A functional-structural model of upland rice root systems reveals the importance of laterals and growing root tips for phosphate uptake from wet and dry soils

Abstract

Background and aims: Upland rice is often grown where water and phosphorus (P) are limited. To better understand the interaction between water and P availability, functional-structural models that mechanistically represent small-scale nutrient gradients and water dynamics in the rhizosphere are needed.

Methods: Rice was grown in large columns using a P-deficient soil at three P supplies in the topsoil (deficient, sub-optimal and non-limiting) in combination with two water regimes (field capacity vs. drying periods). Root system characteristics, such as nodal root number, lateral types, interbranch distance, root diameters and the distribution of biomass with depth, as well as water and P uptake, were measured. Based on the observed root data, 3-D root systems were reconstructed by calibrating the structural architecure model CRootBox for each scenario. Water flow and P transport in the soil to each of the individual root segments of the generated 3-D root architectures were simulated using a multiscale flow and transport model. Total water and P uptake were then computed by adding up the uptake by all the root segments.

Key results: Measurements showed that root architecture was significantly affected by the treatments. The moist, high P scenario had 2.8 times the root mass, double the number of nodal roots and more S-type laterals than the dry, low P scenario. Likewise, measured plant P uptake increased >3-fold by increasing P and water supply. However, drying periods reduced P uptake at high but not at low P supply. Simulation results adequately predicted P uptake in all scenarios when the Michaelis-Menten constant (Km) was corrected for diffusion limitation. They showed that the key drivers for P uptake are the different types of laterals (i.e. S- and L-type) and growing root tips. The L-type laterals become more important for overall water and P uptake than the S-type laterals in the dry scenarios. This is true across all the P treatments, but the effect is more pronounced as the P availability decreases.

Conclusions: This functional-structural model can predict the function of specific rice roots in terms of P and water uptake under different P and water supplies, when the structure of the root system is known. A future challenge is to predict how the structure root systems responds to nutrient and water availability.

Keywords: 3-D root architecture; CRootBox; branching density; lateral root types; phosphorus acquisition; soil–root modelling; upland rice (Oryza spp.); water uptake.

Fig. 1.

Simulated root systems (3-D architture) from CRootBox of upland rice grown on a P-deficient soil with three P treatments in the topsoil [no P amendment (NoP), a sub-optimal rate (SubP) and a non-limiting rate (PlusP)] and two water regimes (field capacity (FC) and drying periods (DP)]. The colour of the root represents the P uptake rate of the root, while the colour on the discs represents the soil P sink. The transition between topsoil and sub-soil (at 20 cm depth) can be observed by the shift in P uptake rate in SubP and PlusP.

Fig. 2.

The simulated P uptake per plant (mg per plant or mg per pot, one plant per pot) vs. the measured P uptake per plant in the lab experiment (including the s.e.m.). The latter value of total P uptake was calculated for the shoot and the root after measuring shoot P concentration, and assuming an equal P concentration in the root. Rice root systems were grown and simulated on a P-deficient soil with three P treatments in the topsoil [no P amendment (NoP), a sub-optimal rate (SubP) and a non-limiting rate (PlusP)] and two water regimes [field capacity (FC) and drying periods (DP)].

Fig. 3.

The simulated cumulative water uptake per plant (in kg) of rice roots during a growing period of 52 d. Rice roots were grown under contrasting P rates [P deficiency (NoP), a sub-optimal P amendment in the topsoil (SubP) and a non-limiting P rate in the topsoil (PlusP)] and contrasting water regimes [field capacity (FC) vs. drying periods (DP)]. These simulations enable the differentiation among total water uptake, water uptake from the topsoil (0–20 cm depth) and water uptake from the sub-soil (>20 cm depth).

Fig. 4.

The simulated cumulative P uptake per plant of upland rice roots during a growing period of 52 d. Rice roots were grown under contrasting P rates [P deficiency (NoP), a sub-optimal P amendment in the topsoil (SubP) and a non-limiting P rate in the topsoil (PlusP)] and contrasting water regimes [field capacity (FC) vs. drying periods (DP)]. These simulations enable the differentiation among total P uptake, P uptake from the topsoil (0–20 cm depth) and P uptake from the sub-soil (>20 cm depth).

Fig. 5.

The cumulative water uptake per plant (top) and cumulative P uptake per plant (bottom) by the different root types of rice during a growing period of 52 d. Rice roots were grown under contrasting P rates [P deficiency (NoP), a sub-optimal P amendment in the topsoil (SubP) and a non-limiting P rate in the topsoil (PlusP)] and contrasting water regimes [field capacity (FC) vs. drying periods (DP)]. These simulations enable the differentiation among the water and P uptake by the nodal roots, the S-type lateral roots and the L-type lateral roots.

Fig. 6.

The simulated cumulative P uptake per unit of root mass for each root type (top) and the cumulative P uptake per root tip in each soil layer (bottom). When root tips start entering the sub-soil (approx. 10 d), segment co-ordinates may be assigned to the sub-soil, while a large part of the segment is still located in the topsoil. Therefore. the P uptake is larger due to a larger availability, and hence peaks can be observed.

Fig. 7.

The simulated cumulative water uptake per unit of root mass for each root type (top) and cumulative water uptake per unit surface for each root type (bottom).