Genetic variation in rhizosheath, root hair, root system architecture, and phosphorus use efficiency among cowpea genotypes (Vigna unguiculata (L.) Walp.)

Abstract

Cowpea (Vigna unguiculata (L) Walp) is an economically important legume crop of vital importance to the livelihood of several millions of people. Even so, cowpea yields on African farmers’ fields are still below the potential yield of the crop. This is, largely, as a result of the use of unimproved genotypes and farming on phosphorus (P)-poor soils, which are pervasive in the tropics. Plants have evolved several strategies to obtain adequate P for their growth under P limiting conditions. These strategies include modification of root system architecture (RSA) and longer root hairs. Root hairs can be measured using rhizosheath, sheet of soil retained on roots after excavation and shaking. Breeding for cowpea genotypes with superior RSA, rhizosheath and root hair traits will help in achieving food security. The variation in rhizosheath, RSA and root hair were quantified among sixty (60) cowpea genotypes grown up to 21 d under greenhouse conditions. Twenty (20) genotypes were selected for further screening on three (3) external P concentrations [P]ext (i.e.: 0, 250 and 500 mg P/kg soil). Analyses were conducted on rhizosheath, root hair, RSA and biomass traits. The cowpea genotypes were also analysed for variation in P use efficiency (PUE) parameters including agronomic P use efficiency (APE), P uptake efficiency (PUpE), and P efficiency ratio (PER). There were genetic variations among cowpea genotypes in almost all the traits examined. Genotypes with longer root hairs produced larger rhizosheath mass compared to genotypes with larger root hair density. Increasing [P]ext resulted in a significant increase in rhizosheath mass, root hair density, biomass, shoot P concentration and content except for root hair length, which was reduced with increasing [P]ext. Substantial variation was observed for shoot-P, root-P and various measures of PUE among the cowpea genotypes. Some genotypes, including MU9, IT91, Sunshine and WC10*WC36 developed longer root hairs under low P conditions and these were categorised as P-efficient genotypes. Root system and root hair traits including root hair density, root hair length and total root length correlated with PUE in cowpea. The results could be used to select for cowpea -genotypes with improved PUE for use on P-poor soils and provide potential materials and targets for breeding new cowpea cultivars better adapted to P-poor soils in Ghana.