A survey was carried out to assess the farmers’ production constraints, traits and preferred cowpea varieties. A semi-structured questionnaire was used in a survey in Buhera District, Zimbabwe, in March and April of 2018. Women farmers dominated the survey as they were 52% of the surveyed population, while men occupied 48% of the total population. All participants concurred that cowpeas were used for domestic consumption. Eighty-three percent of the farmers cited shortage, unavailability, and cost of fertiliser, 16% of the farmers acknowledged that they do not have access to quality seed, and 1% cited labour as the major constraints in cowpea production. Farmers ranked heat (86%), drought (10%), and soil fertility (4%) as the most important abiotic factors. Ninety-one percent of the farmers’ ranked rust as the major disease, while 2% ranked storage rot, 1% ranked anthracnose, and 1% ranked downy mildew. Eighty-one percent of the farmers ranked aphids as the main pests, while 3% ranked thrips, 3% ranked legume borers, and 2% ranked pod borers. Fifty-two percent of the farmers preferred varieties that are resistant to diseases such as rust, whereas 48% were not concerned about diseases. For qualitative traits, 50% of the farmers had no specific colour preference, 32% preferred white colour, 14% brown colour, 3% red colour, and 1% tan colour. Ninety-four percent of interviewed farmers were not concerned about the pod shape, 3% preferred the kidney shape, 2% preferred the spherical shape and 1% preferred the globular shape. Ninety-nine percent of farmers agreed that they needed high yielding varieties per unit area and only 1% were unsure. For quantitative traits such as grain size, pod size, plant height, and head size, the preferences of farmers varied. Forty-four percent of the farmer respondents preferred larger cowpea grains, while 56% were not concerned about the size of the grain. A paltry 2% of the farmers were interested in pod size, while 98% did not regard it as important. Thirteen percent of the farmers were interested in climbing varieties, while 87% considered high grain yield as of the utmost importance.

Abiotic stresses cause extensive loss to agriculture production worldwide. Cowpea is an important legume crop grown widely in tropical and subtropical regions where high temperature, ultraviolet-B (UVB) radiation and drought are the common stress factors limiting production. Various vegetative, physiological, biochemical and reproductive plant attributes were assessed under a range of UVB radiation levels in Experiment I and in a combination with two doses of each carbon dioxide concentration [CO2], temperature, and UVB radiation and their interactions in Experiment II by using six cowpea genotypes and sunlit plant growth chambers. The dynamics of photosynthesis and fluorescence processes were assessed in 15 cowpea genotypes under drought condition in Experiment III in pot-grown plants under sunlit conditions. A distinct response pattern was not observed in cowpea in response to UVB radiation from 0 to 15 kJ; however, plants grown under elevated UVB showed reduced photosynthesis resulting in shorter plants and produced smaller flowers and lower seed yield. Increased phenolic compounds appeared to be a defense response to UVB radiation. The growth enhancements observed by doubling of [CO2] were not observed when plants were grown in combination with elevated UVB or temperature which also showed the most detrimental effects on plant growth and seed yield. Results from Experiment I and II revealed that cowpea reproductive traits were highly sensitive to abiotic stresses compared to the vegetative growth and development. A total stress response index (TSRI) technique, derived from all vegetative and reproductive parameters, was used to screen genotypes for their stress tolerance to UVB or combination of stresses. An increase in water use efficiency while maintaining higher rate of photosynthesis was an important drought tolerance mechanism in tolerant cowpea genotypes. Using principal component analysis technique, four groups of the genotypes were identified for their drought tolerance. Evaluating same genotypes across stress conditions revealed that no single genotype has the absolute tolerance characters to all stress conditions. The identified diversity for abiotic stress tolerance among cowpea genotypes and associated traits can be used to develop tolerant genotypes suitable for an agro-ecological niche though traditional breeding or genetic engineering methods.

Cowpea [Vigna unguiculata (L.) Walp.] is a diploid and nutrient-dense legume species. It provides affordable source of protein to human. Cowpea cultivation is prevalent in Africa, Asia, the western and southern U.S., and Central and South America. However, earlier reports have shown that drought and salt stress can be devastating to cowpea production. The objectives of this study were to screen for salt and drought tolerance in cowpea and to identify molecular markers associated with these traits. Simple methodologies to screen for drought (Chapter 2) and salt tolerance were developed (Chapter 3). Results suggested that: 1) a total of 14, 18, 5, 5, and 35 SNPs were associated with plant growth habit change due to drought stress, drought tolerance index for maturity, flowering time, 100-seed weight, and grain yield respectively in a MAGIC cowpea population, the network-guided approach revealed clear interactions between the loci associated with the drought tolerance traits, and GS accuracy varied from low to moderate for this population, 2) a total of 7, 2, 18, 18, 3, 2, 5, 1, and 23 SNPs were associated with various traits evaluated for salt tolerance in a MAGIC cowpea population, some of these SNPs were in the vicinity of potassium channel and biomolecule transporters, and significant epistatic interactions were found 3) a large variation of salt tolerance and drought tolerance was found in the panel involving 331 cowpea genotypes which were genotyped with 14,465,516 SNPs obtained from whole-genome resequencing, 4) tolerance to salt and drought-related traits seemed to be associated with the geographical origins of the cowpea genotypes, 5) a significant GWAS peak defined by a cluster of 196 significant SNPs and mapped on a 210-kb region of chromosome 5 was identified to be a good locus candidate for tolerance to trifoliate leaf chlorosis under drought stress in cowpea and harbored hormone-induced genes, and 6) a strong candidate locus for tolerance to leaf score injury under salt stress and defined by a cluster of 1,400 significant SNPs on chromosome 3 was identified and this region harbored a potassium channel gene. The results from this study could contribute to a better understanding of salt and drought tolerance in cowpea. The salt- and drought-tolerant genotypes could be used as parents in cowpea breeding programs.

In the wake of rising temperatures, erratic rainfall, and declining ground water table, breeding for drought tolerance in food crops has become a top priority throughout the world. Phenotyping a large population of breeding lines for drought tolerance is time-consuming and often unreliable due to multiple possible mechanisms involved. In cowpea (Vigna unguiculata L. Walp), a box-screening method has been used to partition the confounding effects that shoot and root traits have on drought tolerance by restricting root growth and providing a homogeneous soil moisture environment across genotypes. Nonetheless, multiple mechanisms of shoot drought tolerance have been reported which further complicate phenotyping. In winter wheat (Triticum aestivum L.), canopy temperature depression (CTD) has been proposed as a good indicator of drought tolerance. The recent development of low-cost thermal imaging devices could enable high-throughput phenotyping of canopy temperature. While CTD can be an indicator of overall plant water status, it can be confounded by high stomatal resistance, which is another seemingly contradictory mechanism of drought tolerance. The objectives of this study were to explore the physiological basis and genetics of the two mechanisms of shoot drought tolerance previously reported in cowpea and to develop and evaluate a method of high-throughput phenotyping of drought tolerance in winter wheat using thermal imaging. In cowpea, a legume well known for its tight stomatal control, no differences in gas exchange between drought tolerant and susceptible genotypes were observed. A unifoliate stay-green trait was discovered that segregates as a single recessive gene. However, it did not correlate with trifoliate necrosis or overall drought tolerance. In winter wheat, CTD did not always correlate with yield under rainfed conditions. One drought-tolerant cultivar, in particular, had the hottest canopy temperature, possibly because it was able to conserve moisture by closing its stomata whereas another closely related drought-tolerant cultivar had the coolest canopy temperature. Therefore, it appears that no single method of phenotyping for drought tolerance can be broadly applied across all genotypes of a given species due to possible contrasting mechanisms of drought-tolerance and environmental differences. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/152439

Cowpea (Vigna unguiculata), an indigenous legume to sub-Saharan Africa, is mainly grown in the dry savanna areas as an intercrop with millets, sorghum, groundnut and maize. Cowpea grains rich in protein are consumed in different forms in several parts of the tropics. The average grain yield of cowpea in West Africa is approximately 492kg/ha, which is much lower than its potential yields. This low productivity is due to a host of diseases, insects, pests, parasitic weeds, drought, poor soils and low plant population density in farmers’ fields. Ex situ collection of over 15,000 accessions of cowpea and wild Vigna germplasm from different parts of the world were assembled in the IITA gene bank. These genetic resources have been explored to identify new traits and to develop elite cowpea varieties. Many cowpea varieties with high yield potential have been developed and adopted by the farmers. Efforts are continuing to develop better performing varieties using conventional breeding procedures, while molecular tools are being developed to facilitate progress in cowpea breeding.