Comparative phenomics of annual grain legume root architecture

Suboptimal water and P availability are primary limitations to grain legume production. Root architecture influences water and P acquisition, but tradeoffs need to be better understood and mitigated. We hypothesized that tradeoffs in root class investment and resource acquisition strategy would be observable in a variety of grain legumes. Diversity panels of common bean (Phaseolus vulgaris L.), tepary bean (Phaseolus acutifolius A. Gray), cowpea [Vigna unguiculata (L.) Walp.], soybean [Glycine max (L.) Merr.], chickpea (Cicer arietinum L.), groundnut (Arachis hypogaea L.), and single accessions of other legumes were phenotyped in the field. We identified inverse relationships among investments in different root classes in most species, and between indicators of deep and shallow exploration in all species. Bean and tepary bean showed particularly strong tradeoffs in investment patterns among root classes, whereas chickpea and groundnut show less pronounced tradeoffs. We found that legume root architectural phenotypes can be placed on a root system architecture (RSA) spectrum, and that root phenotypes of epigeal and hypogeal taxa present distinct adaptive mechanisms. These life strategies integrating resource acquisition, use, and phenology are exemplified by contrasting chickpea, with many root axes, to tepary bean with few root axes and a contrasting water use strategy. We propose several RSA ideotypes and highlight how dimorphic root architecture may co-optimize resource acquisition.