Survival analysis of germination data in response to temperature for Ornithopus species and other temperate pasture legumes

Cool temperatures can limit productivity of temperate grazing systems as poor pasture growth rates in winter create feed shortages for livestock. Ornithopus spp. (serradella) are broadly adapted annual pasture legumes that produce high-quality forage in soil types considered marginal for other temperate legume species. However, serradella establishment is perceived to be difficult in cool-season environments. We used survival analysis to compare germination rate and seedling emergence for two serradella species (yellow serradella and French serradella) against three reference species (Medicago sativa, M. polymorpha and Trifolium subterraneum ) in four temperature treatments (10/5, 15/10, 20/15 and 25/20°C; max/min). We also compared shoot relative growth rate and photosynthetic rate at 15/10°C (cool) and 23/18°C (warm). Cool temperatures (10/5, 15/10°C) did not slow germination rates for serradella relative to the reference species, but warm temperatures (20/15, 25/20°C) delayed emergence and reduced post-emergent shoot growth rates. Once established, Ornithopus spp. had similar mean photosynthetic rates and stomatal conductance at cool temperatures to the reference species. We conclude that, contrary to common perception, cool temperatures did not adversely influence germination, emergence, or early growth of Ornithopus spp. relative to the reference species.

Niche differentiation of Mucoromycotinian and Glomeromycotinian arbuscular mycorrhizal fungi along a 2-million-year soil chronosequence

Current literature suggests ecological niche differentiation between co-occurring Mucoromycotinian arbuscular mycorrhizal fungi (M-AMF) and Glomeromycotinian AMF (G-AMF), but experimental evidence is limited. We investigated the influence of soil age, water availability (wet and dry), and plant species (native Microlaena stipoides and exotic Trifolium subterraneum) on anatomical root colonisation and DNA profiles of M-AMF and G-AMF under glasshouse conditions. We grew seedlings of each species in soils collected from the four stages of a soil chronosequence, where pH decreases from the youngest to oldest stages, and phosphorus (P) is low in the youngest and oldest, but high in the intermediate stages. We scored the percentage of root length colonised and used DNA metabarcoding to profile fungal richness and community composition associated with treatment combinations. Soil age, water availability, and plant species were important influencers of root colonisation, although no M-AMF were visible following staining of M. stipoides roots. Soil age and host plant influenced fungal richness and community composition. However, response to soil age, potential host species, and water availability differed between M-AMF and G-AMF. Root colonisation of T. subterraneum by M-AMF and G-AMF was inversely correlated with soil P level. Community composition of M-AMF and G-AMF was structured by soil age and, to a lesser extent, plant species. Richness of M-AMF and G-AMF was negatively, and positively, correlated with available P, respectively. These findings are experimental evidence of ecological niche differentiation of M-AMF and G-AMF and invite further exploration into interactive effects of abiotic and biotic factors on their communities along successional trajectories.

Leaf formononetin content of Trifolium subterraneum increases in response to waterlogging but its proportion of total isoflavones is little changed

The isoflavone formononetin (F) impacts livestock fertility and cultivars of the pasture legume Trifolium subterraneum L. (subclover) have been selected for F levels ≤0.2% of leaf dry weight. However, the impact of waterlogging (WL) on isoflavones is little studied. We investigated the response of isoflavones, biochanin A (BA), genistein (G) and F, to WL for: (1) Yarloop (high F) and eight low F cultivars each from subspecies subterraneum, brachycalycinum and yanninicum (Experiment 1); and (2) four cultivars and 12 ecotypes of ssp. yanninicum (Experiment 2). WL impacted F: estimated means increased from 0.19% (control) to 0.31% (WL) in Experiment 1 and from 0.61% to 0.97% in Experiment 2. Isoflavones under WL were highly heritable, particularly F (H2=95%). The proportions of BA, G and F were little changed by WL, with strong positive correlations between free-drained and waterlogged treatments. Isoflavone contents were not related to WL tolerance, as assessed by shoot relative growth rate. In conclusion, isoflavones varied among genotypes and increased with WL, but the proportion of individual isoflavones in each genotype was stable. High F under WL was unrelated with genotype tolerance to WL. Instead, it was a consequence of inherently high F for that particular genotype.

Low-moderate dietary phytoestrogens transiently disrupt spermatogenesis and the seminal plasma proteome in the ram

In brief

Dietary phytoestrogens disrupt a specific stage of ram spermatogenesis, causing subtle decreases in sperm quality by affecting the expression of pathways involved in the structural integrity of the spermatozoa. This paper demonstrates for the first time that ram reproduction is compromised by oestrogenic pasture, whilst also providing a longitudinal model for the impact of phytoestrogens on male fertility.

Abstract

Compounds with oestrogen-like actions are now common in both the Western diet. The long-term impacts and underlying mechanisms by which oestrogenic compounds alter male reproduction, however, are unclear. To investigate this, we used a longitudinal sheep model examining the impact of oestrogenic pasture consumption on semen quality and production, testicular size, sexual behaviour and the seminal plasma proteome of Merino rams (n = 20), over a full spermatogenic cycle and in the subsequent breeding season. Throughout the study period, sexual behaviour, sperm production and motility were similar between the exposed and non-exposed rams (P > 0.05). However, between 5 and 8 weeks of exposure to dietary phytoestrogens, rams produced a higher percentage of spermatozoa with a specific malformation of the sperm midpiece and reduced DNA integrity, compared to non-exposed rams (P < 0.001). Investigation into the seminal plasma proteome revealed 93 differentially expressed proteins between phytoestrogen-exposed and control rams (P < 0.05). Exposure to phytoestrogens increased the expression of proteins involved in cellular structure development, actin cytoskeleton reorganisation, regulation of cell function and decreased expression in those related to catabolic processes. The greatest fold changes were in proteins involved in the assembly of the sperm flagella, removal of cytoplasm, spermatid development and maintenance of DNA integrity. After returning to non-oestrogenic pasture, no differences in any measure were observed between treatment groups during the subsequent breeding season. We conclude that dietary phytoestrogens can transiently disrupt specific stages of ram spermatogenesis, causing subtle decreases in sperm quality by affecting the expression of pathways involved in the structural integrity of the spermatozoa.

Differences in foliar phosphorus fractions, rather than in cell-specific phosphorus allocation, underlie contrasting photosynthetic phosphorus use efficiency among chickpea genotypes

Although significant intraspecific variation in photosynthetic phosphorus (P) use efficiency (PPUE) has been shown in numerous species, we still know little about the biochemical basis for differences in PPUE among genotypes within a species. Here, we grew two high PPUE and two low PPUE chickpea (Cicer arietinum) genotypes with low P supply in a glasshouse to compare their photosynthesis-related traits, total foliar P concentration ([P]) and chemical P fractions (i.e. inorganic P (Pi), metabolite P, lipid P, nucleic acid P, and residual P). Foliar cell-specific nutrient concentrations including P were characterized using elemental X-ray microanalysis. Genotypes with high PPUE showed lower total foliar [P] without slower photosynthetic rates. No consistent differences in cellular [P] between the epidermis and mesophyll cells occurred across the four genotypes. In contrast, high PPUE was associated with lower allocation to Pi and metabolite P, with PPUE being negatively correlated with the percentage of these two fractions. Furthermore, a lower allocation to Pi and metabolite P was correlated with a greater allocation to nucleic acid P, but not to lipid P. Collectively, our results suggest that a different allocation to foliar P fractions, rather than preferential P allocation to specific leaf tissues, underlies the contrasting PPUE among chickpea genotypes.

Enhanced nodulation and phosphorus acquisition from sparingly-soluble iron phosphate upon treatment with arbuscular mycorrhizal fungi in chickpea

The coordination/trade-off among below-ground strategies for phosphorus (P) acquisition, including root morphology, carboxylate exudation and colonisation by arbuscular mycorrhizal fungi (AMF), is not well understood. This is the first study investigating the relationships between root nodulation, morphology, carboxylates and colonisation by an indigenous community of AMF under varying P levels and source. Two chickpea genotypes with contrasting amounts of rhizosheath carboxylates were grown in pots at six P levels (from 0 to 160 μg g^-1 ) as KH₂ PO₄ (KP, highly soluble) or FePO₄ (FeP, sparingly soluble), with or without AMF (±AMF) treatment. Under both FeP and KP, the presence of AMF inhibited shoot growth and shoot branching, decreased total root length and specific root length, increased mean root diameter and root tissue density and reduced carboxylates. However, the role of AMF in acquiring P differed between the two P sources, with the enhanced P acquisition under FeP while not under KP. Co-inoculation of AMF and rhizobia enhanced nodulation under FeP, but not under KP. Our results suggest that the effects of AMF on shoot branching were mediated by cytokinins as the reduced shoot branching in FeP40 and KP40 under +AMF relative to -AMF coincided with a decreased concentration of cytokinins in xylem sap for both genotypes.

Phosphorus fractions in leaves

Leaf phosphorus (P) comprises four major fractions: inorganic phosphate (P_i ), nucleic acids, phospholipids, P-containing metabolites and a residual fraction. In this review paper, we investigated whether allocation of P fractions varies among groups of terrestrial vascular plants, and is indicative of a species' strategy to use P efficiently. We found that as leaf total P concentration increases, the P_i fraction increases the most, without a plateau, while other fractions plateau. Variability of the concentrations of leaf P fractions is greatest among families > species(family) > regions > plant life forms. The percentage of total P allocated to nucleic acid-P (20-35%) and lipid-P (14-34%) varies less among families/species. High photosynthetic P-use efficiency is associated with low concentrations of all P fractions, and preferential allocation of P to metabolite-P and mesophyll cells. Sequential resorption of P from senescing leaves starts with P_i , followed by metabolite-P, and then other organic P fractions. Allocation of P to leaf P fractions varies with season. Leaf phytate concentrations vary considerably among species, associated with variation in photosynthesis and defence. Plasticity of P allocation to its fractions is important for acclimation to low soil P availability, and species-specific P allocation is needed for co-occurrence with other species.

Phosphorus uptake and toxicity are delimited by mycorrhizal symbiosis in P-sensitive Eucalyptus marginata but not in P-tolerant Acacia celastrifolia

Many plant species from regions with ancient, highly weathered nutrient-depleted soils have specialized adaptations for acquiring phosphorus (P) and are sensitive to excess P supply. Mycorrhizal associations may regulate P uptake at high external P concentrations, potentially reducing P toxicity. We predicted that excess P application will negatively impact species from the nutrient-depleted Jarrah forest of Western Australia and that mycorrhizal inoculation will reduce P toxicity by regulating P uptake. For seedlings of the N₂-fixing legume Acacia celastrifolia and the tree species Eucalyptus marginata, we measured growth at P concentrations of 0-90 mg kg^-1 soil and in relation to inoculation with the arbuscular mycorrhizal fungus (AMF) Rhizophagus irregularis. Non-inoculated A. celastrifolia maintained leaf P concentrations at <2 mg g^-1 dry mass (DM) across the range of external P concentrations. However, for non-inoculated E. marginata, as external P concentrations increased, leaf P also increased, reaching >9 mg g^-1 DM at 30 mg P kg^-1 soil. Acacia celastrifolia DM increased with increasing external P concentrations, while E. marginata DM was maximal at 15 mg P kg^-1 soil, declining at higher external P concentrations. Neither DM nor leaf P of A. celastrifolia was affected by inoculation with AMF. For E. marginata, even at 90 mg P kg^-1 soil, inoculation with AMF resulted in leaf P remaining <1 mg g^-1 DM, and DM being maintained. These data strengthen the evidence base that AMF may not only facilitate P uptake at low external P concentrations, but are also important for moderating P uptake at elevated external P concentrations and maintaining plant P concentrations within a relatively narrow concentration range.

Agricultural land-use favours Mucoromycotinian, but not Glomeromycotinian, arbuscular mycorrhizal fungi across ten biomes

Globally, agricultural land-use negatively affects soil biota that contribute to ecosystem functions such as nutrient cycling, yet arbuscular mycorrhizal fungi (AMF) are promoted as essential components of agroecosystems. Arbuscular mycorrhizal fungi include Glomeromycotinian AMF (G-AMF) and the arbuscule-producing fine root endophytes, recently re-classified into the Endogonales order within Mucoromycotina. The correct classification of Mucoromycotinian AMF (M-AMF) and the availability of new molecular tools can guide research to better the understanding of their diversity and ecology. To investigate the impact on G-AMF and M-AMF of agricultural land-use at a continental scale, we sampled DNA from paired farm and native sites across 10 Australian biomes. Glomeromycotinian AMF were present in both native and farm sites in all biomes. Putative M-AMF were favoured by farm sites, rare or absent in native sites, and almost entirely absent in tropical biomes. Temperature, rainfall, and soil pH were strong drivers of richness and community composition of both groups, and plant richness was an important mediator. Both fungal groups occupy different, but overlapping, ecological niches, with M-AMF thriving in temperate agricultural landscapes. Our findings invite exploration of the origin and spread of M-AMF and continued efforts to resolve the phylogeny of this newly reclassified group of AMF.

Natural attenuation of legacy hydrocarbon spills in pristine soils is feasible despite difficult environmental conditions in the monsoon tropics

The Kimberley region of Western Australia is a National Heritage listed region that is internationally recognised for its environmental and cultural significance. However, petroleum spills have been reported at a number of sites across the region, representing an environmental concern. The region is also characterised as having low soil nutrients, high temperatures and monsoonal rain - all of which may limit the potential for natural biodegradation of petroleum. Therefore, this work evaluated the effect of legacy petroleum hydrocarbons on the indigenous soil microbial community (across the domains Archaea, Bacteria and Fungi) at three sites in the Kimberley region. At each site, soil cores were removed from contaminated and control areas and analysed for total petroleum hydrocarbons, soil nutrients, pH and microbial community profiling (using16S rRNA and ITS sequencing on the Illumina MiSeq Platform). The presence of petroleum hydrocarbons decreased microbial diversity across all kingdoms, altered the structure of microbial communities and increased the abundance of putative hydrocarbon degraders (e.g. Mycobacterium, Acremonium, Penicillium, Bjerkandera and Candida). Microbial community shifts from contaminated soils were also associated with an increase in soil nutrients (notably Colwell P and S). Our study highlights the long-term effect of legacy hydrocarbon spills on soil microbial communities and their diversity in remote, infertile monsoonal soils, but also highlights the potential for natural attenuation to occur in these environments.

Critical phosphorus requirements of Trifolium species: The importance of root morphology and root acclimation in response to phosphorus stress

Differences in root morphology and acclimation to low-phosphorus (P) soil were examined among eight legume species from the Trifolium Section Tricocephalum to understand how these root attributes determine P acquisition. Ornithopus sativus was included as a highly P-efficient benchmark species. Plants were grown as microswards in pots with five rates of P supplied in a topsoil layer to mimic uneven P distribution within a field soil profile. Topsoil and subsoil roots were harvested separately to enable measurement of the nutrient-foraging responses. Critical P requirement (lowest P supply for maximum yield) varied over a threefold range, reflecting differences in root morphology and acclimation of nutrient-foraging roots to P stress. Among the species, there was a 3.2-fold range in root length density, a 1.7-fold range in specific root length, and a 2.1-fold range in root hair length. O. sativus had the lowest critical P requirement, displayed a high root length density, the highest specific root length, and the longest root hairs. Acquisition of P from P-deficient soil was facilitated by development of a large root hair cylinder (i.e. a large root-soil interface). This, in turn, was determined by the intrinsic root morphology attributes of each genotype, and the plasticity of its root morphology response to internal P stress. Root acclimation in low-P soil by all species was mostly associated with preferential allocation of mass to nutrient-foraging roots. Only O. sativus and four of the Trifolium species adjusted specific root length beneficially, and only O. sativus increased its root hair length in low-P soil.

Novel Genes and Genetic Loci Associated With Root Morphological Traits, Phosphorus-Acquisition Efficiency and Phosphorus-Use Efficiency in Chickpea

Chickpea-the second most important grain legume worldwide-is cultivated mainly on marginal soils. Phosphorus (P) deficiency often restricts chickpea yields. Understanding the genetics of traits encoding P-acquisition efficiency and P-use efficiency will help develop strategies to reduce P-fertilizer application. A genome-wide association mapping approach was used to determine loci and genes associated with root architecture, root traits associated with P-acquisition efficiency and P-use efficiency, and any associated proxy traits. Using three statistical models-a generalized linear model (GLM), a mixed linear model (MLM), and a fixed and random model circulating probability unification (FarmCPU) -10, 51, and 40 marker-trait associations (MTAs), respectively were identified. A single nucleotide polymorphism (SNP) locus (Ca1_12310101) on Ca1 associated with three traits, i.e., physiological P-use efficiency, shoot dry weight, and shoot P content was identified. Genes related to shoot P concentration (NAD kinase 2, dynamin-related protein 1C), physiological P-use efficiency (fasciclin-like arabinogalactan protein), specific root length (4-coumarate-CoA ligase 1) and manganese concentration in mature leaves (ABC1 family protein) were identified. The MTAs and novel genes identified in this study can be used to improve P-use efficiency in chickpea.

Evidence for Niche Differentiation in the Environmental Responses of Co-occurring Mucoromycotinian Fine Root Endophytes and Glomeromycotinian Arbuscular Mycorrhizal Fungi

Fine root endophytes (FRE) were traditionally considered a morphotype of arbuscular mycorrhizal fungi (AMF), but recent genetic studies demonstrate that FRE belong within the subphylum Mucoromycotina, rather than in the subphylum Glomeromycotina with the AMF. These findings prompt enquiry into the fundamental ecology of FRE and AMF. We sampled FRE and AMF in roots of Trifolium subterraneum from 58 sites across temperate southern Australia. We investigated the environmental drivers of composition, richness, and root colonization of FRE and AMF by using structural equation modelling and canonical correspondence analyses. Root colonization by FRE increased with increasing temperature and rainfall but decreased with increasing phosphorus (P). Root colonization by AMF increased with increasing soil organic carbon but decreased with increasing P. Richness of FRE decreased with increasing temperature and soil pH. Richness of AMF increased with increasing temperature and rainfall but decreased with increasing soil aluminium (Al) and pH. Aluminium, soil pH, and rainfall were, in decreasing order, the strongest drivers of community composition of FRE; they were also important drivers of community composition of AMF, along with temperature, in decreasing order: rainfall, Al, temperature, and soil pH. Thus, FRE and AMF showed the same responses to some (e.g. soil P, soil pH) and different responses to other (e.g. temperature) key environmental factors. Overall, our data are evidence for niche differentiation among these co-occurring mycorrhizal associates.

Accumulation of phosphorus and calcium in different cells protects the phosphorus-hyperaccumulator Ptilotus exaltatus from phosphorus toxicity in high-phosphorus soils

Ptilotus exaltatus accumulates phosphorus (P) to > 40 mg g^-1 without toxicity symptoms, while Kennedia prostrata is intolerant of increased P supply. What physiological mechanisms underlie this difference and protect P. exaltatus from P toxicity? Ptilotus exaltatus and K. prostrata were grown in a sandy soil with low-P, high-P and P-pulse treatments. Both species hyperaccumulated P (>20 mg g^-1) under high-P and P-pulse treatments; shoot dry weight was unchanged for P. exaltatus, but decreased by >50% for K. prostrata. Under high-P, in young fully-expanded leaves, both species accumulated P predominantly as inorganic P. However, P. exaltatus preferentially allocated P to mesophyll cells and stored calcium (Ca) as occasional crystals in specific lower mesophyll cells, separate from P, while K. prostrata preferentially allocated P to epidermal and spongy mesophyll cells, but co-located P and Ca in palisade mesophyll cells where granules with high [P] and [Ca] were evident. Mesophyll cellular [P] correlated positively with [potassium] for both species, and negatively with [sulfur] for P. exaltatus. Thus, P. exaltatus tolerated a very high leaf [inorganic P] (17 mg g^-1), associated with P and Ca allocation to different cell types and formation of Ca crystals, thereby avoiding deleterious precipitation of Ca₃(PO₄)₂. It also showed enhanced [potassium] and decreased [sulfur] to balance high cellular [P]. Phosphorus toxicity in K. prostrata arose from co-location of Ca and P in palisade mesophyll cells. This study advances understanding of leaf physiological mechanisms for high P tolerance in a P-hyperaccumulator and indicates P. exaltatus as a promising candidate for P-phytoextraction.

First Cryo-Scanning Electron Microscopy Images and X-Ray Microanalyses of Mucoromycotinian Fine Root Endophytes in Vascular Plants

AIMS

Arbuscule-producing fine root endophytes (FRE) (previously incorrectly Glomus tenue) were recently placed within subphylum Mucoromycotina; the first report of arbuscules outside subphylum Glomeromycotina. Here, we aimed to estimate nutrient concentrations in plant and fungal structures of FRE and to test the utility of cryo-scanning electron microscopy (cryoSEM) for studying these fungi.

METHODS

We used replicated cryoSEM and X-ray microanalysis of heavily colonized roots of Trifolium subterraneum.

RESULTS

Intercellular hyphae and hyphae in developed arbuscules were consistently very thin; 1.35 ± 0.03 μm and 0.99 ± 0.03 μm in diameter, respectively (mean ± SE). Several intercellular hyphae were often adjacent to each other forming "hyphal ropes." Developed arbuscules showed higher phosphorus concentrations than senesced arbuscules and non-colonized structures. Senesced arbuscules showed greatly elevated concentrations of calcium and magnesium.

CONCLUSION

While uniformly thin hyphae and hyphal ropes are distinct features of FRE, the morphology of fully developed arbuscules, elevated phosphorus in fungal structures, and accumulation of calcium with loss of structural integrity in senesced arbuscules are similar to glomeromycotinian fungi. Thus, we provide evidence that FRE may respond to similar host-plant signals or that the host plant may employ a similar mechanism of association with FRE and AMF.

Root microbiomes as indicators of seagrass health

The development of early warning indicators that identify ecosystem stress is a priority for improving ecosystem management. As microbial communities respond rapidly to environmental disturbance, monitoring their composition could prove one such early indicator of environmental stress. We combined 16S rRNA gene sequencing of the seagrass root microbiome of Halophila ovalis with seagrass health metrics (biomass, productivity and Fsulphide) to develop microbial indicators for seagrass condition across the Swan-Canning Estuary and the Leschenault Estuary (south-west Western Australia); the former had experienced an unseasonal rainfall event leading to declines in seagrass health. Microbial indicators detected sites of potential stress that other seagrass health metrics failed to detect. Genera that were more abundant in 'healthy' seagrasses included putative methylotrophic bacteria (e.g. Methylotenera and Methylophaga), iron cycling bacteria (e.g. Deferrisoma and Geothermobacter) and N2 fixing bacteria (e.g. Rhizobium). Conversely, genera that were more abundant in 'stressed' seagrasses were dominated by putative sulphur-cycling bacteria, both sulphide-oxidising (e.g. Candidatus Thiodiazotropha and Candidatus Electrothrix) and sulphate-reducing (e.g. SEEP-SRB1, Desulfomonile and Desulfonema). The sensitivity of the microbial indicators developed here highlights their potential to be further developed for use in adaptive seagrass management, and emphasises their capacity to be effective early warning indicators of stress.

Tradeoffs among root morphology, exudation and mycorrhizal symbioses for phosphorus-acquisition strategies of 16 crop species

Plant roots exhibit diverse root functional traits to enable soil phosphorus (P) acquisition, including changes in root morphology, root exudation and mycorrhizal symbioses. Yet, whether these traits are differently coordinated among crop species to enhance P acquisition is unclear. Here, eight root functional traits for P acquisition were characterized in 16 major herbaceous crop species grown in a glasshouse under limiting and adequate soil P availability. We found substantial interspecific variation in root functional traits among species. Those with thinner roots showed more root branching and less first-order root length, and had consistently lower colonization by arbuscular mycorrhizal fungi (AMF), fewer rhizosheath carboxylates and reduced acid phosphatase activity. In response to limiting soil P, species with thinner roots showed a stronger response in root branching, first-order root length and specific root length of the whole root system, Conversely, species with thicker roots exhibited higher colonization by AMF and/or more P-mobilizing exudates in the rhizosheath. We conclude that, at the species level, tradeoffs occur among the three groups of root functional traits we examined. Root diameter is a good predictor of the relative expression of these traits and how they change when P is limiting.

Globular structures in roots accumulate phosphorus to extremely high concentrations following phosphorus addition

Crops with improved uptake of fertilizer phosphorus (P) would reduce P losses and confer environmental benefits. We examined how P-sufficient 6-week-old soil-grown Trifolium subterraneum plants, and 2-week-old seedlings in solution culture, accumulated P in roots after inorganic P (Pi) addition. In contrast to our expectation that vacuoles would accumulate excess P, after 7 days, X-ray microanalysis showed that vacuolar [P] remained low (<12 mmol kg^-1 ). However, in the plants after P addition, some cortex cells contained globular structures extraordinarily rich in P (often >3,000 mmol kg^-1 ), potassium, magnesium, and sodium. Similar structures were evident in seedlings, both before and after P addition, with their [P] increasing threefold after P addition. Nuclear magnetic resonance (NMR) spectroscopy showed seedling roots accumulated Pi following P addition, and transmission electron microscopy (TEM) revealed large plastids. For seedlings, we demonstrated that roots differentially expressed genes after P addition using RNAseq mapped to the T. subterraneum reference genome assembly and transcriptome profiles. Among the most up-regulated genes after 4 hr was TSub_g9430.t1, which is similar to plastid envelope Pi transporters (PHT4;1, PHT4;4): expression of vacuolar Pi-transporter homologs did not change. We suggest that subcellular P accumulation in globular structures, which may include plastids, aids cytosolic Pi homeostasis under high-P availability.

Oxygen loss from seagrass roots coincides with colonisation of sulphide-oxidising cable bacteria and reduces sulphide stress

Seagrasses thrive in anoxic sediments where sulphide can accumulate to phytotoxic levels. So how do seagrasses persist in this environment? Here, we propose that radial oxygen loss (ROL) from actively growing root tips protects seagrasses from sulphide intrusion not only by abiotically oxidising sulphides in the rhizosphere of young roots, but also by influencing the abundance and spatial distribution of sulphate-reducing and sulphide-oxidising bacteria. We used a novel multifaceted approach combining imaging techniques (confocal fluorescence in situ hybridisation, oxygen planar optodes, and sulphide diffusive gradients in thin films) with microbial community profiling to build a complete picture of the microenvironment of growing roots of the seagrasses Halophila ovalis and Zostera muelleri. ROL was restricted to young root tips, indicating that seagrasses will have limited ability to influence sulphide oxidation in bulk sediments. On the microscale, however, ROL corresponded with decreased abundance of potential sulphate-reducing bacteria and decreased sulphide concentrations in the rhizosphere surrounding young roots. Furthermore, roots leaking oxygen had a higher abundance of sulphide-oxidising cable bacteria; which is the first direct observation of these bacteria on seagrass roots. Thus, ROL may enhance both abiotic and bacterial sulphide oxidation and restrict bacterial sulphide production around vulnerable roots, thereby helping seagrasses to colonise sulphide-rich anoxic sediments.

Implications of co-contamination with aged heavy metals and total petroleum hydrocarbons on natural attenuation and ecotoxicity in Australian soils

The bioremediation of historic industrial contaminated sites is a complex process. Co-contamination, often with lead which was commonly added to gasoline until 16 years ago is one of the biggest challenges affecting the clean-up of these sites. In this study, the effect of heavy metals, as co-contaminant, together with total petroleum hydrocarbons (TPH) is reported, in terms of remaining soil toxicity and the structure of the microbial communities. Contaminated soil samples from a relatively hot and dry climate in Western Australia were collected (n = 27). Analysis of soils showed the presence of both contaminants, TPHs and heavy metals. The Microtox test confirmed that their co-presence elevated the remaining ecotoxicity. Toxicity was correlated with the presence of lead, zinc and TPH (0.893, 0.599 and 0.488), respectively, assessed using Pearson Correlation coefficient factor. Next Generation Sequencing of soil bacterial 16S rRNA, revealed a lack of dominate genera; however, despite the variation in soil type, a few genera including Azospirillum spp. and Conexibacter were present in most soil samples (85% and 82% of all soils, respectively). Likewise, many genera of hydrocarbon-degrading bacteria were identified in all soil samples. Streptomyces spp. was presented in 93% of the samples with abundance between 7% and 40%. In contrast, Acinetobacter spp. was found in only one sample but was a dominant member of (45%) of the microbial community. In addition, some bacterial genera were correlated to the presence of the heavy metals, such as Geodermatophilus spp., Rhodovibrio spp. and Rubrobacter spp. which were correlated with copper, lead and zinc, respectively. This study concludes that TPH and heavy metal co-contamination significantly elevated the associated toxicity. This is an important consideration when carrying out risk assessment associated with natural attenuation. This study also improves knowledge about the dynamics of microbial communities in mixed contamination scenarios.

Little evidence that farmers should consider abundance or diversity of arbuscular mycorrhizal fungi when managing crops

Contents Summary 1092 I. Introduction 1093 II. Investigating activity of AMF in agroecosystems 1093 III. Crop benefit from AMF: agronomic and mycorrhizal literature differ 1094 IV. Flawed methodology leads to benefits of mycorrhizas being overstated 1094 V. Rigorous methodology suggests low colonisation by AMF can sometimes reduce crop yield 1095 VI. Predicting when mycorrhizas matter for crop yield 1096 VII. Crop genotype 1099 VIII. Fungal genotype 1100 IX. Complex interactions between the mycorrhizal fungal and soil microbial communities 1102 X. Phosphorus-efficient agroecosystems 1102 XI. Conclusions 1103 Acknowledgements 1104 References 1104 SUMMARY: Arbuscular mycorrhizal fungi (AMF) are ubiquitous in agroecosystems and often stated to be critical for crop yield and agroecosystem sustainability. However, should farmers modify management to enhance the abundance and diversity of AMF? We address this question with a focus on field experiments that manipulated colonisation by indigenous AMF and report crop yield, or investigated community structure and diversity of AMF. We find that the literature presents an overly optimistic view of the importance of AMF in crop yield due, in part, to flawed methodology in field experiments. A small body of rigorous research only sometimes reports a positive impact of high colonisation on crop yield, even under phosphorus limitation. We suggest that studies vary due to the interaction of environment and genotype (crop and mycorrhizal fungal). We also find that the literature can be overly pessimistic about the impact of some common agricultural practices on mycorrhizal fungal communities and that interactions between AMF and soil microbes are complex and poorly understood. We provide a template for future field experiments and a list of research priorities, including phosphorus-efficient agroecosystems. However, we conclude that management of AMF by farmers will not be warranted until benefits are demonstrated at the field scale under prescribed agronomic management.

Phosphorus acquisition and utilisation in crop legumes under global change

Improving phosphorus (P)-use efficiency in legumes is a worldwide challenge in the face of an increasing world population, dwindling global rock phosphate reserves, the relatively high P demand of legumes and global change. This review focuses on P acquisition of crop legumes in response to climate change. We advocate further studies on: firstly, the response of carboxylate exudation, mycorrhizas and root morphology to climate change and their role in P acquisition as dependent on edaphic factors; secondly, developing intercropping systems with a combination of a legume and another crop species to enhance P acquisition; and thirdly, the impact of the interactions of the major climate change factors on P acquisition in the field.

Leaf transpiration plays a role in phosphorus acquisition among a large set of chickpea genotypes

Low availability of inorganic phosphorus (P) is considered a major constraint for crop productivity worldwide. A unique set of 266 chickpea (Cicer arietinum L.) genotypes, originating from 29 countries and with diverse genetic background, were used to study P-use efficiency. Plants were grown in pots containing sterilized river sand supplied with P at a rate of 10 μg P g^-1 soil as FePO₄ , a poorly soluble form of P. The results showed large genotypic variation in plant growth, shoot P content, physiological P-use efficiency, and P-utilization efficiency in response to low P supply. Further investigation of a subset of 100 chickpea genotypes with contrasting growth performance showed significant differences in photosynthetic rate and photosynthetic P-use efficiency. A positive correlation was found between leaf P concentration and transpiration rate of the young fully expanded leaves. For the first time, our study has suggested a role of leaf transpiration in P acquisition, consistent with transpiration-driven mass flow in chickpea grown in low-P sandy soils. The identification of 6 genotypes with high plant growth, P-acquisition, and P-utilization efficiency suggests that the chickpea reference set can be used in breeding programmes to improve both P-acquisition and P-utilization efficiency under low-P conditions.

A mycorrhizal revolution

It has long been postulated that symbiotic fungi facilitated plant migrations onto land through enhancing the scavenging of mineral nutrients and exchanging these for photosynthetically fixed organic carbon. Today, land plant-fungal symbioses are both widespread and diverse. Recent discoveries show that a variety of potential fungal associates were likely available to the earliest land plants, and that these early partnerships were probably affected by changing atmospheric CO₂ concentrations. Here, we evaluate current hypotheses and knowledge gaps regarding early plant-fungal partnerships in the context of newly discovered fungal mutualists of early and more recently evolved land plants and the rapidly changing views on the roles of plant-fungal symbioses in the evolution and ecology of the terrestrial biosphere.

The carboxylate-releasing phosphorus-mobilizing strategy can be proxied by foliar manganese concentration in a large set of chickpea germplasm under low phosphorus supply

Root foraging and root physiology such as exudation of carboxylates into the rhizosphere are important strategies for plant phosphorus (P) acquisition. We used 100 chickpea (Cicer arietinum) genotypes with diverse genetic backgrounds to study the relative roles of root morphology and physiology in P acquisition. Plants were grown in pots in a low-P sterilized river sand supplied with 10 μg P g^-1 soil as FePO₄ , a poorly soluble form of P. There was a large genotypic variation in root morphology (total root length, root surface area, mean root diameter, specific root length and root hair length), and root physiology (rhizosheath pH, carboxylates and acid phosphatase activity). Shoot P content was correlated with total root length, root surface area and total carboxylates per plant, particularly malonate. A positive correlation was found between mature leaf manganese (Mn) concentration and carboxylate amount in rhizosheath relative to root DW. This is the first study to demonstrate that the mature leaf Mn concentration can be used as an easily measurable proxy for the assessment of belowground carboxylate-releasing processes in a range of chickpea genotypes grown under low-P, and therefore offers an important breeding trait, with potential application in other crops.

Phosphorus- and nitrogen-acquisition strategies in two Bossiaea species (Fabaceae) along retrogressive soil chronosequences in south-western Australia

During long-term ecosystem development and its associated decline in soil phosphorus (P) availability, the abundance of mycorrhizal plant species declines at the expense of non-mycorrhizal species with root specialisations for P-acquisition, such as massive exudation of carboxylates. Leaf manganese (Mn) concentration has been suggested as a proxy for such a strategy, Mn concentration being higher in non-mycorrhizal plants that release carboxylates than in mycorrhizal plants. Shifts in nitrogen (N)-acquisition strategies also occur; nodulation in legumes is expected at low N availability, when sufficient P is available. We investigated whether two congeneric legume species (Bossiaea linophylla and Bossiaea eriocarpa) occurring along two long-term chronosequences on the south-western Australian coast and grown in a glasshouse at varying N and P supply exhibited plasticity in nutrient-acquisition strategies. We hypothesised that the shifts in nutrient limitation and nutrient-acquisition strategies at the community level would also be found at the species level. Leaf N: P ratios and the responses to nutrient availability suggested that growth of both species exhibited P-limitation in all treatments, due to the very high leaf [N] of legumes afforded by symbiotic N-fixation. Mycorrhizal colonisation was not greater at higher P supply, and root exudation of carboxylates was not stimulated at low P supply; both were unrelated to leaf [Mn]. However, nodule production declined with increasing N supply. We conclude that intraspecific variation in nutrient-acquisition and use is low in these species, and that the variation at the community level, observed in previous studies, is likely driven by high-species turnover.

Low Light Availability Alters Root Exudation and Reduces Putative Beneficial Microorganisms in Seagrass Roots

Seagrass roots host a diverse microbiome that is critical for plant growth and health. Composition of microbial communities can be regulated in part by root exudates, but the specifics of these interactions in seagrass rhizospheres are still largely unknown. As light availability controls primary productivity, reduced light may impact root exudation and consequently the composition of the root microbiome. Hence, we analyzed the influence of light availability on root exudation and community structure of the root microbiome of three co-occurring seagrass species, Halophila ovalis, Halodule uninervis and Cymodocea serrulata. Plants were grown under four light treatments in mesocosms for 2 weeks; control (100% surface irradiance (SI), medium (40% SI), low (20% SI) and fluctuating light (10 days 20% and 4 days 100%). 16S rDNA amplicon sequencing revealed that microbial diversity, composition and predicted function were strongly influenced by the presence of seagrass roots, such that root microbiomes were unique to each seagrass species. Reduced light availability altered seagrass root exudation, as characterized using fluorescence spectroscopy, and altered the composition of seagrass root microbiomes with a reduction in abundance of potentially beneficial microorganisms. Overall, this study highlights the potential for above-ground light reduction to invoke a cascade of changes from alterations in root exudation to a reduction in putative beneficial microorganisms and, ultimately, confirms the importance of the seagrass root environment - a critical, but often overlooked space.

A review of germination and early growth as a proxy for plant fitness under petrogenic contamination - knowledge gaps and recommendations

Germination-an important stage in the life cycle of plants-is susceptible to the presence of soil contaminants. Since the early 1990s, the use of germination tests to screen multiple plant species to select candidates for phytoremediation has received much attention. This is due to its inexpensive methodology and fast assessment relative to greenhouse or field growth studies. Surprisingly, no comprehensive synthesis is available of these studies in the scientific literature. As more plant species are added to phytoremediation databases, it is important to encapsulate the knowledge thus far and revise protocols. In this review, we have summarised previously-documented effects of petroleum hydrocarbons on germination and seedling growth. The methods and materials of previous studies are presented in tabulated form. Common practice includes the use of cellulose acetate filter paper, plastic Petri dishes, and low numbers of seeds and replicates. A general bias was observed for the screening of cultivated crops as opposed to native species, even though the latter may be better suited to site conditions. The relevance of germination studies as important ecotoxicological tools is highlighted with the proposed use of root imaging software. Screening of novel plant species, particularly natives, is recommended with selection focussed on (i) species phylogeny, (ii) plant morphological and functional traits, and (iii) tolerance towards harsh environmental stresses. Recommendations for standardised protocols for germination and early growth monitoring are made in order to improve the robustness of statistical modelling and species selection in future phytoremediation evaluations and field programs.

Fine root endophytes under scrutiny: a review of the literature on arbuscule-producing fungi recently suggested to belong to the Mucoromycotina

Fine root endophytes (FRE) are arbuscule-forming fungi presently considered as a single species-Glomus tenue in the Glomeromycota (Glomeromycotina)-but probably belong within the Mucoromycotina. Thus, FRE are the only known arbuscule-forming fungi not within the arbuscular mycorrhizal fungi (AMF; Glomeromycotina) as currently understood. Phylogenetic differences between FRE and AMF could reflect ecological differences. To synthesize current ecological knowledge, we reviewed the literature on FRE and identified 108 papers that noted the presence of FRE and, in some, the colonization levels for FRE or AMF (or both). We categorized these records by geographic region, host-plant family and environment (agriculture, moderate-natural, low-temperature, high-altitude and other) and determined their influence on the percentage of root length colonized by FRE in a meta-analysis. We found that FRE are globally distributed, with many observations from Poaceae, perhaps due to grasses being widely distributed. In agricultural environments, colonization by FRE often equalled or exceeded that of AMF, particularly in Australasia. In moderate-natural and high-altitude environments, average colonization by FRE (~10%) was lower than that of AMF (~35%), whereas in low-temperature environments, colonization was similar (~20%). Several studies suggested that FRE can enhance host-plant phosphorus uptake and growth, and may be more resilient than AMF to environmental stress in some host plants. Further research is required on the functioning of FRE in relation to the environment, host plant and co-occurring AMF and, in particular, to examine whether FRE are important for plant growth in stressful environments. Targeted molecular primers are urgently needed for further research on FRE.

Plants in constrained canopy micro-swards compensate for decreased root biomass and soil exploration with increased amounts of rhizosphere carboxylates

Root traits related to phosphorus (P) acquisition are used to make inferences about a species' P-foraging ability under glasshouse conditions. However, the effect on such root traits of constrained canopy spread, as occurs in dense pasture swards, is unknown. We grew micro-swards of Trifolium subterraneum L. and Ornithopus compressus L. at 15 and 60mg kg-1 soil P in a glasshouse. Shoots either spread beyond the pot perimeter or were constrained by a cylindrical sleeve adjusted to canopy height. After 8 weeks, shoot and root dry mass (DM), shoot tissue P concentration, rhizosphere carboxylates, arbuscular mycorrhizal (AM) fungal colonisation, total and specific root length (TRL and SRL respectively), average root diameter (ARD) and average root hair length (ARHL) were measured. In all species and treatments, constrained canopy spread decreased root DM (39-59%), TRL (27-45%) and shoot DM (10-28%), and increased SRL (20-33%), but did not affect ARD, ARHL and AM fungal colonisation. However, shoot P concentration and content increased, and rhizosphere carboxylates increased 3.5 to 12-fold per unit RL and 2.0- to 6.5-fold per micro-sward. Greater amounts of rhizosphere carboxylates when canopy spread was constrained appeared to compensate for reduced root growth enabling shoot P content to be maintained.

Root morphological traits that determine phosphorus-acquisition efficiency and critical external phosphorus requirement in pasture species

Annual pasture legume species can vary more than 3-fold in their critical external phosphorus (P) requirement (i.e. P required for 90% of maximum yield). In this work we investigated the link between root morphology, P acquisition and critical external P requirement among pasture species. The root morphology acclimation of five annual pasture legumes and one grass species to low soil P availability was assessed in a controlled-environment study. The critical external P requirement of the species was low (Dactylis glomerata L., Ornithopus compressus L., Ornithopus sativus Brot.), intermediate (Biserrula pelecinus L., Trifolium hirtum All.) or high (Trifolium subterraneum L.). Root hair cylinder volumes (a function of root length, root hair length and average root diameter) were estimated in order to assess soil exploration and its impact on P uptake. Most species increased soil exploration in response to rates of P supply near or below their critical external P requirement. The legumes differed in how they achieved their maximum root hair cylinder volume. The main variables were high root length density, long root hairs and/or high specific root length. However, total P uptake per unit surface area of the root hair cylinder was similar for all species at rates of P supply below critical P. Species that maximised soil exploration by root morphology acclimation were able to prolong access to P in moderately P-deficient soil. However, among the species studied, it was those with an intrinsic capacity for a high root-hair-cylinder surface area (i.e. long roots and long root hairs) that achieved the lowest critical P requirement.

Physiological and morphological adaptations of herbaceous perennial legumes allow differential access to sources of varyingly soluble phosphate

The aim of this study was to investigate the capacity of three perennial legume species to access sources of varyingly soluble phosphorus (P) and their associated morphological and physiological adaptations. Two Australian native legumes with pasture potential (Cullen australasicum and Kennedia prostrata) and Medicago sativa cv. SARDI 10 were grown in sand under two P levels (6 and 40 µg P g(-1) ) supplied as Ca(H2 PO4 )2 ·H2 O (Ca-P, highly soluble, used in many fertilizers) or as one of three sparingly soluble forms: Ca10 (OH)2 (PO4 )6 (apatite-P, found in relatively young soils; major constituent of rock phosphate), C6 H6 O24 P6 Na12 (inositol-P, the most common form of organic P in soil) and FePO4 (Fe-P, a poorly-available inorganic source of P). All species grew well with soluble P. When 6 µg P g(-1) was supplied as sparingly soluble P, plant dry weight (DW) and P uptake were very low for C. australasicum and M. sativa (0.1-0.4 g DW) with the exception of M. sativa supplied with apatite-P (1.5 g). In contrast, K. prostrata grew well with inositol-P (1.0 g) and Fe-P (0.7 g), and even better with apatite-P (1.7 g), similar to that with Ca-P (1.9 g). Phosphorus uptake at 6 µg P g(-1) was highly correlated with total root length, total rhizosphere carboxylate content and total rhizosphere acid phosphatase (EC 3.1.3.2) activity. These findings provide strong indications that there are opportunities to utilize local Australian legumes in low P pasture systems to access sparingly soluble soil P and increase perennial legume productivity, diversity and sustainability.

Mechanisms for tolerance of very high tissue phosphorus concentrations in Ptilotus polystachyus

Study of plants with unusual phosphorus (P) physiology may assist development of more P-efficient crops. Ptilotus polystachyus grows well at high P supply, when shoot P concentrations ([P]) may exceed 40 mg P g(-1) dry matter (DM). We explored the P physiology of P. polystachyus seedlings grown in nutrient solution with 0-5 mM P. In addition, young leaves and roots of soil-grown plants were used for cryo-scanning electron microscopy and X-ray microanalysis. No P-toxicity symptoms were observed, even at 5 mM P in solution. Shoot DM was similar at 0.1 and 1.0 mM P in solution, but was ∼14% lower at 2 and 5 mM P. At 1 mM P, [P] was 36, 18, 14 and 11 mg P g(-1) DM in mature leaves, young leaves, stems and roots, respectively. Leaf potassium, calcium and magnesium concentrations increased with increasing P supply. Leaf epidermal and palisade mesophyll cells had similar [P]. The root epidermis and most cortical cells had senesced, even in young roots. We conclude that preferential accumulation of P in mature leaves, accumulation of balancing cations and uniform distribution of P across leaf cell types allow P. polystachyus to tolerate very high leaf [P].

Moderating mycorrhizas: arbuscular mycorrhizas modify rhizosphere chemistry and maintain plant phosphorus status within narrow boundaries

Pastures often experience a pulse of phosphorus (P) when fertilized. We examined the role of arbuscular mycorrhizal fungi (AMF) in the uptake of P from a pulse. Five legumes (Kennedia prostrata, Cullen australasicum, Bituminaria bituminosa, Medicago sativa and Trifolium subterraneum) were grown in a moderate P, sterilized field soil, either with (+AMF) or without (-AMF) addition of unsterilized field soil. After 9-10 weeks, half the pots received 15 mg P kg(-1) of soil. One week later, we measured: shoot and root dry weights; percentage of root length colonized by AMF; plant P, nitrogen and manganese (Mn) concentrations; and rhizosphere carboxylates, pH and plant-available P. The P pulse raised root P concentration by a similar amount in uncolonized and colonized plants, but shoot P concentration increased by 143% in uncolonized plants and 53% in colonized plants. Inoculation with AMF decreased the amount of rhizosphere carboxylates by 52%, raised rhizosphere pH by ∼0.2-0.7 pH units and lowered shoot Mn concentration by 38%. We conclude that AMF are not simply a means for plants to enhance P uptake when P is limiting, but also act to maintain shoot P within narrow boundaries and can affect nutrient uptake through their influence on rhizosphere chemistry.

The role of root exuded low molecular weight organic anions in facilitating petroleum hydrocarbon degradation: current knowledge and future directions

Rhizoremediation is a bioremediation technique whereby enhanced microbial degradation of organic contaminants occurs within the plant root zone (rhizosphere). It is considered an effective and affordable 'green technology' for remediating soils contaminated with petroleum hydrocarbons (PHCs). This paper critically reviews the potential role of root exuded compounds in rhizoremediation, with emphasis on commonly exuded low molecular weight aliphatic organic acid anions (carboxylates). The extent to which remediation is achieved shows wide disparity among plant species. Therefore, plant selection is crucial for the advancement and widespread adoption of this technology. Root exudation is speculated to be one of the predominant factors leading to microbial changes in the rhizosphere and thus the potential driver behind enhanced petroleum biodegradation. Carboxylates can form a significant component of the root exudate mixture and are hypothesised to enhance petroleum biodegradation by: i) providing an easily degradable energy source; ii) increasing phosphorus supply; and/or iii) enhancing the contaminant bioavailability. These differing hypotheses, which are not mutually exclusive, require further investigation to progress our understanding of plant-microbe interactions with the aim to improve plant species selection and the efficacy of rhizoremediation.

Commensalism in an agroecosystem: hydraulic redistribution by deep-rooted legumes improves survival of a droughted shallow-rooted legume companion

We investigated commensalism of water use among annual shallow-rooted and perennial deep-rooted pasture legumes by examining the effect of hydraulic lift by Cullen pallidum (N.T.Burb.) J.W.Grimes and Medicago sativa on growth, survival and nutrient uptake of Trifolium subterraneum L. A vertically split-root design allowed separate control of soil water in top and bottom soil. Thirty-five days after watering ceased in the top tube, but soil remained at field capacity in the bottom tube, an increase in shallow soil water content by hydraulic lift was 5.6 and 5.9 g kg(-1) soil overnight for C. pallidum and M. sativa, respectively. Trifolium subterraneum in this treatment maintained higher leaf water potentials (with M. sativa) or exhibited a slower decline (with C. pallidum) than without companion perennial plants; and shoot biomass of T. subterraneum was 56% (with C. pallidum) and 67% (with M. sativa) of that when both top and bottom tubes were at field capacity. Uptake of rubidium (a potassium analog) and phosphorus by T. subterraneum was not facilitated by hydraulic lift. Interestingly, phosphorus content was threefold greater, and shoot biomass 1.5-3.3-fold greater when T. subterraneum was interplanted with C. pallidum compared with M. sativa, although dry weight of C. pallidum was much greater than that of M. sativa. This study showed that interplanting with deep-rooted perennial legumes has benefited the survival of T. subterraneum.

Seasonal and diurnal variation in the stomatal conductance and paraheliotropism of tedera (Bituminaria bituminosa var. albomarginata) in the field

The mechanisms of drought resistance in perennial legumes are poorly understood. We explored the diurnal and seasonal variation (May, August, February) in stomatal conductance (gs) and paraheliotropism of three tedera accessions (Bituminaria bituminosa (L.) C.H. Stirton var. albomarginata) and lucerne (Medicago sativa L.), both perennial legumes, grown in the field. For the tedera accessions, there was a significant reduction in gs during the day in May (late autumn) and February (summer), but there was little reduction for lucerne. The peak leaf angle in the tedera accessions ranged from <40° to 70°, whereas for lucerne, the leaf angle was nearly parallel to incident light at 85°. Leaf water-use efficiency, relative leaf water content and leaf retention were higher for the tedera accessions than for lucerne in February. These results highlight the superior drought resistance of tedera compared with lucerne. The reduction in gs over the day in tedera shows the capacity of this species to reduce water loss quickly when conditions for CO2 fixation relative to water loss are highly unfavourable. The high retention of leaves in summer by tedera is a valuable trait for a perennial pasture plant in Mediterranean environments. Leaf folding, combined with effective stomatal control in summer, provides tedera with a set of physiological responses that confer high drought resistance.

Carbon trading for phosphorus gain: the balance between rhizosphere carboxylates and arbuscular mycorrhizal symbiosis in plant phosphorus acquisition

Two key plant adaptations for phosphorus (P) acquisition are carboxylate exudation into the rhizosphere and mycorrhizal symbioses. These target different soil P resources, presumably with different plant carbon costs. We examined the effect of inoculation with arbuscular mycorrhizal fungi (AMF) on amount of rhizosphere carboxylates and plant P uptake for 10 species of low-P adapted Kennedia grown for 23 weeks in low-P sand. Inoculation decreased carboxylates in some species (up to 50%), decreased plant dry weight (21%) and increased plant P content (23%). There was a positive logarithmic relationship between plant P content and the amount of rhizosphere citric acid for inoculated and uninoculated plants. Causality was indicated by experiments using sand where little citric acid was lost from the soil solution over 2 h and citric acid at low concentrations desorbed P into the soil solution. Senesced leaf P concentration was often low and P-resorption efficiencies reached >90%. In conclusion, we propose that mycorrhizally mediated resource partitioning occurred because inoculation reduced rhizosphere carboxylates, but increased plant P uptake. Hence, presumably, the proportion of plant P acquired from strongly sorbed sources decreased with inoculation, while the proportion from labile inorganic P increased. Implications for plant fitness under field conditions now require investigation.

Adaptive shoot and root responses collectively enhance growth at optimum temperature and limited phosphorus supply of three herbaceous legume species

BACKGROUND AND AIMS

Studies on the effects of sub- and/or supraoptimal temperatures on growth and phosphorus (P) nutrition of perennial herbaceous species at growth-limiting P availability are few, and the impacts of temperature on rhizosphere carboxylate dynamics are not known for any species.

METHODS

The effect of three day/night temperature regimes (low, 20/13 °C; medium, 27/20 °C; and high, 32/25 °C) on growth and P nutrition of Cullen cinereum, Kennedia nigricans and Lotus australis was determined.

KEY RESULTS

The highest temperature was optimal for growth of C. cinereum, while the lowest temperature was optimal for K. nigricans and L. australis. At optimum temperatures, the relative growth rate (RGR), root length, root length per leaf area, total P content, P productivity and water-use efficiency were higher for all species, and rhizosphere carboxylate content was higher for K. nigricans and L. australis. Cullen cinereum, with a slower RGR, had long (higher root length per leaf area) and thin roots to enhance P uptake by exploring a greater volume of soil at its optimum temperature, while K. nigricans and L. australis, with faster RGRs, had only long roots (higher root length per leaf area) as a morphological adaptation, but had a higher content of carboxylates in their rhizospheres at the optimum temperature. Irrespective of the species, the amount of P taken up by a plant was mainly determined by root length, rather than by P uptake rate per unit root surface area. Phosphorus productivity was correlated with RGR and plant biomass.

CONCLUSIONS

All three species exhibited adaptive shoot and root traits to enhance growth at their optimum temperatures at growth-limiting P supply. The species with a slower RGR (i.e. C. cinereum) showed only morphological root adaptations, while K. nigricans and L. australis, with faster RGRs, had both morphological and physiological (i.e. root carboxylate dynamics) root adaptations.

Growth, yield and seed composition of native Australian legumes with potential as grain crops

BACKGROUND

Many Australian native legumes grow in arid and nutrient-poor environments. Yet few Australian herbaceous legumes have been investigated for domestication potential. This study compared growth and reproductive traits, grain yield and seed composition of 17 native Australian legumes with three commercial grain legumes.

RESULTS

Seed yields of seven native legumes were > 40% of Cicer arietnum, with highest seed yields and harvest indices in Glycine sp. (14.4 g per plant, 0.54 g g(-1) ) and Lotus cruentus (10.2 g per plant, 0.65 g g(-1) ). Five native species flowered earlier than field pea (Pisum sativa) (109 days), though many were slower to flower and set seed. Largest seeds were found in Glycine canescens (17 mg), with seed of other native species 14 times smaller than commercial cultivars. Seed composition of many native legumes was similar to commercial cultivars (200-330 g protein kg(-1) dry weight (DW), 130-430 g dietary fibre kg(-1) DW). Two Cullen species had high fat content (>110 g kg(-1) DW) and Trigonella sauvissima had the highest crude protein content (370 g kg(-1) DW).

CONCLUSION

The seed composition and reproductive traits of some wild native Australian legumes suggest they could offer potential as grain crops for soils and environments where the current grain legumes are uneconomic. Further evaluation of genetic diversity, especially for seed size, overall productivity, and reproductive development is needed.

Multiple adaptive responses of Australian native perennial legumes with pasture potential to grow in phosphorus- and moisture-limited environments

BACKGROUND AND AIMS

Many Australian legumes have evolved in low-phosphorus (P) soils and low-rainfall areas. Therefore a study was made of the interaction of soil [P] and water availability on growth, photosynthesis, water-use efficiency (WUE) and P nutrition of two Australian native legumes with pasture potential, Cullen australasicum and C. pallidum, and the widely grown exotic pasture legume, lucerne (Medicago sativa).

METHODS

Plants were grown in a glasshouse at 3, 10 and 30 mg P kg(-1) dry soil for 5 months. At week 10, two drought treatments were imposed, total pot dried (all-dry) and only top soil dried (top-dry), while control pots were maintained at field capacity.

KEY RESULTS

Shoot dry weight produced by lucerne was never higher than that of C. australasicum. For C. pallidum only, shoot dry weight was reduced at 30 mg P kg(-1) dry soil. The small root system of the Cullen species was quite plastic, allowing plants to access P and moisture efficiently. Lucerne always had a higher proportion of its large root system in the top soil layer compared with Cullen species. All species showed decreased photosynthesis, leaf water potential and stomatal conductance when exposed to drought, but the reductions were less for Cullen species, due to tighter stomatal control, and consequently they achieved a higher WUE. All species showed highest rhizosphere carboxylate concentrations in the all-dry treatment. For lucerne only, carboxylates decreased as P supply increased. Citrate was the main carboxylate in the control and top-dry treatments, and malate in the all-dry treatment.

CONCLUSIONS

Multiple adaptive responses of Cullen species and lucerne favoured exploitation of low-P soils under drought. The performance of undomesticated Cullen species, relative to that of lucerne, shows their promise as pasture species for environments such as in south-western Australia where water and P are limiting, especially in view of a predicted drying and warming climate.

Putting the P in Ptilotus: a phosphorus-accumulating herb native to Australia

BACKGROUND AND AIMS

Ptilotus polystachyus (green mulla mulla; ptilotus) is a short-lived perennial herb that occurs widely in Australia in arid and semi-arid regions with nutrient poor soils. As this species shows potential for domestication, its response to addition of phosphorus (P) and nitrogen (N) was compared to a variety of the domesticated exotic perennial pasture herb Cichorium intybus (chicory), 'Puna'.

METHODS

Pots were filled with 3 kg of an extremely nutrient-deficient sterilized field soil that contained 3 mg kg(-1) mineral N and 2 mg kg(-1) bicarbonate-extractable P. The growth and P and N accumulation of ptilotus and chicory in response to seven rates of readily available phosphorus (0-300 mg P pot(-1)) and nitrogen (N) (0-270 mg N pot(-1)) was examined.

KEY RESULTS

Ptilotus grew extremely well under low P conditions: shoot dry weights were 23, 6 and 1.7 times greater than for chicory at the three lowest levels of P addition, 0, 15 and 30 mg P pot(-1), respectively. Ptilotus could not downregulate P uptake. Concentrations of P in shoots approached 4% of dry weight and cryo-scanning electron microscopy and X-ray microanalysis showed 35-196 mM of P in cell vacuoles in a range of tissues from young leaves. Ptilotus had a remarkable tolerance of high P concentrations in shoots. While chicory exhibited symptoms of P toxicity at the highest rate of P addition (300 mg P pot(-1)), no symptoms were present for ptilotus. The two species responded in a similar manner to addition of N.

CONCLUSIONS

In comparison to chicory, ptilotus demonstrated an impressive ability to grow well under conditions of low and high P availability. Further study of the mechanisms of P uptake and tolerance in ptilotus is warranted.

Prospects for three Dorycnium species as forage plants in agricultural systems: a review of their agronomic characteristics

Perennial legumes in the Dorycnium genus may have potential as forage plants that could reduce the seasonality of feed production and improve the sustainability of agricultural systems. However, Dorycnium species are not currently used commercially and little is known about their agronomic characteristics. This review covers the current knowledge on Dorycnium distribution, taxonomy and the agronomic performance of Dorycnium hirsutum, Dorycnium rectum and Dorycnium pentaphyllum, including adaptation, establishment, biomass production, water use, grazing management and nitrogen fixation, along with considerations for animal production. Dorycnium originate from temperate Europe and the Mediterranean basin and may be suitable for other regions with similar climatic conditions. Little data exist on the climatic and edaphic conditions to which Dorycnium species are best adapted. Current evidence suggests that D. hirsutum is widely adapted and might be suitable as a forage plant for acid soils in drier and frost-prone agricultural regions. D. hirsutum also persists well in low rainfall environments (down to 300 mm mean annual rainfall), can produce up to 21 t dry matter(DM)/ha in its first 3 years and, by utilising extra water compared with annual pastures, can reduce water leakage below the root zone, thereby slowing development of dryland salinity. The use of D. rectum would be limited to high rainfall or water-accumulating sites. D. pentaphyllum is a diverse species, yet available material appears to be less productive but has better forage quality than D. hirsutum. Currently, establishment reliability and/or forage digestibility are major limitations of the tested Dorycnium species that restrict their potential role and challenge the feasibility of their future use. Breeding may overcome or minimise these limitations and improved agronomic management might also enhance their usefulness. However, current collected genetic resources of Dorycnium are very limited and targeted collections would be needed to yield better adapted germplasm. Breeding to reduce the high levels of condensed tannins (>13% of DM) to moderate concentrations in Dorycnium might improve forage digestibility and could have positive implications for animal performance and health. Despite the poor digestibility of some Dorycnium species (<60% DM digestibility), these plants may still play a significant role as drought forage to provide feed when other forage sources are in limited supply. Further research is required to quantify the potential of Dorycnium species for commercial release and to determine how these plants should be best managed and integrated into livestock and mixed cropping systems.

Relative amounts of soluble and insoluble forms of phosphorus and other elements in intraradical hyphae and arbuscules of arbuscular mycorrhizas

Transport of phosphorus (P) into host plants and its release to root cells is an important function of arbuscular mycorrhizal fungi (AMF). However, relatively little is known about the forms and water solubilities of P compounds in specific locations in the intraradical fungal structures. We determined concentrations and solubility of P components in these structures in white clover (Trifolium repens L.). Plants were grown in the field (colonised by indigenous AMF) or in the glasshouse (inoculated with Glomus intraradices). Mycorrhizas were cryo-fixed in liquid nitrogen immediately (control) or after treatments designed to destroy cell membranes and extract solubles. Thirty to 70% of total P in hyphae and 100% in arbuscules was not extracted. The unextracted proportion of P was higher in the inoculated plants suggesting an environmental effect. It is proposed that the large component of non-extractable P in the arbuscules is involved in the tight regulation of inorganic P release to the host cells. In control roots magnesium, potassium and P were present in hyphae in molar ratios 1 : 2 : 4, further evidence that this relationship may be universal for AMF, and that other P-balancing cations are present but undetectable by the analytical technique.