Root trait variation and its contribution to drought tolerance in bambara groundnut (Vigna subterranea (L.) Verdc.)

Bambara groundnut (Vigna subterranea (L) Verdc), is an exemplar neglected African grain legume that thrives under strikingly contrasted environments relative to other grain legumes. Originating in West Africa, its distribution spans across aridity gradients from tropical dry climates in Senegal and Kenya, respectively, down to arid and semi-arid regions in sub-Saharan Africa. This is on soils more or less poor in nutrients and formed under variable pedoclimatic conditions. In these contrasting habitats, it is generally agreed that bambara groundnut has diversified due to domestication from its wild relative, Vigna subterranea var. spontanea (Harms) Hepper, as a result of steady changes through natural and artificial selection. Bambara groundnut is a close relative of cowpea (Vigna unguiculata) and morphologically fits into the same niche as groundnut (Arachis hypogaea L.). The wide distribution in natural environments and ability to tolerate both biotic and abiotic stresses better than cowpea and groundnut, make bambara groundnut an interesting model for examining diversification in response to ephemeral soil water resources. Although important, comprehensive variation assessment on below ground (root) traits in bambara groundnut have not been pursued. The hypothesis was that by focusing on naturally occurring genotypic variation in root system architecture and rooting distribution, bambara groundnut genotypes from dry agroecologies with periodic drought stress had developed root system traits that improved water foraging in deeper soil depths over time. This could be visualised and quantified using a low-cost polyvinyl chloride column (PVC) phenotyping system and image analysis. To test this hypothesis, morphological variability in root system architecture was characterized in eight bambara groundnut parental lines of varying geographic origin (Gresik, LunT, IITA-686, DodR, S19-3, Tiga nicuru, Ankpa-4, DipC1). The experiment was conducted over two seasons (2018 and 2019) under fixed rainout shelter at the Crops For the Future-Field Research Center (CFF-FRC) in Semenyih, Malaysia. Results revealed that in deeper (60-90cm) soil depths, genotypes S19-3 and DipC1 from drier regions of Sub-Saharan Africa had longer tap roots and greater root length distribution. Bambara groundnut genotypes from wetter regions in Southeast Asia and West Africa (i.e., Gresik, LunT, and IITA-686), on the other hand, had shallower and more branched root growth closer to the soil surface. Genotypes generally displayed two extremes in root foraging patterns and branching habits i.e., deep-cheap rooting in genotypes sourced from dry regions and shallow-costly rooting in genotypes adapted to higher rainfall areas with shallow soils. Next, the natural genotypic diversity revealed in the eight genotypes was then investigated to detect adaptive changes in tap root length and root length density in response to periodic drought stress. Genotypes were grown in PVC columns in well-watered and 30-day drought stress (DS) treatments for two seasons (2018 and 2019). DS significantly (P < 0.05 - < 0.001) reduced average shoot height, number of leaves, and delayed flowering in 2018 and 2019. In 2018, the average root-to-shoot ratio was significantly higher (P < 0.001; 22%) under DS treatment. On average tap root length at 55 days after emergence (DAE) i.e., end of 30-d DS, was reduced by 14% and 22% in 2018 and 2019, respectively, and by 5% and 11% at 105 DAE (50-days of DS recovery) in 2018 and 2019, respectively, with some genotypes in 2018: DodR (55 DAE); LunT and Ankpa-4 (105 DAE) and in 2019: IITA-686 (105 DAE), increasing to measurements comparable to the well-watered (WW) treatment. In 2018 and 2019, root length density in the DS treatment was associated with significant grain yield advantage (R2 = 0.27 and R2 = 0.49) in 2018 and 2019, respectively. This indicates that the various agroecological conditions to which bambara groundnut has been exposed in its natural setting may have induced phenotypic differentiation to adapt to ecotypic conditions, which may help offset the impact of adverse events like regular drought stress. When looking for superior genotypes, ecotypic distinction can be an interesting aspect to remember. Finally, root traits such as tap root length and root length density in the 60-90cm soil layer were shown to be beneficial in screening and selecting superior lines from a bambara groundnut population. The population was derived from a cross between two parental lines i.e., S19-3 (maternal) × DodR (paternal). Across replicates, 100-seed weight had the lowest average repeatability (0.62), while high repeatability values were observed for root length density in the 60-90cm soil depth (0.99). Under DS environment (50-d of DS recovery), root length density in the 60-90cm soil depth was strongly correlated (P < 0.05 - P < 0.001) with shoot traits such as number of leaves (r = 0.69), shoot dry weight (r = 0.78), and shoot height (r = 0.67). This indicates that shoot traits are useful traits that can also be used as proxies to make estimations of root length density. According to a regression analysis, root length density in the 60-90cm soil depth was associated with grain yield (R2 = 42%; P < 0.001). According to biplot analysis, the top three bambara groundnut lines in terms of yield under drought stress were ‘Line12′, ‘Line35′, and ‘Line41′. Overall, the work provides a novel and in-depth examination of bambara groundnut below-ground (root trait variation) and its role to drought tolerance. According to this research, bambara groundnut possess differential deep root foraging and density patterns with two extremes i.e., deep-cheap rooting in the genotypes mainly sourced from dry regions and a shallow-costly rooting system in genotypes suited to higher rainfall areas. Farmers have inadvertently selected for these variations over time due to their effect on yield in both dry and wet conditions. Drought tolerance breeding for bambara groundnut will more likely accelerate as a consequence of a better understanding of root systems and foraging patterns. Selected high yielding lines from the S19-3 (maternal) × DodR (paternal) cross i.e., ‘Line12′, ‘Line35′ and ‘Line41′ — all exhibiting deep and extensive rooting in deeper soil depths, will be advanced as part of the current Future Food Beacon: Bambara Groundnut breeding (BamBREED) research project. Elite lines generated from this breeding programme could be registered as improved varieties and released to the general public for cultivation in drought-prone areas. This is projected to boost dietary diversity and significantly increase the nutritional value of people's diets