Characterization and potential for the genetic improvement of pigeonpea (cajanus cajan ) landraces
Pigeonpea (Cajanus cajan) is an important grain legume that provides highly nutritious food for human consumption, livestock feeds and fixes considerable amounts of atmospheric nitrogen, thus improving soil fertility. Local farmers utilize traditional unimproved cultivars that are inherently low yielding limiting the adoption of the crop. The narrow genetic base of pigeonpea limits plant breeding efforts aimed at improving the species. Determining the genotypic variation in agronomic and nutritional attributes among pigeonpea genotypes is essential for improving crop productivity and nutritional content of the crop. Nonetheless, there is a dearth of information regarding the characterization and the genetic diversity in agronomic and nutritive attributes in the pigeonpea germplasm pool maintained at the University of Venda. Therefore, the aim of this study was to characterize and evaluate the potential for the genetic improvement of the agronomic and nutritive attributes of pigeonpea landraces. The specific objectives of the study were to: (i) determine the genetic diversity in agro-morphological attributes (ii) determine the genetic diversity in selected nutritive attributes among pigeonpea landraces (iii) characterize the seed storage proteins in pigeonpea landraces and (iv) determine the general and specific combining abilities for agronomic attributes among pigeonpea landraces. For the first objective of the study, the genetic variation for agronomic and morphological traits among fourteen pigeonpea landraces from the germplasm pool maintained at the University of Venda was evaluated. The genotypes were raised in pots filled with top (up to 25.0 cm deep) soil from the field in the shadehouse for two consecutive seasons. In each season, the experiment was laid out in a completely randomized design replicated three times. Qualitative traits such as the branching pattern, stem colour, flower colour, pod colour and testa colour were recorded. The quantitative traits that were recorded included plant height at flowering (PHF), number of primary branches (NPB), number of secondary branches (NSB), number of seeds per pod (NSP), number of pods per plant (NPP), pod length (PL), hundred seed weight (100-SW) and grain yield per plant (GYPP). The Shannon Weaver diversity index (H’) revealed that the testa colour (1.254) and seed colour pattern (1.301) were the most divergent qualitative traits. There were marked differences in the duration to flowering ranging from 84.67 to 132.83 days with a mean of 117.05 days. The genotype ‘T1’ required <90 days to flower. The local genotype ‘L3’ was among the large seeded (>16.0 g per 100 seeds). The grain yield per plant varied between 19.9 – 51.7 g. The best performing genotypes (with mean GYPP > 45.0 g) were ‘L1’ and ‘L5’. The principal component analysis (PCA) revealed that the first two principal components with eigenvalues of more than one accounted for 58.04% of the variation among the pigeonpea genotypes. Positive correlations observed between most secondary traits showed that multiple trait selection is possible. The NPP, 100-SW and GYPP were the most important agronomic attributes for assessing genetic variation due to their high contribution to the variation accounted on PC1 and PC2. The findings revealed the potential for exploiting genetic diversity to improve pigeonpea agronomic performance. The second study objective focussed on determining the diversity in nutritional attributes among pigeonpea landraces. Seed samples (5.0 g each) from each of the genotypes harvested from the study of the first objective were harvested and processed for determining mineral composition using the Inductively Coupled Plasma Emission Spectrometer. Relatively high amounts of calcium (2103.43 mg/kg) and manganese (73.11 mg/kg) were observed in ‘L3’. High amounts of zinc (38.56 mg/kg) were observed for ‘L9’ whereas ‘L4’ produced high levels of phosphorus (4945.12 mg/kg). The PCA showed that the first two principal components cumulatively explained 62.06% of the total variation among the pigeonpea genotypes. On the PCA biplot, Ca, Mg, Mn and P were associated with ‘L3’, ‘L4’ and ‘L5’ respectively. Genotypes producing high crude protein and high mineral concentrations in most nutritional traits (i.e., ‘L1’, ‘L3’, ‘T1’ and ‘T4) could be used as parental lines for the genetic improvement of nutritional attributes in pigeonpea. The variability in the range of mineral elements among pigeonpea genotypes indicated that there was genetic potential for selection of parental lines for nutritional quality improvement of the crop. The third study objective was designed to characterize the seed storage proteins of pigeonpea using initially the sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS PAGE) in a 12.0% separating gel in a discontinuous buffer system. The SDS PAGE resolved the pigeonpea seed storage proteins into 21 protein bands with molecular weights ranging from 10 to 250 kDa. The absence of the 42, 58, 130 and 250 kDa subunits in genotype ‘L2’ and the presence of the 58 kDa in genotype ‘T5’ could be used for genotypic identification. Thereafter, gel fractions were excised and prepared for the Liquid Chromatography – Mass Spectrometry analysis (LCMS) to evaluate the proteomic variation in seed and determine their functional spectrum. The spectroscopic analysis identified 446 proteins from pigeonpea seed constituted mainly by stress related proteins such as heat shock protein, proteases and chaperones amongst others. Proteases were localized in the nucleus, cytoplasm, proteasome regulatory particle and complex. The amino acid sequence alignment revealed functional motifs such as the Walker A, Walker B and the DEVD. The MPN (Mpr1, Pad1 N-terminal), PCI (Proteasome, COP9, Initiation factor 3) and AAA+ ATPase were the protein superfamilies recognized from the proteases. There was 97.31% similarity between the 26S proteasome regulatory subunit 4 homolog A (A0A1S2XSJ5) and the 26S proteasome regulatory subunit 4 homolog A-like (A0A1S2XWG8) when the amino acid residues were aligned with each other. The proteomic analysis identified large number of stress related proteins probably due to pigeonpea’s ability to withstand harsh environmental conditions such as drought, salinity and extreme temperatures. The final study objective estimated the general and specific combining ability effects as well as the gene action controlling agronomic traits in pigeonpea at the vegetative stage. Nine progenies derived from three lines and three testers in a line x tester mating design were evaluated for agronomic traits at the vegetative growth stage together with their parents in the shadehouse. Crosses were carried out manually by emasculation using forceps prior to flower opening. Each cross was labelled for subsequent identification. The six parents and their nine F1 progenies were raised simultaneously in the shadehouse in pots (as described above). Agronomic traits such as the PH, NPB, and the branching angle (BA) were recorded. The experiment was laid as a completely randomized design with three replications. Line ‘L3’ was the best general combiner for the NPB and BA exhibiting desirable positive GCA effects of 4.00 and 5.37 in the desirable direction. The tester ‘T4’ had positive and significant GCA effects of 11.815 for PH. The crosses ‘L3xT5’, ‘L4xT2’, ‘L5xT4’ and ‘L5xT5’ were good specific combiners for PH with significant and positive SCA estimates of 13.730, 48.396, 25.352 and 15.185, respectively. The best specific combiners for NPB were ‘L4xT2’ (4.778) and ‘L5xT5’ (5.556). For canopy width, the cross ‘L4xT2’ produced a SCA value of 16.363 followed by ‘L5xT5’ with 15.196 in a desirable direction. The narrow sense heritability for all the traits was low (<12.0%) indicating that there is a predominant role of non-additive gene actions for the traits evaluated in this study
Agronomic attribute, Combining ability, Genetic diversity, Minerals, Seed storage, Proteins, UCTD
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