Abstract:

Hybridization among ten pure breeding lines of soybean was made in 1997
to study the inheritance of pod shattering in soybean. Lines Nam 2, Roan, Duiker,
TGx 1448-2E and GC 81090-2E were resistant, AGS 292 and TGm 737P
susceptible while Kabanyolo 1, Nam 1 and Samsoy 1 were intermediate to pod
shattering. Preliminary evaluations for pod shattering among F2 segregating
populations were carried out in oven conditions at 60°C for 12 hours, 80°C for 6
segregating populations into their shattering phenotypes. The 1 - 5 scale was
modified into 1-3 scale in which 1 was regarded as resistant, 2 intermediate and 3
susceptible. The pods of parental lines, Fi, F2, F3 and F« progenies were assessed
for shattering using an oven set at 80°C for 12 hours. A Chi square test was used
to determine the departure of the observed frequencies from the expected ones.
Maternal influence on the inheritance of pod shattering was determined using
crosses of TGm 737P x Duiker, TGm 737P x Roan and AGS 292 x TGx 1448-2E
and their reciprocals. The original Castle-Wright’s method used by Zeng et al.
(1990) was used to estimate the number of loci and Bjarco and Line(1987) method
was used to determine the number of genes. The half diallei mating design was
used to estimate genetic parameters and to detect presence of non allelic
interaction of genes affecting pod shattering following analyses by Hayman (1954)
and Griffing(1956).
Six lines, Nam 2, TGx 1448-2E, Duiker, Nam 1, TGm 737P and Kabanyolo
1 were grown at three locations during first and second seasons of 1997 and first
season, 1998 to determine amount of seed yield lost in the field due to pod
shattering and effect of G X E interactions. Weather parameters were recorded at
the locations starting from physiological maturity stage. Basing on the number of
shattered and unshattered pods, an extrapolation model was developed and
applied to determine the amount of soybean seed yield lost in the field due to pod
shattering.
The phenotype of Fi plants was closer to that of susceptible parents
indicating partial dominance and the distribution of F2 population was not discrete
and could not fit Mendelian ratios. Based on a 1 - 3 scale, the distribution best
fitted the 1:3:12 genetic model suggesting pod-shattering trait to be conditioned by
2 genes and classical dominant epistasis. The ratios obtained from the single
plant progenies advanced to F3 and F« were in agreement with expected ratios of
16:0:0, 0:16:0, 0:0:16, 0:4:12, 4:0:12, 4:12:0 and 1:3:12 for resistant: intermediate:
and susceptible phenotypic ratios respectively, confirming dominant epistasis. The
improvement made on the oven method to evaluate resistance to pod shattering
was effective in terms of reduced time for the experiment to be accomplished and
in identification of consistent genetic ratios. There was no significant difference
between reciprocal crosses of susceptible and resistant varieties suggesting
absence of maternal effects. The parent offspring regression technique indicated
that heritability in the narrow sense was 0.73 while estimates from Hayman’s and
Griffings’ diallei was 0.70. The mean number of segregating genes was estimated
crosses was 1.65.
Hayman’s diallei analysis indicated significant variation of Wr + Vr and Wr -
Vr over arrays suggesting epistatic gene action. Similarly results from a joint
unity and zero which further indicated presence of non allelic interaction of genes.
The intercept was positive suggesting partial dominance. Varieties AGS 292 and
TGm 737P contained more dominant alleles while Nam 2 and Roan contained
more recessive alleles than the other parental lines. Both general combining ability
the ratio of GCA to SCA was significant (P
effect was important for the shattering trait. Parental lines with high negative GCA
effects were Nam 2, Roan and TGx 1448-2E qualifying them as suitable parents
for transferring resistance to pod shattering in their progenies and subsequent
selected lines.
Yield losses in susceptible and intermediate susceptible varieties ranged
from 57 - 175 kg ha -1 and 0 - 186 kg ha-1 respectively depending on genotype,
location, season and harvesting date. Delay of harvesting for 14 days from
harvesting maturity stage does not lead to significant yield loss due to pod
shattering when intermediate varieties are grown as opposed to susceptible ones
which start shattering at R8 maturity stage. It was demonstrated that the
developed extrapolation method was accurate in estimation of field yield loss due
to pod shattering in soybean and produced estimates, which were not significantly
different from actual yield loss. The resistant variety did not shatter even when
harvested after 21 days. Due to significant influence of genotype, location and
xv
< 0.05) indicating that additive gene
(GCA) and specific combining ability (SCA) effects were significant (P < 0.05) and
regression coefficient over replications were significantly (P < 0.05) different from season on pod shattering in soybean, It is recommended that selections for
resistance to pod shattering be carried out in different agro- ecological zones over
several seasons to cater for the genotype x environment interactions.