West African Okra (Abelmoschus caillei [A. Chevy] Stevels) is a common household vegetable crop, cultivated primarily for its fresh pods and leaves. Nutritional importance of okra pods have been documented (Adeniji, 2003). Developing early flowering and consequently early maturing genotypes of West African Okra with short growth cycle, high pod and seed yield has become an important breeding objective in this crop. West African Okra is photoperiod sensitive (short day), with number of days to flowering ranging between 57 and 105 days in some accessions. Variation among accessions for days to flower bud appearance and flowering has been reported (Adeniji, 2003). This apparently makes it difficult cultivate this crop twice during the growing season. Early cultivation in March/April often results into excessive vegetative growth as compared with planting in July/August. A number of factors have been identified to influence flower bud initiation in crops, hence maturity and adaptation. Among these are photoperiod, temperature and intensity of radiation, insects and disease (Adeniji, 2003).
In the process of transition from late flowering to early flowering genotypes, important issues to be considered include the relationship among the parents for flowering and the genetic action involved. The economic importance of West African Okra makes developing hybrid of improved genetic potential ideal. Technological options for evolving early flowering genotypes of West African Okra will require explanation of inheritance pattern, heterotic components and heritability.
Developing early flowering and early maturing genotypes of crop, is costly and time-consuming phase of crop improvement. In maize (Zea mays), early maturing and extra-early varieties have been developed (IITA, 2000; Ogunbodede et al., 2000). Presents investigation intend to evaluate the genetic action underlying inheritance of earliness and its heritability. This will provide genetic information necessary to advance improvement in this specie.
MATERIALS AND METHODS
Six accessions of West African Okra were crossed, F1, F2,
backcross progenies (Bc1 and Bc2) were produced for each
of the five crosses. Parents, F1 and segregating generations were
evaluated at the Teaching and Research Farm, University of Agriculture, Abeokuta
(Latitude 7.35° N, 3.83° E, 450 m as1). The soil analysis of the experimental
site collected at 1-15 cm indicated a pH 5.10, total N (%) = 0.06, organic matter
(%) = 1.10 Available P (mg kg-1) = 2.80, Exchangeable Ca = (Cmo1kg-1)
= 1.51, Exchangeable (Cmo1 kg-1) = 0.23, Exchangeable Mg. (Cmo1 kg-1)
= 0.48, Exchangeable Na (Cmo1 kg-1) = 0.09, Total P (mg kg-1)
= 91.5, Texture is a loamy sand.
Six generation from the six crosses were planted for field evaluation in a
randomized complete block design with three replications. Between and within
row spacing was 1 m, respectively. Two seeds of each generation were planted
per hole, 2-3 cm deep; a total of 560 plants were established for each cross.
Compound fertilizer NPK 15: 15: 15 was applied to the plant at the rote of 60
kg N ha-1 at 2-3weeks after planting and at flowering. Data was collected
from 64 plants for each parent and F1 generation, 96 stands for each
backcross generation and 128 stand for each F2 generation. The individual
scaling test of Mather (1949) was used to test the adequacy of the additive-dominance
model for earliness in each case i.e.,
Generation mean analysis was conducted under the assumptions that observed
variation was due to additive and dominance effects, with no epitasis or linkage
(Mather and Jinks, 1971). The mean effect (m), pooled additive effect (d), pooled
dominance gene effects (h), pooled additivexadditive (i) and pooled additivexdominance
interaction effects (j) and pooled dominancex dominance interaction effects
(l) are related to the generation means by the following equations:
The estimates were tested for significance by their respective standard errors.
Degree of dominance (H) was completed as a ratio between the dominant genetic
effects and additive gene effects across loci. Heritability estimates in broad
and narrow sense were estimated as specified by Ketata (1952). Mid parent heterosis
was computed as F1-Mp/Mp
RESULTS AND DISCUSSION
The number of days to flowering among the accession of West African okra varied between 58 days in accession 2 and 105 days in accession 1. The F1 mean for earliness was intermediate between the two parents involved in each cross. The mid parent value for days to flowering was least (74 days) in the cross Acc3xAcc 6. F1 mean were greater than the mid parent value in three crosses (Acc1xAcc2, Acc1xAcc3, Acc3xAcc4). The mean values of BC1 and BC2 were close in Acc3xAcc4 and Acc4xAcc5 for earliness (Table 1).
The A, B and C scaling test were statistically significant (p<0.05 and p<0.01) in two crosses (Acc1xAcc2, Acc1xAcc3). Significant and non-significant A, B and C scaling tests were recorded in Acc4xAcc5, Acc3xAcc6 and Acc3xAcc4. Significant scaling tests suggest adequacy of the additive-dominance scale in understanding the inheritance of earliness. Conversely, A non significant tests a clear indication of the inadequacy of the additive-dominance model in explaining the inheritance of earliness among the generation evaluated (Table 2).
|| Estimation of generation means and within variance for days
to plot flowering in West African Okra
|| A,B and C scaling tests for earliness
|*Statistically significant at p<0.05; ns: non significant
||Estimates of gene effects using the six parameter model of
parents F1, F2, BC1 and BC2
for number of days to flowering
|*Statistically significant at p<0.05; ns: non significant
Both additive and dominant gene effects were significant for earliness. Estimates of additive effects were positive and statistically significant (p<0.05) in the crosses evaluated. Dominance, additivexadditive interaction effects and dominancexdominance digenic interaction effect were positive and negative in direction and statistically significant. But the additivexdominance digenic interaction effect was positive in direction. The measures of the relative contribution of additive and non-additive gene effects showed that additive gene effects made the major contribution to earliness in most of the crosses evaluated. The relative proportion of additive gene effects suggests that their contribution to inheritance of earliness is high enough to be significant in developing early maturing hybrids and purelines (Table 3).
The analysis indicate that of the individual type of digenic epistatic effects, dominancex dominance interaction effects though positive were relatively large in magnitude in two crosses (Acc1xAcc2) (Acc1xAcc3), suggesting an increasing effects. The additivex additive interaction effects on the other hand was statistically significant (Acc1xAcc2, Acc4xAcc5, Acc3xAcc4) suggesting an enhancing effects in the inheritance of earliness. The additivexdominance gene effects was positive and statistically significant (p<0.05) (Acc1xAcc2, Acc1xAcc3, Acc3xAcc6, Acc4xAcc3) and negative in Acc4xAcc5 implying the potentials for enhancement and depression of characters as the case may be. The results of the analysis are indicators assuming that a negligible epistasis would bias the dominance components to some extent. Therefore, inheritance of earliness has been found to be moderated by varying proportion of additive and non-additive gene effects.
A non significant I, J, l interaction effects revealed absence of non-allelic gene interaction in the Acc4xAcc5. This agrees with the conclusion of the individual scaling tests of Mather (1949). A significantly negative (h) and positive (l) observed in Acc1xAcc3, Acc1xAcc2 suggest involvement of duplicate epitasis. This precludes complication factors such as trigenic interaction and linkages. This cross combinations is highly undesirable for selection and consequently for genetic improvement.
This evaluation revealed that the additive gene effect was important for inheritance
of earliness. This was evidenced by (i) The estimates of the degree of dominance
which was invalidated due to a negative estimates of dominance (H) across loci
in all the crosses evaluated; (ii) estimates of additive gene effects were larger,
positive and greater in magnitude as compared with dominant genetic effects
|| Components of genetic variation for days to first flowering
in the crosses of West African Okra
|E = Environmental variance; D = Additive variance; H = Dominant
variance; Hb = Broad sense heritability; Hn = Narrow sense heritability;
MPH = Mid parent heterosis; vH/D = Dominance ratio
Broad sense heritability estimates were moderate (Acc1xAcc2, Acc1xAcc3, Acc3xAcc4) to high (Acc4xAcc5). The computation of the narrow sense was impossible due to a negative estimate of dominance (H). High estimates of broad sense heritability as found in this study suggest that the earliness among the generations was highly heritable. Similar result was obtained for maize (Ogunbodede et al., 2000). The presence of duplicate epistasis as found in this study might have inflated the estimates of heritability recorded. The estimates of mid parent heterosis were high, this range between 85 and 1.13% in cross combinations Acc4xAcc5 and Acc1xAcc2, respectively. This imply a high genetic determination for earliness among the crosses combinations Positive estimates of MPH for earliness suggest the presence of genes for development of earliness in the cross combinations. This present evaluation indicated that direct selection among segregating population may advance genetic improvement for earliness as well in incorporating earliness in elite lines.