Phosphate solubilization by organic anion excretion from rice growing in aerobic soil: rates of excretion and decomposition, effects on rhizosphere pH and effects on phosphate solubility and uptake

Abstract

Rice (Oryza sativa) plants were grown with their roots sandwiched between thin layers of phosphorus-deficient soil from which they were separated by fine mesh, and root-induced changes in the soil affecting phosphate solubility were measured. The concentrations of low molecular weight organic anions in the thin layers, particularly citrate, increased in the presence of the plants. Apparent rates of citrate excretion from the roots, calculated from the quantities in the soil and rates of decomposition calculated with a first order rate constant measured independently, varied from 337-155 nmol g −" root f. wt h −" over the course of plant growth, equivalent to 2-3% of plant d. wt. Rates of excretion were similar for NH % + and NO $ − -fed plants. The soil pH decreased from its initial value by up to 0.6 units for the NH % + -fed plants and increased by up to 0.4 units for the NO $ − -fed ones. The contribution of organic anion excretion to the pH changes was small compared with that of the inorganic cation-anion balance in the plants. The extent to which the observed excretion of citrate and root-induced pH changes could account for the observed phosphate solubilization and uptake was assessed using a mathematical model. Previous work had shown that phosphate solubilization by rice in this soil could not be explained by enhanced phosphatase activity in the rhizosphere, and the roots were not infected with mycorrhizas. The model allows for the diffusion of the solubilizing agent (citrate or H + ) away from the roots, its decomposition by soil microbes (citrate only) ; its reaction with the soil in solubilizing phosphate and diffusion of the solubilized phosphate to the roots. The model contains no arbitrary assumptions and uses only independently measured parameter values. The agreement between the measured time course of phosphorus uptake and that predicted for solubilization by citrate was good. Root-induced acidification by NH % + -fed plants resulted in additional solubilization, the acidification enhancing the solubilizing effect of citrate. However, the final phosphorus uptake by NH % + -fed plants was no greater than that of NO $ − -fed plants, presumably because the acidification inhibited plant growth. The mechanism of solubilization by citrate involved formation of soluble metal-citrate chelates rather than displacement of phosphate from adsorption sites.Key words : citric acid, diffusion, rice (Oryza sativa), organic acid, phosphate, pH change, solubilization, rhizosphere. Certain plant species are particularly good at extracting phosphate from phosphorus (P)-deficient soils, and one of the main mechanisms behind this is thought to be the excretion of P-solubilizing organic anions from the roots. However only in a few extreme cases (Dinkelaker et al., 1989 ;Hoffland et al., 1989) have the amounts of organic anions *Author for correspondence (fax j63 2 845 0606 ; e-mail g.kirk!cgiar.org).excreted and their P-solubilizing effects been shown to quantitatively explain the amounts of P solu...

Organic phosphorus in Madagascan rice soils

“…This means they are relatively unavailable to most crops (Richardson et al, 2005) and there is no evidence to suggest that they are available to rice growing in aerobic soils. It is worth noting, however, that the secretion of organic anions from rice roots under aerobic conditions (Kirk et al, 1999;Huguenin-Elie et al, 2003) could release inositol phosphates from binding sites (Hens et al, 2003).…”

Stress Response Versus Stress Tolerance: A Transcriptome Analysis of Two Rice Lines Contrasting in Tolerance to Phosphorus Deficiency

“…The exudation of LMWOAs capable of releasing soilbound P is considered to be a highly effective way of increasing the substrate (Pi) availability for uptake (Kirk et al 1999;Lambers et al 2006T). P-deficiency-induced alterations in gene expression that would increase carbon supply for organic acid synthesis have been reported for rice (Wasaki et al 2003) and Arabidopsis (Morcuende et al 2007).…”

Quantification of oxalate ions and protons released by ectomycorrhizal fungi in rhizosphere soil

“…Plants, whether mycorrhizal or not, were grown in mini rhizoboxes similar to those described by Kirk et al [16], that are presented in Figure 1. They were made from 2 glass plates (100 ´ 200 ´ 2 mm) separated by a PVC band.…”

“…The growth of the trees relies on the root capacity to use natural soil resources such as mineral P. Among the factors governing P dissolution, rhizosphere pH modifications and organic anion release are thought to be the most important [9]. Medium acidification can dramatically increase the dissolution of apatites or natural phosphates in the plant rhizosphere [6,10,11,15,16,20,24,30]. In plants, nitrogen supply is a key factor for rhizosphere pH, as a supply of NH 4 + will result in acidification, whereas NO 3 -will result in alkalisation of the medium [21].…”