A field experiment was conducted at Savanna Agricultural Research Institute in 2015 cropping season to examine the inheritance of early maturity among an extra-early maturing landrace
Cowpea (
Cowpea production is constrained by several biotic and a biotic stress factors. Among the abiotic factors, terminal drought which usually occurs during flowering and pod filling stage of the crop could cause as high as 80% yield loss [
Genetic variation and the heritability of that trait are among the key considerations for genetic improvement of crops for any trait [
The study was conducted at the research fields of the CSIR-Savanna Agricultural Research Institute, Nyankpala. SARI is in the Guinea Savanna agroecological zone of Ghana, located on latitude 9°, 25′, 41N, longitude 0°, 58′, 42W, and about 183 m above sea level. The rainfall which normally begins mid-May and ends in early October is unimodal with an average annual rainfall of about 1200 mm [
Two cultivated varieties
Three experiments were conducted between May 2014 and June 2015. The parents were planted in pots and direct and reciprocal crosses made to produce enough hybrid seeds. Some of the hybrid seeds and their parents were planted. The parents were then backcrossed to the F1s and RF1s plants to generate BC1, RBC1, BC2, and RBC2 populations. Some of the F1 and RF1 plants were allowed to produce F2 and RF2 seeds, respectively, by themselves. In the third experiment, the eight developed populations together with their parents were evaluated under rain fed condition between March 2015 and May 2015.
The experimental materials were arranged in a randomized complete block design (RCBD) with three replications. The sample size (number of plants studied) varied as follows: 30 plants for P1, P2, F1, and RF1 generations, 90 plants for the BC1, BC2, RBC1, and RBC2 generations, and 300 plants for each of the F2 and RF2 generations. The plot size comprised one row each for nonsegregating populations (P1, P2, F1, and RF1), three rows each for BC1, BC2, RBC1, and RBC2 generations, and 10 rows each for F2 and RF2 generations. Each row (constituting 10 plants) was 2.7 m long with row spacing of 100 cm and a distance of 30 cm between plants. One seed was planted per hill; refilling was done immediately after emergence. Field pests were controlled using K-Optimal (Cyhalothrin 15 g/l + Acetamiprid 20; EC) at the rate of 500 ml per ha at vegetative stage, flowering, and podding. Weeds were manually controlled as and when necessary.
Data were collected on maturity indices, namely, number of days to first flower initiation (DFFI), as number from day of planting to the day each plant on each plot flowered, number of days to 50% flowering (DFF) from day of planting to the day 50% of the plants on each plot flowered, and number of days to first pod maturity (DFPM) were determined from the day of planting to complete yellowing of first pods on each plant and the number of days to 90% pod maturity (DNDM) was determined from the day of planting to 90% yellowing of pods on each plot. Plant height at flowering (H@F) was measured from the base of each plant to the last terminal bud on the main stem at flowering; plant height at maturity (H@M) was also measured from the base of each plant to the last terminal bud on the main stem at pod maturity; canopy width (CW) was measured as an average of the shortest and the longest canopy width of each plant. While yield components such as number of pods per plant (N_PLT) were counted, number of seeds per pod (N_SPOD), 10 pods, were randomly selected from each plant and the number of seeds was counted; 100-seed weight (H_WT) was determined in grams from the weight of 100 randomly selected dried seeds and grain yield per plant (G_YD) was determined as average weight of seeds harvested from each plant.
Data for all the measured variables were subjected to analyses of variance (ANOVA) using Genstat version 12 [
The following traits DFFI, DFF, DFPM, and DNPM were taken on individual plants and the variances calculated using Genstat version 12 [ Broad sense heritability ( Narrow sense heritability (
The maternal effect was investigated by comparing the mean values of F1 with the mean values of RF1 using a mean-difference test at 5% level of significance [
Chi-square test of significance was used to investigate gene interactions for the F2 generations. The segregation ratios were analyzed using the
Analysis of variance showed highly significant (
Summary statistics for mean of phenotypic traits for the parents and their different generations.
Genotype | DFFI | DFF | DFPM | DNPM | H@F | H@M | CW | P_PLT | S_POD | H_WT (g) | G_YD t/ha |
---|---|---|---|---|---|---|---|---|---|---|---|
P1 | 41.37 | 44.29 | 63.53 | 70.67 | 17.2 | 40.26 | 39.3 | 24.71 | 15.23 | 23.00 | 2.413 |
P2 | 29.16 | 33.11 | 43.73 | 49.33 | 7.37 | 10.51 | 104.83 | 38.50 | 16.24 | 8.91 | 0.520 |
F1 | 32.12 | 36.48 | 51.56 | 55.00 | 11.2 | 26.05 | 53.13 | 39.40 | 17.97 | 16.83 | 3.163 |
RF1 | 31.70 | 36.11 | 51.07 | 54.67 | 10.7 | 26.48 | 53.20 | 39.33 | 17.23 | 16.61 | 2.890 |
F2 | 39.75 | 43.50 | 55.70 | 68.33 | 11.31 | 29.60 | 75.98 | 21.82 | 16.24 | 15.93 | 2.420 |
RF2 | 39.03 | 42.03 | 55.43 | 69.00 | 10.39 | 28.11 | 74.77 | 25.53 | 16.50 | 15.78 | 2.157 |
BC1 | 37.37 | 41.55 | 56.07 | 61.00 | 12.88 | 31.84 | 59.23 | 27.06 | 16.47 | 16.49 | 2.603 |
BC2 | 36.82 | 40.67 | 53.20 | 58.33 | 9.89 | 24.96 | 53.57 | 42.56 | 17.03 | 15.61 | 2.713 |
RBC1 | 37.07 | 40.04 | 54.73 | 59.67 | 9.47 | 22.54 | 64.70 | 23.34 | 17.26 | 16.02 | 2.713 |
RBC2 | 36.16 | 38.75 | 51.10 | 57.00 | 9.89 | 23.37 | 62.53 | 27.96 | 17.70 | 12.15 | 2.333 |
| |||||||||||
Mean | 36.054 | 39.65 | 53.61 | 60.30 | 11.05 | 26.37 | 64.1 | 31.02 | 16.920 | 15.73 | 2.377 |
S.E.D | 0.43 | 0.69 | 1.79 | 1.41 | 0.73 | 3.09 | 4.50 | 3.83 | 0.37 | 0.49 | 0.13 |
CV (%) | 1.5 | 2.1 | 4.1 | 2.9 | 8.1 | 14.4 | 8.6 | 15.1 | 2.7 | 3.8 | 6.7 |
| 163.09 | 52.63 | 15.89 | 49.82 | 25.32 | 11.93 | 31.87 | 8.61 | 9.64 | 107.84 | 59.73 |
DFFI = days to first flower initiation, DFF = days to 50% flowering, DFPM = days to first pod maturity, DNPM = days to 90% pod maturity, H@F = plant height at flowering, H@M = plant height at maturity, CDM = canopy width, P_PLT = pod per plant, S_POD = seed per pod, H_WT = hundred-seed weight (g), G_YD t/Ha = grain yield (tons per hectare), SED = standard error of difference, and CV% = coefficient of variation.
Grain yield and yield components showed highly significant (
The analysis of variance indicated that there was no significant difference (
Phenotypic variances ranged from the minimum of 4.89 average number of days to 69.63 for days to first flower initiation (DFFI) and number of days to first pod maturity (DFPM), respectively. Genotypic variances on the other hand varied from 4.84 (DFF) to 51.90 (DFPM). Environmental variance ranged between 1.96 and 17.73 for DFF and DFPM, respectively, while additive variance ranged from 4.84 (DFF) to 51.24 (DFPM), respectively. Dominance variances also ranged from 1.95 (0) to 1.56 for DFF and DFFI, respectively. Estimated heritability values were very high and varied slightly between parameters studied. Broad-sense heritability (H2b) ranged from 74.50 to 99%, for DFFI, DFF, DFPM, and DNPM, while the narrow-sense heritability (h2n) varied from 73.60% to 99% (Table
Estimates of heritability based on variances of DFFI, DFF, DFPM, and DNPM.
Parameter | Variance components | Heritability estimates | |||||
---|---|---|---|---|---|---|---|
| | | | | H2b | h2n | |
DFFI | 44.88 | 40.24 | 38.68 | 1.56 | 4.64 | 89.7 | 82.2 |
DFF | 4.89 | 4.84 | 4.84 | −1.95 (0) | 1.96 | 99 | 99 |
DFPM | 69.63 | 51.90 | 51.24 | 0.66 | 17.73 | 74.5 | 73.6 |
DNPM | 7.65 | 7.01 | 7.03 | −2.27 (0) | 2.92 | 91.6 | 91.6 |
DFFI = days to first flower initiation, DFF = days to 50% flowering, DFPM = days to first pod maturity, DNPM = days to 90% pod maturity, df = degrees of freedom,
Segregation ratios, chi-square, and
Segregation pattern for maturity indices in F2 and BC1 populations.
Early | Medium | |||||||
---|---|---|---|---|---|---|---|---|
Observe | Expected | Observe | Expected | | | |||
DFFI | F2 | 221 | 225 | 79 | 75 | 3 : 1 | 0.27 | 3.84 |
DFPM | F2 | 228 | 225 | 72 | 75 | 3 : 1 | 0.20 | 3.84 |
DFFI | BC1 | 76 | 67.5 | 14 | 22.5 | 3 : 1 | 1.03 | 3.84 |
DFPM | BC1 | 64 | 67.5 | 26 | 22.5 | 3 : 1 | 0.43 | 3.84 |
Days to first flower initiation varied between minimum of 30 days to maximum of 55 days and DNPM had minimum of 45 days to maximum of 68 days among the F2 population evaluated. In the BC1 generation, the maturity indices ranged between minimum of 31 days and 45 days and maximum of 46 days and 62 days for DFFI and DNPM, respectively (Figure
Frequency distribution analysis of DFFI and DNPM for F2 and BC1.
Frequency distribution analysis of DFFI and DNPM for F1 and Sanzi.
Frequency distribution analysis of DFFI and DNPM for Padi-Tuya.
Pearson correlation analysis of maturity, grain yield, and yield-related traits showed significant (
Correlation among maturity indices, yield, and yield components.
DFI | DFF | DFPM | DNPM | H@F | H@M | CW | P_PLT | S_DP | HSWT | G_YD | |
---|---|---|---|---|---|---|---|---|---|---|---|
DFI | 1 | ||||||||||
DFF | 0.974 | 1 | |||||||||
DFPM | 0.834 | 0.813 | 1 | ||||||||
DNM | 0.916 | 0.894 | 0.820 | 1 | |||||||
H@F | 0.611 | 0.651 | 0.761 | 0.614 | 1 | ||||||
H@M | 0.685 | 0.735 | 0.787 | 0.696 | 0.85128 | 1 | |||||
CW | −0.39 | −0.418 | −0.612 | −0.326NS | −0.691 | −0.706 | 1 | ||||
P_PLT | −0.66 | −0.642 | −0.422 | −0.645 | −0.305NS | −0.303NS | −0.019 | 1 | |||
S_PD | −0.314NS | −0.334NS | −0.505 | −0.464 | −0.486 | −0.545 | 0.319NS | 0.158NS | 1 | ||
HSWT | 0.613 | 0.648 | 0.836 | 0.653 | 0.848 | 0.803 | −0.786 | −0.271NS | −0.582NS | 1 | |
G_YD | 0.307NS | 0.361 | 0.446 | 0.263NS | 0.394 | 0.571 | −0.801 | 0.059 | −0.240NS | 0.609 | 1 |
Early maturity constitutes an important adaptation in agroecological zones with short growing seasons particularly in the arid and semiarid tropics where such cowpea genotypes which mature between 55 and 60 days are ideal for cultivation [
The highly significant differences in the maturity traits (DFFI, DFF, DFPM, and DNPM) among the parents and their generations indicated a high level of genetic variation. This further implies a large scope for breeding and therefore could provide the necessary genetic information for selection for early maturity in cowpea. The phenotypic difference between the parents and their progeny is very important for genetic inheritance studies, aiming to obtain the precise heritability estimates [
The genetic variance components for number of days to first flower initiation and first pod maturity were positive, except for variance due to dominance for number of days to 50% flowering and that of 90% pod maturity which were negative values and for practical purposes were considered as zero. Negative values for dominance variances which imply the masking effects of genes were also observed in previous studies using populations of cowpea [
High narrow sense and broad sense heritability estimates observed suggest that the phenotypes of the traits strongly reflect their genotypes [
The observed segregation frequency in the F2 generation was 3 : 1 for early to medium, respectively. This was significant according to the chi-squares analysis. This implies that inheritance of extra-early maturity in cowpea is under monogenic dominant control. Similar observations were made by Brittingham [
In addition, the frequency distribution analysis of the 300 F2 plants evaluated also indicated that 229 representing 76.33% fell within early maturity group while 71 representing 23.66% were in medium maturity group for DFFI. Similar observation was made for BC1 for the trait. In DFPM, 222 out of the 300 F2 plants representing 74% were early maturing and the others were medium maturing; BC1 population was not much different from that of F2.
All the F1 population were early with means of 32.12 days and 51.56 days for DFFI and DFPM, respectively; these were closer to the early maturing parent (
The absence in significant differences between the F1 and the RF1 populations suggests the absence of maternal effects for inheritance of the maturity indices (DFF, DFFI, DNPM, and DFPM) in cowpea [
Correction analysis suggested that grain yield is significant positively correlated with DFF, DFPM, HSWT, and P_PLT, indicating that the longer the genotypes takes to flower and/or mature, the higher the grain yield is. Hundred-seed weight was also highly positively correlated with grain yield and DFF and DFPM and DNPM. This implies that longer grain filling period enables bigger grains to be developed and hence high grain yield. Similar observations were made by Nakawuka and Adipala [
High heritability estimates for the maturity indices indicate possible transfer of the genes that control early maturity to a medium or late maturing variety. The segregation ratio in the F2 population for early and medium maturity fitted into the ratio, 3 : 1, indicating that maturity in cowpea is conditioned by single dominant gene. Inheritance of early maturity in cowpea is not influenced by maternal effects; hence any of the parents can be used as a maternal parent in hybridization programme for improving early maturity in cowpea. Selection criteria to improve maturity of cowpea should focus on number of days to first flower initiation, number of days to 50% flowering, and number of days to first pod maturity. Grain yield was positively correlated with DFF, DFPM, HSWT, and P_PLT, indicating that the longer the genotypes takes to flower and/or mature the higher the grain yield is. The study provides understanding of the genetic basis of inheritance of early maturity in cowpea which can be useful in future breeding programmes.
First filial generation
Reciprocal cross of first filial generation
Second filial generation
Reciprocal cross of second filial generation
First backcross generation
First reciprocal backcross generation
Second backcross
Second reciprocal backcross generation
Number of days to first flower initiation
Number days to 50% flowering
Number of days to first pod maturity
Number of days to 90% maturity
Height at maturity
Pod per plant
Seed per pod
Hundred-seed weight
Grain yield.
The authors declare that they do not have any conflicts of interest.
The authors are grateful to CSIR-Savanna Agricultural Research Institute and Kwame Nkrumah University of Science and Technology for providing facilities for this experiment. Special thanks and appreciations are due to Dr. S. K. Nutsugah (The Director, CSIR-SARI) for his technical assistance. The efforts of Mr. Emmanuel Kofi Sie for helping in the data collection are well appreciated.