Topsoil foraging and its role in plant competitiveness for phosphorus in common bean

We evaluated the effect of root shallowness on interplant competition for phosphorus in common bean (Phaseolus vulgaris L.). Recombinant inbred lines (RILs) segregating for basal root gravitropism were evaluated in monogenetic and polygenetic stands with varying phosphorus availability in the field in South China and in solution culture and solid media in controlled environments. In the field, shallow-rooted RILs were more productive than deep-rooted RILs. Shoot biomass of these RILs almost doubled the deep-rooted ones when in competition. In the greenhouse, three treatments representing different soil phosphorus distributions were compared. Root shallowness did not confer any competitive advantage when phosphorus availability was uniformly low or uniformly high, but did confer a competitive advantage when phosphorus availability was concentrated in the topsoil. Shallow and deep-rooted RILs did not differ in response to phosphorus availability in solution culture where phosphorus is mixed and uniformly available. Our results demonstrate that basal root gravitropism, which is a specific root architectural trait under genetic control, is important for belowground competition in low phosphorus soils.