Genotypic Variation in Root System Architecture in Okra (Abelmoschus esculentus (L.) Moench) for Adaptive Breeding in Soil Resource-Limited Environments

Root system architecture (RSA) is crucial to plant adaptation and the efficiency of soil resource acquisition. However, the RSA variation in okra remains fundamentally uncharacterised. This study aimed to fill this knowledge gap by investigating genetic variability, heritability, and trait associations of RSA characteristics across sixty okra genotypes using a rhizobox-based phenotyping system that evaluated over 30 RSA traits. There was genotypic variation with coefficients of variation ranging from 5% to 70%. Most traits (76%) demonstrated high broad-sense heritability (> 60%), particularly those important for capturing soil resources, including total root length, surface area, and volume. Genetic and phenotypic coefficients of variation were predominantly intermediate (10%–20%) to high (> 20%), except for lateral root angle and primary root length, which showed low variation (< 10%). The first four principal components explained 81.7% of the total genotypic variation, with root perimeter, surface area, and volume as the primary contributors to the diversity in the RSA. There were two genotype groups with contrasting RSA ideotypes independent of the geographical origin of the germplasm. There were moderate to very strong, significant positive associations among many RSA and biomass traits (r = 0.51–0.99; p < 0.001). However, the mean root diameter exhibited weak negative but non-significant correlations with several characteristics. Notable genotypes were identified for specific RSA traits: VI063895 (0.39 and 0.40 g), VI060692 (0.32 and 0.33 g), and GH154 (0.30 and 0.34 g) for superior root biomass allocation; GH108 (2032.28 and 1895.14 cm) for maximum root length; GH111 (25.14 and 20.80 cm³), GH121 (23.56 and 24.59 cm³), and GH157 (18.54 and 19.06 cm³) for enhanced root volume; VI060691 (60° and 62°) and GH125 (61° and 59°) for steep lateral root angles; and V1063895 (10,899 and 10,873), GH135 (9464 and 9330) and GH102 (9303 and 9441) for branching architecture across the two trials. This study advances our understanding of okra RSA diversity, laying the groundwork for trait-based breeding strategies that enhance adaptation to resource-limited environments. The identified genotypes represent diverse RSA ideotypes that offer the potential for improving nutrient and water use efficiency.