The experiment was carried out at the experimental farm of Gemmeiza Agricultural Research Station,
Egypt during the two successive seasons, 2003/2004 and 2004/2005. Diallel crosses among eight bread
wheat genotypes were used to establish the experimental materials for this investigation. The aim of the
present investigation was to determinate the magnitude of both general and specific combining ability
and their interactions under three nitrogen fertilizer levels for heading date, plant height, spike length,
number of spikes per plant, number of kernels per spike,1000-kernel weight and grain yield per plant.
The mean squares associated with general and specific combining abilitiy were highly significant for all
studied traits under the three different nitrogen fertilizer levels and their combined data. Results also
showed high GCA/SCA variance ratios, which exceeded the unity, suggesting that selection based on
phenotype could be effective to improve and develop wheat genotypes concerning these characters under
these conditions, where the additive genes were dominant. The mean squares of interaction between
nitrogen levels and both general and specific combining ability were significant for all studied traits,
except for plant height for general combining ability. Generally, the obtained results showed that CHAM-
6 / Mayon"s", Gimmeza 9 and Gimmeza 10 wheat genotypes proved to be good combiners for improving
grain yield under three nitrogen levels and Gemmeiza 10 for spike length and Sakha 94 for grain yield
under low nitrogen level. Some crosses had significant desirable SCA effects for yield, yield components
and earliness, and most of these crosses might be of prime important in breeding program for traditional
breeding procedures under low nitrogen level for decreasing nitric compound pollution.