Evaluation of Cowpea (Vigna unguiculata (L.) Walp) Genotypes for Drought Tolerance

Cowpea is multipurpose, leguminous, high protein crop in the tropics that provides food for humans and fodder for animals and adds nitrogen and other nutrients to the soil through symbiosis with rhizobia. Despite its multiple benefits for humankind, the yield of cowpea is far below its potential in Africa due to soil moisture deficit or drought. Even though cowpea is a drought tolerant legume, different genotypes respond differently to drought, resulting in up to 100% or more yield increases in the case of resistant genotypes or 50% or more yield loss in the case of susceptible types. The aim of this study was to identify drought tolerant, high N2-fixing and salt tolerant genotypes and to carry out Quantitative Trait Loci (QTL) mapping. Cowpea genotypes were subjected drought stress conditions in the Northern Region of Ghana, and salt stress in hydroponic conditions in a greenhouse at the Tennessee State University, TN, USA. Ground cowpea samples were subjected to 15N and 13C natural abundance analysis to assess N2 fixation and water use efficiency, respectively. The experiment was a factorial design laid in randomized complete block design with three replications per treatment. Morphological and yield data were taken, and drought tolerance was evaluated based on indices that were derived from grain yield. Most of the parameters measured showed significant differences (p ≤ 0.05). The test genotypes could produce grain yields ranging from 627 – 748 kg/ha under drought imposition, and also elicited high symbiotic dependence by deriving 55% to 98% of their N requirements from symbiosis. The amounts of N-fixed ranged from 37 kg N-fixed ha-1 to 337 kg N-fixed ha-1. Eleven (11) genotypes from USDA and 70 genotypes from RIL population were shown to vary in their salt tolerance levels. Eight thousand two hundred and eighty nine (8,289) Single nucleotide polymorphisms (SNPs) were used for QTL mapping using the RIL population and 10 QTLs were identified. These genotypes and the QTLs identified can be exploited for use in future breeding programs to gain maximum genetic recombination for drought tolerance in subsequent generations of the lines