Genetic improvement of yam bean (Pachyrhizus species) for high storage root yield, dry matter content and nutritional quality in Uganda
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 eco-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 the need genetic studies for improvement of RDM and associated traits as a foundation for yam bean breeding programs. This study presents the first comprehensive results on genetic characterization, 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 yield and quality traits among the introduced yam bean germplasm in Uganda, ii) determine combining abilities and gene action for RDM and associated traits in F1 and F2 yam bean populations and iii) assess response to selection for RDM and associated traits in selected F3 yam bean populations. Four trials involving 26 accessions of the three cultivated yam bean species were conducted at two locations over two seasons for genetic characterization of root dry matter, yield and associated traits 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 and involved to generate F1 populations. Field evaluations were done involving parental lines, F1 plants, F2 segregating populations, and 83 F3 families selected at 10% on the basis of high RDM. A randomized complete block design (RCBD) was used in all field experiments with spacing of 0.3m between plants and 1m 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 genetical analyses were done through analysis of variance, variance component analyses, combining ability analyses and correlational analyses. The results from genetic characterization of the 26 yam bean accessions revealed significant (p<0.01) genetic variation (σ2G) for RFY, RDM, RDY, VNY, FBY, STA, iron (Fe) and high broad sense heritability 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 were observed amongst quality traits. The results of study two on combining ability and gene action revealed significant (p<0.01) mean squares of crosses for RDM, STA, RFY, RDY and FBY in F1 and all traits in F2. The GCA mean squares for females (GCAf) and males (GCAm) 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 F1 generation. In F2 generation, GCAf were significant (p<0.01) for all traits except RDM and VNY while GCAm were significant for RDM, RFY, RDY and FBY. Likewise, the SCA mean squares were significant (P<0.01) for RDM, RFY, HI, FBY and STA suggesting both additive and non-additive gene action. The narrow sense heritability and baker ratio were moderately high (>0.5) for all the traits except VNY and PRO in F1 and VNY in F2 indicating the predominance of additive genetic effects and suggesting that performance of progeny could be reliably predicted from parents for most traits. The GCA effects were positive and significant (P<0.01) for RDM and STA among parents 209014 and 209018 in F1 and F2 respectively and for RDM, RFY, RDY and FBY of parent 209016 in F2. The SCA effects were positive and significant (p<0.01) for RDM in crosses; 209022 x 209014 and 209019 x 209013 in F1 and crosses 209016 x 209013 and 209031 x 209014 in F2. These parents and crosses are promising for improvement of dry matter and associated traits such as RFY, RDY, FBY, and STA in yam beans. The analysis of variance for the F3 data revealed significant genetic variation (p<0.001) among the selected 83 families for all the eight traits. Additive genetic variances (σ2A) were high and above unity for all the traits except VNY and PRO. Moderately high GCV values were observed for STA (10.28), RDM (13.42), RFY (18.8), RDY (23.71), FBY (19.91) and VNY (25.25). High narrow sense heritability (0.53 - 0.74%) and percentage response to selection (15.5 – 33.12%) were obtained for RDM, RFY, RDY, VNY, FBY and STA indicating high potential for genetic improvement of the yam bean germplasm in Uganda. In conclusion, this study demonstrated that the yam bean germplasm in Uganda possesses considerable genetic variation that should allow genetic gains for RFY, RDM, RDY, FBY, STA and perhaps root Fe and PRO contents. The observed genetic variation ( σ2G = 9.3%2 ) for RDM suggested that it is possible to breed for high dry matter yam beans by using low dry matter accessions without having access to high dry matter P. tuberous-chuins. The inheritance of RDM, RFY, RDY, FBY and STA was controlled by both additive and non-additive gene effects but with predominance of additive genetic effects. Lastly, the combination of high genetic variance, heritability and response to selection obtained in F3 indicated potential to obtain high genetic gains for RDM, RFY, RDY, VNY, FBY, STA and Fe in a systematic breeding program utilizing the available yam bean germplasm to develop well adapted, protein and nutrient rich root crop in East and Central Africa.