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 (L.) Walp.) is a warm-season, multi-purpose legume that is well-adapted to the southeastern USA and has many traits that make it an attractive forage or cover crop for integration into organic production systems, including high rates of nitrogen (N) fixation, phosphorus (P) use efficiency, regrowth ability, and high digestibility. Eight cowpea cultivars were evaluated under organic management at two locations in summer 2014 for stand establishment, forage yield and quality, and weed biomass. Charcoal rot [Macrophomina phaseolina (Tassi) Goid.] is a fungal disease that is economically important to many host plant species. High temperatures and drought conditions favor disease development making it difficult to predict when disease outbreak will occur. Cowpea (Vigna unguiculata L. Walp.) is an important crop for many regions of the globe and is a host species for M. phaseolina. Efforts have been made to breed genetic lines that are resistant to M. phaseolina but little research has been done to screen many popular cowpea cultivars for resistance. This study includes an inoculated field trial and greenhouse seedling screening of twenty-six cowpea lines to identify resistance to charcoal rot.