Root architecture and phosphorus acquisition efficiency in common bean (Phaseolus vulgaris L.)

Low phosphorus availability in soils is often a constraint to plant productivity. Root growth and architecture are important for phosphorus acquisition due to the relative immobility of P in the soil. An obstacle to the study of root architecture is the difficulty of measuring and quantifying the three-dimensional configuration of roots in soil. Fractal geometry might be useful in estimating the three-dimensional complexity of root architecture from more accessible measurements. Carbon/phosphorus budgets and fractal geometry of contrasting simulated root architectures was estimated using SimRoot, a deformable geometric model of root growth. Carbon allocation to biomass, respiration, exudation and phosphorus acquisition efficiency varied significantly among architectures. In a sand culture experiment phosphorus efficient genotypes had a higher root:shoot ratio compared to inefficient genotypes. This could be due to the observed lower root respiration per unit dry weight in these genotypes thereby allowing a larger fraction of the carbon allocated to the root system to be used for growth. A log-linear relationship was found between fractal dimension of simulated root systems and spatial dimension. This log-linear relationship suggests that the three-dimensional fractal dimension of root systems may be estimated from tracing of root intersections on exposed planes. In a field study it was found that planar fractal dimension of root systems increased over time and correlated well with shoot P content indicating fractal dimension to be a possible descriptor of root branching complexity and P uptake.