A diallel cross involving six bread wheat genotypes was evaluated to determine the genetic behaviour of earliness components as well as yield and its components in the F1 and F2 of bread wheat under optimum and late sowing dates conditions. In addition to thermal unit and susceptibility index, the resultant hybrids along (F1 and F2) with their parents were evaluated in two experiments. The first experement was planted in optimum sowing date on November 20th and the second one was planted in late sowing dates (Jan 1st) at field experement in Sids station, Agriculture Research Centre. Genotypes mean squares were significant for most studied characters in both experiments as well as thermal unit and susceptibility index. The obtained results showed that positive heterosis values over better parent values were detected for different studied characters. Inbreeding depression estimates were also significant and positive for grain filling rate and grain yield/plant in most crosses under both optimum and late sowing conditions.
GCA variance values were two or more times higher than the SCA variance ones for days to maturity under both sowing dates, thermal unit under optimum sowing, grain filling period, grain filling rate and for grain yield/plant under both two sowing dates, suggesting the predominant of additive and additive x additive gene actions in controlling the studied characters. The early line (P2) could be considered as a good combiners for the early maturity, numbers of days and thermal unit under optimum and late sowing conditions. Parent Giza168 (P5) is a good combiner for the number of spikes/plant, grain yield/plant and its susceptibility index under late sowing condition while parent pastor (P4) could be considered as a good combiner for thermal unit under optimum sowing date and number of kernels/spike under late sowing and for grain filling period, grain filling rate and grain yield/plant under both optimum and late sowing dates. The best SCA values were detected by the cross P3xP5 for the earliness of maturity (number of days and thermal unit), grain filling period, grain filling rate, number of spikes/plant, susceptibility index of number of number kernels/spike, and grain yield/plant under optimum sowing date, also, cross P1xP4 was superior in number of spike/plant under both optimum and late, 100-kernel weight under optimum date and grain yield under late sowing. Moreover, cross P2 x P5 for grain yield/plant under optimum and late dates and its susceptibility, cross P3 x P4 for susceptibility index (SI) of No. of spikes/plant and for grain yield/plant under optimum and late dates cross P2 x P6 for grain yield under optimum sowing, cross P3 x P6 for days and thermal unit of heading and grain yield/plant and cross P4 x P5 gave the best SCA effects for grain yield/plant under late sowing condition.