Eudicots from severely phosphorus-impoverished environments preferentially allocate phosphorus to their mesophyll

Photosynthetic physiological characteristics, growth performance, and element concentrations reveal the calcicole-calcifuge behaviors of three Camellia species

We grew three yellow Camellia species (the calcifuge C. nitidissima and C. tunghinensis, and the calcicole C. pubipetala) in acidic and calcareous soils for 7 months and assessed their photosynthetic physiological characteristics, growth performance, and element concentrations in this developmental context. The calcifuge C. nitidissima and C. tunghinensis species exhibited poor growth with leaf chlorosis, growth stagnation, and root disintegration in calcareous soils, and with their P n, G s, T r, F v/F m, ΦPSII, ETR, qP, leaf Chla, Chlb, and Chl(a + b) concentrations, and root, stem, leaf, and total biomass being significantly lower when grown in calcareous soils relative to in acidic soils. In contrast, the calcicole C. pubipetala grew well in both acidic and calcareous soils, with few differences in the above parameters between these two soil substrates. The absorption and/or transportation of nutrient elements such as N, K, Ca, Mg, and Fe by the two calcifuge Camellia species plants grown in calcareous soils were restrained. Soil type plays a major role in the failure of the two calcifuge Camellia species to establish themselves in calcareous soils, whereas other factors such as competition and human activity are likely more important limiting factors in the reverse case. This study furthers our understanding of the factors influencing the distribution of these rare and endangered yellow Camellia species, allowing for improved management of these species in conservation projects and horticultural production.

Correlations between root phosphorus acquisition and foliar phosphorus allocation reveal how grazing promotes plant phosphorus utilization

Overgrazing and phosphorus (P) deficiency are two major factors limiting the sustainable development of grassland ecosystems. Exploring plant P utilization and acquisition strategies under grazing can provide a solid basis for determining a reasonable grazing intensity. Both foliar P allocation and root P acquisition are crucial mechanisms for plants to adapt to environmental P availability; however, their changing characteristics and correlation under grazing remain unknown. Here, we investigated foliar P fractions, root P-acquisition traits and gene expression, as well as rhizosphere and bulk soil properties of two dominant plant species, Leymus chinensis (a rhizomatous grass) and Stipa grandis (a bunchgrass), in a field grazing intensity gradient site in Inner Mongolia. Grazing induced different degrees of compensatory growth in the two dominant plant species, increased rhizosphere P availability, and alleviated plant P limitation. Under grazing, the foliar metabolite P of L. chinensis increased, whereas the nucleic acid P of S. grandis increased. Increased P fractions in L. chinensis were positively correlated with increased root exudates and rapid inorganic P absorption. For S. grandis, increased foliar P fractions were positively correlated with more fine roots, more root exudates, and up-regulated expression of genes involved in defense and P metabolism. Overall, efficient root P mobilization and uptake traits, as well as increases in leaf metabolic activity-related P fractions, supported plant compensatory growth under grazing, a process that differed between tiller types. The highest plant productivity and leaf metabolic activity-related P concentrations under medium grazing intensity clarify the underlying basis for sustainable livestock production.

Leaf phosphorus fractions vary with leaf economic traits among 35 Australian woody species

Adaptations of plants to phosphorus (P) deficiency include reduced investment of leaf P in storage (orthophosphates in vacuoles), nucleic acids and membrane lipids. Yet, it is unclear how these adaptations are associated with plant ecological strategies. Five leaf P fractions (orthophosphate P, Pi ; metabolite P, PM ; nucleic acid P, PN ; lipid P, PL ; and residual P, PR ) were analysed alongside leaf economic traits among 35 Australian woody species from three habitats: one a high-P basalt-derived soil and two low-P sandstone-derived soils, one undisturbed and one disturbed by human activities with artificial P inputs. Species at the undisturbed low-P site generally exhibited lower concentrations of total leaf P ([Ptotal ]), primarily associated with lower concentrations of Pi , and PN . The relative allocation of P to each fraction varied little among sites, except that higher PL per [Ptotal ] (rPL ) was recorded at the undisturbed low-P site than at the high-P site. This higher rPL , reflecting relative allocation to membranes, was primarily associated with lower concentrations of leaf nitrogen at the undisturbed low-P site than at the high-P site. Associations between leaf P fractions and leaf nitrogen may provide a basis for understanding the variation in plant ecological strategies dependent on soil P availability.

Leaf phosphorus allocation to chemical fractions and its seasonal variation in south-western Australia is a species-dependent trait

South-western Australia is a global biodiversity hotspot and has some of the oldest and most phosphorus (P)-impoverished soils in the world. Proteaceae is one of the dominant P-efficient plant families there, but it is unknown how leaf P concentrations and foliar P allocation of Proteaceae and coexisting dominant plant families vary between seasons and habitats. To investigate this, we selected 18 species from Proteaceae, Myrtaceae and Fabaceae, six from each family, in two habitats from Alison Baird Reserve (32°1'19''S 15°58'52''E) in Western Australia. Total leaf P and nitrogen (N) concentrations, leaf mass per area, photosynthetic rate, pre-dawn leaf water potential and foliar P fractions were determined for each species both at the end of summer (March 2019 and early April 2020) and at the end of winter (September 2019). Soil P availability was also determined for each site. This is the very first study that focused on seasonal changes of foliar P fractions from different P-impoverished environments in three plant families. However, contrary to our expectation, we found little evidence for convergence of foliar P allocation within family, season or habitat. Each species exhibited a specific species-dependent pattern of foliar P allocation, and many species showed differences between seasons. Native plants in south-western Australia converged on a high photosynthetic P-use efficiency, but each species showed its own unique way associated with that outcome.

Fine-root traits are devoted to the allocation of foliar phosphorus fractions of desert species under water and phosphorus-poor environments

Traits of leaves and fine roots are expected to predict the responses and adaptation of plants to their environments. Whether and how fine-root traits (FRTs) are associated with the allocation of foliar phosphorus (P) fractions of desert species in water- and P-poor environments, however, remains unclear. We exposed seedlings of Alhagi sparsifolia Shap. (hereafter Alhagi) treated with two water and four P-supply levels for three years in open-air pot experiments and measured the concentrations of foliar P fractions, foliar traits, and FRTs. The allocation proportion of foliar nucleic acid-P and acid phosphatase (APase) activity of fine roots were significantly higher by 45.94 and 53.3% in drought and no-P treatments relative to well-watered and high-P treatments, whereas foliar metabolic-P and structural-P were significantly lower by 3.70 and 5.26%. Allocation proportions of foliar structural-P and residual-P were positively correlated with fine-root P (FRP) concentration, but nucleic acid-P concentration was negatively correlated with FRP concentration. A tradeoff was found between the allocation proportion to all foliar P fractions relative to the FRP concentration, fine-root APase activity, and amounts of carboxylates, followed by fine-root morphological traits. The requirement for a link between the aboveground and underground tissues of Alhagi was generally higher in the drought than the well-watered treatment. Altering FRTs and the allocation of P to foliar nucleic acid-P were two coupled strategies of Alhagi under conditions of drought and/or low-P. These results advance our understanding of the strategies for allocating foliar P by mediating FRTs in drought and P-poor environments.

Improved chloroplast Pi allocation helps sustain electron transfer to enhance photosynthetic low-phosphorus tolerance of wheat

Phosphorus (P) deficit limits high wheat (Triticum aestivum L.) yields. Breeding low-P-tolerant cultivars is vital for sustainable agriculture and food security, but the low-P adaptation mechanisms are largely not understood. Two wheat cultivars, ND2419 (low-P-tolerant) and ZM366 (low-P-sensitive) were used in this study. They were grown under hydroponic conditions with low-P (0.015 mM) or normal-P (1 mM). Low-P suppressed biomass accumulation and net photosynthetic rate (A) in both cultivars, whereas ND2419 was relatively less suppressed. Intercellular CO2 concentration did not decrease with the decline of stomatal conductance. Additionally, maximum electron transfer rate (Jmax) decreased sooner than maximum carboxylation rate (Vcmax). Results indicate that impeded electron transfer is directly responsible for decreased A. Under low-P, ND2419 exhibited greater PSII functionality (potential activity (Fv/Fo), maximum quantum efficiency (Fv/Fm), photochemical quenching (qL) and non-photochemical quenching (NPQ) required for electron transfer than ZM366, resulting more ATP for Rubisco activation. Furthermore, ND2419 maintained higher chloroplast Pi concentrations by enhancing chloroplast Pi allocation, compared with ZM366. Overall, the low-P-tolerant cultivar sustained electron transfer under low-P by enhancing chloroplast Pi allocation, allowing more ATP synthesis for Rubisco activation, ultimately presenting stronger photosynthesis capacities. The improved chloroplasts Pi allocation may provide new insights into improve low-P tolerance.

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 Acquisition and Utilization in Plants

Tremendous progress has been made on molecular aspects of plant phosphorus (P) nutrition, often without heeding information provided by soil scientists, ecophysiologists, and crop physiologists. This review suggests ways to integrate information from different disciplines. When soil P availability is very low, P-mobilizing strategies are more effective than mycorrhizal strategies. Soil parameters largely determine how much P roots can acquire from P-impoverished soil, and kinetic properties of P transporters are less important. Changes in the expression of P transporters avoid P toxicity. Plants vary widely in photosynthetic P-use efficiency, photosynthesis per unit leaf P. The challenge is to discover what the trade-offs are of different patterns of investment in P fractions. Less investment may save P, but are costs incurred? Are these costs acceptable for crops? These questions can be resolved only by the concerted action of scientists working at both molecular and physiological levels, rather than pursuing these problems independently.

Salt tolerance in relation to elemental concentrations in leaf cell vacuoles and chloroplasts of a C4 monocotyledonous halophyte

Halophytes accumulate and sequester high concentrations of salt in vacuoles while maintaining lower levels of salt in the cytoplasm. The current data on cellular and subcellular partitioning of salt in halophytes are, however, limited to only a few dicotyledonous C₃ species. Using cryo-scanning electron microscopy X-ray microanalysis, we assessed the concentrations of Na, Cl, K, Ca, Mg, P and S in various cell types within the leaf-blades of a monocotyledonous C₄ halophyte, Rhodes grass (Chloris gayana). We also linked, for the first time, elemental concentrations in chloroplasts of mesophyll and bundle sheath cells to their ultrastructure and photosynthetic performance of plants grown in nonsaline and saline (200 mM NaCl) conditions. Na and Cl accumulated to the highest levels in xylem parenchyma and epidermal cells, but were maintained at lower concentrations in photosynthetically active mesophyll and bundle sheath cells. Concentrations of Na and Cl in chloroplasts of mesophyll and bundle sheath cells were lower than in their respective vacuoles. No ultrastructural changes were observed in either mesophyll or bundle sheath chloroplasts, and photosynthetic activity was maintained in saline conditions. Salinity tolerance in Rhodes grass is related to specific cellular Na and Cl distributions in leaf tissues, and the ability to regulate Na and Cl concentrations in chloroplasts.

Mechanisms for improving phosphorus utilization efficiency in plants

BACKGROUND

Limitation of plant productivity by phosphorus (P) supply is widespread and will probably increase in the future. Relatively large amounts of P fertilizer are applied to sustain crop growth and development and to achieve high yields. However, with increasing P application, plant P efficiency generally declines, which results in greater losses of P to the environment with detrimental consequences for ecosystems.

SCOPE

A strategy for reducing P input and environmental losses while maintaining or increasing plant performance is the development of crops that take up P effectively from the soil (P acquisition efficiency) or promote productivity per unit of P taken up (P utilization efficiency). In this review, we describe current research on P metabolism and transport and its relevance for improving P utilization efficiency.

CONCLUSIONS

Enhanced P utilization efficiency can be achieved by optimal partitioning of cellular P and distributing P effectively between tissues, allowing maximum growth and biomass of harvestable plant parts. Knowledge of the mechanisms involved could help design and breed crops with greater P utilization efficiency.

AusTraits, a curated plant trait database for the Australian flora

We introduce the AusTraits database - a compilation of values of plant traits for taxa in the Australian flora (hereafter AusTraits). AusTraits synthesises data on 448 traits across 28,640 taxa from field campaigns, published literature, taxonomic monographs, and individual taxon descriptions. Traits vary in scope from physiological measures of performance (e.g. photosynthetic gas exchange, water-use efficiency) to morphological attributes (e.g. leaf area, seed mass, plant height) which link to aspects of ecological variation. AusTraits contains curated and harmonised individual- and species-level measurements coupled to, where available, contextual information on site properties and experimental conditions. This article provides information on version 3.0.2 of AusTraits which contains data for 997,808 trait-by-taxon combinations. We envision AusTraits as an ongoing collaborative initiative for easily archiving and sharing trait data, which also provides a template for other national or regional initiatives globally to fill persistent gaps in trait knowledge.

Phosphorus toxicity, not deficiency, explains the calcifuge habit of phosphorus-efficient Proteaceae

The calcifuge habit of plants is commonly explained in terms of high soil pH and its effects on nutrient availability, particularly that of phosphorus (P). However, most Proteaceae that occur on nutrient-impoverished soils in south-western Australia are calcifuge, despite their ability to produce cluster-roots, which effectively mobilize soil P and micronutrients. We hypothesize that the mechanism explaining the calcifuge habit in Proteaceae is their sensitivity to P and calcium (Ca), and that soil-indifferent species are less sensitive to the interaction of these nutrients. In this study, we analyzed growth, gas-exchange rate, and chlorophyll fluorescence of two soil-indifferent and four calcifuge Hakea and Banksia (Proteaceae) species from south-western Australia, across a range of P and Ca concentrations in hydroponic solution. We observed Ca-enhanced P toxicity in all analyzed species, but to different extents depending on distribution type and genus. Increasing P supply enhanced plant growth, leaf biomass, and photosynthetic rates of soil-indifferent species in a pattern largely independent of Ca supply. In contrast, positive physiological responses to increasing [P] in calcifuges were either absent or limited to low Ca supply, indicating that calcifuges were far more sensitive to Ca-enhanced P toxicity. In calcifuge Hakeas, we attributed this to higher leaf [P], and in calcifuge Banksias to lower leaf zinc concentration. These differences help to explain these species' contrasting sensitivity to Ca-enhanced P toxicity and account for the exclusion of most Proteaceae from calcareous habitats. We surmise that Ca-enhanced P toxicity is a major factor explaining the calcifuge habit of Proteaceae, and, possibly, other P-sensitive plants.

Recent insights into the metabolic adaptations of phosphorus-deprived plants

Inorganic phosphate (Pi) is an essential macronutrient required for many fundamental processes in plants, including photosynthesis and respiration, as well as nucleic acid, protein, and membrane phospholipid synthesis. The huge use of Pi-containing fertilizers in agriculture demonstrates that the soluble Pi levels of most soils are suboptimal for crop growth. This review explores recent advances concerning the understanding of adaptive metabolic processes that plants have evolved to alleviate the negative impact of nutritional Pi deficiency. Plant Pi starvation responses arise from complex signaling pathways that integrate altered gene expression with post-transcriptional and post-translational mechanisms. The resultant remodeling of the transcriptome, proteome, and metabolome enhances the efficiency of root Pi acquisition from the soil, as well as the use of assimilated Pi throughout the plant. We emphasize how the up-regulation of high-affinity Pi transporters and intra- and extracellular Pi scavenging and recycling enzymes, organic acid anion efflux, membrane remodeling, and the remarkable flexibility of plant metabolism and bioenergetics contribute to the survival of Pi-deficient plants. This research field is enabling the development of a broad range of innovative and promising strategies for engineering phosphorus-efficient crops. Such cultivars are urgently needed to reduce inputs of unsustainable and non-renewable Pi fertilizers for maximum agronomic benefit and long-term global food security and ecosystem preservation.

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.

OCBIL theory examined: reassessing evolution, ecology and conservation in the world’s ancient, climatically buffered and infertile landscapes

OCBIL theory was introduced as a contribution towards understanding the evolution, ecology and conservation of the biological and cultural diversity of old, climatically buffered, infertile landscapes (OCBILs), especially in the Southern Hemisphere. The theory addresses some of the most intransigent environmental and cultural trends of our time – the ongoing decline of biodiversity and cultural diversity of First Nations. Here we reflect on OCBILs, the origins of the theory, and its principal hypotheses in biological, anthropological and conservation applications. The discovery that threatened plant species are concentrated in the Southwest Australian Floristic Region (SWAFR) on infertile, phosphorous-impoverished uplands within 500 km of the coast formed the foundational framework for OCBIL theory and led to the development of testable hypotheses that a growing literature is addressing. Currently, OCBILs are recognized in 15 Global Biodiversity Hotspots and eight other regions. The SWAFR, Greater Cape Floristic Region of South Africa and South America’s campos rupestres (montane grasslands) are those regions that have most comprehensively been investigated in the context of OCBIL theory. We summarize 12 evolutionary, ecological and cultural hypotheses and ten conservation-management hypotheses being investigated as recent contributions to the OCBIL literature.

Tightening the Phosphorus Cycle through Phosphorus-Efficient Crop Genotypes

We are facing unprecedented phosphorus (P) challenges, namely P scarcity associated with increasing food demand, and an oversupply of P fertilisers, resulting in eutrophication. Although we need a multidisciplinary approach to systematically enhance P-use efficiency, monodisciplinary studies still prevail. Here, we propose to tighten the P cycle by identifying P-efficient crop genotypes, integrating four plant strategies: increasing P-acquisition efficiency, photosynthetic P-use efficiency and P-remobilisation efficiency, and decreasing seed phytate P concentrations. We recommend P-efficient genotypes together with diversified cropping systems involving complementary P-acquisition strategies as well as smart P-fertiliser management to enhance P-use efficiency in agriculture dependent on soil P status. These strategies will reduce P-fertiliser requirements and offsite environmental impacts, while enhancing seed quality for human and livestock nutrition.

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.

Calcicole behaviour of Callisthene fasciculata Mart., an Al-accumulating species from the Brazilian Cerrado

Most aluminium (Al)-accumulating species are found on soils with high Al saturation and low Ca availability (Ca poor). Callisthene fasciculata Mart. (Vochysiaceae), however, is an Al-accumulating tree restricted to Ca-rich soils with low Al saturation in the Brazilian Cerrado savanna. Here we tested its calcicole behaviour, and the possible role of organic acids in detoxification of Al during the early stages of plant development. We assessed growth, dry mass, nutrients, Al and organic acids in seedlings grown for 50 days on two contrasting Cerrado soils; one with high Ca concentrations and low Al saturation and the other with low Ca availability and high Al saturation. Relative to plants on Ca-rich soil, plants on Ca-poor soil had necrotic spots and bronzing of leaves. Roots and shoots contained reduced concentrations of P and Cu, but higher concentrations of Fe, Al and citrate. Despite lower concentrations in the soil, Ca and Mg increased in shoots. Shoot concentrations of oxalate were also higher. We confirmed C. fasciculata as an Al-accumulating species with calcicole behaviour. The increased concentrations of organic acids in plants with higher Al accumulation suggest that high availability of soluble Al does not prevent occurrence of this species on soils with high Al saturation. Instead, the absence of C. fasciculata from Ca-poor soils is probably due to imbalances in tissue Fe, Cu and Zn imposed by this soil type.

Divergent responses of above- and below-ground chemical defence to nitrogen and phosphorus supply in waratahs (Telopea speciosissima)

Plant nutrition can affect the allocation of resources to plant chemical defences, yet little is known about how phosphorus (P) supply, and relative nitrogen (N) and P supply, affect chemical defences, especially in species with intrinsically conservative nutrient use adapted to P-impoverished soils. Waratah (Telopea speciosissima (Sm.) R.Br.), like other Proteaceae, is adapted nutrient-poor soils. It was identified as having cyanogenic glycosides (CNglycs) throughout the plant. T. speciosissima seedlings were grown for 15 weeks under two N and P concentrations. CNglycs (N-based defence) and nutrients were quantified in above- and below-ground organs; foliar carbon (C)-based phenolics and tannins were also quantified. CNglyc concentrations in roots were on average 51-fold higher than in above-ground tissues and were affected by both N and P supply, whereas foliar CNglyc concentrations only responded to N supply. Leaves had high concentrations of C-based defences, which increased under low N, and were not correlated with N-based defences. Greater root chemical defence against herbivores and pathogens may be important in a non-mycorrhizal species that relies on basal resprouting following disturbance. The differing responses of secondary chemistry in above- and below-ground organs to P and N demonstrate the importance of broadening the predominantly foliar focus of plant defence studies.

Calcium modulates leaf cell-specific phosphorus allocation in Proteaceae from south-western Australia

Over 650 Proteaceae occur in south-western Australia, contributing to the region's exceptionally high biodiversity. Most Proteaceae occur exclusively on severely nutrient-impoverished, acidic soils (calcifuge), whilst only few also occur on young, calcareous soils (soil-indifferent), higher in calcium (Ca) and phosphorus (P). The calcifuge habit of Proteaceae is explained by Ca-enhanced P toxicity, putatively linked to the leaf cell-specific allocation of Ca and P. Separation of these elements is essential to avoid the deleterious precipitation of Ca-phosphate. We used quantitative X-ray microanalysis to determine leaf cell-specific nutrient concentrations of two calcifuge and two soil-indifferent Proteaceae grown in hydroponics at a range of Ca and P concentrations. Calcium enhanced the preferential allocation of P to palisade mesophyll (PM) cells under high P conditions, without a significant change in whole leaf [P]. Calcifuges showed a greater PM [P] compared with soil-indifferent species, corresponding to their greater sensitivity. This study advances our mechanistic understanding of Ca-enhanced P toxicity, supporting the proposed model, and demonstrating its role in the calcifuge distribution of Proteaceae. This furthers our understanding of nutrient interactions at the cellular level and highlights its importance to plant functioning.

Responses of foliar phosphorus fractions to soil age are diverse along a 2 Myr dune chronosequence

Plants respond to soil phosphorus (P) availability by adjusting leaf P among inorganic P (Pi) and organic P fractions (nucleic acids, phospholipids, small metabolites and a residual fraction). We tested whether phylogenetically divergent plants in a biodiversity hotspot similarly adjust leaf P allocation in response to P limitation by sampling along a 2 Myr chronosequence in southwestern Australia where nitrogen (N) limitation transitions to P limitation with increasing soil age. Total P and N, and P allocated to five chemical fractions were determined for photosynthetic organs from Melaleuca systena (Myrtaceae), Acacia rostellifera (Fabaceae) and Hakea prostrata (Proteaceae). Soil characteristics were also determined. Acacia rostellifera maintained phyllode total P and N concentrations at c. 0.5 and 16 mg g^-1 DW, respectively, with a constant P-allocation pattern along the chronosequence. H. prostrata leaves allocated less P to Pi, phospholipids and nucleic acids with increasing soil age, while leaf N concentration was constant. M. systena had the greatest variation in allocating leaf P, whereas leaf N concentration decreased 20% along the chronosequence. Variation in P-allocation patterns was only partially conserved among species along the chronosequence. Such variation could have an impact on species distribution and contribute to species richness in P-limited environments.

How Does Evolution in Phosphorus-Impoverished Landscapes Impact Plant Nitrogen and Sulfur Assimilation?

Phosphorus (P) fertilisers, made from rock phosphate, are used to attain high crop yields. However, rock phosphate is a finite resource and excessive P fertilisers pollute our environment, stressing the need for more P-efficient crops. Some Proteaceae have evolved in extremely P-impoverished environments. One of their adaptations is to curtail the abundance of ribosomal RNA, and thus protein, and tightly control the acquisition and assimilation of nitrogen (N) and sulfur. This differs fundamentally from plants that evolved in environments where N limits plant productivity, but is likely common in many species that evolved in P-impoverished landscapes. Here, we scrutinise the relevance of these responses towards developing P-efficient crops, focusing on plant species where 'P is in the driver's seat'.

Molecular mechanisms underpinning phosphorus-use efficiency in rice

Orthophosphate (H₂ PO₄^- , Pi) is an essential macronutrient integral to energy metabolism as well as a component of membrane lipids, nucleic acids, including ribosomal RNA, and therefore essential for protein synthesis. The Pi concentration in the solution of most soils worldwide is usually far too low for maximum growth of crops, including rice. This has prompted the massive use of inefficient, polluting, and nonrenewable phosphorus (P) fertilizers in agriculture. We urgently need alternative and more sustainable approaches to decrease agriculture's dependence on Pi fertilizers. These include manipulating crops by (a) enhancing the ability of their roots to acquire limiting Pi from the soil (i.e. increased P-acquisition efficiency) and/or (b) increasing the total biomass/yield produced per molecule of Pi acquired from the soil (i.e. increased P-use efficiency). Improved P-use efficiency may be achieved by producing high-yielding plants with lower P concentrations or by improving the remobilization of acquired P within the plant so as to maximize growth and biomass allocation to developing organs. Membrane lipid remodelling coupled with hydrolysis of RNA and smaller P-esters in senescing organs fuels P remobilization in rice, the world's most important cereal crop.

Effects of calcium and its interaction with phosphorus on the nutrient status and growth of three Lupinus species

Phosphorus (P)-deficiency symptoms are known for Lupinus species grown in calcareous soil, but we do not know if this is due to a high calcium (Ca) availability or a low P availability in the soil. To address this problem, we explored both the effects of Ca and its interactions with P on nutrient status and growth of three Lupinus species. Two Ca-sensitive genotypes (L. angustifolius L. P26723 and L. cosentinii Guss. P27225) and two Ca-tolerant genotypes (L. angustifolius L. cv Mandelup and L. pilosus Murr. P27440) were grown hydroponically at two P (0.1 and 10 μM) and three Ca (0.1, 0.6 and 6 mM) levels. Leaf symptoms and biomass were recorded, whole leaf and root nutrient concentrations were analysed, and leaf cellular P and Ca concentrations were determined. Phosphorus-deficiency symptoms were only observed in the Ca-sensitive genotypes. Among all the genotypes in this study, the Ca-tolerant L. pilosus showed an ability to maintain stable leaf Ca and P concentrations whereas the Ca-tolerant L. angustifolius cv Mandelup did not maintain a stable whole leaf Ca concentration, but maintained a low cytosolic Ca2+ concentration through effective Ca compartmentation. However, the two Ca-sensitive genotypes, L. angustifolius P26723 and L. cosentinii, did not exhibit an ability to maintain a stable whole leaf Ca concentration or effectively compartmentalise Ca. Therefore, having the capacity to maintain a stable whole leaf Ca concentration or effectively compartmentalising Ca in leaves are likely critical for Lupinus species to be Ca tolerant.