Identifying key contributing root system traits to genetic diversity in field grown cowpea (Vigna unguiculata L. Walp.) genotypesying key contributing root system traits to genetic diversity in field grown cowpea (Vigna unguiculata L. Walp.) genotypes

Abstract

Cowpea is a grain legume that is grown extensively as an alternate protein and income source for many smallholder farmers. Characterizing cowpea root phenotypes could provide the basis for developing genotypes with root system architecture (RSA) traits that increase soil resource acquisition. Measuring RSA traits of any field crop is a demanding task, in terms of expediency, time, cost, and competence. Targeted root phenotyping strategies focusing on a few relevant traits might aid in side-stepping some of the challenges associated with phenotyping roots of field crops. The objectives of this study were to (i) measure genotypic variation for RSA and shoot traits of cowpea genotypes; and (ii) identify candidate variables and genotypes that contribute the largest share of variance. Sixty cowpea accession were grown in field trials at the University of Cape Coast, Ghana. Seventy variables, mostly quantitative RSA traits were measured. Multivariate analysis was used to determine major traits contributing to variation. There were significant differences (P < 0.05) for the majority of traits evaluated. Fifty-nine traits had coefficients of variation of ≥0.3 among genotypes and were selected for further examination. Broad-sense heritability (H2) estimates were generally intermediate to high and ranged from 0.11 to 0.96. The Shannon-Weaver diversity index (H’) was variable among traits and ranged from 0.00 to 0.88. Shoot and root biomass correlated closely and positively with count- and length- and diameter-related traits. Cluster analysis identified three homogeneous genotype groups and identified groups of cowpea genotypes that could be exploited in breeding programs to improve the genetic basis of root traits. The first nine principal components explained over 74% of total genotypic variation for the twenty-nine traits included in the PCA. Sixteen traits contributed more than other traits to the variability in PC1 and PC2. Soil and root tissue angle-related traits, shoot and root diameter-related traits, root biomass, hypocotyl root length, root count and lateral root density -related traits were among the top 50% of the most important traits contributing to variation and thus warrant consideration in efforts to breed for improved genotypes in cowpea. The workflow presented offers a robust, cost-effective and simple approach to identifying focal root traits that contribute to diversity in grain legumes. The results could potentially facilitate the characterization of specific traits suitable for targeted genotype selection and breeding of new cowpea varieties for efficient RSAs.