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


ZhenYang, G.; Rubio, G.; Lynch, J.P.


Plant and Soil, Volume 218, Issue 1/2, p.159-171 (2000)

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Low soil phosphorus availability alters the gravitropic response of basal roots in common bean ( Phaseolus vulgaris ), resulting in a shallower root system. A geometric model was used 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 ( D e ), with root age, and with increasing root gravitropism. In heterogeneous 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 r 2 between observed and predicted values of 0.75. The 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 thegreatest P availability.