| As a member of the Euphorbiaceae, Cassava (Maninot esculenta Crantz), is referred to as the "starch king", and is one the world’s three largest potato crops together with potato and sweet potato. Cassava shows high genomic heterozygosity and genetic diversity, providing good opportunities for cassava crossbreeding. However, cassava breeding faces many difficulties, such as high cost, long generation time, large sample population and low breeding efficiency. In addition, the genetic relationships among cassava germplasms introduced from outside of China are not clear. This could cause problems for parent selection in cassava breeding. To clarify the genetic relationships and genetic diversity in the National Cassava Germplasm Repository, this study performed comprehensive identification and evaluation of more than200cassava accessions from all over the world were performed for botany traits, pest and disease resistance, starch traits, genetic diversity and genetic relationships. In addition, germplasm innovation and breeding of new varieties were conducted. This study provided a theoretical basis and guidance for selection of parents for cassava hybrid breeding and directed breeding.The main result obtained was as follows:1. Identification and evaluation of botany propertiesFifteen quality traits and seven quantitative traits for228accessions were studied. The frequency of distribution and index of genetic diversity of the15quality traits were analyzed. The result showed that the diversity index was low, ranging from0.435to1.889, with an average of0.957. The diversity index of mature stem rind color was the largest at1.889. The frequency distribution was more dispersed for the inner and outer skins of the root, but more concentrated for traits such as plant type and root epidermis, where the number of open type plants and root epidermal were enough. The frequency distribution coefficient of variation and index of genetic diversity of the seven quantitative traits were also analyzed. The coefficient of variation for populations from different geographical origins was small. The coefficients of variation for traits such as plant fresh weight stem height, and harvest index were large (44.60,29.70and20.19%, respectively). The diversity index of quantitative traits was larger, ranging from1.895to2.073, with an average of2.013. The diversity index of root dry matter content was the largest at2.073. These results showed that the228accessions of cassava resources had high genetic diversity. Using a probability grading method, the seven quantitative characters were divided into five levels:very low, low, medium, high and very high. Accessions with higher scores for a particular trait could be used as parent materials for cassava breeding. For example, for fresh tuber weight there were22accessions with high scores (Ruishi J17, ZM95111, SC5, ZM9713, ZM99229, Hainanxiye, CMR36-60-12, R90, ZM9244, Ba-1, ZM99200, Baodao9-1, COL523-7, Wenchang, Zhedongzhenjiexingshepojiao, SM2300-1, E25, ZM99250, ZM9495, MCR142, Hainanhongxin and4363), which represented9.95%of the test resources. For the harvest index trait,18accessions had high scores (SM1747, CM92-56-1, ZM9710, Ruishi M14, ZM9079, CMR35-70-1, OMR32-29-1, OMR36-34-6, ZM8625, CMR35-70-6, ZM9066, ZM9679, Ruishi H19, ZM8316, CMR37-14-9, CMR36-60-12, KM98-1, SG424-19), which represented8.22%of the test resources. For dry matter content trait,17accessions had high scores (ZM98173, CM2399-4, Ruishi No.12, KM98-7, ZM9066, KU50, R5, Huayemushu(bian), R80, Xinxuan048, R72, SM2300-1, Guire No.5, ZM9710, CMR35-22-196, Gungxi No.4, Taiyin No.2), which represented8.02%of the test resources.Three dimensional principal component analyses showed that all228cassava accessions comprised two gene pools representing indigenous and foreign accessions. Unweighted pair group method with arithmetic mean (UPGMA) cluster analysis based on the genetic distance obtained a score of six. The results showed that the domestic and foreign resources penetrated each other.2. Identification and evaluation of starch propertiesFive starch traits of212cassava landraces and varieties were measured, include root starch content, amylose content, amylopectin content, peak viscosity and gelatinization temperature. The result showed that the starch index of different cassavas varied greatly, and there were certain correlations between different indexes. The probability grading method mentioned in section1was used to classify the starch traits. For amylose content, the17accessions showing high scores, which could be used as parental lines in breeding, were Nanzhi199, Guire No.5, Ruishi No.12, CM2399-4, Yun nan8, CM6740-7, OMR35-2-6, Guangxi No4, Huayemushu (bian), R5, R90, Zhedongzhenjiexingshepojiao, Taiyin No.2, ZM9710, Guizhou No.2, CMR35-22-196and Xinxuan048. Eighteen accessions showed high scores for amylopectin content (OMR36-40-1, R72, SM2323-6, Ruishi No12, ZM9066, CMR26-07-15, Nanzhi188, R5, Xinxuan048, ZM9710, Guangxi No4, CMR35-22-196, CMR36-63-4, KU50, R80, Guire No5, SM2300-1and Taiyin No2). Using principle component analysis, the first (starch content, amylose content and amylopectin content) and the second component (peak viscosity, gelatinization temperature) explained83.79%of the observed variation. Under the comprehensive evaluation by subordinate function,12cassava accessions (SM2300-1, Hainnanhongxin,4363, Zhedongzhenjiexingshepojiao, MCR142, E25, ZM99250, COL523-7, Ba-1, R90, Wenchanghongxin and ZM99200) were considered more suitable for deep processing and utilization of cassava starch.3. Identification and evaluation of resistance to plant diseases and insect pestsDuring the period of cassava’s susceptibility to bacterial wilt and mite injury, resistance to bacterial wilt disease and resistance to mite disease in the field were assessed. According to the separate disease index and the mite index, the data could were divided into five levels: high resistance, resistance, moderate resistance, susceptibility and high susceptibility. Six cassavas (ZM9932, Yunnan8, CM92-56-1, COL523-7, CM965-3, SM2300-1) showed high resistance to bacterial wilt disease. Two cassavas, ZMF701and Miandian cassava, showed high resistance to mite disease. ZMF701, ZM9932and ZM8752were resistant to bacterial wilt disease and resistance to mite disease.4. Genetic diversity and the genetic relationships among cassava germplasmsOne hundred and sixty pairs of primers were screened, and44pairs were observed as polymorphic. On average, each pair of primers detected4.23allelic loci. The average gene diversity was0.56. The average value of Shannon’s index of diversity was1.01, and the polymorphism information content score was0.49. Using a genetic similarity coefficient of0.71as the threshold,195accessions could be divided into seven groups. Group A included most of the cultivars, Landraces and bred strains. Group B included15accessions from different areas. Group C comprised Brazil accessions and some Chinese landraces. Most Swiss accessions appeared in group D. Group E comprised CM483-2, CM769-2and Ruishi J17. Most of the Columbian and Thai accessions comprised group F. Accession DCU81appeared as a separate category (Group G).5. Germplasm innovationThe fertility rate at different hybrid times, seedling emergence rate and offspring were analyzed in six reciprocal test crosses from SC5, SC6and SC7varieties were studied. The results showed that the fertility rates for cross pollination from12:30pm to6:00pm were normal. The fertility rates and seedling emergence rates for different parents varied. The fertility rates and seedling emergence rates of different parents were positively correlated. In addition,22excellent strains from six reciprocal crosses were obtained. Eleven of the22excellent strains were higher than SC205in terms of tuber yield, dry matter content and harvest index. The remaining11excellent strains were higher than SC205in terms of tuber yield and dry matter content. All22excellent strains were subjected to a preliminary yield trial. As a result, we recommend that the combination SC5(♀) and SC6(♂) could be used as the main parent combination in future breeding.6. Breeding of a new cassava cultivar, SC No.12SC No.12, a new cassava cultivar (Maninot esculenta Crantz.cv), was selected from the Fl generation from hybridization experiments between cassava OMR36-34-1(♀) and ZM99247(♂), via seedling selection; primary, intermediate and advanced testing systems; regional, and production tests. SC No.12had a high yield and wide adaptability to various environments. It also showed strong resistance to wind damage. The average root yield was 39.37t/hm2, which was28.65%higher than that of the leading cultivar, SC No.205. The average dry matter percentage and starch content were higher than those in SC No.205. The cyanide (HCN) content in tuberous roots for SC No.12on average content in tuberous roots in SC No.12, were42.19and31.11%, respectively,2.51and1.86%was42.20mg/Kg. Thus, SC No.12is a good cultivar for food, animal feed, modified starch and biofuel. |
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