Genetic Improvement Of Cowpea (Vigna Unguiculata (L.) Walp) For Phosphorus Use Efficiency

ABSTRACT

Cowpea is an important grain legume crop for millions of humans, fodder for livestock and source of income for all the value chain actors. Its productivity is constrained by several biotic and abiotic stresses such as drought and poor soil fertility. Aligned with poor soil fertility in most growing areas, this thesis describes phosphorus (P) use and acquisition of elite cowpea lines from different breeding programmes. In the first chapter, a brief overview was provided about cowpea as an important multi-purpose legume, constraints to its production including P deficiency and knowledge of farmers on using P based fertilizers. A participatory survey of farmers was conducted across 36 villages of cowpea growing areas in northern Nigeria, using a semi-structured questionnaire and focus group discussions to determine farmers’ perceptions on phosphorus fertilization, prevailing cowpea cropping systems, use of improved varieties and farmers’ perceived production constraints and preferred traits. Results showed that farmers were aware of fertilizers as important for growth and increased yield but did not know the major need of P for cowpeas. Intercropping with cereals was the most popular cropping system. A little above 20% used improved varieties of cowpea. Farmers identified insects and yield as the major constraint and preferred trait, respectively. Screening experiments, both in the screenhouse and low P field, to identify and group elite cowpea lines based on performance under low P and high soil P conditions using shoot dry weight and other parameters were conducted. There was significant diversity among the elite lines for adaptation to low P and response to applied P fertilizer. A few cowpea lines such as IT97K-556-6, IT84S-2246-4 and IT89KD-288 produced above average yield under sub-optimal and high P conditions. The relative reduction in yield as a result of soil P deficiency compared to high P performance was over 50%. Cowpea lines with above average performance under the contrasting P soils were grouped as efficient and responsive lines and are suitable for cultivation under limited and optimum P input systems. There was a significant reduction in days to flowering and maturity among cowpea lines under high

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P conditions relative to low P. To understand the genetics underlying P use and uptake efficiency, a quantitative trait loci (QTLs) mapping study was undertaken through marker-trait analysis with a biparental recombinant inbred lines (RIL) mapping population. A total of 27 QTLs were detected across 7 of 11 linkage groups of cowpea. These QTLs were different from the previously identified ones, indicating they are new QTLs under varying P conditions. These genomic regions were associated with flowering time, yield components and P use efficiency traits under low and high P conditions. In addition, a genome-wide association mapping study (GWAS) using DArTseq derived SNP markers on 400 diverse cowpea lines to identify QTLs and SNP markers based on historical recombination events underlying the genetics of tolerance to low P and response to applied mineral P fertilizer was conducted. The GWAS mapping resulted in the identification of over 60 SNP markers significantly associated with adaptation to low P conditions and response to P application as measured by differences in shoot dry weight, P use and uptake efficiency under two soil P conditions. In conclusion, this research revealed that farmers did not use the recommended fertilizer types and rates for cowpea production, use of mixed intercropping was the popular cropping system and cultivation of landraces was most prominent over improved varieties. The screening of cowpea lines under different P concentrations showed varied performance, and lines were grouped into efficient responsive, efficient non-responsive, inefficient responsive and inefficient non-responsive classes based on their performance in low and high P growth media. Marker-trait analysis with a biparental RIL population led to the identification of QTLs and SNPs that will lay the foundation for marker-assisted selection, that will fast-track the development of P efficient varieties. The study reported for the first time on cowpea use of high-density SNP markers to identify QTLs and markers associated with P traits under field conditions using genetic materials relevant to sub-Saharan African breeding programmes. The validation of SNP markers identified from this study before their use in marker-assisted selection is highly recommended.