SUMMARYRape {Brassica napus L.) plants are efficient users of rock phosphates. This has been ascribed to the excretion of malic and citric acids from the roots during phosphorus deficiency, resulting in a decrease of the pH in the rhizosphere and a consequent solubilization of the rock phosphate. In this paper the biosynthesis and excretion of organic acids were investigated, partly by comparing physiological reactions to P deficiency of rape with those of an inefficient user of rock phosphate from the same family, hedge mustard (Sisymbrium officinale Scop.). In the leaves of both species, both citrate levels and phosphoenolypyruvate carboxylase were nearly doubled in P-deficient plants, and the citrate/sugar ratio in the phloem exudate into EDTA solution was doubled as well.After exposure of the shoots of rape plants to "COj in the light, the specific activities of citrate in the root and in the nutrient solution were ten times higher than those of malate, and also much higher than the respective activities of citrate in P-sufficient controls. It is suggested that citrate produced in the leaf is exported to the roots, where it is accumulated in the excretion region. Malic acid which is prevailing in the exudate is probably newly synthesized within the excreting root segment. When no phosphate is supplied, this excretion zone is located 1-2 cm behind the root tips. However, when rock phosphate is applied locally this zone shifts along the root to that part which is in direct contact with the rock phosphate particles. No accumulation and excretion of organic acids could be observed with the roots of P-deficient Sisymbrium.

Summary Rhizosphere processes strongly influence the availability of phosphorus (P) to plants. Organic ligands that are exuded from the root surface mobilize phosphorus by dissolution of P minerals or by desorption of adsorbed phosphate. We developed a mechanistic model to study the mobilization of phosphate sorbed on goethite by the exudation of citrate and consequent uptake of phosphate by the root. The use of a model allows the effects of the organic anion and pH on P desorption to be separated. The model is also used to predict concentration profiles developing around the root for phosphate, citrate (with or without accounting for degradation) and pH, providing insight into the processes that occur in the rhizosphere. Results of model calculations show that with larger rates of citrate exudation, greater P availability is predicted. Exudation at a rate of 0.5 μmol citrate m–1 root day–1, which is in the range found for P‐deficient plants, increased P availability almost 2‐fold at fairly large phosphate loading of goethite (1.9 μmol m–2) and almost 30‐fold at small phosphate loading (1.3 μmol m–2). Competitive adsorption causes a much greater relative increase in the phosphate concentration in solution at small than at large phosphate loading, which explains this result. Simultaneous acidification of the rhizosphere results in a smaller P mobilization than at a fixed pH of 5, as a result of the pH dependence of phosphate adsorption in the presence of citrate. Sorption of citrate increases its persistence against microbial decay, and hence has a positive effect on the mobilization of adsorbed phosphate.

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The effect of adding phytase to the root medium of maize plants on the P-availability of added myo-inositol hexaphosphate (phytin) has been studied in pot experiments. When 40 mM phytin-P in nutrient solution was incubated in quartz-sand for 15 days in the absence of plants, 80% of it could be recovered from the solution as soluble organic P. Maize plants growing on this mixture assimilated P from phytin at rates comparable to those from inorganic phosphate (Pi). At a lower addition rate (2 mM phytin-P) only 10% was recovered in the soil solution, and plant growth was severely limited by P. At this low phytin level, the addition of phytase (10 enzyme units per kg sand) increased the plants' dry weight yield by 32%. The relative increases of the Pi concentration in the solution and of the amount of P in the plants were even higher, indicating that the observed growth stimulation was due to an increased rate of phytin hydrolysis. The enzyme-induced growth stimulation was also observed with plants growing in pots filled with soil low in P, when phytin was added. However, on three different soils the addition rates of phytin and phytase necessary for obtaining a significant phytase effect were both about 10 times higher than those required in quartzsand. It is concluded that the P-availability from organic sources can be limited by the rate of their hydrolytic cleavage.Abbreviation: Pi -inorganic phosphate