SUMMARY The assimilationof ammonium ion in plant cell cytoplasm produces at least one H+ per NH+4; N2 fixation generates 0.1‐0.2 H+ per N assimilated; NO‐3 assimilation produces almost one OH‐ per NO‐3. H+ or OH‐ produced in excess of that required to maintain cytoplasmic pH for H+ or OH‐, the major process involved is H+ efflux (frequently by active transport) from the cell. IN higher land plants, much of assimilated N occurs as hoot protein; the shoot cells have no direct acess to the H+ and OH‐ sink of the soil solution. When ammonium ion is the N source it is assimilated into organic‐N in the roots. The shoot is supplied with a mixture of amino‐acids, amides and organic acids which an be incorporated (with neutral photosynthate) into cell material without damaging pH changes. Similar considerations apply to symbiotic N2 assimilation in root nodules. IN both cases the excess H+ generated in the root cell cytoplasm is exerted is excreted to the soil solution; there is no mechanism whereby photolithotrophic plant can, in the long term, counter intracellular acidity without resort to active H+ efflux to an extracellular sink. When nitrate is reduced in roots, the organic compounds involved in N transportged to the shoot are similar to those used when ammonium or N2 is the N source with similar implications for the regulation of shoot pH. The excess OH‐ generated in the roots is partly excreted to the soil solution, and partly neutralized by the ‘biochemical pH stat’ which produces strong organic acids from essentially neutral precursors. When nitrate is assimilated solely in shoots, the excess OH‐ is initially neutralized by the operation of the biochemical pH state. Storage of the inorganic cation‐organate in shoot cell vacuoles could lead to turgor and volume regulation problems in these cells. These are avoided when an insoluble salt (calcium oxalate) is the product of the pH stat, or when the cation organate is translocated to the roots where organate breakdown regenerates OH‐, whcih is lost to the soil solution. This mixture of biochemical, and long and short distance transport processes, enables cells remote from a large sink for H+ or OH‐ to produce protein without unfavourable pH changes. These processes related to pH regulation during N assimilation have important consequences for the carbon and energy economy of the plant.

Background Agricultural production is often limited by low phosphorus (P) availability. In developing countries, which have limited access to P fertiliser, there is a need to develop plants that are more efficient at low soil P. In fertilised and intensive systems, P-efficient plants are required to minimise inefficient use of P-inputs and to reduce potential for loss of P to the environment. Scope Three strategies by which plants and microorganisms may improve P-use efficiency are outlined: (i) Root-foraging strategies that improve P acquisition by lowering the critical P requirement of plant growth and allowing agriculture to operate at lower levels of soil P; (ii) P-mining strategies to enhance the desorption, solubilisation or mineralisation of P from sparingly-available sources in soil using root exudates (organic anions, phosphatases), and (iii) improving internal P-utilisation efficiency through the use of plants that yield more per unit of P uptake.Conclusions We critically review evidence that more P-efficient plants can be developed by modifying root growth and architecture, through manipulation of root exudates or by managing plant-microbial associations such as arbuscular mycorrhizal fungi and microbial inoculants. Opportunities to develop P-efficient plants through breeding or genetic modification are described and issues that may limit success including potential trade-offs and trait interactions are discussed. Whilst demonstrable progress has been made by selecting plants for root morphological traits, the potential for manipulating root physiological traits or selecting plants for low internal P concentration has yet to be realised.

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A compartmented soil-glass bead culture system was used to investigate characteristics of iron plaque and arsenic accumulation and speciation in mature rice plants with different capacities of forming iron plaque on their roots. X-ray absorption near-edge structure spectra and extended X-ray absorption fine structure were utilized to identify the mineralogical characteristics of iron plaque and arsenic sequestration in plaque on the rice roots. Iron plaque was dominated by (oxyhydr)oxides, which were composed of ferrihydrite (81-100%), with a minor amount of goethite (19%) fitted in one of the samples. Sequential extraction and XANES data showed that arsenic in iron plaque was sequestered mainly with amorphous and crystalline iron (oxyhydr)oxides, and that arsenate was the predominant species. There was significant variation in iron plaque formation between genotypes, and the distribution of arsenic in different components of mature rice plants followed the following order: iron plaque > root > straw > husk > grain for all genotypes. Arsenic accumulation in grain differed significantly among genotypes. Inorganic arsenic and dimethylarsinic acid (DMA) were the main arsenic species in rice grain for six genotypes, and there were large genotypic differences in levels of DMA and inorganic arsenic in grain.

Summary In this review we compare the structure and function of the interfaces between symbionts in biotrophic associations. The emphasis is on biotrophic fungal parasites and on mycorrhizas, although necrotrophic parasitic associations and the Rhizobium/legume symbiosis are mentioned briefly. We take as a starting point the observations that in the parasitic associations nutrient transport is polarized towards the parasite, whereas in mutualistic associations it is bidirectional. The structure and function of the interfaces are then compared. An important common feature is that in nearly all cases the heterotrophic symbiont (whether mutualistic or parasitic) is located topologically outside the cytoplasm of the host cells, in an apoplastic compartment. This means that nutrient movements across the interface must involve transport into and out of this apoplastic region through membranes of both organisms. Basic principles of membrane transport in uninfected cells are briefly reviewed to set the scene for a discussion of transport mechanisms which may operate in parasitic and mycorrhizal symbioses. The presence and possible roles of ATPases associated with membranes at the interfaces are discussed. We conclude that cytochemical techniques (used to demonstrate the activity of these enzymes) need to he extended and complemented by biochemical and biophysical studies in order to confirm that the activity is due to transport ATPases. Nevertheless, the distribution of activity appears to he in accord with polarized transport mechanisms in some pathogens and with bidirectional transport in mycorrhizas. The absence of ATPases on many fungal membranes needs re‐examination. We emphasize that transport mechanisms between mycorrhizal symbionts cannot be viewed simply as the exchange of carbon for phosphate. Additional features include provision for transport of carbon and nitrogen as amino acids or amides and for ions such as K+ and H+ involved in the maintenance of charge balance and pH regulation, processes which also occur in parasitic associations. Interplant transport of nutrients via mycorrhizal hyphae is discussed in the context of these complexities. Some suggestions for the directions of future work are made.

Phosphorus (P)-deficiency is a significant challenge for agricultural productivity on many highly P-sorbing weathered and tropical soils throughout the world. On these soils it can be necessary to apply up to five-fold more P as fertiliser than is exported in products. Given the finite nature of global P resources, it is important that such inefficiencies be addressed. For low P-sorbing soils, P-efficient farming systems will also assist attempts to reduce pollution associated with P losses to the environment. P-balance inefficiency of farms is associated with loss of P in erosion, runoff or leaching, uneven dispersal of animal excreta, and accumulation of P as sparingly-available phosphate and organic P in the soil. In many cases it is possible to minimise P losses in runoff or erosion. Uneven dispersal of P in excreta typically amounts to~5% of P-fertiliser inputs. However, the rate of P accumulation in moderate to highly P-sorbing soils is a major contributor to inefficient P-fertiliser use. We discuss the causal edaphic, plant and microbial factors in the context of Plant Soil (2011) 349:89-120 soil P management, P cycling and productivity goals of farms. Management interventions that can alter P-use efficiency are explored, including better targeted P-fertiliser use, organic amendments, removing other constraints to yield, zone management, use of plants with low critical-P requirements, and modified farming systems. Higher productivity in low-P soils, or lower P inputs in fertilised agricultural systems can be achieved by various interventions, but it is also critically important to understand the agroecology of plant P nutrition within farming systems for improvements in P-use efficiency to be realised.

Recent research on arbuscular mycorrhizas has demonstrated that AM fungi play a significant role in plant phosphorus (P) uptake, regardless of whether the plant responds positively to colonization in terms of growth or P content. Here we focus particularly on implications of this finding for consideration of the balance between organic carbon (C) use by the fungi and P delivery (i.e. the C-P trade between the symbionts). Positive growth responses to arbuscular mycorrhizal (AM) colonization are attributed frequently to increased P uptake via the fungus, which results in relief of P deficiency and increased growth. Zero AM responses, compared with non-mycorrhizal (NM) plants, have conventionally been attributed to failure of the fungi to deliver P to the plants. Negative responses, combined with excessive C use, have been attributed to this failure. The fungi were viewed as parasites. Demonstration that the AM pathway of P uptake operates in such plants indicates that direct P uptake by the roots is reduced and that the fungi are not parasites but mutualists because they deliver P as well as using C. We suggest that poor plant growth is the result of P deficiency because AM fungi lower the amount of P taken up directly by roots but the AM uptake of P does compensate for the reduction. The implications of interplay between direct root uptake and AM fungal uptake of P also include increased tolerance of AM plants to toxins such as arsenate and increased success when competing with NM plants. Finally we discuss the new information on C-P trade in the context of control of the symbiosis by the fungus or the plant, including new information (from NM plants) on sugar transport and on the role of sucrose in the signaling network involved in responses of plants to P deprivation.

Summary• A hydroponic experiment was conducted to investigate the effect of phosphorus (P) nutrition and iron plaque on root surfaces on arsenate uptake by, and translocation within, the seedlings of three cultivars of rice ( Oryza sativa ).• Supply of 0.5 mg As l − 1 had no significant effects on dry weights of shoots or roots, but resulted in elevated concentrations of As in tissues, particularly in roots. Rice roots appeared reddish after 24 h in -P solution (without P), indicating the formation of iron plaque.• Arsenic concentrations in iron plaque (determined in dithionite-citratebicarbonate (DCB)-extracts) were significantly higher in -P plants (up to 1180 mg kg − 1 in cultivar CDR22) than in +P plants. Concentrations of arsenic in shoots were significantly lower in -P plants than in +P plants. This indicates that iron plaque might sequestrate As, and consequently reduce the translocation of arsenic from roots to shoots.• Values for the total uptake of As show that As in -P rice plants was mainly concentrated in the DCB-extracts or on the surface of rice roots, whereas most arsenic in +P plants was accumulated in the roots. Arsenic significantly decreased the concentrations of iron (Fe) in roots and shoots ( P < 0.001) and slightly reduced P concentrations in shoots, except for the -P cultivar CDR22.