Root gravitropism and below-ground competition among neighbouring plants: A modelling approach

Competition for nutrients among neighbouring roots occurs when their individual depletion volumes overlap, causing a reduction in nutrient uptake. By exploring different spatial niches, plants with contrasting root architecture may reduce the extent of competition among neighbouring root systems. The main objectives of this study were: (1) to evaluate the impact of root architecture on competition for phosphorus among neighbouring plants; and (2) to compare the magnitude of competition among roots of the same plant vs. roots of neighbouring plants. SimRoot, a dynamic geometric model, was used to simulate common bean root growth and to compare the overlap of depletion volumes. By varying the gravitropism of basal roots, we simulated three distinct root architectures: shallow, intermediate and deep, corresponding to observed genetic variation for root architecture in this species. Combinations of roots having the same architecture resulted in more intense inter-plant competition. Among them, the deep-deep combination had the most intense competition. Competition between deep root systems and shallow root systems was only half, that of deep root systems competing with other deep root systems. Inter-plant root competition increased as soil diffusivity increased and the distance among plants decreased. In heterogeneous soils, co-localization of soil resources and roots was more important in determining resource uptake than inter-plant root competition. Competition among roots of the same plant was three- to five-times greater than competition among roots of neighbouring plants. Genetic variation for root architecture in common bean may be related to adaptation to diverse competitive environments.