an evergreen shrub. Juice is very sour, free from cold, and some strains are resistant to mosquito nematodes and gummosis, and their childhood is short. For example, one-year-old seedlings of M.papuana can blossom and bear fruit in greenhouse.
studies in the United States show that the genus Citropsis, Hespeyethusa, Citrus grandis, Clausena, Clymenia, Atalantia and Pleiospermium are the most promising ones to obtain new citrus. It is promising to try to use related species of citrus as germplasm for breeding. However, due to the long generation, apogamy and heterozygosity, citrus hybrid breeding can not achieve rapid results and is often frustrated. Schwinger and others have bred hundreds of hybrids, only a small part of which can be used as rootstocks. More distant relatives, such as Cherry Citrus, Clycosmis and Severinia, failed to cross with Citrus and Citrus. Australian sand lemon has been successfully crossed with lime, orange, sweet orange, kumquat and Australian orange lemon.
research on citrus germplasm resources
investigation and collection of resources
with the development of modern industrial and agricultural production, wild citrus germplasm resources and ancient cultivation types have been seriously lost. It is an important work to accelerate the investigation and collection of citrus germplasm resources. In November, 1981, the International Commission on Plant Genetic Resources (IBPGR) held a meeting of citrus professional group in Tsukuba, Japan. Nine countries and regions, including China, Brazil, Indonesia, Italy, France, Japan, Spain and the United States, attended the meeting. After the meeting, two regional international collaborative surveys were organized, namely, the 1983 survey of citrus germplasm resources in Southeast Asia and the 1985 survey of citrus germplasm resources in Nepalese mountainous areas.
China is an important center of citrus origin in the world. Since the founding of the People's Republic of China, fruit tree resources including citrus have been investigated one after another. In 198s, the investigation of crop variety resources was widely carried out, and the investigation of citrus resources was listed as an important content. Through investigation, a number of original wild types of citrus have been discovered in Tibet, Sichuan, Guangxi, Hunan, Yunnan and Jiangxi, and some new species have been identified, and a number of local variety resources have been collected. At the same time, the collection of foreign germplasm resources was also carried out.
classification
there are two main classification systems for citrus. The classification of citrus plants, especially citrus, has great differences in subgenus division, classification level and species status determination. According to the identified species, W.T.Swingle identified 16 species of citrus. In 1954, Tanaka Saburo was designated as more than 15 species, and in 1977, it developed into 162 species on this basis. The focus of the differences between the two classification systems lies in the different understanding of species and the basis for their division. In 1961, R.W.Hodgson of the United States put forward an intermediate classification scheme, which was mainly based on Schwinger's classification system and recognized some species in Tanaka. The divergence of classification system requires us to find scientific methods and new ways to identify the genetic relationship on the basis of previous classification studies. At present, scholars at home and abroad are mainly devoted to the research and application of gas chromatography analysis of essential oil and other components in citrus organs, observation and comparison of pollen morphology under electron microscope, isozyme analysis, numerical classification by computer and so on.
Preservation
Establishing a germplasm resource garden to preserve citrus germplasm resources is the key to research and utilization. The Citrus Research Institute of China Academy of Agricultural Sciences in Chongqing, Sichuan Province has been designated as the national citrus germplasm resource garden by the Chinese people and the Ministry of Agriculture, and the garden now holds 1,2 citrus germplasm resources. Its tasks are: ① properly preserve citrus germplasm resources; (2) Obtain the objective evaluation of biological characters and responses to environmental conditions; (3) provide wax leaves and various organ specimens; (4) provide cross breeding parents and all kinds of test materials; ⑤ Provide pure breeding materials without dangerous pests and diseases. In the long run, the preservation of citrus germplasm resources should help to solve various problems that have not been recognized at present. In addition to widely preserving citrus germplasm resources, special attention should be paid to collecting germplasm with special resistance or high yield and high quality.
Citrus is a perennial plant with a long young age. It mainly relies on grafting and propagation to preserve its variety characteristics, so it is not suitable for seed preservation. Citrus germplasm resources are mainly preserved by planting. At the same time, research on other preservation methods will be carried out. In order to obtain resource materials that conform to genetic characteristics without dangerous virus diseases, countries all over the world are exploring the establishment of reasonable propagation and preservation procedures.
evaluation and data processing
evaluation is an important daily scientific and technological work of citrus germplasm garden, and the evaluation results and data processing are the basis of archival work of citrus germplasm garden and the basis of germplasm resources utilization. There is a two-level evaluation system for the evaluation of citrus germplasm resources, that is, the basic evaluation by the germplasm resources nursery and the utilization evaluation by the utilization unit after further experiments and utilization. The data obtained from the latter's further evaluation should be fed back to the germplasm resources nursery and compiled into the data file system. The main contents of the basic evaluation are morphological characteristics, flowering and fruiting habits, stress resistance, genetic traits, gene identification and biochemical analysis. Utilization evaluation includes natural conditions and plant growth performance, fruit quality, stress resistance, rootstock test, breeding utilization results, etc. In order to facilitate international communication, the International Commission on Plant Genetic Resources put forward the Catalogue of Main Characters of Citrus Data Processing in 1985 for countries to try to modify in data processing.