Abstract Soybean is an important food crop and oil crop for humans. Based on the analysis of current production and research of soybean in Shandong Province and other provinces and cities， this paper clarified the background of soybean germplasm resources， evaluated the genetic diversity of soybean germplasm resources， revealed the evolution pattern of soybean varieties， summarized the utilization of wild-type soybean resources， and analyzed the prospects of research and application of soybean germplasm resources in Shandong Province.
　　Key words Soybean； Germplasm resources； Shandong Province； Identification
　　Soybean is an important oil crop and food crop for humans， which is rich in fats and proteins. Soybean is commonly used to extract soybean oil， make tofu and soy products， extract proteins and brew soy sauce. Soybean residues and soybean meal are commonly used as livestock feed[1].
　　At present， the major soybean producing countries in the world are the United States， Brazil， China， and Argentina successively. In China， soybean is mainly distributed in the northeast China， Huang-Huai-Hai region and south of the Yangtze River. In Shandong Province， soybean is mainly planted along the Yellow River， along the Weishan Lake， along the Dongping Lake and in the Jiaodong Hilly Region[2].
　　The number and scale of soybean processing enterprises in Shandong Province rank first in China. The consumption of soybean for food and protein processing is nearly 4 million tons per year， whereas the planting area of soybean in Shandong Province is only 0.2 million ha and the total annual yield is only about 0.6 million tons， resulting in a large gap.
　　Stresses， diseases and pests， such as soil salinization， soybean mosaic virus disease （SMV）， soybean cyst nematode （SCN） and bacterial spot disease， can lead to a reduction in soybean yield[3]. In addition to strengthening the field management， prevention and control of pests and diseases， and improvement of stress resistance， cultivating high-yield， high-quality， and high-resistant soybean varieties is also an effective measure to solve these problems. Soybean germplasm resources are an important material basis for the cultivation of new soybean varieties.
　　According to differences in topography， climate， soil quality and cultivation conditions， Shandong Province can be roughly divided into five soybean ecoregions， including the northwest and southwest of the Yellow River Flood Plain， saline alkali area of Northern Shandong， mountain region of Central-south Shandong， mountain plain and hilly area of Southern Shandong， and Jiaodong hilly ecological area[4]. Cultivated soybeans and wild-type soybeans in each of the ecoregions can adapt to specific local natural conditions. 　　The collection and inspection of crop germplasm resources were conducted in China during 1956， 1979 and 1990， respectively[5]. The landraces and wild-type varieties scattered throughout the country were collected and preserved in genebanks. By 2017， more than 25 000 cultivated soybean resources and more than 7 000 wild-type soybean resources were preserved in the Chinese National Genebank[6]， which ranks first in the world by the amount of soybean resources preserved.
　　Crop germplasm resource experts had collected and preserved landraces and wild-type varieties in Shandong Province. At present， 4 423 soybean resources were collected and preserved in Shandong Province， accounting for 13.4% of the total amount in China， including 835 landraces， 1 357 germplasm resources from other provinces and cities in China， 696 foreign germplasm resources[7]， and 1 535 wild-type germplasm resources. Among them， there are 2 948 yellow soybean varieties， 180 black soybean varieties， 65 tawny soybean varieties， 157 green soybean varieties and 33 bicolourable soybean varieties. Specifically， 1 800 cultivated soybean varieties and 1 535 wild-type soybean varieties were preserved in Shandong Provincial Germplasm Resource Center of Crop[8].
　　These resources have been used as important materials for soybean breeding and basic research and are provided to many research institutes such as Shandong Academy of Agricultural Sciences， Dezhou Academy of Agricultural Sciences， Heze Academy of Agricultural Sciences， Weifang Academy of Agricultural Sciences， Linyi Academy of Agricultural Sciences and Shandong Shengfeng Seed Industry Scientific and Technological Co.， Ltd.
　　According to seed coat color， growth period， grain size and sowing date， Chinese soybean varieties can be divided into 480 types[9]. Based on the sowing date， soybeans can be divided into spring soybeans and summer soybeans. Summer soybeans are the dominant varieties in Shandong Province. Specifically， Pingdinghuang， Tiejiaohuang， Yaojiaohuang， Tianehuang， Gulihun， Shuilizhan， Qisiwa， Dawuye， Xiaowuye， Sijiaoqi， Bayuezha， Lianyezi and Xiaohuapi are main yellow soybean varieties； Daqingdou， Xiaoliqing and Jiangnanqing are main green soybean varieties； Daheidou， Xiaoheidou and Gunlongzhu are main black soybean varieties； Dahongdou， Xiaohongdou， Hongzhadou， Honghuadou， Hongnidou and Hongyoudou are main brown soybean varieties.
　　Zhang et al.[9] analyzed the agronomic traits and quality of 1 069 soybean varieties and found that several agronomic traits of soybean varieties in Shandong Province varied greatly， such as growth period， plant height， and 100-grain weight. 　　Spring soybean plants in Northeast China are relatively tall， 41.56% of which has a plant height of above 91 cm， whereas most of the southern soybean plants are shorter than 60 cm. The heights of most soybean plants in Shandong Province range from 41 cm to 80 cm. Northeastern soybeans are rich in fat， whereas southern soybeans have low fat contents. Shandong soybeans exhibit moderate fat contents， half of which is higher than 18%[10].
　　During the 7th and 8th Five-Year Plan period， the Chinese Academy of Agricultural Sciences conducted a crude protein assay on 16 022 soybean germplasms in China and found that 40.4% of the soybean varieties contained more than 45% protein. Southern soybeans are rich in proteins， whereas northeastern soybeans have low protein contents. Shandong soybeans exhibit moderate protein contents， most of which ranges from 40% to 48%.
　　Soybean breeding in Shandong Province started in the early 1950s. The breeding methods have been developed from landrace selection and systematic breeding to current hybrid breeding combined with radiation breeding and biotechnology breeding. So far， more than 100 soybean varieties have been cultivated and popularized[11-12].
　　Landraces played an important role in soybean production before the founding of the Peoples Republic of China， most of which were subsequently replaced by bred varieties. However， the landraces are the most important parental sources for bred varieties. Especially， at the early stage of hybrid breeding， most parents are excellent landraces. For instance， the female parent and male parent of Qihuang 5 are Xinhuangdou and Tiejiaohuang， respectively.
　　In the early 1950s， a number of excellent soybean varieties were identified by landrace selection in Shandong Province[13]， including Pamanqing in Haiyang City， Pingdinghuang in Yidu County， Tiezhugan in Huimin County and Dezhou City， Bayuezha in Taian City， Hongyingzi in Linyi City， Niumaohuang， Tiejiaohuang and Yaojiaohuang in southwest Shandong Province. Qihuang 1， Ju 23， Xinhuangdou， Jinan 1， Jinan 2， Jinan 3， Jinan 4 and Jinan 5 were selected by systematic breeding. The selected landraces played an important role in soybean production and the first-generation soybean variety update was completed. Most of the soybean varieties in this period were mid-late-maturing with a growth period of about 110 days， which are suitable for two-year three-cropping cultivation system. Qihuang 1 is a medium-maturing and high-yield soybean variety， which is suitable for one-year two-cropping cultivation system with the largest planting area of more than 1.3×105 ha. The yield of these varieties ranged from 1 125 to 1 500 kg/ha. Although the yield was improved， the growth period of these varieties was relatively long. 　　From the late 1960s to the 1970s， soybean varieties Xiangyang 1， Fengshouhuang， Yuejin 4 and Wenfeng were bred in Shandong Province. The growth period of these varieties was shortened to 95-100 days， whereas the yield was improved to 1 800-2 250 kg/ha. Specifically， the planting area of Xiangyang 1 exceeded 1.3×105 ha. Thus， the second-generation soybean variety update was completed. These medium-maturing varieties promoted the development of one-year two-cropping cultivation system.
　　The soybean varieties bred in the 1980s， such Yuejin 5， Ludou 4， Ludou 2， Ludou 1， Ludou 5， Ludou 6 and Ludou 7， had more pods per node， high pod density per plant and strong resistance to lodging， diseases， insects and stresses. Specifically， Yuejin 5 is a variant strain selected from Dingtaopingdingdahuangdou by Heze Academy of Agricultural Sciences. The planting area of Yuejin 5 had reached 4.7×106 ha during 1980-1987. Ludou 4 is an early-maturing variety with high yield， good quality， resistance to various diseases and insect pests， strong adaptability and great potential for yield increase. The yield of Ludou 4 was about 2 250-3 000 kg/ha. In 1993， the planting area of Ludou 4 reached more than 2.8×105 ha， accounting for 50% of soybean planting area in Shandong Province. By 1994， the planting area of Ludou 4 was more than 2.0×106 ha.
　　Since the 1990s， in order to adapt to the requirement for agricultural production and market economy， a number of new soybean varieties （lines） with high protein content， high yield， resistance to pests and diseases and special uses were bred in Shandong Province， such as Ludou 11， Ludou10， He 84-5， Ludou 9， 883060，8504（17）-2， Lin 502， Qihuang 25， Luqingdou 1， Luheidou 1， Luheidou 2 and Qichadou 1 （Ji 3045）. Thus， the fourth-generation soybean variety update was completed.
　　Soybean cultivars change constantly in Shandong Province. The overall trend is to cultivate soybean varieties with larger seeds or grains， shorter plants， stronger disease resistance and higher contents of fat and protein， thereby gradually enhancing per unit area yield under improving agricultural conditions[14].
　　Excellent Traits and Utilization Value of Wild Soybean
　　Annual Glycine soja Sieb. & Zucc.， perennial G. tabacina （Labill.） Benth. and G. tomentella Hayata are wild relatives of soybean[15].
　　Wild soybeans are widely distributed throughout China with high protein contents. The average protein content in wild soybeans is 45% and the maximum protein content reaches above 55%， which is far higher than that of cultivated soybeans （40%）. Half the weight of soybean seeds is protein， suggesting that wild soybeans are rare and excellent germplasms[16]. The high protein content in wild soybeans is the most desirable trait for breeding high-protein soybean varieties in the future. 　　Wild soybeans have more flowers， more pods and high seed propagation coefficient[17]. These traits are fundamental to the high yield of legumes. The pod number per plant of wild soybeans is 400-500 in general， which can reach 1 000 or even 4 000 at most. Such high seed propagation coefficient is unmatched by soybean cultivars.
　　In addition， wild soybeans have strong environmental adaptability and strong tolerance against adverse natural environments such as pests and diseases， which can grow in in depressions， saline lands， infertile soils and dry soils. So far， stable immune genes for several races of soybean cyst nematode， immune genes for mosaic virus disease， resistance genes to aphid and drought tolerance genes have been isolated from wild soybean resources In China[18]， Special trait genes required by humans can be isolated from wild soybeans. For instance， linolenic acid is a fatty acid beneficial to humans. The average content of linolenic acid in cultivated soybeans is 3%-5%， whereas linolenic acid content in wild soybeans is about 17%， which can reach 23% at most. Therefore， special commercial varieties can be cultivated using soybean germplasms with high linolenic acid content. Moreover， a large number of excellent genes can be obtained from wild soybeans.
　　Wild soybeans are the most favorable resources for expanding the genetic basis of cultivated soybeans. The introduction of wild soybean germplasms is one of the effective ways to expand the genetic basis of soybean. Wild soybeans and cultivated soybeans have the same chromosome number （2n = 40）， resulting in no reproductive isolation during interspecific hybridization[19]. Introducing wild soybean germplasms into cultivated soybeans has the following advantages： ① Wild soybeans can provide useful traits or gene sources for cultivated soybeans； ② wild soybeans can provide genetic germplasm and broaden the genetic basis of soybeans； ③ interspecific hybridization can be conducted between wild soybeans and cultivated soybeans to create new genotypes.
　　At present， 7 000 wild soybean resources have been preserved in the Chinese National Genebank[20]. The worlds largest wild soybean community is located in the Yellow River estuary in Shandong Province. So far， 1 535 wild soybean varieties have been collected in Shandong Province， accounting for 22% of the total number in China. Wild soybean resources with excellent traits should be screened based on the requirement for agricultural production and breeding. High-protein genes， high-yield genes and stress-resistance genes should be identified and excavated to analyze their genetic mechanisms， which can be introduced into cultivars by biotechnology and traditional hybrid recombination methods[21]. It is necessary to create new soybean germplasms by hybridization between wild soybeans and cultivars[22]. 　　Sun et al.[23] prepared the hybrid combination with cultivated soybean 167 and wild-type soybean 035. Through four backcrosses， a cytoplasmic-nuclear male sterile line was obtained for the first time. A soybean hybrid was selected and approved by the Variety Examination Approval Committee of Jilin Province， of which the yield was improved by 20%. Currently， the difficulty of variety popularization lies in seed production. A series of small-grain soybean varieties were bred via hybridization between wild soybeans and cultivated soybeans by Jilin Academy of Agricultural Sciences[24]. Using wild soybeans， the Chinese Academy of Agricultural Sciences had bred Zhongye 1 and Zhongye 2 with good comprehensive traits[25].
　　8255， an excellent soybean variety with high yield， disease resistance and lodging resistance， was obtained via hybridization between cultivated soybean 7518 （female parent） and wild soybean and backcross using cultivated soybean. By distant hybridization and mixed selection technique， Lusidou 2 and Lusidou 3 were bred， of which the forage yield was improved by 15% and 12% compared to wild soybean， respectively[26]. Dongsidou 1， a forage variety that is erect and easy to harvest with high yield， strong saline-alkali resistance and good palatability， was bred via hybridization between wild soybean and cultivated soybean Binzhi 1 in 2014[27]. Lu et al.[28] found that crude protein content， crude ash content and calcium content in the hybrid offspring with wild soybean as the female parent were higher than those in the hybrid offspring with cultivated soybean as the female parent， which could be used as the parent materials for breeding forage soybeans.
　　There are many excellent traits in foreign soybean germplasms. For instance， Japanese variety Tokachi-Nagaha has large grain size and resistance to lodging； American soybean varieties Amsoy and Williams have good plant type and high oil content， which can be used to cultivate soybean varieties suitable for the environment and production in Shandong Province. At present， 121 new soybean varieties have been bred in China by using foreign resources. Specifically， Japanese variety Tokachi-Nagaha is the parent of Hefeng 25； American semi-dwarf soybean variety Hobbit and Beifeng11 are the parents of Hefeng 42 that was approved in 2002； Liaodou 10 and Mecury are the parents of Liaodou 14； Williams is the parent of Jidou 7. It is necessary to strengthen cooperation with foreign soybean research institutions and make full use of the available excellent soybean resources to enrich the genetic basis of soybean germplasm resources in China. 　　In addition， a large number of studies have been carried out on gene mapping for salt tolerance， high yield， resistance to diseases， pests and weeds， and good quality of soybean， in order to promote the cloning of important genes and molecular marker-assisted breeding of soybean. Wang et al.[29-30] cloned soybean P450 / CYP78A gene GmCYP78A10 and identified two alleles. It was found that soybeans containing the allele GmC-YP78AIOb had higher grain weight， grain width and grain thickness than those containing GmCYP78Al0a， which could explain 7.2% of variations in grain weight. Liu et al.[31] identified transcription factor gene GmMYB73 that was highly expressed in soybean grains. Overexpressing GmMYB73 gene could increase the size and 1 000-grain weight of transgenic Arabidopsis seeds， and improve fatty acid content in soybean seeds， leaves and hairy roots. GmMYB73 could inhibit the expression of GL2 in combination with GL3 and EGL3， and reduce the inhibition effects of GL2 transcription factor on PLDal expression. PLDal could hydrolyze phosphatidylcholine in grains into phosphatidic acid and promote the synthesis and accumulation of DAG and TAG， which revealed the mechanism of how GmMYB73 promoted lipid synthesis by releasing PLDal in the late period of soybean oil accumulation. It is necessary to strengthen cooperation among domestic soybean research institutions in the identification of important genes and sharing of identification data， thus improving research efficiency.
　　The disease resistance， stress resistance， and quality of some soybean varieties preserved were preliminarily identified. Repeated identification of high resistance and good quality of a few varieties was performed. In addition to the repeated identification of excellent resources that have been preliminarily screened， research on unascertained germplasms should be strengthened in order to provide materials for in-depth analysis and solve new problems in the development of soybean industry effectively.
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