Combining ability for nodulation in common bean (Phaseolus vulgaris L.) genotypes from Andean and Middle American gene pools

Ten F-1's obtained from crosses among five common bean genotypes of Andean (WAF 15, Mineiro Precoce and Batatinha) and Middle American (BAT 304 and Ou ro) gene pools were assessed for their combining abilities for root nodulat ion with Rhizobium tropici strain CIAT 899. The plants were grown under con trolled conditions and evaluated for number of nodules per plant (NN), nodu le dry weight (NDW), mean nodule weight (MNW) and plant fresh weight (PFW). The subdivision of the treatment effects on the general (GCA) and specific combining effects (SCA) were performed according to Griffing's diallel ana lysis method 2, model 1. The analyses of variance and estimates of quadrati c components showed that non-additive gene effects were more important in t he expression of NN and PFW, whereas additive gene effect was predominant f or NDW and MNW. A close association was observed between high number of nod ules and GCA. Generally, crosses involving parents of different gene pools yielded hybrids with high positive estimates of SCA for all assessed traits . The genotypes of Andean origin WAF 15 and Mineiro Precoce are the most pr omising parents for breeding programs to increase NN and NDW in common bean s.