Abstract:

Yam bean (Pachyrhizus species) is a legume root crop newly introduced in East and Central Africa for food production. The crop has nutritious storage roots rich in protein, iron, zinc and vitamins in addition to wide cco-geographical adaptation and biological nitrogen fixation. However, the acceptance, adoption and utilization of yam bean is constrained by low storage root dry matter content (RDM) and yield yet high RDM is preferred for consumption and industrial applications among staple root crops in Africa. Thus genetic studies to improve RDM, yield and associated traits were needed as a basis for designing yam bean improvement programs. This study presents the first comprehensive results on genetic characterization, gene action, inheritance and response to selection for yield, RDM and nutritional quality traits in the yam beans. The objectives of this study were to: i) estimate genetic variability for RDM, yield and quality traits among the introduced yam bean germplasm in Uganda, ii) determine combining abilities and gene action for inheritance of RDM and associated traits in Fi and F2 yam bean populations and iii) assess response to selection for RDM and associated traits in selected F3 yam bean populations. Four trials were conducted for genetic characterization of RDM, yield and associated traits in 26 accessions of the three cultivated yam bean species at two locations over two seasons as a basis for parental selection. Then hybridisation was done by crossing three high RDM P. tuberosus - chuin cultivar and six low RDM accessions of P. erosus and P. ahipa using North Carolina II mating design (NCD II) and to generate Fi populations. Field evaluations were done for parental lines, Fi plants, F2 segregating populations, and 83 F3 families selected at 10% on the basis of high RDM. A randomised complete block design (RCBD) was used in all field experiments with spacing of 0.3m between plants and Im between ridges. Data was collected on storage root fresh yield (RFY), storage root dry yield (RDY), vine yield (VNY), fresh biomass yield (FBY), harvest index (HI), protein (PRO), starch (STA), iron (Fe), zinc (Zn), potassium (K) and phosphorus (P) content of storage roots. The Near-infrared reflectance spectroscopy (NIRS) was used to determine the nutritional quality of storage root flour samples. Statistical and genetic analyses were done through analysis of variance, variance component analyses, combining ability and correlational analyses. The results from genetic characterization of the 26 yam bean accessions revealed significant (p<0.01) genetic variation (g2g) for RFY, RDM, RDY, VNY, FBY, STA, iron (Fe) and high broad sense heritabilities ranging from 58.4% to 83.6%. Similarly, the phenotypic coefficients of variation were high for RFY (66%), RDY (53.3%), VNY (60.5%), and FBY (59%), but medium for RDM (22.6%), STA (15.1%) and Fe (21.3%). Strong positive correlations were observed between RFY and both RDY (r = 0.926) and FBY (r = 0.962), besides RDY and FBY (r = 0.899) but low to moderate correlations amongst quality traits. Conversely, the results of study two on combining abilities and gene action revealed significant (p<0.01) mean squares of crosses for RDM, STA, RFY, RDY and FBY in Fi and all traits in F2. The GCA mean squares for females (GCAf) and males (GCA,n) were also significant (p<0.01) for RFY, RDY, FBY, STA, PRO and RDM, RDY, STA respectively while SCA mean squares were only significant (p<0.05) for RDM, RFY and STA in F, generation. In F2 generation, GCAf were significant (p<0.01) for all traits except RDM and VNY but GCA,„ were significant for RDM, RFY, RDY and FBY.