A ten – parent diallel experiment was carried out at the Teaching and Research Farms of the Federal University of Agriculture, Makurdi, Nigeria, to estimate combining ability and heterosis in rice. The design was a 10x10 alpha lattice with three replications and data was collected on plant height (PH), tiller numbers/plant (TN), leaf length (LL) and width (LW), days to flowering (DF), panicle length (PL), panicle exsertion (PE), spikelet numbers/panicle (SN), 100 seed weight (100SW) and days to maturity(DM). A preponderance of both additive and non – additive gene action with both negative and positive GCA, SCA and % heterosis was observed in the control of yield component traits in the studied rice varieties and their hybrids. Non – additive variance was higher, except for DF, SN and DM where additive variance was higher compared to non – additivity. Six parents, namely MGD 101, FARO 44, FARO 52, FARO 57, STRASSA 58 and IR 72 recorded the highest GCA values for most of the traits and the least values in terms of days to flowering and maturity. These parents were the most frequent in crosses with the highest SCA effects and % heterosis for Tiller number, Leaf length, leaf width, panicle length and Spikelet numbers/panicle. Crosses involving these six parents also recorded the least SCA effects and % heterosis for days to flowering and maturity. Based on the results of GCA, SCA and % heterosis, backcross breeding of F1s to their respective parents (the six parents), was recommended for improvement of yield component traits.

A six parent F 2 diallel of soybeans was evaluated in potted acid sand culture with the objective of estimating combining ability and heterosis for aluminium stress tolerance. Highly significant differences were observed among the genotypes, crosses (F 2 /R), Parents, F 2 , Reciprocals (R) and Parents Vs (F 2 /R) for the root dry weight, shoot dry weight and relative root surface area. Both general combining ability (GCA) and specific combining ability (SCA) mean squares were highly significant for the three traits, except the root dry weight, where the SCA was not significant. The result also showed the presence of both additive and dominance gene action and the preponderance of the former compared to the later, indicating the possibility of selection of pure lines from the genotypes studied. Selection from TGX 1896-3F and TGX 1844-18E and crosses involving these two genotypes on acid soil would enhance a rapid progress in the breeding of aluminium tolerant genotypes of soybeans.

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