Exudation Of Ligands Research Articles

Constraints to Synergistic Fe Mobilization from Calcareous Soil by a Phytosiderophore and a Reductant

Synergistic effects between ligand- and reductant-based Fe acquisition strategies can enhance the mobilization of Fe, but also of competing metals from soil. For phytosiderophores, this may alter the time and concentration window of Fe uptake during which plants can benefit from elevated Fe concentrations. We examined how the size of this window is affected by the ligand and reductant concentration and by non-simultaneous addition. To this end, a series of kinetic batch experiments was conducted with a calcareous clay soil to which the phytosiderophore 2′-deoxymugineic acid (DMA) and the reductant ascorbate were added at various concentrations, either simultaneously or with a one- or two-day lag time. Both simultaneous and non-simultaneous addition of the reductant and the phytosiderophore induced synergistic Fe mobilization. Furthermore, initial Fe mobilization rates increased with increasing reductant and phytosiderophore concentrations. However, the duration of the synergistic effect and the window of Fe uptake decreased with increasing reductant concentration due to enhanced competitive mobilization of other metals. Rate laws accurately describing synergistic mobilization of Fe and other metals from soil were parameterized. Synergistic Fe mobilization may be vital for the survival of plants and microorganisms in soils of low Fe availability. However, in order to optimally benefit from these synergistic effects, exudation of ligands and reductants in the rhizosphere need to be carefully matched.

Root-zone modeling of heavy metal uptake and leaching in the presence of organic ligands

We present a mechanistic model which describes root uptake and leaching of heavy metals in the plant root zone, accounting for solution- and surface-complexation, (kinetic) mineral dissolution, heavy metal diffusion towards the root, root uptake, root exudation, ligand degradation and convective-dispersive transport of the soluble species. The model was used to simulate the influence of EDTA addition on Cu transport and plant uptake and the effect of oxalate exudation by roots on Cu transport and bioavailability using parameter values from the literature. In the simulations we assumed that free Cu2+ is the bioavailable form. Under slightly acidic conditions (pH 6) the model predicted that EDTA stabilizes Cu while at a slightly alkaline pH (pH 7.5), EDTA mobilizes Cu. The addition of EDTA approximately halved the cumulative Cu uptake after 360 days at pH 4.5, and reduced the uptake by a factor of 100 and 1000 at pH 6 and 7.5, respectively. Although the total dissolved concentration was increased, plant uptake was reduced by the formation of bio-inavailable complexes. The exudation of oxalate resulted in a decrease of the Cu concentration breaking through below the root zone, due to sorption of Cu-oxalate. In the presence of dissolved organic carbon (DOC), the exudation of oxalate increased Cu leaching considerably at pH 6 and 7.5. In the absence of DOC, the exudation of oxalate reduced Cu uptake due to the formation and adsorption of Cu-oxalate on goethite surface sites. Exudation of oxalate in the presence of DOC resulted in a further decrease of Cu uptake. Oxalate gradually takes over from DOC in binding Cu due to simultaneous production of oxalate and leaching of DOC. The simulations show that addition or exudation of ligands does not necessarily increase the solubility, transport and bioavailability of metals. Depending on the conditions (mainly the pH), also reduced transport and uptake can be observed, either by formation of ternary surface complexes or reduction of free metal concentration. The model can be easily extended to include further processes.

Role of exudation of organic acids and phosphate in aluminum tolerance of four tropical woody species.

Responses of Melaleuca leucadendra (L.) L., Melaleuca cajuputi Powell, Acacia auriculiformis A. Cunn. ex Benth. and Eucalyptus camaldulensis Dehnh. to aluminum (Al) toxicity at low pH are poorly understood. We investigated effects of low pH and exudation of ligands by roots on Al tolerance of these species. Seedlings were grown hydroponically in nutrient solutions at pH 4.2 or 3.5 containing AlCl3 at concentrations ranging from 0 to 4 mM Al. The presence of 4 mM Al at pH 3.5 depressed growth in all species. Growth depression was greatest in E. camaldulensis, least in A. auriculiformis. In the low Al treatment (0.5 mM Al), roots of M. cajuputi tended to have the highest Al concentration among species, whereas in the 4 mM Al treatment, the highest Al concentration was found in roots of E. camaldulensis. Aluminum application enhanced root exudation of citrate in all species, with the enhancement in M. cajuputi, M. leucadendra and A. auriculiformis being similar and much greater than in E. camaldulensis. Exudation of oxalate and phenolic compounds was greater in E. camaldulensis than in the other species. The presence of Al enhanced phosphate exudation in all species, particularly in A. auriculiformis. Acacia auriculiformis was tolerant to low pH, probably because the presence of an unknown substance increased the pH. Application of 0.38 mM Al alleviated the toxicity of the pH 3.5 treatment in E. camaldulensis and M. cajuputi, whereas low pH alleviated Al toxicity in A. auriculiformis. We conclude that exudation of ligands such as citrate and phosphate only partly accounts for interspecific differences in Al tolerance among the tropical woody plants studied, whereas the reciprocal alleviation of Al toxicity and low pH differed considerably among the species.