In most soils, inorganic phosphorus occurs at fairly low concentrations in the soil solution whilst a large proportion of it is more or less strongly held by diverse soil minerals. Phosphate ions can indeed be adsorbed onto positively charged minerals such as Fe and Al oxides. Phosphate (P) ions can also form a range of minerals in combination with metals such as Ca, Fe and Al. These adsorption/desorption and precipitation/dissolution equilibria control the concentration of P in the soil solution and, thereby, both its chemical mobility and bioavailability. Apart from the concentration of P ions, the major factors that determine those equilibria as well as the speciation of soil P are (i) the pH, (ii) the concentrations of anions that compete with P ions for ligand exchange reactions and (iii) the concentrations of metals (Ca, Fe and Al) that can coprecipitate with P ions. The chemical conditions of the rhizosphere are known to considerably differ from those of the bulk soil, as a consequence of a range of processes that are induced either directly by the activity of plant roots or by the activity of rhizosphere microflora. The aim of this paper is to give an overview of those chemical processes that are directly induced by plant roots and which can affect the concentration of P in the soil solution and, ultimately, the bioavailability of soil inorganic P to plants. Amongst these, the uptake activity of plant roots should be taken into account in the first place. A second group of activities which is of major concern with respect to P bioavailability are those processes that can affect soil pH, such as proton/bicarbonate release (anion/cation balance) and gaseous (O2/CO2) exchanges. Thirdly, the release of root exudates such as organic ligands is another activity of the root that can alter the concentration of P in the soil solution. These various processes and their relative contributions to the changes in the bioavailability of soil inorganic P that can occur in the rhizosphere can considerably vary with (i) plant species, (ii) plant nutritional status and (iii) ambient soil conditions, as will be stressed in this paper. Their possible implications for the understanding and management of P nutrition of plants will be briefly addressed and discussed.

Bioavailability of soil inorganic P in the rhizosphere as affected by root-induced chemical changes: A review

The rhizosphere is a complex environment where roots interact with physical, chemical and biological properties of soil. Structural and functional characteristics of roots contribute to rhizosphere processes and both have significant influence on the capacity of roots to acquire nutrients. Roots also interact extensively with soil microorganisms which further impact on plant nutrition either directly, by influencing nutrient availability and uptake, or indirectly through plant (root) growth promotion. In this paper, features of the rhizosphere that are important for nutrient acquisition from soil are reviewed, with specific emphasis on the characteristics of roots that influence the availability and uptake of phosphorus and nitrogen. The interaction of roots with soil microorganisms, in particular with mycorrhizal fungi and non-symbiotic plant growth promoting rhizobacteria, is also considered in relation to nutrient availability and through the mechanisms that are associated with plant growth promotion.

Acquisition of phosphorus and nitrogen in the rhizosphere and plant growth promotion by microorganisms

Phosphate (Pi) is considered to be one of the least available plant nutrients in the soil. High-affinity Pi transporters are generally accepted as entry points for Pi in the roots. The physiological, genetic, molecular and biochemical analysis of phosphate starvation response mechanisms highlight the ability of plants to adapt and thrive under phosphate limiting conditions. These responses help them enhance the availability of Pi, increase its uptake and improve the use-efficiency of Pi within a plant. Enhanced ability to acquire Pi appears to be regulated at the level of transcription of high-affinity phosphate transporters. These transporters are encoded by a family of small number of genes having characteristic tissue and organ associated expression patterns. Many of them are strongly induced during phosphate deficiency thus providing plants with enhanced ability to acquire and transfer phosphate. In addition, plants also activate biochemical mechanisms that could lead to increased acquisition of phosphate from both inorganic and organic phosphorus sources in the soil. Furthermore, altered root morphology and mycorrhizal symbiosis further enhance the ability of plants to acquire Pi. Interestingly most of these responses appear to be coordinated by changes in cellular phosphate levels. It is becoming apparent that phosphate acquisition and utilization are associated with activation or inactivation of a host of genes in plants. In this article we describe molecular, biochemical and physiological factors associated with phosphate acquisition by plants.

Phosphate Acquisition

The aim of the present review is to define the various origins of root-mediated changes of pH in the rhizosphere, i.e., the volume of soil around roots that is influenced by root activities. Root-mediated pH changes are of major relevance in an ecological perspective as soil pH is a critical parameter that influences the bioavailability of many nutrients and toxic elements and the physiology of the roots and rhizosphere microorganisms. A major process that contributes root-induced pH changes in the rhizosphere is the release of charges carried by H+ or OH− to compensate for an unbalanced cation–anion uptake at the soil–root interface. In addition to the ions taken up by the plant, all the ions crossing the plasma membrane of root cells (e.g., organic anions exuded by plant roots) should be taken into account, since they all need to be balanced by an exchange of charges, i.e., by a release of either H+ or OH−. Although poorly documented, root exudation and respiration can contribute some proportion of rhizosphere pH decrease as a result of a build-up of the CO2 concentration. This will form carbonic acid in the rhizosphere that may dissociate in neutral to alkaline soils, and result in some pH decrease. Ultimately, plant roots and associated microorganisms can also alter rhizosphere pH via redox-coupled reactions. These various processes involved in root-mediated pH changes in the rhizosphere also depend on environmental constraints, especially nutritional constraints to which plants can respond. This is briefly addressed, with a special emphasis on the response of plant roots to deficiencies of P and Fe and to Al toxicity. Finally, soil pH itself and pH buffering capacity also have a dramatic influence on root-mediated pH changes.

Origins of root-mediated pH changes in the rhizosphere and their responses to environmental constraints: A review

The beneficial effects of mycorrhizae on plant growth have often been related to the increase in the uptake of immobile nutrients, especially phosphorus (P). In this review the mechanisms for the increase in the uptake of P by mycorrhizae and the sources of soil P for mycorrhizal and non-mycorrhizal plants are examined. Various mechanisms have been suggested for the increase in the uptake of P by mycorrhizal plants. These include: exploration of larger soil volume; faster movement of P into mycorrhizal hyphae; and solubilization of soil phosphorus. Exploration of larger soil volume by mycorrhizal plants is achieved by decreasing the distance that P ions must diffuse to plant roots and by increasing the surface area for absorption. Faster movement of P into mycorrhizal hyphae is achieved by increasing the affinity for P ions and by decreasing the threshold concentration required for absorption of P. Solubilization of soil P is achieved by the release of organic acids and phosphatase enzymes. Mycorrhizal plants have been shown to increase the uptake of poorly soluble P sources, such as iron and aluminium phosphate and rock phosphates. However, studies in which the soil P has been labelled with radioactive 32P indicated that both mycorrhizal and non-mycorrhizal plants utilized the similarly labelled P sources in soil.

A critical review on the role of mycorrhizal fungi in the uptake of phosphorus by plants

Rape (Brassica napus, var. Emerald) plants were grown in thin layers of a phosphate-deficient soil supplied with a P-free nutrient solution containing N solely as NO3- (3 5 x 10-3 M). Inter-root competition (excluding root hairs) for soil P began on the 14th day of growth, after which all soil in the thin layer was considered to be rhizosphere soil. During subsequent plant growth the pH in the rhizosphere decreased as much as 2-4 units (from pH 6&5 to 4-1) before rising slightly towards the end of the experiment (35th day). The pH decrease was associated with an increase in solution P concentration in the depleted rhizosphere soils, compared to the P concentration attained when similar amounts of P were desorbed from unplanted control soils by shaking them in 102 M Ca(N03)2 at the original soil pH of 6-1. P desorption in 10-2 M Ca(NO3)2 at pH values between 6&2 and 4-5 indicated that a decrease in the soil pH from 6-5 to 4-1 could result in at least a 10-fold increase in the P released into solution, which would be available for plant uptake. These results may explain reports in the literature that rape is efficient in absorbing P from P-deficient soils.

Plant-induced changes in the rhizosphere of rape (brassica napus var.

Summary
Additions of soluble phosphate to P-deficient Begbroke sandy loam delayed or completely prevented the fall in rhizosphere pH observed when rape was grown at high root densities in this soil (Parts I to III, this series). Whereas the pH of the unamended rhizosphere soil fell from 6–6.5 to 5.1–5.3 by day 41, the rhizosphere pH of soil with an extra 2 μmol P g−1 remained steady (6 to 6.5) until day 35, and that of soil with an extra 20 μmol P g−1 increased slightly from pH 6 to 6.4 during 41 days of growth. The increase in rhizosphere phosphatase activity with increasing severity of P deficiency appeared to be a response to increasing root density and decreasing concentration of soluble inorganic P in the soil. No significant change in levels of soil organic P was detected.

Plants that acidified their rhizosphere (low P status) depleted acid-soluble forms of soil P and absorbed twice the amount of P which could be desorbed from the control soil in 10−2 M Ca(NO3)2at pH 6.1. Uptake of P by plants which did not acidify their rhizosphere (high P status) was never greater than the amount of P desorbable in 10−2m Ca(NO3)2 at pH 6.1, and was derived from alkali-soluble and resin-extractable forms of soil P. Cation uptake exceeded anion uptake in both low and high P plants, but the difference was greater in low P plants. These differences in cation and anion uptake (mEq) were approximately equal to the milliequivalents of H+ (OH−) required to produce the observed changes in rhizosphere pH. These results explain why models for P uptake based on in vitro measurements of physicochemical parameters governing phosphate diffusion in soil work well for P sufficient plants, but tend to underestimate P uptake by P deficient plants growing in soils in which soluble P levels are sensitive to pH change.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1982.tb03289.x

Plant‐induced changes in the rhizosphere of rape (brassica napus var. emerald) seedlings

Plant-induced changes in the rhizosphere of rape (Brassica napus var. Emerald) seedlings. III. Changes in L value, soil phosphate fractions and phosphatase activity.

Rape (Brassica napus, var. Emerald) plants were grown in thin layers of a phosphate-deficient soil supplied with a P-free nutrient solution containing N solely as NO3(3 5 x 10-3 M). Inter-root competition (excluding root hairs) for soil P began on the 14th day of growth, after which all soil in the thin layer was considered to be rhizosphere soil. During subsequent plant growth the pH in the rhizosphere decreased as much as 2-4 units (from pH 6&5 to 4-1) before rising slightly towards the end of the experiment (35th day). The pH decrease was associated with an increase in solution P concentration in the depleted rhizosphere soils, compared to the P concentration attained when similar amounts of P were desorbed from unplanted control soils by shaking them in 102 M Ca(N03)2 at the original soil pH of 6-1. P desorption in 10-2 M Ca(NO3)2 at pH values between 6&2 and 4-5 indicated that a decrease in the soil pH from 6-5 to 4-1 could result in at least a 10-fold increase in the P released into solution, which would be available for plant uptake. These results may explain reports in the literature that rape is efficient in absorbing P from P-deficient soils.

Plant induced changes in the rhizosphere of rape brassica napus cultivar emerald seedlings 1. ph change and the increase in phosphorus concentration in the soil solution

SUMMARY Additions of soluble phosphate to P-deficient Begbroke sandy loam delayed or completely prevented the fall in rhizosphere pH observed when rape was grown at high root densities in this soil (Parts I to III, this series). Whereas the pH of the unamended rhizosphere soil fell from 6-6'5 to 5·1-5·3 by day 41, the rhizosphere pH of soil with an extra 2 pmol P g-l remained steady (6 to 6'5) until day 35, and that of soil with an extra 20 pmol P g-l increased slightly from pH 6 to 6'4 during 41 days of growth. The increase in rhizosphere phosphatase activity with increasing severity of P deficiency appeared to be a response to increasing root density and decreasing concentration of soluble inorganic P in the soil. No significant change in levels of soil organic P was detected. Plants that acidified their rhizosphere (low P status) depleted acid-soluble forms of soil P and absorbed twice the amount of P which could be desorbed from the control soil in 10-· M Ca(NOa). at pH 6'1. Uptake of P by plants which did not acidify their rhizosphere (high P status) was never greater than the amount of P desorbable in 10-· M Ca(NOa). at pH 6'1, and was derived from alkali-soluble and resin-extractable forms of soil P. Cation uptake exceeded anion uptake in both low and high P plants, but the difference was greater in low P plants. These differences in cation and anion uptake (mEq) were approximately equal to the milliequivalents of H+ (OH-) required to produce the observed changes in rhizosphere pH. These results explain why models for P uptake based on in vitro measurements of physicochemical parameters governing phosphate diffusion in soil work well for P sufficient plants, but tend to underestimate P uptake by P deficient plants growing in soils in which soluble P levels are sensitive to pH change.

Plant-induced changes in the rhizosphere of rape (Brassica napus var. Emerald) seedlings. IV: The effect of rhizosphere phosphorus status on the pH, phosphatase activity and depletion of soil phosphorus fractions in the rhizosphere and on the cation-anion balance in the plants

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