Adaptation and genetic analysis of earliness and yield component traits of yam bean (Pachyrhizus Spp.) in Rwanda
The yam bean (Pachyrizhus spp) is a high yielding storage root legume, rich in protein and micronutrients. The crop is constituted by three species, P. ahipa, P. erosus and P. tuberosus that are in cultivation but remain orphaned in research. The yam bean was recently introduced into Rwanda to determine its adaptability, adoption and improvement potential for integration into the diverse farming agro-ecologies and improvement of diets of root crop dependent communities. Field evaluation of 22 yam bean accessions was done across three experimental sites (Karama, Musanze and Rubona) representing the major root crop growing regions of Rwanda. The study involved crossing three yam bean accessions of early maturing P. ahipa and three late maturing P. tuberosus which were crossed using North Carolina II mating design followed by field evaluation of F1 and their parents to determine genetic control for earliness. Seven F2 progenies were further evaluated at Rubona research station located in southern province of Rwanda and assessed for genetic variability, heritability and genetic advance for yield traits to determine potential for yam bean improvement. A randomized completed block design (RCBD) with 3 replications was used and data collected on yield and yield components of the progenies. The GGE bi-plot revealed that EC209018 was high yielding but unstable. However, genotypes, AC209034, AC209035 and EC209046, were outstanding in terms of adaptation and relative stability across the 3 locations, suggesting consistent root yields irrespective of location and environmental conditions. The GGE scatter plot showed that all environments formed one mega-environment for storage root yield and two mega-environments for biomass suggesting that yam bean is adapted in the Rwandan environment and can easily be integrated into the farming systems of Rwanda to supplement diets of communities with root crop rich in protein, zinc and iron. The crosses were significantly (P<0.01) different for key earliness traits, days to emergence, days to start flowering, days to 50% flowering and days to maturity in addition to storage root yield, plant height, number of clusters, and seed yield. High and significant General Combining Ability (GCA) and Specific Combining Ability (SCA) were observed for several traits, with a predictability ratio (σ2gca / σ2sca) being higher than unity for all earliness traits, indicating predominance of additive gene actions. These findings suggested that earliness traits could be improved through selection during early generation phase which would effectively lead to selection of lines with short time to 50% flowering, and days to maturity. Cultivars with high GCA can be used to transfer suitable genes of earliness into yam bean genotypes as our effective breeding strategy for earliness in new lines. The results demonstrated highly significant (p<0.01) genetic variability for yield and yield traits; plant vigor (PV), weight of pods (WOP), plant height (PHT), 100 seed weight (100SW), total biomass yield (TBY), storage root yield (SRY), dry matter content (DMC), and starch content (STA). High genetic and phenotypic variances and genotypic coefficients of variation (GCV> 25%) were observed for WOP, PHT, 100SW, TBY, DMC, and STA. Estimates of heritability were greater than 0.5 for all traits except seed yield, number of roots and number of pods. The genetic advance as percentage of the mean (GAM%) was high for PV (91.39%), WOP (133.88%), PHT (73.56%), 100SW (17.78), TBY (25.15%) and DMC (33.95%). Correlational analysis revealed positive and significant (p<0.05) associations between PV and PHT, protein content (PRO), 100SW, TBY, SRY, and DMC while DMC had positive and significant (p<0.01) correlations with STA. These results indicated presence of high genetic variability, heritability and expected genetic gain and confirmed potential for genetic improvement of plant vigor, weight of pods, plant height, 100 seed weight, total biomass yield, storage root fresh yield, dry matter content and starch content of yam beans in Rwanda. The high yielding and well adapted genotypes that were identified in this study viz, AC 209033, AC 209035 and EC209018 should be assessed through participatory farmer’s selection for quick adoption. In medium term, several crosses should be initiated using the few parental germplasm to form base populations for recurrent selection of yam bean for earliness and high yield. A breeding program should be initiated targeting the development of high yielding and early maturing yam bean accessions by utilizing the high genetic variability and predominance of additive genetic effects that were found to control these traits.