Genetic analysis of earliness and its components in yam bean (Pachyrhizus spp.)

Yam bean (Pachyrhizus spp.) comprises three closely related cultivated species: P. ahipa grown locally in the Andean highlands, P. tuberosus grown throughout the Amazon Basin and P. erosus grown in Central America and Asia. The objectives of this study were to determine gene actions, combining ability and genetic parameters for earliness in F₁ (P. ahipa × P. tuberosus) hybrids under Rwandan East African highland conditions. Three P. ahipa accessions (early maturing, tropical highlands) as females and three accessions of P. tuberosus (late maturing and high yields) as males were crossed using a North Carolina II mating design. Evaluation of F₁ progeny and their parents revealed highly significant differences among crosses. Significant (P<0.01) mean squares were recorded for female, male and interactions (female × male) for nearly all observed traits (the exception was number of roots, NR): storage root yield (SRY), plant height (PH), number of clusters (NC), seed yield (SYP), days to emergence (DE), days to start flowering (BF), days to flowering 50% (DtF) and days to maturity (DM). The P. hybrids were observed for all earliness traits (i. e. DE, BF, DtF and DM). There were high and positive correlation coefficients between BF and NR (0.89), and BF and DtF (0.81). However, there were low to moderate positive correlations between SRY and BF (0.39); NR and PH (0.32); NC (0.59), SYP (0.36), and DtF (0.64); SYP and BF (0.61), DtF (0.64) and DM (0.63); DE and BF (0.34) and DM (0.49) and BF and DM (0.51). No strong negative correlations were observed. Low to medium negative correlations were observed between SRY and NC (−0.37), PH and DE (−0.39), and PH and DM (−0.37). Thus, it should be possible to recombine the earliness attribute from P. ahipa with SRY and SYP attributes of P. tuberosus in new F₁ hybrid lines from which segregating populations will be developed in further studies.