Basal root whorl number: a modulator of phosphorus acquisition in common bean (Phaseolus vulgaris L.)

Root architectural phenes enhancing topsoil foraging are important for phosphorus acquisition. Here we describe the utility of a novel phene, basal root whorl number (BRWN), that has significant effects on topsoil foraging in common bean (Phaseolus vulgaris L.). Whorls are defined as distinct tiers of basal roots that emerge in a tetrarch fashion along the base of the hypocotyl. In this study, wild and domesticated bean taxa as well as a Recombinant Inbred Line (RIL) population were screened for BRWN and basal root number (BRN). A set of 6 RILs contrasting for BRWN was evaluated for performance under low phosphorus availability in the greenhouse and in the field. In the greenhouse, plants were grown in a sand-soil media with low or high phosphorus availability. In the field, plants were grown in an Oxisol in Mozambique under low and moderate phosphorus availability. Wild bean accessions tended to have a BRWN of one or two, whereas cultivated accessions had BRWN reaching 4 and sometimes 5. BRWN and BRN did not vary with phosphorus availability, i.e. BRWN was not a plastic trait in these genotypes. Greater BRWN was beneficial for phosphorus acquisition in low phosphorus soil. Genotypes with three whorls had almost twice the shoot biomass, greater root length, and greater leaf area than related genotypes with two whorls. In low phosphorus soil, shoot phosphorus content was strongly correlated with BRWN (r^2= 0.64 in the greenhouse and r^2=0.88 in the field). Genotypes with three whorls had shallower root systems with a greater range of basal root growth angles (from 10 to 45 degrees from horizontal) than genotypes with two whorls (angles ranged from 60 to 85 degrees from horizontal). Our results indicate that BRWN is associated with increased phosphorus acquisition and that this trait may have value for selection of genotypes with better performance in low phosphorus soils.