Genetic analysis of cowpea (Vigna unguiculata L. WALP) agronomic variables in phosphorus limiting soil.

Abstract

Cowpea is an important cultivated grain legume pulse, vegetable and fodder crop of African origin. However, yields have been characteristically low as a result of both abiotic and biotic stresses. Among the abiotic stresses, yield losses due to phosphorus (P) deficiency is an important factor. Identification and selection of cowpea genotypes possessing suitable agronomic traits is key for cultivar design and development to increase production. Accordingly, genetic improvement in cowpea aims to develop cultivars with improved grain yields and tolerance to abiotic and biotic stress factors. The objectives of the study were to (i) identify high yielding cowpea genotypes when exposed to P-limiting soil, (ii) determine the cowpea heritability response of yield and its associated variables when exposed to P-limiting soil and (iii) identify the variables with high discriminating capability among cowpea genotypes in P-limiting soil. Twenty cowpea genotypes comprising of twelve crosses and eight progenitors were evaluated for agronomic variables performance in P limited soil at the University of Zambia Greenhouse in Lusaka. The experiment was arranged as a two set experiment (i.e. applied rate of i) 0kg P2O5 /ha and ii) 60kg P2O5/ha [a control]) and laid as a completely randomized design (CRD) with 3 replications. Comparisons using a paired t- test revealed a significant increase in performance in a control (60kg P2O5 /ha) for all measured variables across genotypes when compared to a 0 Kg P2O5 / ha experimental set. In this study, genotype [LT 11- 3-3-12] and a cross [LT 3-8-4-1 X LT 11-5-1-1] were identified as the highest performers exhibiting a mean yield of 1994.90 kg/ha and 1984 kg/ha respectively in a P-limiting soil. The narrow sense heritability for measured variables ranged from 11- 35 % and 12 -72% for 0 Kg P2O5 and 60 Kg P2O5 experimental set respectively. The highest narrow sense heritability scores in a limiting P experimental set were for pod length (h2= 0.35) and root biomass (h2= 0.28) while in the control set the variable yield (h2= 0.72) had the highest score. Variables plant biomass, root biomass, number of pods and pod length were identified as ideal at discriminating genotypic performance responses in P- limiting soil. These findings imply that high yielding cowpea genotypes with acceptable P deficiency can be developed. Keywords: Cowpea, yield, phosphorus, discriminating, principal component analysis, Heritability