Breeding for Tolerance of Cowpea to Low Phosphorus Soil Conditions through Physiological and Genetic Studies

Cowpea (Vigna unguiculata (L.) Walp.) is a major food legume across Sub- Saharan West Africa where its leaves, pods and seeds are consumed as food and its residues are fed to livestock as protein rich fodder. However, soils of West Africa are poor in phosphorus (P), a soil macro-nutrient all crops need for growth. Fertilizer with P is not readily available and is too expensive for West African farmers. This research was therefore, undertaken to identify cowpea lines that inherently grow well in P-deficient soils and use them to breed improved cowpea varieties that require less phosphorus fertilization. A hydroponic phenotypic screening method with silica sand was used to identify cowpea varieties that have tolerance to low soil P as measured by shoot dry biomass production. Both tolerant and susceptible varieties from the screen were further analyzed for root biomass, internal shoot P content, and internal root P content. Seed P, particularly the effect of cotyledon P, and total root production were investigated as physiological sources of tolerance. Tolerant cowpea varieties were crossed with susceptible varieties, and the resulting F_(1), F_(2) and BC_(1) seeds were screened to determine the inheritance and genetic control of tolerance. A Recombinant Inbred Line (RIL) population of a tolerant by susceptible cross was mapped using SSR markers to identify linkage groups or QTL for tolerance to low soil P.

Phenotypic screening results identified four cowpea varieties to have P-deficiency tolerance (Big John, IT97K-1069-6, IT98K-476-8, and TX2028-1-3-1) and three cowpea varieties (Big John, CB-46, and Golden Eye Cream) to have partial P-deficiency tolerance via high seed P content. All varieties experienced increases in root production under low P treatments relative to normal P treatments. Phenotyping of F_(1), F_(2), and BC_(1) populations showed that low P tolerance is a heritable trait in cowpea with significant additive effects and narrow-sense heritability. Estimates of gene number suggested the tolerance to be a single-gene trait. Mapping linkage groups or QTL for low P tolerance identified QTL in which three SSR markers ? CLM0269, 221/222, and CLM0298 ? were significantly associated with tolerance and are potential candidates for marker-assisted selection (MAS).