Plant Exudates for Nutrient Uptake

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Plants require nutrients for unimpaired growth. Many plant strategies for acquiring nutrients from the soil involve root exudates that facilitate the detachment from the soil solid phase and the transport to the plant root. In this report, root exudation related to acquisition of nutrients other than nitrogen (N) and phosphate (P) has been considered. In this context, three important classes of root exudates can be identified: low molecular weight organic acids (LMWOA), phytosiderophores (PS) and reductants. The mechanisms by which these exudates can enhance bioavailability include ligand exchange, ligand-promoted dissolution, mineral dissolution by lowering solution saturation state through complexation, co-exudation of protons and chemical reduction. These mechanisms are not specific to a certain class of exudates, and a single class of exudates can be involved in multiple mechanisms. The efficiency of exudates in mobilizing nutrients from soil depends on the chemical affinity of the exudate ligand for the targeted nutrient element (the denticity of the exudate ligand plays an important role in this respect), the characteristics of the soil, and the susceptibility of the exudate to microbial degradation, adsorption and binding of non-targeted elements. A meta-analysis of available literature data on the response of root exudation levels by different crop species to the availability of specific nutrients was carried out. The relative change in root exudation level as a result of a decrease in the availability of specific nutrients was investigated. The responsiveness and the magnitude of these responses seem to be strongly plant species, cultivar and nutrient specific. Available data on exudation proved biased towards certain nutrients, specifically iron (Fe) and zinc (Zn), and comparisons between studies were often complicated due to differences in experimental approach. Furthermore, at present there are very few published data on exudation under actual rhizosphere conditions. Despite the shortage of data, the potential for utilizing root exudates for making better use of soil nutrient reserves and improving nutrient acquisition, e.g., in intercropping systems, looks promising and needs to be further explored.
D.H. Keuskamp, R. Kimber, P. Bindraban, C. Dimkpa and W.D.C. Schenkeveld, 2015. Plant Exudates for Nutrient Uptake. VFRC Report 2015/4. Virtual Fertilizer Research Center, Washington, D.C. 53 pp.; 10 tables; 18 figs.; 179 ref