title: The importance of root gravitropism for inter-root competition and phosphorus acquisition efficiency: results from a geometric simulation model.
abstract: We have observed that low soil phosphorus availability alters the gravitropic response of basal roots in common bean (Phaseolus vulgaris L.), resulting in a shallower root system. In this study we use a geometric model to test the hypotheses that a shallower root system is a positive adaptive response to low soil P availability by (1) concentrating root foraging in surface soil horizons, which generally have the highest P availability, and (2) reducing spatial competition for P among roots of the same plant. The growth of nine root systems contrasting in gravitropic response over 320 h was simulated in SimRoot, a dynamic three-dimensional geometric model of root growth and architecture. Phosphorus acquisition and inter-root competition were estimated with Depzone, a program that dynamically models nutrient diffusion to roots. Shallower root systems had greater P acquisition per unit carbon cost than deeper root systems, especially in older root systems. This was due to greater inter-root competition in deeper root systems, as measured by the volume of overlapping P depletion zones. Inter-root competition for P was a significant fraction of total soil P depletion, and increased with increasing values of the P diffusion coefficient (De), with root age, and with increasing root gravitropism. In heterogenous soil having greater P availability in surface horizons, shallower root systems had greater P acquisition than deeper root systems, because of less inter-root competition as well as increased root foraging in the topsoil. Root P acquisition predicted by SimRoot was validated against values for bean P uptake in the field, with an r2 between observed and predicted values of 0.75. Our results support the hypothesis that altered gravitropic sensitivity in P-stressed roots, resulting in a shallower root system, is a positive adaptive response to low P availability by reducing inter-root competition within the same plant and by concentrating root activity in soil domains with the greatest P availability.
authors.affiliation: College of Polytechnic, South China Agricultural University, P.R. China.
authors.forename: Z
authors.initials: Z
authors.lastname: Ge
authors.forename: G
authors.initials: G
authors.lastname: Rubio
authors.forename: J P
authors.initials: JP
authors.lastname: Lynch
mesh_terms: D003198:Computer Simulation; D007887:Fabaceae / Q000254:growth & development / Q000378:metabolism; D018522:Gravitropism / Q000502:physiology*; D008954:Models, Biological; D010758:Phosphorus / Q000032:analysis / Q000493:pharmacokinetics*; D018517:Plant Roots / Q000254:growth & development / Q000378:metabolism*; D010946:Plants, Medicinal; D015203:Reproducibility of Results; D012984:Software; D012987:Soil / Q000032:analysis; D013997:Time Factors
chemical_list: D012987:Soil; D010758:Phosphorus