Agriculture and Natural Resources Research Center of Mazandran
The combining ability of quantitative and qualititative traits has been studied quite frequently in rapeseed (Brassica napus L.), but rarely at different nitrogen levels. Twenty-one genotypes, including 6 parents and their 15 F2 diallel progenies, were evaluated at two nitrogen rates, N0 and N150. A combined analysis of variance revealed significant mean squares of general (GCA) and specific (SCA) combining abilities for all of the traits. The nitrogen × GCA mean square was not significant for all of the traits, indicating the stability of additive genetic effects in different environments. Days to end of flowering, days to maturity, oil content and oil yield had significant nitrogen × SCA mean squares, which showed significant variation of SCA effects on these traits at different nitrogen levels. Days to flowering, days to end of flowering and days to maturity showed high narrow-sense heritability at two nitrogen levels, but plant height and seed yield had high narrow-sense heritability estimates at N0. Most of the crosses had significant positive or negative SCA effects for seed yield at N0 however, at N150 only two crosses had significant SCA effects. Therefore, selection of hybrids based on SCA effects at N0 would be more effective.