Vm Dunbabin

Modelling root–soil interactions using three–dimensional models of root growth, architecture and function

BackgroundThree–dimensional root architectural models emerged in the late 1980s, providing an opportunity to conceptualise and investigate that all important part of plants that is typically hidden and difficult to measure and study. These models have progressed from representing pre–defined root architectural arrangements, to simulating root growth in response to heterogeneous soil environments. This was done through incorporating soil properties and more complete descriptions of plant function, moving into the realm of functional-structural plant modelling. Modelling studies are often designed to investigate the relationship between root architectural traits and root distribution in soil, and the spatio–temporal variability of resource supply. Modelling root systems presents an opportunity to investigate functional tradeoffs between foraging strategies (i.e. shallow vs deep rooting) for contrasting resources (immobile versus mobile resources), and their dependence on soil type, rainfall and other environmental conditions. The complexity of the interactions between root traits and environment emphasises the need for models in which traits and environmental conditions can be independently manipulated, unlike in the real world.ScopeWe provide an overview of the development of three–dimensional root architectural models from their origins, to their place today in the world of functional–structural plant modelling. The uses and capability of root architectural models to represent virtual plants and soil environment are addressed. We compare features of six current models, RootTyp, SimRoot, ROOTMAP, SPACSYS, R-SWMS, and RootBox, and discuss the future development of functional-structural root architectural modelling.ConclusionFunctional-structural root architectural models are being used to investigate numerous root–soil interactions, over a range of spatial scales. They are not only providing insights into the relationships between architecture, morphology and functional efficiency, but are also developing into tools that aid in the design of agricultural management schemes and in the selection of root traits for improving plant performance in specific environments.

Modelling the interactions between water and nutrient uptake and root growth

A model of three-dimensional root growth has been developed to simulate the interactions between root systems, water and nitrate in the rooting environment. This interactive behaviour was achieved by using an external-supply/internal-demand regulation system for the allocation of endogenous plant resources. Data from pot experiments on lupins heterogeneously supplied with nitrate were used to test and parameterise the model for future simulation work. The model reproduced the experimental results well (R2 = 0.98), simulating both the root proliferation and enhanced nitrate uptake responses of the lupins to differential nitrate supply. These results support the use of the supply/demand regulation system for modelling nitrate uptake by lupins. Further simulation work investigated the local uptake response of lupins when nitrate was supplied to a decreasing fraction of the root system. The model predicted that the nitrate uptake activity of lupin roots will increase as the fraction of root system with access to nitrate decreases, but is limited to an increase of around twice that of a uniformly supplied control. This work is the first example of a modelled root system responding plastically to external nutrient supply. This model will have a broad range of applications in the study of the interactions between root systems and their spatially and temporally heterogeneous environment.

Simulation of field data by a basic three-dimensional model of interactive root growth

Published field data for lupins grown in a deep sandy soil in the wheatbelt of south-western Australia were used to test the predictive ability of a model of three-dimensional root growth. The model has the capacity to simulate the growth of individual root sections in response to the supply and demand for water and nitrate. N mineralisation was not modelled explicitly, but was accounted for through the use of a seasonally variable mineralisation input derived from the field data. Simulated nitrogen and water contents and root length densities in the soil profile agreed well with observed profiles, although all were slightly under-predicted. A sensitivity analysis revealed that model predictions were most sensitive to the drained upper limit values (v/v) and the mineralisation rates (μgN m−3 s−1) incorporated as external inputs to the model, along with the unit rate of N2 fixation (mol nodule−1 s−1) and unit root growth rates (μm mol−1 s−1) which are physiological parameters previously calibrated for lupins. The amount of nitrate leached was predicted well. Spatial plots of nitrate leaching were a close inverse of the root length density plots, with the highest nitrate leaching below the inter-plant zones, and the least nitrate leaching directly below each plant. These results suggest that the root distribution of a legume species such as lupin can have an effect on the leaching of nitrate to depth. It may thus be possible to reduce the total amount of nitrate leached under lupin crops by investigating factors such as the spatial deployment of roots, planting densities and intercropping.

Development of a novel semi-hydroponic phenotyping system for studying root architecture

A semi-hydroponic bin system was developed to provide an efficient phenotyping platform for studying root architecture. The system was designed to accommodate a large number of plants in a small area for screening genotypes. It was constructed using inexpensive and easily obtained materials: 240L plastic mobile bins, clear acrylic panels covered with black calico cloth and a controlled watering system. A screening experiment for root traits of 20 wild genotypes of narrow-leafed lupin (Lupinus angustifolius L.) evaluated the reliability and efficiency of the system. Root architecture, root elongation rate and branching patterns were monitored for 6 weeks. Significant differences in both architectural and morphological traits were observed among tested genotypes, particularly for total root length, branch number, specific root length and branch density. Results demonstrated that the bin system was efficient in screening root traits in narrow-leafed lupin, allowing for rapid measurement of two-dimensional root architecture over time with minimal disturbance to plant growth and without destructive root sampling. The system permits mapping and digital measurement of dynamic growth of taproot and lateral roots. This phenotyping platform is a desirable tool for examining root architecture of deep root systems and large sets of plants in a relatively small space.

The root growth response to heterogeneous nitrate supply differs for Lupinus angustifolius and Lupinus pilosus

Little is known about the ability of legume root systems to respond to the heterogeneous supply of nitrate. A split-root nutrient solution experiment was set up to compare the root growth response of 2 lupin species, Lupinus angustifolius L. (dominant tap root and primary lateral system) and L. pilosus Murr. (minor tap root and well-developed lateral root system), to differentially supplied nitrate. These 2 species represent the extremes of the root morphology types present across the lupin germplasm. Nutrient solution containing low (250 M) or high (750 M) nitrate was supplied either uniformly, or split (high and low) between the upper and lower root system. The average growth rate and total root length of L. pilosus was 1.7 times that of L. angustifolius. For both species, the increased proliferation of roots in a high nitrate zone was accompanied by a decrease in root growth in the low nitrate zone, giving approximately the same total growth as the uniform low nitrate treatment. This correlative growth rate response was 15% larger for the first-order branches of L. pilosus than L. angustifolius. While few second-order branches grew for L. angustifolius, the second-order laterals of L. pilosus showed a 2-fold correlative root growth and branching response to the split treatments, with no difference in growth between the uniform high and low nitrate treatments. The second-order laterals thus proliferated in response to the differential supply of nitrate and not the absolute concentration. While the growth rate and branching of the second-order laterals of L. pilosus exhibited a typical correlative response, first-order branching was inhibited in all split treatments, regardless of whether the roots were in the high or low nitrate zone. This response was not seen in L. angustifolius. The difference in the root growth response of the 2 root system types to differentially supplied nitrate suggests a potential in the lupin germplasm for developing a line capable of greater nitrate capture from the soil profile.

Phenotypic variability and modelling of root structure of wild Lupinus angustifolius genotypes

Background and aimsRoot plasticity in response to the edaphic environment represents a challenge in the quantification of phenotypic variation in crop germplasm. The aim of this study was to use various growth systems to assess phenotypic variation among wild genotypes of Lupinus angustifolius.MethodsTen wild genotypes of L. angustifolius selected from an earlier phenotyping study were grown in three different growth systems: semi-hydroponics, potting-mix filled pots, and river-sand filled pots.ResultsMajor root-trait data collected in the present study in the semi-hydroponic growth system were strongly correlated with those from the earlier large phenotyping trial. Plants grown in the two solid media had some of the measured parameters significantly correlated. Principal component analysis captured the major variability in three (semi-hydroponics) or four (solid media) principal components. The genotypes were grouped into five clusters for each growth media, but cluster composition varied among the media. We found genetic variation and phenotypic plasticity in some root traits among tested genotypes. Using input parameters derived from the semihydroponic phenotyping system, simulation models (ROOTMAP and SimRoot) closely reproduced the root systems of a diverse range of lupin genotypes.ConclusionsWild L. angustifolius genotypes displayed genetic variation and phenotypic plasticity when exposed to various growth conditions. The consistent ranking of genotypes in the semihydroponic phenotyping system and the two solid media confirmed the capacity of the semihydroponic phenotyping system of providing simple and relevant growing conditions. The results demonstrated the utility of this system in gathering the data for parameterising the simulation models of root architecture.

Lupinus angustifolius has a plastic uptake response to heterogeneously supplied nitrate while Lupinus pilosus does not

Uptake rates calculated from plants uniformly supplied with a nutrient will underestimate uptake under heterogeneous conditions. A split-root nutrient solution experiment was set up to compare the uptake rate of 2 lupin species (Lupinus angustifolius L., L. pilosus Murr.) under conditions of uniform and heterogeneous nitrate supply. Nitrate was supplied uniformly to the root system at 250 μM (low), 750 μM (high), or 1500 μM (high), or in a split low/high or high/low combination between the upper and lower root system. While L. pilosus had a greater total nitrate uptake over the treatment period due to a higher total root length, L. angustifolius had 1.5-2.5 times greater nitrate uptake rate per unit of root length. L. angustifolius also had the capacity to increase the nitrate uptake rate in sections of the root system supplied locally with high nitrate, compared with a root system uniformly supplied with high nitrate. This increased uptake rate under heterogeneous supply enabled the plant to take up 74-94% of the total nitrate taken up when uniformly supplied with high nitrate, while only 58-72% would have been taken up without such a compensation mechanism. L. pilosus did not show this response. The difference between the response of these 2 species suggests that a range of nitrate uptake responses may exist across the lupin germplasm, and that it may be possible to select a lupin species with an enhanced ability to capture nitrate from the profile, thus decreasing nitrate losses from leaching.

Identifying fertiliser management strategies to maximise nitrogen and phosphorus acquisition by wheat in two contrasting soils from Victoria, Australia

Crop yield and profitability in the dryland production systems of southern Australia are directly affected by the application of nitrogen (N) and phosphorus (P) fertilisers. How efficiently a crop utilises applied fertiliser is affected by several factors that interact in a complex way, including: nutrient mobility, soil type and soil physicochemical and biological factors, season (including rainfall amount and distribution), and crop physiology. In addition, nutrient supply and crop demand need to synchronise both temporally and spatially if nutrient use efficiency is to be optimised. In this study, the mechanistic simulation model, ROOTMAP, was used to investigate and generate hypotheses about the implications of a range of fertiliser management strategies on the nutrient utilisation of wheat. A range of seasons and 2 commercially important soil types (a Wimmera Vertosol and a Mallee Sodosol) were considered. Simulation results showed a strong interaction between the timing and placement of N and P fertiliser, soil type, seasonal conditions, root growth, and nutrient uptake by wheat. This suggests that region-specific recommendations for fertiliser management may be superior to the ‘one size fits all’ approach typically adopted over the Wimmera/Mallee region. Fertiliser use efficiency differed between the 2 soil types, primarily because physicochemical subsoil constraints were present in the Sodosol, but not the Vertosol. These affected rooting depth, total root system size, and root distribution—notably root growth and hence foraging in the topsoil layer. The root growth response to fertiliser management strategies and seasonal rainfall was also reduced on the Sodosol compared with the Vertosol. Simulated fertiliser uptake was responsive to the placement strategy in a dry year characterised by small rainfall events, typical for the Wimmera and Mallee regions. Shallow placement (0.05 or 0.025 m) of N and P in the topsoil utilised topsoil moisture from these small rainfall events, improving crop N and P uptake. The degree of benefit differed between the 2 soil types, and placement of fertiliser was more effective than topdressing. The simulation approach used here provides a preliminary assessment of a range of fertiliser strategies for different soil type and seasonal conditions. However, because ROOTMAP does not provide direct predictions of grain yield response, simulation results need subsequent validation under field conditions before they can be used by growers.

Phosphorus starvation boosts carboxylate secretion in P-deficient genotypes of Lupinus angustifolius with contrasting root structure

Abstract. Lupinus angustifolius L. (narrow-leafed lupin) is an important grain legume crop for the stockfeed industry in Australia. This species does not form cluster roots regardless of phosphorus (P) nutrition. We hypothesise that this species may have adaptive strategies for achieving critical P uptake in low-P environments by altering shoot growth and root architecture and secreting carboxylates from roots. Three wild genotypes of L. angustifolius with contrasting root architecture were selected to investigate the influence of P starvation on root growth and rhizosphere carboxylate exudation and their relationship with P acquisition. Plants were grown in sterilised loamy soil supplied with zero, low (50 μm) or optimal (400 μm) P for 6 weeks. All genotypes showed a significant response in shoot and root development to varying P supply. At P deficit (zero and low P), root systems were smaller and had fewer branches than did roots at optimal P. The amount of total carboxylates in the rhizosphere extracts ranged from 3.4 to 17.3 μmol g–1 dry root. The total carboxylates comprised primarily citrate (61–78% in various P treatments), followed by malate and acetate. Genotype #085 (large root system with deep lateral roots) exuded the greatest amount of total carboxylates to the rhizosphere for each P treatment, followed by #016 (medium root system with good branched lateral roots) and #044 (small root system with short and sparse lateral roots). All genotypes in the low-P treatment significantly enhanced exudation of carboxylates, whereas no significant increase in carboxylate exudation was observed in the zero-P treatment. Small-rooted genotypes had higher P concentration than the medium- and large-rooted genotypes, although larger plants accumulated higher total P content. Large-rooted genotypes increased shoot P utilisation efficiency in response to P starvation. This study showed that narrow-leafed lupin genotypes differing in root architecture differed in carboxylate exudation and P uptake. Our finding suggested that for L. angustifolius there is a minimum plant P concentration below which carboxylate exudation is not enhanced despite severe P deficiency. The outcomes of this study enhance our understanding of P acquisition strategies in L. angustifolius genotypes, which can be used for the selection of P-efficient genotypes for cropping systems.

Wheat roots proliferate in response to nitrogen and phosphorus fertilisers in Sodosol and Vertosol soils of south-eastern Australia

Root growth responses to separately placed of bands of N and P fertiliser were examined at the 3-leaf (GS13) and stem extension growth stages (GS30) for wheat (Triticum aestivum L. cv. Yitpi) growing in 2 major alkaline soil types from the rainfed (375–420 mm) grain production regions of south-eastern Australia. Intact cores of a Sodosol and a Vertosol were destructively sampled and changes in root length density (RLD) and root diameter distribution within the soil profile were examined using restricted maximum likelihood analysis and principal component analysis, respectively. At GS13, RLD increased in the Vertosol when only P was applied, although there was no shoot growth response. The root response to P consisted of a spatially generalised increase in RLD, rather than a specific increase in the vicinity of the P fertiliser band. There was a substantially greater, but still generalised, increase in RLD in the Vertosol when both N and P fertiliser were applied, although there was no response to N fertiliser (without P). The distribution of root length in diameter classes changed with depth in the profile at GS13 but was otherwise similar, regardless of soil types and fertiliser treatment. The root responses to fertiliser at GS30 also consisted of a generalised proliferation of RLD in the topsoil, with no detectable fertiliser-specific changes in the location or structure of the root system. Shoot and root growth increased to a similar level at GS30 when plants were supplied with N, irrespective of P, and root diameter distributions were again insensitive to fertiliser treatment. Plants responded to N by increasing the RLD of relatively fine roots (100–250 μm), which was a P style of acquisition strategy that was possibly triggered by moisture limitations. Consequently, the root responses to fertiliser under realistic semi-arid conditions did not follow expectations based on nutrient acquisition studies. Instead, wheat plants responded to N or P fertiliser with a generalised proliferation of fine roots, apparently to better compete for finite water and nutrients.