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Materials and methods
Six varieties of spring wheat (Table
1) were crossed
in a 6 x 6 diallel system including reciprocals. The parents and
their F1's were grown during Rabi 1990-91 in the
experimental field of Agriculture Research Sub-Station, Kot Deji,
Khairpur, Sindh (Pakistan). The varieties had different parentages
and wide ecogeographic diversity for their origin (Table
1).
Seeds were sown in a randomized block design with three replications
in 10 feet long rows with 30 and 15 cms distance between rows and
between plants respectively. Data on eight quantitative traits were
collected from 10 sample plants selected randomly from each parent
and their F1 hybrids. Breeding value of the material was
evaluated by analyzing the data on heterosis and combining ability
for yield and yield components in F1 generation. The
method of analysis of variance of combining ability with model-2 of
Griffing (1956) was used. Heterotic values were calculated by using
the formula as reported earlier (Larik et a]. 1992).
Results and discussion
The degree and the direction of heterosis and combining ability
in the F1 hybrids of six cultivated spring wheat genotype
crossed in a complete 6 x 6 diallel cross including reciprocals were
investigated.
1. Heterosis
There was general decrease in plant height for most of the hybrids,
indicating higher frequency of short statured plants which will
resist lodging and give better response to higher fertilizer inputs
(Table
2). It may be
pointed out here that only certain genes in the heterozygous
condition produce heterosis and that homozygosity due to selfing does
not produce the desired effects. Out of 30 crosses, 8 crosses in
F1 generation produce mid parent (MP) heterosis. It can be
concluded that increased height in case of these hybrids over MP may
be due to the interaction of complementary growth genes for tallness
and these hybrids could be exploited for developing varieties for
Dobari, Bosi and rainfed areas where tall varieties appear to surpass
weeds to some extent and produce more straw that can be fed to cattle
and used for other purposes. The expression of positive heterosis in
these hybrids, indicates the preponderance of additive gene action
for this trait (Liu et al. 1989). Significant mean squares for this
trait also indicates the presence of additive and non-additive gene
action (Table
3).
MP heterosis and heterobeltiosis (BP) in F1 for tillers
per plant, spike length, spikelets per spike, seeds per spike, yield
per spike, seed index and single plant yield were significantly
positive (Table
2). Among the
crosses P1x
P2 and
P6x
P4 each
had more than 75.4% to 80.2% MP heterosis and 63.4% to 66.8%
heterobeltiosis for yield per plant (Table
2), indicating
that parents of these crosses are genetically more diverse than the
parents of other crosses.
Grain yield is a total sum of the genetic expression of all the yield
components, being polygenic (Larik 1978, 1979; Larik et al. 1978) and
is greatly influenced by environmental factors (Kheradnan and
Nikhejad 1974). The overall performance of a hybrid, therefore, may
vary due to changes in environment. The selection of population
simply on the basis of yield may not be beneficial and may lead to
incorrect conclusions. In R generation, heterosis for tiller per
plant ranged from 6.3% to 28.6%. The range of heterosis for other
quantitative traits were 10.0% to 5 1.0% for spike length, 7. 1 % to
25.0% for spikelets per spike, 0% to 65.2% for seeds per spike, 0% to
60.5% for yield per spike, 0.5% to 16.8% for seed index
(Table
2).
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