Trait decoupling and phenotypic diversity in farmer-managed bambara groundnut ecotypes from Niger

Bambara groundnut (Vigna subterranea (L.) Verdc.) is a drought-tolerant grain legume widely cultivated in low-input systems across sub-Saharan Africa, yet its phenotypic diversity remains incompletely characterized under farmer-managed conditions. This study evaluated 20 local ecotypes from the Dosso region, Niger, in a randomized complete block design during the 2023 rainy season. Fourteen agromorphological traits were recorded at plot and plant level. Friedman tests identified significant varietal differentiation for days to flowering (DF; χ² = 53.56, df = 19, p < 0.001) and hundred-seed weight (HSW_g; χ² = 42.79, df = 19, p < 0.01), while all other traits, including grain yield and all plant-level morphological traits, did not reach statistical significance under this conservative blocked design. Conover post-hoc tests with Holm correction identified 104 significant pairwise contrasts for days to flowering (DF) and 18 for hundred-seed weight (HSW_g). Spearman rank correlations on ecotype means revealed a strongly integrated biomass-yield axis (PTFg-GY_kgha: ρ = 0.945, p < 0.001 after BH-FDR correction), with days to flowering and hundred-seed weight operating as independent dimensions. Principal component analysis on all 14 standardized traits explained 40.0% and 20.3% of variance on PC1 and PC2, respectively. Ward clustering separated the 20 ecotypes into four preliminary phenotypic groups, though sensitivity analysis showed moderate cluster instability when V3 was excluded (Adjusted Rand Index = 0.536). Three ecotypes merit targeted follow-up: V4 for early flowering, V28 for structural architecture, and V18 for pod production. These results argue for multi-trait, multi-environment evaluation before any selection decision.