Interactions of Fusarium Crown Rot of Wheat with Nitrogen.

The cereal disease Fusarium crown rot (FCR), caused by the fungal pathogen Fusarium pseudograminearum (Fp), is a major constraint to cereal production worldwide. Nitrogen (N) fertilizer is estimated to be approximately 30% of the input costs for grain growers in Australia and is the primary driver of yield and grain protein levels. When targeting high yield and protein, generous nitrogen fertilizer applications are thought to result in large biomass production, which exacerbates FCR severity, reducing grain yield and quality. This research was undertaken to investigate the effect of temporal N availability in high-protein bread and durum wheat varieties on FCR severity. Laboratory and controlled environment experiments assessed the relationship between FCR and N at a mechanistic and plant level. An in vitro study demonstrated an increase in Fp mycelial growth under increased N availability, especially when N was supplied as urea compared with ammonium nitrate. Similarly, under controlled environmental conditions, increased soil N availability promoted FCR severity within infected plants. Stem N transfer efficiency was significantly decreased under FCR infection in both bread and durum wheat varieties by 4.5% and 10.2%, respectively. This new research demonstrates that FCR not only decreases yield and grain quality but appears to have previously unrecognised detrimental impacts on nitrogen-use efficiency in wheat. This indicates that the current impact of losses from FCR may also decrease N-use inefficiencies, as well as yield and quality penalties. An improved understanding of the interactions and restrictions of FCR infection may allow growers to better manage the disease through manipulation of the soil's temporal N availability.

Intrinsic root morphology determines the phosphorus acquisition efficiency of five annual pasture legumes irrespective of mycorrhizal colonisation.

Mycorrhizal fungi are ubiquitous in agroecosystems and form symbiotic associations that contribute to the phosphorus (P) acquisition of many plants. The impact of mycorrhizas is most pronounced in P-deficient soil and commonly involves modifications to the root morphology of colonised plants. However, the consequences of mycorrhizal colonisation on root acclimation responses to P stress are not well described. Five annual pasture legumes, with differing root morphologies, were grown to determine the effect of mycorrhizal colonisation on shoot yield, root morphology and P uptake. Micro-swards of each legume were established in pots filled with a topsoil layer that had been amended with five rates of P fertiliser. The topsoil overlaid a low-P subsoil that mimicked the stratification of P that occurs under pasture. Mycorrhizal colonisation improved P acquisition and shoot yield in the low-P soil treatments, but did not reduce the critical external P requirement of the legumes for near-maximum yield. The yield responses of the mycorrhizal plants were associated with reduced dry matter allocation to topsoil roots, which meant that the P acquisition benefit associated with mycorrhizal colonisation was not additive in the P-deficient soil. The contribution of the mycorrhizal association to P acquisition was consistent among the legumes when they were compared at an equivalent level of plant P stress, and was most pronounced below a P stress index of ~0.5. The intrinsic root morphology of the legumes determined their differences in P-acquisition efficiency irrespective of mycorrhizal colonisation.

Mitigating the open vessel artefact in centrifuge-based measurement of embolism resistance.

Centrifuge-based techniques to assess xylem vulnerability to embolism are increasingly being used, although we are yet to reach a consensus on the nature and extent of artefactual embolism observed in some angiosperm species. In particular, there is disagreement over whether these artefacts influence both the spin (Cavitron) and static versions of the centrifuge technique equally. We tested two methods for inducing embolism: bench dehydration and centrifugation. We used three methods to measure the resulting loss of conductivity: gravimetric flow measured in bench-dehydrated and centrifuged samples (static centrifuge), in situ flow measured under tension during spinning in the centrifuge (Cavitron) and direct imaging using X-ray computed microtomography (microCT) observations in stems of two species of Hakea that differ in vessel length. Both centrifuge techniques were prone to artefactual embolism in samples with maximum vessel length longer than, or similar to, the centrifuge rotor diameter. Observations with microCT indicated that this artefactual embolism occurred in the outermost portions of samples. The artefact was largely eliminated if flow was measured in an excised central part of the segment in the static centrifuge or starting measurements with the Cavitron at pressures lower than the threshold of embolism formation in open vessels. The simulations of loss of conductivity in centrifuged samples with a new model, CAVITOPEN, confirmed that the impact of open vessels on the vulnerability to embolism curve was higher when vessels were long, samples short and when embolism is formed in open vessels at less negative pressures. This model also offers a robust and quantitative tool to test and correct for artefactual embolism at low xylem tensions.

Plant roots redesign the rhizosphere to alter the three-dimensional physical architecture and water dynamics.

The mechanisms controlling the genesis of rhizosheaths are not well understood, despite their importance in controlling the flux of nutrients and water from soil to root. Here, we examine the development of rhizosheaths from drought-tolerant and drought-sensitive chickpea varieties; focusing on the three-dimensional characterization of the pore volume (> 16 µm voxel spatial resolution) obtained from X-ray microtomography, along with the characterization of mucilage and root hairs, and water sorption. We observe that drought-tolerant plants generate a larger diameter root, and a greater and more porous mass of rhizosheath, which also has a significantly increased water sorptivity, as compared with bulk soil. Using lattice Boltzmann simulations of soil permeability, we find that the root activity of both cultivars creates an anisotropic structure in the rhizosphere, in that its ability to conduct water in the radial direction is significantly higher than in the axial direction, especially in the drought-tolerant cultivar. We suggest that significant differences in rhizosheath architectures are sourced not only by changes in structure of the volumes, but also from root mucilage, and further suggest that breeding for rhizosheath architectures and function may be a potential future avenue for better designing crops in a changing environment.

A 3D anatomical atlas of appendage musculature in the chelicerate arthropod Limulus polyphemus.

Limulus polyphemus, an archetypal chelicerate taxon, has interested both biological and paleontological researchers due to its unique suite of anatomical features and as a useful modern analogue for fossil arthropod groups. To assist the study and documentation of this iconic taxon, we present a 3D atlas on the appendage musculature, with specific focus on the muscles of the cephalothoracic appendages. As L. polyphemus appendage musculature has been the focus of extensive study, depicting the muscles in 3D will facilitate a more complete understanding thereof for future researchers. A large museum specimen was CT scanned to illustrate the major exoskeletal features of L. polyphemus. Micro-CT scans of iodine-stained appendages from fresh, non-museum specimens were digitally dissected to interactively depict appendage sections and muscles. This study has revealed the presence of two new muscles: one within the pushing leg, located dorsally relative to all other patella muscles, and the other within the male pedipalp, located in the modified tibiotarsus. This atlas increases accessibility to important internal and external morphological features of L. polyphemus and reduces the need for destructive fresh tissue dissection of specimens. Scanning, digitally dissecting, and documenting taxa in 3D is a pivotal step towards creating permanent digital records of life on Earth.

An image processing and analysis tool for identifying and analysing complex plant root systems in 3D soil using non-destructive analysis: Root1.

The objective of this study was to develop a flexible and free image processing and analysis solution, based on the Public Domain ImageJ platform, for the segmentation and analysis of complex biological plant root systems in soil from x-ray tomography 3D images. Contrasting root architectures from wheat, barley and chickpea root systems were grown in soil and scanned using a high resolution micro-tomography system. A macro (Root1) was developed that reliably identified with good to high accuracy complex root systems (10% overestimation for chickpea, 1% underestimation for wheat, 8% underestimation for barley) and provided analysis of root length and angle. In-built flexibility allowed the user interaction to (a) amend any aspect of the macro to account for specific user preferences, and (b) take account of computational limitations of the platform. The platform is free, flexible and accurate in analysing root system metrics.

Visualization of xylem embolism by X-ray microtomography: a direct test against hydraulic measurements.

X-ray microtomography (microCT) is becoming a valuable noninvasive tool for advancing our understanding of plant-water relations. Laboratory-based microCT systems are becoming more affordable and provide better access than synchrotron facilities. However, some systems come at the cost of comparably lower signal quality and spatial resolution than synchrotron facilities. In this study, we evaluated laboratory-based X-ray microCT imaging as a tool to nondestructively analyse hydraulic vulnerability to drought-induced embolism in a woody plant species. We analysed the vulnerability to drought-induced embolism of benchtop-dehydrated Eucalyptus camaldulensis plants using microCT and hydraulic flow measurements on the same sample material, allowing us to directly compare the two methods. Additionally, we developed a quantitative procedure to improve microCT image analysis at limited resolution and accurately measure vessel lumens. Hydraulic measurements matched closely with microCT imaging of the current-year growth ring, with similar hydraulic conductivity and loss of conductivity due to xylem embolism. Optimized thresholding of vessel lumens during image analysis, based on a physiologically meaningful parameter (theoretical conductivity), allowed us to overcome common potential constraints of some lab-based systems. Our results indicate that estimates of vulnerability to embolism provided by microCT visualization agree well with those obtained from hydraulic measurements on the same sample material.

Ecological succession, hydrology and carbon acquisition of biological soil crusts measured at the micro-scale.

The hydrological characteristics of biological soil crusts (BSCs) are not well understood. In particular the relationship between runoff and BSC surfaces at relatively large (>1 m(2)) scales is ambiguous. Further, there is a dearth of information on small scale (mm to cm) hydrological characterization of crust types which severely limits any interpretation of trends at larger scales. Site differences and broad classifications of BSCs as one soil surface type rather than into functional form exacerbate the problem. This study examines, for the first time, some hydrological characteristics and related surface variables of a range of crust types at one site and at a small scale (sub mm to mm). X-ray tomography and fine scale hydrological measurements were made on intact BSCs, followed by C and C isotopic analyses. A 'hump' shaped relationship was found between the successional stage/sensitivity to physical disturbance classification of BSCs and their hydrophobicity, and a similar but 'inverse hump' relationship exists with hydraulic conductivity. Several bivariate relationships were found between hydrological variables. Hydraulic conductivity and hydrophobicity of BSCs were closely related but this association was confounded by crust type. The surface coverage of crust and the microporosity 0.5 mm below the crust surface were closely associated irrespective of crust type. The δ (13)C signatures of the BSCs were also related to hydraulic conductivity, suggesting that the hydrological characteristics of BSCs alter the chemical processes of their immediate surroundings via the physiological response (C acquisition) of the crust itself. These small scale results illustrate the wide range of hydrological properties associated with BSCs, and suggest associations between the ecological successional stage/functional form of BSCs and their ecohydrological role that needs further examination.

Non-destructive quantification of cereal roots in soil using high-resolution X-ray tomography.

One key constraint to further understanding plant root development is the inability to observe root growth in situ due to the opaque nature of soil. Of the present non-destructive techniques, computed tomography (CT) is best able to capture the complexities of the edaphic environment. This study compared the accuracy and impact of X-ray CT measurement of in situ root systems with standard technology (soil core washing and WinRhizo analysis) in the context of treatments that differed in the vertical placement of phosphorus fertilizers within the soil profile. Although root lengths quantified using WinRhizo were 8% higher than that observed in the same plants using CT, measurements of root length by the two methodologies were highly correlated. Comparison of scanned and unscanned plants revealed no effect of repeated scanning on plant growth and CT was not able to detect any changes in roots between phosphorus treatments that was observed using WinRhizo. Overall, the CT technique was found to be fast, safe, and able to detect roots at high spatial resolutions. The potential drawbacks of CT relate to the software to digitally segment roots from soil and air, which will improve significantly as automated segmentation algorithms are developed. The combination of very fast scans and automated segmentation will allow CT methodology to realize its potential as a high-throughput technique for the quantification of roots in soils.