الفهرس 
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Abstract
The present work aims to estimate the mid and better parent heterosis and combining ability for grain yield and its components using 6x6 half diallel crosses of bread wheat under normal and late planting dates.
The experiment was carried out at El-Mattana Agricultural Research Station, Agricultural Research Center, Egypt during the two successive seasons of 2019/2020 and 2020/2021. The randomized complete block design with three replications was used to evaluate the 15 F1’s and their 6 parental cultivars.
Results could be summarized as the following main topics:
1- Mean performance and variances
2- Heterosis
3- Combining ability
4- Genetic components
5- Correlations
6- Heat stress tolerance indices
Significant differences (p≤ 0.05 or 0.01) were found for the most studied traits due to genotypes, parents, crosses and parents vs crosses under both two planting dates.
Significant differences (p≤ 0.05 or 0.01) were found for the most studied traits due to general and specific combining abilities under both planting dates.
Gemmiza-11 was the earliest in days to heading (82.33 days) and gave the highest cultivar spike length (14.27 cm.), harvest index (32.50 %), number of grains / spike (53.00) and weight of grains / spike (2.91 g) under normal planting date in addition biological yield / plant (50.00 gm) and NG/S (55.10) under late planting date.
Misr-2 cultivar was latest parent in DH (93.67) and the tallest plant height (115.00 cm.) while, recorded the shortest spike length (10.23 cm.) under normal planting date.
Sids-14 cultivar recorded the highest spikes/plant (11.67) and biological yield/plant (92.00 g), while gave the lowest values for HI (22.00 %), NG/S (41.53) and WG/S (2.14 gm) under normal date. Moreover, Sids-14 cultivar recorded the lowest value for PH (91.33 cm.) SL (11.47 cm.), BY/P (40.00 gm), GY/P (10.17 gm.), HI (25.56%), WG/S (2.18 gm.) and 1000GW (44.35 g), and was the latest parent in DH (77.33 days) under late planting date.
Giza-171 cultivar recorded the highest 1000GW (52.00 gm.) and the lowest number of spikes/plant (6.03) and NG/S (45.35) under late planting date. Shandwel-1 cultivar gave the lowest values for each of PH (105.00 cm.), GY/P (19.50 gm.) and 1000GW (46.33gm.) under normal date, while, it was the highest parent for PH (101.33 cm) , NS/P (9.63), GY/P (21.33 gm.), HI (46.07%) and WG/S (2.63 g).
Under normal planting date, P1XP4 was the earliest in cross DH (83.33) and was the highest for WG/S (3.66 g). Meanwhile, this cross recorded the lowest NS/P (7.40) and GY/P (23.75 g) because of the earliness in DH. On contrast the cross P1XP3 was the high yielding GY/P (32.75 g). The cross P1XP6 recorded the shortest plant height (105.00 cm.) and the tallest spike length (14.60 cm.). Moreover, the contrary was showed with cross P2XP4 that gave the tallest plant height (122.50 cm.) and the shortest spike length (10.33 cm.). It`s worth noted that, P2XP4 gave the lowest NG/S (50.25) and WG/S (2.59 g) as a result of short spike length. While, the highest NG/S (64.90) was observed for the cross P1XP5. Cross P3XP4 was recorded the highest NS/P (11.43) and BY/P (101.90 g).
Under late planting date, the cross P1XP3 gave the highest WG/S (3.42 gm.) and 1000GW (60.55g.), while P1XP5 gave the highest SL (13.67 cm.), NS/P (8.80) and BY/P (67.00 g). The cross P2XP4 gave the lowest values for each of GY/P (10.25 g), HI (15.85%) and 1000GW (48.15gm), while the high yielding cross was P2XP6 by 22.50 gm. The lowest values of PH (89.00cm.) and NS/P (5.63) were recorded for P3XP5 and BY/P (34.00gm.), NG/S (44.43) and WG/S (2.36g) for cross P3XP6.).