The importance of root gravitropism for inter-root competition and phosphorus acquisition efficiency: results from a geometric simulation model

We have observed that low soil phosphorus availability alters the gravitrop ic response of basal roots in common bean (Phaseolus vulgaris L.), resultin g in a shallower root system. In this study we use a geometric model to tes t the hypotheses that a shallower root system is a positive adaptive respon se to low soil P availability by (1) concentrating root foraging in surface soil horizons, which generally have the highest P availability, and (2) re ducing spatial competition for P among roots of the same plant. The growth of nine root systems contrasting in gravitropic response over 320 h was sim ulated in SimRoot, a dynamic three-dimensional geometric model of root grow th and architecture. Phosphorus acquisition and inter-root competition were estimated with Depzone, a program that dynamically models nutrient diffusi on to roots. Shallower root systems had greater P acquisition per unit carb on cost than deeper root systems, especially in older root systems. This wa s 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 (D-e), with root age, and with increasing root gravitropism. In heterogenous soil having greater P a vailability in surface horizons, shallower root systems had greater P acqui sition than deeper root systems, because of less inter-root competition as well as increased root foraging in the topsoil. Root P acquisition predicte d by SimRoot was validated against values for bean P uptake in the field, w ith an r(2) between observed and predicted values of 0.75. Our results supp ort 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 ava ilability.