Five Egyptian and exotic wheat cultivars and their all half diallel possible
cross combinations were used to generate combining ability information and identify
suitable parents and cross combinations for further exploitation under normal (01) and
late (02) planting dates. Mean squares of genotype x environment interactions were
highly significant, suggesting a differential response of the genotypes under normal
and stress environments. Heat stress conditions caused reduction about 7.69%,
5.84% and 19.37% in the F1 hybrids average for heading date, 1000 grain weight and
grain yield per plant, res
pectively. The results also revealed that all cross combinations including
00vin-2 (P3), Giza 164 (P4) and two out of four crosses including Sakha 69 (P1) gave
susceptibility index (S) values less than the unity. This result that indicated that they
transmitted their genes controlling heat tolerance. It was also noticed that the two
crosses (P1XP3) and (P1XP4) gave the highest yield under both the two environments
with susceptibility index less than unity. Correlation coefficients between stress
susceptibility index (S) and each of grain yield per plant and heading date were -
0.34 and 0.84, respectively. Estimates of general combining ability of each parent
revealed that the parents Sakha 69 (P1), Giza 164 (P4) and 00vin-2 (P3) possessed
more desirable additive genes for all studied traits under each of the two
environments and their combined data. whereas, Gemmeiza 5 (P2) and Bau'S' (Ps)
were the poorest general combiners for the same traits. The results indicated that the
cross combination (P1XP3) exhibited Significant SCA effects for earliness and high
yielding ability under normal and stress conditions. While, the two crosses (P2XP4) and
(P3XPS) revealed Significant SCA effects for grain yield per plant and 1000 grain
weight, respectively, under each of the two environments and their combined data.
The results also indicated that the non additive gene action including dominance (ifo)
played a major role in the inheritance of days to heading. On the other hand, the
estimates of additive variance (if A) were higher than those of non additive ones (ifo)
for 1000 grain weight and grain yield per plant at optimum and stress conditions as
well as their combined data, verifying by the ratios (rlrJifA)112 which were less than
unity. The interaction if AxE variance was positive and lower than those of rloxE
ones for heading date, verifying by the ratios ~oXElrlAXE)112 which were more than
unity. In contrast, the ratio (ifoxElifAxE)1 were less than unity for 1000 grain
weight and grain yield per plant. This finding indicated that the additive gene effects
were more influenced by heat stress than non additive ones. The results showed that
the largest values of broad sense heritability were observed for heading date (87.22%
and 93.26%) under normal and stress conditions, respectively. While, the largest
estimates of narrow sense heritability were obtained for 1000 grain weight (44.46%
and 49.93%) and grain yield per plant ( 55.84% and 62.00%) under normal and stress
conditions, respectively. Estimates of nature of gene action and narrow sense
heritability in these promising populations proved that selection for heat tolerance
could be effective in early segregating generations.