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PREMISE OF THE STUDY Cluster-root (CR) formation is a desirable trait to improve phosphorus (P) acquisition as global P resources are dwindling. CRs in some lupine species are suppressed at higher P status. Whether increased growth rate enhances CR formation due to a "dilution" of leaf P concentration is unknown. We investigated interactive effects of leaf P status and relative growth rate (RGR) on CR formation in two Lupinus species, which differ in their CR biomass investment. Variation in RGR was imposed by varying day length. METHODS Lupinus albus and L. pilosus were grown hydroponically with KH2PO4 at a day length of 6, 10, or 14 h. We used a slightly higher P supply at longer day lengths to avoid a decline in leaf P concentration, which would induce CRs. Cluster-root percentage, leaf P concentrations, and RGR were determined at 22, 38, and 52 d after sowing. KEY RESULTS Lupinus species grown at similar root P availability, but with a faster growth rate, as dependent on day length, showed a greater CR percentage. Because our aim to achieve exactly the same leaf P concentrations at different day lengths was only partially achieved, we carried out a multiple regression analysis. This analysis showed the CR percentage was strongly and negatively correlated with plant P status and only marginally and positively correlated with RGR. CONCLUSIONS The two Lupinus species invariably formed fewer cluster roots at higher leaf P status, irrespective of RGR. Differences in RGR or leaf P concentration cannot explain the species-specific variation in cluster-root investment.

In Brazil, the campos rupestres occur over the Brazilian shield, and are characterized by acidic nutrient-impoverished soils, which are particularly low in phosphorus (P). Despite recognition of the campos rupestres as a global biodiversity hotspot, little is known about the diversity of P-acquisition strategies and other aspects of plant mineral nutrition in this region. To explore nutrient-acquisition strategies and assess aspects of plant P nutrition, we measured leaf P and nitrogen (N) concentrations, characterized root morphology and determined the percentage arbuscular mycorrhizal (AM) colonization of 50 dominant species in six communities, representing a gradient of soil P availability. Leaf manganese (Mn) concentration was measured as a proxy for carboxylate-releasing strategies. Communities on the most P-impoverished soils had the highest proportion of nonmycorrhizal (NM) species, the lowest percentage of mycorrhizal colonization, and the greatest diversity of root specializations. The large spectrum of leaf P concentration and variation in root morphologies show high functional diversity for nutritional strategies. Higher leaf Mn concentrations were observed in NM compared with AM species, indicating that carboxylate-releasing P-mobilizing strategies are likely to be present in NM species. The soils of the campos rupestres are similar to the most P-impoverished soils in the world. The prevalence of NM strategies indicates a strong global functional convergence in plant mineral nutrition strategies among severely P-impoverished ecosystems.

Some Lupinus species produce cluster roots in response to low plant phosphorus (P) status. The cause of variation in cluster-root formation among cluster-root-forming Lupinus species is unknown. The aim of this study was to investigate if cluster-root formation is, in part, dependent on different relative growth rates (RGRs) among Lupinus species when they show similar shoot P status.Three cluster-root-forming Lupinus species, L. albus, L. pilosus and L. atlanticus, were grown in washed river sand at 0, 7·5, 15 or 40 mg P kg(-1) dry sand. Plants were harvested at 34, 42 or 62 d after sowing, and fresh and dry weight of leaves, stems, cluster roots and non-cluster roots of different ages were measured. The percentage of cluster roots, tissue P concentrations, root exudates and plant RGR were determined.Phosphorus treatments had major effects on cluster-root allocation, with a significant but incomplete suppression in L. albus and L. pilosus when P supply exceeded 15 mg P kg(-1) sand. Complete suppression was found in L. atlanticus at the highest P supply; this species never invested more than 20 % of its root weight in cluster roots. For L. pilosus and L. atlanticus, cluster-root formation was decreased at high internal P concentration, irrespective of RGR. For L. albus, there was a trend in the same direction, but this was not significant.Cluster-root formation in all three Lupinus species was suppressed at high leaf P concentration, irrespective of RGR. Variation in cluster-root formation among the three species cannot be explained by species-specific variation in RGR or leaf P concentration.

Summary Uren & Parsons (2013) criticize our nutrient addition experiment and question the integrity of the Jurien Bay dune chronosequence. Their primary criticisms relate to (i) inconsistencies in parent material along the chronosequence and (ii) the lack of consideration of micronutrients in our glasshouse experiment. We reaffirm that the Quindalup–Spearwood succession is a consistent Holocene–Middle (possibly Early) Pleistocene soil chronosequence in which parent material, topography, and present-day climate and vegetation type (i.e. Mediterranean shrubland) are held relatively constant. The older (Early Pleistocene–Late Pliocene) Bassendean Sand probably contained less carbonate initially, but nevertheless represents the endpoint of an exceptionally strong gradient in soil nutrient availability. The claim that we did not consider the potential importance of micronutrients is unfounded. We included a micronutrient treatment in our experiment and discussed the importance of micronutrient availability in young calcareous substrates. Synthesis. We restate that our experimental results support predictions about changes in nutrient limitation during long-term pedogenesis, and affirm that the Jurien Bay chronosequence is a valuable model system for addressing ecological questions related to pedogenesis, plant diversity and plant–soil interactions.

Environmental monitoring is becoming increasingly sophisticated with the widespread adoption of data loggers, sensor arrays and remote sensing, leading to larger scale, higher resolution and superior quality data. However, interpreting monitoring data and deciding when and how to apply environmental management remains a subjective and underdeveloped area of research. Control charts, developed in industrial settings to identify when manufacturing processes were beyond the acceptable bounds of production quality, represent one solution. Despite their potential utility, control charts have rarely been adopted in environmental monitoring. In theory, they are able to identify undesirable trends early and provide transparent and broadly consensual criteria for defining when management action should take place, that is action is triggered when parameter values are observed beyond the agreed control limits of the process. Once triggered, a predetermined management action is implemented. Possible actions are many and varied, and range from investigation and increased monitoring to intervention in the system. Here, the utility of control charts in monitoring water supply in south-western Australia from 1911 to 2010 is examined, and their ability to provide an early, transparent and easily understandable means of triggering management action is assessed. Two control chart types are applied: the X-bar chart and the CUSUM chart. X-bar charts varied widely in their ability to trigger action and were insensitive to many traditional threshold criteria (of which there are many to choose from). In contrast, standard CUSUM charts are specifically designed to detect subtler shifts away from a mean trend and hence provided a more consistent warning of the decline in water supply. While managers were aware of the decline in water supply from an early stage, we believe that control charts could have clearly communicated this earlier, enabling consensus among decision makers to be reached more rapidly.

†Background and Aims The Australian legume species Viminaria juncea forms both cluster roots and mycorrhizal associations. The aim of this study was to identify if these root specializations are expressed at differential supplies of phosphorus (P) and at different shoot P concentrations [P]. †Methods Seedlings were planted in sand and provided with a mycorrhizal inoculum and basal nutrients plus one of 21 P treatments, ranging from 0 to 50 mg P kg 21 dry soil. Plants were harvested after 12 weeks, and roots, shoots and cluster roots were measured for length and fresh and dry weight. The number of cluster roots, the percentage of mycorrhizal colonization, and shoot [P] were determined. †Key Results Shoot biomass accumulation increased with increasing P supply until a shoot dry weight of 3 g was reached at a P supply of approx. 27. 5m g Pk g 21 dry soil. Neither cluster-root formation nor mycorrhizal colonization was fully suppressed at the highest P supply. Most intriguingly, shoot [P] did not differ across treatments, with an average of 1. 4m g Pk g 21 shoot dry weight. †Conclusions The almost constant shoot [P] in V. juncea over the very wide range of P supplies is, to our knowledge, unprecedented. To maintain these stable values, this species down-regulates its growth rate when no P is supplied; conversely, it down-regulates its P-uptake capacity very tightly at the highest P supplies, when its maximum growth rate has been reached. It is proposed that the persistence of cluster roots and mycorrhizal colonization up to the highest P treatments is a consequence of its tightly controlled shoot [P]. This unusual P physiology of V. juncea is surmised to be related to the habitat of this N2-fixing species. Water and nutrients are available at a low but steady supply for most of the year, negating the need for storage of P which would be metabolically costly and be at the expense of metabolic energy and P available for symbiotic N2 fixation.

†Background and Aims Previous research has suggested a trade-off between the capacity of plants to downregulate their phosphorus (P) uptake capacity and their efficiency of P resorption from senescent leaves in species from P-impoverished environments. †Methods To investigate this further, four Australian native species (Banksia attenuata, B. menziesii, Acacia truncata and A. xanthina) were grown in a greenhouse in nutrient solutions at a range of P concentrations [P]. Acacia plants received between 0 and 500 mM P; Banksia plants received between 0 and 10 mM P, to avoid major P-toxicity symptoms in these highly P-sensitive species. †Key Results For both Acacia species, the net P-uptake rates measured at 10 mM P decreased steadily with increasing P supply during growth. In contrast, in B. attenuata, the net rate of P uptake from a solution with 10 mM P increased linearly with increasing P supply during growth. The P-uptake rate of B. menziesii showed no significant response to P supply in the growing medium. Leaf [P] of the four species supported this finding, with A. truncata and A. xanthina showing an increase up to a saturation value of 19 and 21 mg P g 21 leaf dry mass, respectively (at 500 mM P), whereas B. attenuata and B. menziesii both exhibited a linear increase in leaf [P], reaching 10 and 13 mg P g 21 leaf dry mass, respectively, without approaching a saturation point. The Banksia plants grown at 10 mM P showed mild symptoms of P toxicity, i.e. yellow spots on some leaves and drying and curling of the tips of the leaves. Leaf P-resorption efficiency was 69 % (B. attenuata), 73 % (B. menziesii), 34 % (A. truncata) and 36 % (A. xanthina). The P-resorption proficiency values were 0.08 mg Pg 21 leaf dry mass (B. attenuata and B. menziesii), 0.32 mg P g 21 leaf dry mass (A. truncata) and 0.36 mg Pg 21 leaf dry mass (A. xanthina). Combining the present results with additional information on P-remobilization efficiency and the capacity to downregulate P-uptake capacity for two other Australian woody species, we found a strong negative correlation between these traits. †Conclusions It is concluded that species that are adapted to extremely P-impoverished soils, such as many south-western Australian Proteaceae species, have developed extremely high P-resorption efficiencies, but lost their capacity to downregulate their P-uptake mechanisms. The results support the hypothesis that the ability to resorb P from senescing leaves is inversely related to the capacity to downregulate net P uptake, possibly because constitutive synthesis of P transporters is a prerequisite for proficient P remobilization from senescing tissues.

Summary 1. The classical model of long-term ecosystem development suggests that primary productivity is limited by nitrogen (N) on young substrates and phosphorus (P) on older substrates. Measurements of foliar and soil nutrients along soil chronosequences support this model, but direct tests through nutrient-addition experiments are rare. 2. We conducted a nutrient-limitation bioassay using phytometer species grown in soils from five stages of a >2-million-year dune chronosequence in south-western Australia. This long-term chronosequence is located within a region of exceptionally high plant species diversity and has not been previously studied in the context of ecosystem development. 3. Growth of unfertilized phytometers, a proxy for primary productivity, peaked on young soils (hundreds to a few thousand years) and then declined steadily on older soils. This decline was linked to P limitation, and its rapid appearance ( 9), being of a marine origin (i.e. carbonate dunes). Fe is poorly soluble at high pH and K likely plays a role in the secretion of Fe-mobilizing exudates from wheat roots. 5. Synthesis. Our results provide strong support for the long-term ecosystem-development model, particularly with regard to the appearance of P limitation and associated declines in productivity. However, our study also shows that N cannot be assumed to invariably be the most important limiting nutrient in young soils, and it is unlikely to be the only limiting nutrient in calcareous soils. This south-western Australian long-term chronosequence provides an excellent opportunity to explore edaphic controls over plant species diversity.

Australia harbors some of the most nutrient-impoverished soils on Earth. Southwestern Australian soils are especially phosphorus (P) impoverished, due to the age of this ancient landscape and it being unaffected by major geological disturbance for millions of years ([Hopper, 2009][1]; [Lambers et al

The tropical pasture grass Brachiaria humidiola (Rendle) Schweick releases nitrification inhibitory compounds from its roots, a phenomenon termed ‘biological nitrification inhibition’ (BNI). We investigated the influence of root exudates of B. humidicola on nitrification, major soil microorganisms and plant growth promoting microorganisms using two contrasting soil types, Andosol and Cambisol. The addition of root exudates (containing BNI activity that is expressed in Allylthiourea unit (ATU) was standardized in a bioassay against a synthetic inhibitor of nitrification, allylthiourea, and their function in soil was compared to inhibition caused by the synthetic nitrification inhibitor dicyandiamide. At 30 and 40 ATU g−1soil, root exudates inhibited nitrification by 95% in fresh Cambisol after 60 days. Nitrification was also similarly inhibited in rhizosphere soils of Cambisol where B. humidicola was grown for 6 months. Root exudates did not inhibit other soil microorganisms, including gram-negati...

Nitrification results in poor nitrogen (N) recovery and negative environmental impacts in most agricultural systems. Some plant species release secondary metabolites from their roots that inhibit nitrification, a phenomenon known as biological nitrification inhibition (BNI). Here, we attempt to characterize BNI in sorghum (Sorghum bicolor). In solution culture, the effect of N nutrition and plant age was studied on BNI activity from roots. A bioluminescence assay using recombinant Nitrosomonas europaea was employed to determine the inhibitory effect of root exudates. One major active constituent was isolated by activity-guided HPLC fractionations. The structure was analysed using NMR and mass spectrometry. Properties and the 70% inhibitory concentration (IC(70)) of this compound were determined by in vitro assay. Sorghum had significant BNI capacity, releasing 20 allylthiourea units (ATU) g(-1) root DW d(-1). Release of BNI compounds increased with growth stage and concentration of supply. NH4+ -grown plants released several-fold higher BNI compounds than NO3- -grown plants. The active constituent was identified as methyl 3-(4-hydroxyphenyl) propionate. BNI compound release from roots is a physiologically active process, stimulated by the presence of NH4+. Methyl 3-(4-hydroxyphenyl) propionate is the first compound purified from the root exudates of any species; this is an important step towards better understanding BNI in sorghum.

Using a recombinant luminescent Nitrosomonas europaea assay to quantify biological nitrification inhibition (BNI), we found that a wild relative of wheat (Leymus racemosus (Lam.) Tzvelev) had a high BNI capacity and releases about 20 times more BNI compounds (about 30 ATU g−1 root dry weight 24 h−1) than Triticum aestivum L. (cultivated wheat). The root exudate from cultivated wheat has no inhibitory effect on nitrification when applied to soil; however, the root exudate from L. racemous suppressed \( {\text{NO}}^{ - }_{3} \) formation and kept more than 90% of the soil’s inorganic-N in the \( {\text{NH}}^{ + }_{4} \)-form for 60 days. The high-BNI capacity of L. racemosus is mostly associated with chromosome Lr#n. Two other chromosomes Lr#J, and Lr#I also have an influence on BNI production. Tolerance of L. racemosus to \( {\text{NH}}^{ + }_{4} \) is controlled by chromosome 7Lr#1-1. Sustained release of BNI compounds occurred only in the presence of \( {\text{NH}}^{ + }_{4} \) in the root environment. Given the level of BNI production expressed in DALr#n and assuming normal plant growth, we estimated that nearly 87,500,000 ATU of BNI activity ha−1 day−1 could be released in a field of vigorously growing wheat; this amounts to the equivalent of the inhibitory effect from the application of 52.5 g of the synthetic nitrification inhibitor nitrapyrin (one AT unit of BNI activity is equivalent to 0.6 μg of nitrapyrin). At this rate of BNI production it would take only 19 days for a BNI-enabled wheat crop to produce the inhibitory power of a standard commercial application of nitrapyrin, 1 kg ha−1. The synthetic nitrification inhibitor, dicyandiamide, blocked specifically the AMO (ammonia monooxygenase) pathway, while the BNI from L. racemosus blocked the HAO (hydroxylamine oxidoreductase) pathway in Nitrosomonas. Here we report the first finding of high production of BNI in a wild relative of any cereal and its successful introduction and expression in cultivated wheat. These results demonstrate the potential for empowering the new generation of wheat cultivars with high-BNI capacity to control nitrification in wheat-production systems.

The capacity of plant roots to increase their carboxylate exudation at a low plant phosphorus (P) status is an adaptation to acquire sufficient P at low soil P availability. Our objective was to compare crop species in their adaptive response to a low-P availability, in order to gain knowledge to be used for improving crop P-acquisition efficiency from soils that are low in P or that have a high capacity to retain P. In the present screening study we compared 13 crop species, grown in sand at either 3 or 300 μM of P, and measured root mass ratio, cluster-root development, rhizosphere pH and carboxylate composition of root exudates. Root mass ratio decreased with increasing P supply for Triticum aestivum L., Brassica napus L., Cicer arietinum L. and Lens culinaris Medik., and increased only for Pisum sativum L., while the Lupinus species and Vicia faba L. were not responsive. Lupinus species that had the potential to produce root clusters either increased or decreased biomass allocation to clusters at 300 μM of P compared with allocation at 3 μM of P. All Lupinus species acidified their rhizosphere more than other species did, with average pH decreasing from 6.7 (control) to 4.3 for Lupinus pilosus L. and 5.9 for Lupinus atlanticus L.; B. napus maintained the most alkaline rhizosphere, averaging 7.4 at 300 μM of P. Rhizosphere carboxylate concentrations were lowest for T. aestivum, B. napus, V. faba, and L. culinaris than for the other species. Exuded carboxylates were mainly citrate and malate for all species, with the exception of L. culinaris and C. arietinum, which produced mainly citrate and malonate. Considerable variation in the concentration of exuded carboxylates and protons was found, even with a genus. Cluster-root forming species did not invariably have the highest concentrations of rhizosphere carboxylates. Lupinus species varied both in P-uptake and in the sensitivity of their cluster-root development to external P supply. Given the carbon cost of cluster roots, a greater plasticity in their formation and exudation (i.e. reduced investment in cluster roots and exudation at higher soil P, a negative feedback response) is a desirable trait for agricultural species that may have variable access to readily available P.

• Background Global phosphorus (P) reserves are being depleted, with half-depletion predicted to occur between 2040 and 2060. Most of the P applied in fertilizers may be sorbed by soil, and not be available for plants lacking specific adaptations. On the severely P-impoverished soils of south-western Australia and the Cape region in South Africa, non-mycorrhizal species exhibit highly effective adaptations to acquire P. A wide range of these non-mycorrhizal species, belonging to two monocotyledonous and eight dicotyledonous families, produce root clusters. Non-mycorrhizal species with root clusters appear to be particularly effective at accessing P when its availability is extremely low. • Scope There is a need to develop crops that are highly effective at acquiring inorganic P (Pi) from P-sorbing soils. Traits such as those found in non-mycorrhizal root-cluster-bearing species in Australia, South Africa and other P-impoverished environments are highly desirable for future crops. Root clusters combine a specialized structure with a specialized metabolism. Native species with such traits could be domesticated or crossed with existing crop species. An alternative approach would be to develop future crops with root clusters based on knowledge of the genes involved in development and functioning of root clusters. • Conclusions Root clusters offer enormous potential for future research of both a fundamental and a strategic nature. New discoveries of the development and functioning of root clusters in both monocotyledonous and dicotyledonous families are essential to produce new crops with superior P-acquisition traits.

In soils where “plant-available phosphorus (P)” exists in very low concentrations because of a high capacity of P sorption, plants require specialised mechanisms to make sparingly soluble forms of P available for plant uptake. In this issue, Wang et al. (2011) used an isotopic dilution technique to confirm that cotton (Gossypium hirsutum L.) is relatively inefficient in acquiring P from sparingly soluble forms compared to wheat (Triticum aestivum L.) and white lupin (Lupinus albus L.). This followed a previous pot study where AlPO4, FePO4 and hydroxyapatite were found to be unavailable to cotton (Wang et al. 2010). Previously, field pea (Pisum sativum L.) and chickpea (Cicer arietinum L.) were found to be incapable of acquiring P from both AlPO4 and FePO4, but capable of acquiring P from hydroxyapatite [Ca5OH(PO4)3] (Pearse et al. 2007). Wang et al. (2011) have confirmed that wheat can solubilise AlPO4, and white lupin can solubilise Ca5OH(PO4)3, as found previously (Pearse et al. 2006, 2007; Vu et al. 2010; Wang et al. 2010). Canola (Brassica napus L.) can access hydroxyapatite almost as readily as it utilises soluble P, and can access AlPO4 less readily, but can access FePO4 only to a lesser extent (Hoffland et al. 1989; Pearse et al. 2007). The difference among species in ability to acquire P from one particular sparingly soluble form rather than another, or the complete lack of ability to acquire any of them, remains very much an issue of scientific intrigue. Why can plants with mechanisms to acquire one form of sparingly soluble P, only partially access or not access another? The acquisition of P from sparingly soluble forms can be attributed to a combination of three broader mechanisms working collectively to enable P to be taken up. First, root structural specialisations increase the surface area of the root. These include enhanced formation of root hairs, fine roots and cluster-roots, and increased branching and mycorrhizal associations. Second, modification of the rhizosphere pH by the transport of anions or cations, favour dissolution of P from particular sparingly soluble forms. Third, release of water to moisten dry soil and/or compounds such as carboxylates, phosphatases, phenolics and mucilage, facilitates uptake of P that is bound or exists in unavailable chemical forms (reviewed by Lambers et al. 2006). Certain plant species appear to have ticked all the boxes when it comes to the three broad approaches to P acquisition from soils with a Plant Soil (2011) 348:81–83 DOI 10.1007/s11104-011-0975-8

South-western Australia harbours a biodiversity hotspot on severely phosphorus-impoverished soils. Threats include eutrophication due to phosphorus enrichment, due to increased fire frequency and spraying with phosphite to reduce the impacts of the introduced pathogen Phytophthora cinnamomi. We propose a strategy to work towards alternatives to phosphite for pathogen management., South-western Australia harbours a global biodiversity hotspot on the world's most phosphorus (P)-impoverished soils. The greatest biodiversity occurs on the most severely nutrient-impoverished soils, where non-mycorrhizal species are a prominent component of the flora. Mycorrhizal species dominate where soils contain slightly more phosphorus. In addition to habitat loss and dryland salinity, a major threat to plant biodiversity in this region is eutrophication due to enrichment with P. Many plant species in the south-western Australian biodiversity hotspot are extremely sensitive to P, due to a low capability to down-regulate their phosphate-uptake capacity. Species from the most P-impoverished soils are also very poor competitors at higher P availability, giving way to more competitive species when soil P concentrations are increased. Sources of increased soil P concentrations include increased fire frequency, run-off from agricultural land, and urban activities. Another P source is the P-fertilizing effect of spraying natural environments on a landscape scale with phosphite to reduce the impacts of the introduced plant pathogen Phytophthora cinnamomi, which itself is a serious threat to biodiversity. We argue that alternatives to phosphite for P. cinnamomi management are needed urgently, and propose a strategy to work towards such alternatives, based on a sound understanding of the physiological and molecular mechanisms of the action of phosphite in plants that are susceptible to P. cinnamomi. The threats we describe for the south-western Australian biodiversity hotspot are likely to be very similar for other P-impoverished environments, including the fynbos in South Africa and the cerrado in Brazil.

Summary Proteaceae species in south-western Australia occur on severely phosphorus (P)-impoverished soils. They have very low leaf P concentrations, but relatively fast rates of photosynthesis, thus exhibiting extremely high photosynthetic phosphorus-use-efficiency (PPUE). Although the mechanisms underpinning their high PPUE remain unknown, one possibility is that these species may be able to replace phospholipids with nonphospholipids during leaf development, without compromising photosynthesis. For six Proteaceae species, we measured soil and leaf P concentrations and rates of photosynthesis of both young expanding and mature leaves. We also assessed the investment in galactolipids, sulfolipids and phospholipids in young and mature leaves, and compared these results with those on Arabidopsis thaliana, grown under both P-sufficient and P-deficient conditions. In all Proteaceae species, phospholipid levels strongly decreased during leaf development, whereas those of galactolipids and sulfolipids strongly increased. Photosynthetic rates increased from young to mature leaves. This shows that these species extensively replace phospholipids with nonphospholipids during leaf development, without compromising photosynthesis. A considerably less pronounced shift was observed in A. thaliana. Our results clearly show that a low investment in phospholipids, relative to nonphospholipids, offers a partial explanation for a high photosynthetic rate per unit leaf P in Proteaceae adapted to P-impoverished soils.

A sensitive fluorometric assay for the quantification of phosphite has been developed. The assay uses the enzymatic oxidation of phosphite to phosphate by a recombinant phosphite dehydrogenase with NAD(+) as cosubstrate to produce the highly fluorescent reaction product resorufin. The optimized assay can be carried out in a 96-well microtiter plate format for high-throughput screening purposes and has a detection limit of 0.25 nmol phosphite. We used the method to quantify phosphite levels in plant tissue extracts and to determine phosphite dehydrogenase activity in transgenic plants. The assay is suitable for other biological or environmental samples. Because phosphite is a widely used fungicide to protect plants from pathogenic oomycetes, the assay provides a cost-effective and easy-to-use method to monitor the fate of phosphite following application.

* The relationship between carboxylate release from roots and the ability of the species to utilize phosphorus from sparingly soluble forms was studied by comparing Triticum aestivum, Brassica napus, Cicer arietinum, Pisum sativum, Lupinus albus, Lupinus angustifolius and Lupinus cosentinii. * Plants were grown in sand and supplied with 40 mg P kg(-1) in the sparingly soluble forms AlPO(4), FePO(4) or Ca(5)OH(PO(4))(3), or as soluble KH(2)PO(4); control plants received no P. * The ability to utilize sparingly soluble forms of P differed between forms of P supplied and species. Pisum sativum and C. arietinum did not access AlPO(4) or FePO(4) despite releasing carboxylates into the rhizosphere. * Species accessed different forms of sparingly soluble P, but no species was superior in accessing all forms. We conclude that a single trait cannot explain access to different forms of sparingly soluble P, and hypothesize that in addition to carboxylates, rhizosphere pH and root morphology are key factors.

Summary • The relationship between carboxylate release and the ability of plants to access phosphorus from AlPO4 and to detoxify aluminium was studied by comparing species with a low and high rate of carboxylate release, Triticum aestivum (wheat) and Lupinus albus (white lupin), respectively. • Species were supplied with P at 10, 20, 40 or 100 mg P kg−1 sand in the form of sparingly soluble AlPO4 or soluble KH2PO4; control plants did not receive any P. • Triticum aestivum was significantly better than L. albus at accessing P from AlPO4, despite accumulating fewer carboxylates in its rhizosphere. Rhizosphere pH of L. albus did not vary with form or level of P supply, while the rhizosphere pH of T. aestivum increased with the level of P supplied. • Based on the evidence in the present study, a model is proposed to explain the poor performance of L. albus, whereby the release of carboxylates and associated protons reduces the chelating ability of exuded carboxylates, thus reducing P acquisition and increasing Al toxicity.

Seven Lupinus albus L. landraces were selected, based on their geographic origin and the soil type and pH at the site of collection of the seeds, and compared with the cv. Kiev mutant. We hypothesised that those landraces collected from red/yellow acidic sands (pH 5–5.7) would be better at acquiring P from FePO4 or AlPO4 than those selected from brown neutral (pH 7) or fine, calcareous, alkaline sands (pH 9), and that those selected from fine calcareous sands would be more effective at acquiring P from Ca5OH(PO4)3. Plants were grown in sand and supplied with 40 mg P/kg as the above sparingly soluble forms, or as soluble KH2PO4; control plants received no P. All genotypes were able to use these P sources. Variation in using poorly soluble P was not due to differences in rhizosphere carboxylate concentration, cluster-root development, or rhizosphere-extract pH. L. albus landraces with a better ability to use P from different sparingly soluble forms could be exploited to develop cultivars that are more P-acquisition efficient on soils that are low in [P] or highly P-sorbing; however, desirable genotypes cannot simply be selected based on soil type of origin.