Abstract
The present investigation was undertaken to study the genetics of some morpho-physiological traits
related to heat tolerance in spring wheat conducted during 2001-02 to 2003-2004 at three locations. Combining ability with a 6 x 6 half-diallel showed the involvement of both additive and non-additive components of
heritable variances in the inheritance of biomass plant, grain yield plant, 1000-grain weight (TGW), grain
filling rate (GFR), chlorophyll content at 21 days after anthesis (CHL 21) and canopy temperature depression
(CTD) contributing to heat tolerance with the predominance of additive gene effects. The parents Kanchan,
Gourab and Fang 60 appeared to be good general combiners for most of the traits. Significant and positive SCA
effect was observed in crosses Gourab × Pavon 76, Kanchan × Gourab, Kanchan × Pavon76, Gourab × Fang 60
and Fang 60 × Pavon76 for most of the traits studied. These crosses might have some merits in breeding for
heat tolerance. Genetic variability was studied in 223 F2- derived F3 bulks of six wheat crosses viz. Kanchan
Gourab (1), Gourab × Fang 60 (2), Kanchan × Pavon 76 (3), Gourab × Pavon 76 (4), Fang 60 × Pavon 76 (5)
and Kalyansona × Pavon 76 (6). Mean values of the 223 F2- derived F3 bulk lines from six crosses showed
continuous variation for six morpho-physiological traits suggesting quantitative nature of inheritance. Genetic
variability was found high for biomass m² (BM), grain yield m² (GY), TGW and GFR in crosses, 3, 4 and 5
and for CHL 21 and CTD in crosses 1 and 5. All the six traits showed moderate to high-observed response and
realized heritability in most of the crosses both in F4 and F, generations but with low estimate for TGW and
CTD in few crosses in F4. The estimates of heritability in standard unit were generally higher or closer to the
realized heritability estimates in most cases indicating good efficiency of selection for the traits. Direct selection
for these traits in F3 and F4 brought about significant increase in respective values in the F4 and F5 generations,
in all the six crosses with few exceptions. BM and GY showed significant indirect response to selection for
most of the traits in most cases. Direct selection of TGW significantly improved BM, GY and GFR in crosses 3,
4 and 5, CHL 21 in crosses 1, 4 and 5 and CTD in cross 3. GFR showed significant indirect responses to BМ,
GY and TGW in most of the crosses and CHL 21 and CTD in cross 3 and 6. Selection for CHL 21 significantly
improved BM and GY in crosses 3 and 4, TGW in cross 1 and CTD in cross 5. Direct selection for CTD
significantly improved BM and GY in crosses 3, 4 and 6. The magnitude of indirect response to selection in F,
was lower than that in F4. However, the trend was almost similar. Among the six, crosses 2, 3 and 4 showed
significant intergeneration correlation for most of the traits between F3 and F4 and between F4 and Fs
generations. Genotypic and phenotypic correlations were studied in F4 generations. BM exhibited significant
correlation with GY, GFR, and CTD in four, with TGW, CHL21 and MTS in three crosses both at genotypic
and phenotypic level. The correlations of GY with all the traits were significant at genotypic and phenotypic
levels in six crosses with few exceptions. TGW showed significant correlation with GFR in three crosses, with
CHL 21 in one cross and with MTS in two crosses at both levels. The correlation between TGW and CTD was
not encouraging. GFR showed significant correlation with CHL21 in one, CTD in two and with MTS in four
crosses both at genotypic and phenotypic levels. The correlation of CTD with CHL 21 and MTS was not
significant in most of the crosses. Twenty best Fs bulks along with five parents were tested at three locations
(Dinajpur, Jessore and Joydebpur) under optimum and late seeding during 2003-04. The Heat susceptibility
Index (S) based on both TGW and grain yield showed that the Fs bulks were less sensitive to heat than the
parents. Among the 20 F, bulks, 13 genotypes have "S" values less than unity, of which, the bulks KG-2, KG-3,
GF-1 and GF-3 were more heat tolerant along with high yield potential. The heat tolerant genotypes based on
"S" estimates also associated with relatively cooler canopies i.e. high CTD, high Chlorophyll retention,
stomatal conductance and membrane thermostability. Combining ability analysis with a 6 x 6 half-diallel
showed the involvement of both additive and non-additive gene effects for membrane thermostability (MTS)
with the predominance of additive gene effects. The parents Kanchan, Gourab and Fang 60 appeared to be good
general combiners for this trait. The crosses Kanchan × Gourab, Gourab × Fang 60, Kanchan × Pavon 76 and
Fang 60 × Pavon76 showed significant and positive SCA effect for MTS and might have some merits in
breeding for heat tolerance. Genetic variability with high population mean for MTS in F4 bulks was observed in
crosses 1, 3 and 5. These crosses also exhibited high-observed response to selection, realized heritability and
heritability in standard unit indicating expected good gain through early generation selection. Direct response to
selection for this trait was significant in crosses 1, 3, 5 and 6 in F, generation. Selection for MTS significantly
improved BM, GY, TGW, GFR and CTD in crosses 3, 4, 5 and 6 with few exceptions and CHL21 in crosses 2
and 4. Significant intergeneration correlations (F4 vs. F5) were observed for MTS in all six crosses studied
indicating effective improvement of this trait could be possible through selection in early generation.