Conservation and Utilization of Actinidia Germplasm Resources in China

Abstract China is very rich in germplasm resources of Actinidia Lindl. There are 66 actinidia species in the world， and 62 species are found in China. A. deliciosa， A. chinensis Planch.， A. eriantha Benth. and A. arguta （Sieb. & Zucc） Planch. ex Miq. unique to China all have high economic value and are widely cultivated. In this paper， the origin and cultivation history， nutritional value， preservation and rational use of Actinidia germplasm resources were reviewed， and the development and utilization of Actinidia germplasm resources in China was discussed， aiming to facilitate the sustainable utilization and variety improvement of kiwifruit germplasm resources.

Key words Actinidia Lindl; Germplasm resources; Preservation; Utilization

Received： August 10， 2021  Accepted： October 15， 2021

Supported by Dalian Science and Technology Innovation Fund （2019J13SN120）.

Yilong HOU （1963-）， male， P. R. China， PhD， devoted to research about plant biotechnology.

*Corresponding author.

Actinidia Lindl is aliased as kiwi， and commonly known as Yangtao， Tengli and Houzaili in China. It is called ‘Chinese gooseberry’ in Britain and the United States， and ‘kiwi fruit’ in New Zealand. It is a deciduous vine berry tree， belonging to Actinidia of Actinidiaceae in botany[1-2]. It is one of the four most accomplished tree species for artificial domestication and cultivation of wild fruit trees in the 20th century （the remaining three tree species are American avocado， Eastern American bilberry and macadamia nut）. Because of its unique flavor， rich nutrition， high Vc content， high economy and nutritional value and good medical effects， it has attracted much attention. China is the origin and distribution center of Actinidia， and is extremely rich in its germplasm resources[3]. In the past 20 years， extensive research has been conducted in the investigation， collection， sorting， evaluation and utilization of Actinidia resources， and the focus of collection and utilization has gradually expanded from cultivated species to wild species and wild relatives， especially germplasm resources with specific traits. In addition， comprehensive identification and evaluation of its varieties， distribution and classification and main traits， including quality， stress resistance， cytology， biochemical identification and genetic evaluation， etc.， have been carried out. The current status and prospects of its research and utilization were summarized here to provide references for sustainable utilization and variety improvement of Actinidia germplasm resources.

Germplasm resources are also called genetic resources. Germplasms refer to the genetic materials passed from parents to offspring of crops， which often exist in specific varieties， such as ancient landraces， newly bred varieties， important genetic materials and wild relatives， all belonging to the scope of germplasm resources. Germplasm resources of Actinidia refer to the general term for plants with certain genetic materials that are valuable in kiwi production and breeding[4]. Germplasm resources can be as large as a complex of genetic stocks， as small as a certain gene fragment that controls individual genetic trait. Germplasm resources not only provide basic materials such as food and clothing and a good ecological environment for human society， but also provide rich genetic materials for the selection and breeding of high-quality new varieties， and provide the most basic raw materials for understanding and studying biological species. With the continuous progress of society and the rapid development of science and technology， the protection and development of germplasm resources have been paid more and more attention.

Origin and Cultivation History of Actinidia

Actinidia is an ancient plant in China， and China is the origin center of Actinidia in the world. Guo Shuangxing from Nanjing Institute of Geology and Palaeontology， Chinese Academy of Sciences， discovered several leaf fossils during an inspection in Tiandong County， Guangxi Zhuang Autonomous Region in 1977. According to the analysis of the fossil geological age， it is believed that the fossils are Actinidia leaf fossils， which belong to the early Miocene fossils， about 26 million years ago[2].

Actinidia has a history of more than 2 000 years in Chinese written records， and at least more than 1 000 years of history as a tree species for garden greening. There are many records and descriptions of Actinidia in the early Chinese classical documents such as The Book of Songs and The Classic of Mountains and Seas. In the Poetry·Huifeng about 2 800 years ago， it was recorded that kiwi was found in Mixian County， Henan Province. At that time， people called it Changchu. Cen Shen （714-770 AD）， a poet of the Tang Dynasty， has a poem saying "kiwi vines on a frame on the well rail of the atrium"， which shows that kiwi was used as a tree species for garden greening at that time. In the Materia Medica compiled in the past dynasties， there are mentions about the medicinal and edible usage and morphological characteristics of Actinidia. Chen Zangqi （Tang） recorded in A Supplement to Materia Medica： "Actinidia is salty， warm and non-toxic， and can be used for medicinal purposes. It mainly treats joint wind， paralysis， long-term white hair， hemorrhoids， etc.， and the skin can be used to make paper". Li Shizhen （Ming） recorded in Compendium of Materia Medica： "the fruit of Actinidia looks like a pear， and its color is like a peach. Macaques like eating the fruit of Actinidia， and it is thus called the name Mihoutao". For a long time， Actinidia had been in the wild state. The folks mainly collected the fruit for fresh food and wine making， and used the leaves as feed， and extracted the branches to get a pulp-regulating agent for papermaking. The use of the fruit of Actinidia as a commercial fruit and its artificial cultivation in China began in the late 1970s， with only more than 30 years of cultivation history[4].

Nutritional Value

The fruit of Actinidia is fat and juicy， fragrant and delicious， sweet and sour， and durable in storage. It has rich nutritional value and good medicinal value. It is rich in protein， sugar， vitamins A， C， E， potassium， magnesium， and cellulose， and also contains nutrients that are relatively rare in other fruits——folic acid， carotene， calcium， lutein， amino acids， and natural inositol. In particular， the fruit of Actinidia has a vitamin C content much higher than that of common fruits such as apples， pears， watermelons and citruses， and is thus known as the "king of fruits" or "king of Vc". Because the fruit of Actinidia is rich in vitamin C， it can strengthen the immune system， promote wound healing and the absorption of iron; its rich inositol and amino acids can inhibit depression and supplement the nutrients consumed by brain power; and its perfect ratio of low sodium and high potassium can supplement the physical strength lost by staying up late and working overtime， and has a good effect in maintaining cardiovascular health. Nowadays， the Actinidia industry has made great strides， and the fruit of Actinidia is no longer limited to fresh food. It has also been widely used in food cooking and beverage wine brewing. It is a nourishing and health food that people love. Modern medical research shows that the fruit of Actinidia contains excellent dietary fiber and rich antioxidants， which can clear away heat and reduce internal heat and moisten dryness for relaxing bowels， and can effectively prevent and treat constipation and hemorrhoids. The fruit of Actinidia is rich in arginine， which can effectively improve blood flow， prevent the formation of thrombus， and has certain effects on cardiovascular diseases such as coronary heart disease， hypertension， myocardial infarction， and arteriosclerosis. The fruit juice can effectively block the synthesis of N2 nitrosomorpholine in the human body， and has the effect of inhibiting digestive system cancers （such as esophageal cancer， gastric cancer， rectal cancer， etc.）[5]. In addition， Actinidia contains the anti-mutation ingredient glutathione which is beneficial to inhibit the mutation of cancer genes， and has a certain inhibitory effect on liver cancer， lung cancer， skin cancer， prostate cancer and other cancer cells. Yixing City of Jiangsu Province has made "anti-cancer tablets" with Actinidia roots， and Xi’an Mingde Hospital has made Actinidia oral liquid， which is quite effective in the treatment of liver cancer.

Conservation of Actinidia Germplasm Resources

Species and distribution

At present， except for 4 species， Vietnamese Sabah A. petelotii Diels， Nepal A. strigosa Hook. & Thoms.， and Japanese A. rufa Planch ex Miq and A. Hypoleuca Nakai， other 62 species of  Actinidia plants in the world are distributed in China， that is， most of them are unique to China， so Actinidia is also semi-endemic in China[6]. Except Qinghai， Xinjiang， and Inner Mongolia， Actinidia is distributed everywhere in China （18°-34°N latitude）. It is concentrated in the areas south of the Qinling Mountains and east of the Hengduan Mountains in China （25°-30°N latitude） and in the warm and humid mountain forests of southern China[7].

The plants with higher economic value mainly include A. deliciosa， A. chinensis Planch. and a small amount of A. eriantha Benth. and A. arguta （Sieb. & Zucc） Planch. ex Miq. Other Actinidia species are mostly in the wild or semi-wild state， and few are used for planting and production.

Distribution of main cultivars and variety resources

China is very rich in Actinidia variety resources. More than 60 cultivars and more than 1 400 excellent individual plants have been selected in more than 30 years[4]， most of which are derived from A. deliciosa and A. chinensis， and a small part are derived from A. arguta and A. eriantha.

① A. deliciosa： It is also known as the hard-haired kiwifruit. It is the most economically valuable， most important， directly usable cultivar in the genus Actinidia. Its hromosome is hexaploid， and the number is 174 （2n-174， 6X）. The main distribution areas are Yunnan， Guizhou， Sichuan， Guangxi， Hunan， Henan， Shaanxi， and Gansu， and it is also distributed in the mountainous areas of Guangxi， Yunnan， and Guizhou between 1 300-2 100 m. The main varieties are： Qinmei， Jinkui， Miliang 1， Xuxiang， Wancui， Qinxiang， Humei 1， Qincui， Xiangzhou 83802， Huamei 1 and Bangzeng 1[8-13].

② A. chinensis： It is also known as the soft-haired kiwifruit. It has high economic value. Its chromosome is diploid， and the number is 58 （2n-58， 2X）. The main distribution areas are Jiangxi， Hunan， Hubei， Fujian， Zhejiang， Henan， Anhui， Guangxi， Yunnan， and Guizhou. It grows in areas 200 to 600 m above sea level， and generally grows in mountain forests， shrubs， tall grass shrubs and secondary forests in low mountainous areas. The main varieties are： Zaoxian， Kuimi， Lushanxiang， Jinfeng， Yixiang， Suxiang， Hongyang， Jintao， Wuzhi No.3， Tongshan No.5， Taishanghuang， Cuiyu， Ganki No.5， Jinyang 1 No.， Qingyuan Qiucui， Moshan No.4， Zhengxiong No.1 and Fengxiong No.1[8-13].

③ A. arguta： It is also known as Ruanzaozi. The general chromosome is tetraploid， and the number is 116 （2n-116， 4X）. This species has strong cold resistance， can grow and develop normally even at a low temperature of -39 ℃， and has a long life span （≥150 years）. The fruit can be eaten fresh and processed， and the plant is generally used as a cold-resistant rootstock and parent material for cold-resistant breeding. It is mainly distributed in Anhui， Fujian， Gansu， Guangxi， Hebei， Heilongjiang， Henan， Hubei， Hunan， Jilin， Jiangsu， Jiangxi， Liaoning， Shaanxi， Shandong， Shanxi， Sichuan， Taiwan， Yunnan， Zhejiang， Chongqing. The main varieties are： Kuilu， Fenglu[8-13].

④ A. eriantha： It is also known as Maodonggua. It can be eaten freshly or after processing. This species has strong resistance to humidity and heat， and can also be used as a rootstock. It has great potential for development in southern China. The main distribution areas are in Anhui， Zhejiang， Fujian， Hubei， Hunan， Guangdong， Guangxi， Guizhou， with an altitude of 250-1 000 m. The main varieties are： Shanong No.18， Anzhang Maohua No.2[8-13].

Germplasm preservation

The preservation of plant germplasm resources is first of all the needs of plant breeding. Various germplasm resources are the basis for effective crop improvement[13]. Actinidia， as the precious germplasm resources mainly produced in China， is mostly in the wild or semi-wild state. Due to the deteriorating ecological conditions， some species are already endangered， so they are in urgent need of collection and preservation. The common preservation method is to collect seeds， branches and seedlings to establish a germplasm nursery. At present， the Actinidia germplasm nursery of Wuhan Institute of Botany， Chinese Academy of Sciences has collected and preserved a total of 55 species （variants）， 155 varieties （lines） and 6 endangered species of Actinidia （A. chrysantha， A. grandiflora， A. chinensis， A. indochinensis， A. pilosula， A. henanensis）， has become the world’s largest Actinidia germplasm gene bank[15].

In-vitro preservation includes tissue culture preservation and ultra-low temperature preservation. Test-tube plantlet preservation is a good way for short-term preservation of germplasms. Test-tube plantlets of Actinidia were stored on the medium supplemented with 0.5 mg/L PP333 at room temperature （19 ℃）， and the survival rate was 90% after 11 months. In addition， cryopreservation is a method of long-term preservation of germplasms， which refers to the technique of cryopreservation of germplasms below -80 ℃， usually called liquid nitrogen preservation or LN （-196 ℃） cryopreservation[16]. It is generally believed that there will be no harm in the process of ultra-low temperature storage of germplasms， and as long as the liquid nitrogen is constantly added， the frozen materials can be stored for a long time. There is no correlation between the storage time of kiwi calli in liquid nitrogen and its survival rate[17]. Using A. chinensis No.36 as a test material， after 120 d of liquid nitrogen storage， it showed a high survival rate during the re-cultivation process， and calli could be seen after 25 d of culture on the medium supplemented with 1 mg/L ZT， producing a large number of new plants[18].

Yilong HOU. Conservation and Utilization of Actinidia Germplasm Resources in China

Reasonable Use of Actinidia Germplasm Resources

Actinidia germplasm resources can be reasonably to produce standardized and high-quality fruit. In terms of cultivation， we should use modern and advanced fruit tree management methods， apply standardized and standardized green organic Actinidia production technologies， establish excellent fruit bases， expand organic Actinidia production， limit the use of chemical pesticides， fertilizers， growth regulators， etc.， and use organic fertilizers instead of chemical fertilizers. Actinidia can be developed in the most suitable areas， and good or distinctive varieties can be selected for production and adjustment of the variety structure. It is necessary to strengthen the construction of post-production treatment， storage and processing facilities， increase the added value of Actinidia， and take the road to industrialization. Meanwhile， the promotion of the use value of Actinidia should be strengthened， so that Actinidia from China will become a local characteristic fruit and go to the world.

Actinidia cultivation bases can be established in nature reserves. At present， 5 national nature reserves and 14 provincial nature reserves have been established， with a total area of 328 806 hm2[19]. Making full use of nature reserves and rafting cultivated scions to wild resources as rootstocks is valuable for improving the quality of seedlings.

China is rich in germplasm resources of Actinidia， and their respective characteristics can be fully utilized for development and utilization. For example， the polyploid germplasm resources of A. deliciosa are all natural hexaploid （such as Qinmei， Jinkui， etc.）， which can be directly used for production. Tetraploid A. chinensis （such as Kuimi， Yixiang， etc.） plays an important role in the selection of new Actinidia varieties， and its fruit is larger than that of diploid and has stronger resistance and better storability; the red flesh germplasm resource of A. chinensis ‘Hongyang’ has been promoted and cultivated in production; the special germplasm A. deliciosa varieties ‘Qinmei’ and ‘Jinkui’， A. chinensis varieties ‘Kuimi’ and ‘Taishanghuang’， etc. are all large-fruit germplasms， which are popular because of their large size; A. chinensis variety ‘Zaoxian’ is a very early maturing germplasm， and is the variety maturing earliest at home and abroad， and its fruit can be harvested in mid-to-late August; and the extremely storage-resistant germplasms include the excellent plant of A. chinensis ‘Feng 93-1’， which has a late maturity period （late October） and is storage-resistant， and can be stored for 40 d at room temperature. The excellent plant of A. deliciosa ‘E-30’， matures in early October， has strong storability， and can be stored for more than 60 d under normal temperature conditions; and Actinidia usually has more seeds， and the average number of seeds per fruit is 250-400， which affects its fresh-eating palatability. A. deliciosa ‘Xiangzhou 83802’ is a rare few-seed or seedless germplasm with an average number of seeds per fruit of 27-33， a maximum of 50 seeds and a minimum of 2 seeds. The excellent plant ‘Meiwei Wuzi No.1’ selected by Hunan Jishou University has no seeds in its fruit. Reasonable use of these high-quality Actinidia germplasm resources opens up new and broader prospects for the development of the Actinidia industry[20-23].

Conclusions

Actinidia is a new fruit tree industry originating in China. Although it has a long history of cultivation， the research on the utilization of its germplasm resources is not long. In recent years， it has attracted wide attention due to its high nutritional value， and many research results have been obtained in the preservation， identification， classification and breeding of specific varieties of its germplasm resources. In particular， A. deliciosa， A. chinensis， A. eriantha and A. arguta with high economic value have been produced. The basic research on Actinidia seed resources will lay the foundation for the rapid development of the Actinidia industry.

In order to make full use of China’s rich Actinidia germplasm resources and establish a distinctive Actinidia industry， China should do a well job in ① improving national awareness of the Actinidia industry and strengthening the protection of wild Actinidia germplasm resources， ② establishing a sound germplasm preservation system and focusing on researching specific germplasms such as polyploid， storage-tolerant， red-flesh and large-fruit Actinidia planting resources， ③ using biotechnologies to select germplasm resources with high economic value， large fruit size， few seeds， and good storage tolerance， and ④ strengthening the research of Actinidia resistance， so that Actinidia can be expanded in different areas.

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