الفهرس 
TABLE OF CONTENTSOnly 14 pages are availabe for public view
 |  | from | 146 |  |  |
| | | from | 146 | | |
Abstract
In the present study, a nine-parents half diallel cross of bread wheat (Triticum aestivum L.) was performed. The parents and their F1’s hybrids were evaluated under (1st sowing date) and heat stress (late sowing date) conditions. the genetic system controlling tetrazolium chloride (TTC) reduction and cell membrane thermostability (CMS) were evaluated under heat stress condition. Grain yield per plant, 1000-kernel weight, spike length and stem diameter under favorable and heat stress conditions were measured. A heat tolerance index adjusted based on 1000-kernel weight (g) was also analyzed.
The results could be summarized as follow:
1. A considerable variation among the parental genotypes as well as their F1 hybrids was observed for all the traits studied under favorable and heat stress conditions.
2. Heat stress of the late sowing date caused significant reductions in grain yield per plant (g), 1000-kernel weight (g), spike length (cm) and stem diameter (mm) by 17.07, 9.12, 10.39 and 15.44% overall parental genotypes and 15.15, 13.88, 12.69 and 14.94% overall F1 hybrids, respectively.
3. On average and compared with their parents, the F1 hybrids produced higher CMS (%) under heat stress condition, higher grain yield per plant (g) and 1000-kernel weight (g) under both favorable and heat stress conditions as well as a larger heat tolerance index. Moreover, further dominance effects were due to specific combinations were observed in the F1 hybrids for all the studied traits.
4. The regression analysis of the covariance (Wr) on the variance (Vr) revealed the adequacy of a simple additive-dominance genetic model for all the traits studied under favorable and heat stress conditions, except grain yield per plant (g) which exhibited a non-adequate additive-dominance genetic model under both environmental conditions.
5. Grain yield per plant (g) under favorable condition as well as spike length (cm), TTC (%) and CMS (%) under heat stress exhibited an overdominance, whereas a partial dominance was found for the remaining cases.
6. Highly significant differences due to the general combining ability (GCA) of the parental genotypes and the specific combining ability (SCA) of their 36 F1 hybrids were observed for all the traits, with a predominance of GCA than SCA effects.
7. Both additive and non-additive gene effects were involved in the inheritance of all the traits, with a predominance of the additive gene action. However, the dominance genetic variance of TTC (%), CMS (%) and spike length (g) under heat stress as well as grain yield per plant (g) under favorable condition was greater in magnitude than the additive variance, while a predominance of the additive variance was observed in the remaining cases.
8. High broad-sense heritability (ranging from 0.80 to 0.94) and moderate to high narrow-sense heritability (ranging from 0.51 to 0.81) were observed for all the traits studied. The broad-sense heritability estimates of TTC, CMS and heat tolerance index were 0.94, 0.91 and 0.80; whereas the narrow-sense heritability estimates were 0.57, 0.52 and 0.71, respectively.
9. Highly significant positive correlation was observed between TTC and CMS (r= 0.73). The TTC was significantly correlated with grain yield per plant (r= 0.66) and 1000-kernel weight (r= 0.37) under heat stress condition as well as with heat tolerance index (r= 0.37). Similarly, the CMS was significantly correlated with grain yield per plant (r= 0.55) and 1000-kernel weight (r= 0.33) under heat stress condition as well as with heat tolerance index (r= 0.31).
10. The parents P1, P9 and P2 were the highest general combiners for TTC, CMS and heat tolerance index, respectively. While, the parent P1 and P2 were the highest general combiners for grain yield per plant and 1000-kernel weight under heat stress condition, respectively. In addition, the best promising F1 combinations were (P5×P6) for TTC, (P5×P9) for CMS and (P1×P2 and P8×P9) for heat tolerance index Therefore, the inclusion these superior genotypes in breeding programs could be useful for improvement of heat tolerance in wheat.
11. The high broad- and narrow-sense heritability for stem diameter obtained under heat stress as well as the significant correlation with grain yield per plant and 1000-kernel weight may be promising for selection.