Abstract
Iron (Fe) is an essential plant nutrient usually present in the soil mainly in insoluble Fe (III) forms. Particularly in well aerated soils at neutral to alkaline pH, inorganic Fe forms are scarcely available, such that plants rely mainly on Fe uptake from Fe-organic matter complexes or Fe that has been solubilized by organic compounds released by roots and/or microorganisms. In response to Fe deficiency, plants can be classified in two categories. Strategy I which is typical of dicotyledonous and non-graminaceous monocotyledonous plants, characterised by an increase in ferric (Fe3+) reduction capacity, acidification of the rhizosphere, and the release of organic acid anions and phenolic compounds into the soil solution. Strategy II is confined to graminaceous plant species (cereals and grasses) and characterized by an enhanced release of non-proteinogenic amino acids called phytosiderophores (PS) of the mugineic acid family.
In this research barley (Hordeum vulgare L.), tomato (Lycopersicon esculentum L.) and white lupin (Lupinus albus L.) were grown hydroponically in an Fe free (–Fe) nutrient solution using a plant-based biotest (RHIZOtest). After the first symptoms of chlorosis in Fe-deficient (–Fe) planst, were put in close contact with two different calcareous agricultural soils and root exudates were extracted from the soils (1: 5, w: v) after 6 days. For the duration of the experiment, other–Fe and+ Fe plants were continued to grow hydroponically and root exudates were collected at the beginning and the end of the 6-day period. Root exudates were determined quali-quantitatively using HPLC (High-Performance Liquid Chromatography) and colorimetric analyses, in order to determine the presence of phytosiderophores, amino acids, organic acids and flavonoids. The efficiency of the mobilization and acquisition mechanisms were evaluated measuring the accumulation of nutrients in plant tissues via ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry) measurements after total acid digestion. Soil mineral modifications were assessed by XRD (X-Ray Diffraction) and SEM-EDX (Scanning Electron Microscopy coupled to Energy Dispersive X-ray spectroscopy). XRD quantitative analyses were carried out by using the Rietveld method and a highly crystalline corundum was used as internal standard.