Utility of Basal Root Whorl Number for Phosphorus Acquisition in Common Bean.

The common bean root system consists of the primary root, hypocotyl-borne roots, basal roots, and lateral roots originating from these axial root classes. Basal roots emerge from distinct whorls at the base of the hypocotyls. Bean genotypes vary in the number of basal roots (Basal Root Whorl Number or BRWN) which ranges from one to four, with each whorl giving rise to up to four basal roots. Uppermost whorls produce shallower basal roots and lower whorls produce deeper basal roots and hence plants with greater BRWN have an increased vertical range of soil exploration. Phosphorus is immobile in soil and in general the topsoil has greater phosphorus availability than subsoil.  Shallow basal roots explore more topsoil and hence are associated with increased phosphorus uptake. We used SimRoot, a structural-functional root architectural model to study the performance of bean phenotypes with varying BRWN under different soil phosphorus conditions. Phenotypes with greater BRWN had greater root length and shoot biomass when compared to those with less BRWN. Our results support the hypothesis that plants with greater BRWN perform better under conditions of low P availability