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Organics-Metals-Salts Interactions in Food Safety and Environment Protection: Combined
Experimental and Modelling Approach

Final Report Summary - O-M-S (Organics-Metals-Salts Interactions in Food Safety and Environment Protection: CombinedExperimental and Modelling Approach)

The background of the OMS project
OMS project deals with the current degradation processes in the food-producing agro-systems:
i) metals contamination,
ii) salinization and
iii) organic matter decline.
Metal contamination of the agro-ecosystems is increasing rapidly, and food crops grown in soils containing excessive concentration of potentially toxic metals represent one of the main exposure routes for humans. Only a few metals are essential to plants in low concentration, but these metals can become phytoxic with increasing concentrations. Some metals have no essential bio-function, and some (such as Cd) are biotoxic and cancerogenic. Salinization of the ago-ecosystems is the main cause of salt stress, i.e. one of the widespread and the most harmful abiotic stresses in food production. Besides decreasing crop yields, certain forms of soil salinity may also facilitate a transfer of some toxic metals (e.g. Cd) from soils to crops, thereby enhancing metals entry into the human food chain.
Organic matter (OM), derived from residual bio-material, decomposed and altered by soil biota, is a key variable with multiple benefits in the soil-plant continuum. The OM decline is an indicator of compromised soil quality and is accompanied by a decrease in fertility and loss of structure, which together exacerbate soil degradation. OM is one of the most crucial soil variables affecting availability and phytoextraction of metals (essential and non-essential). OM substances may adsorb/complex metals and salt, thus lessening the salinity impacts and minimizing (in OM-enriched environments) or facilitating (in OM-depleted environments) the entry of metals into the human food chain.

The objectives of the OMS project
Metal contamination and increased salinity occur frequently and to a large extent in arid/semi-arid areas on predominantly light-textured soils low in organic matter. This project is aimed at characterizing the benefits of various soil organic compounds in protecting crops and surface and groundwater from potentially toxic trace metals in salt-affected, metal-contaminated soils used in food production. Specific forms of metals detected in the crop rhizosphere by i) chemical fractionation and ii) modelled by computational approaches will be related with metals visualized and quantified at the nano scale by state-of-the-art in situ microanalysis in plant tissues.

A description of the work performed and main results of the OMS project

OMS project resulted with many preliminary and six (6) principal experiments, performed in controlled conditions of the glasshouse and growth chamber, i.e. with over 3.050 processed samples of the plant material, soil and rhizosphere solution. Obtained results showed that applied humic acids (HA) had significant impact on uptake/accumulation of certain trace metal elements (Cd, Zn, Cu) in tested cultures. In study with ICP-MS technique was revealed that tested metal-sensitive 17-d old radish plant exposed to relatively week equimolar rhizosphere trace element contamination (2.2 microM of Cd and Zn) has a great potential for root absorption and uptake of toxic Cd and micronutrient Zn in a very short period of 24 h.
Furthermore, applied NanoSIMS analysis has been confirmed as one of the most powerful state of the art in situ technique for identifying sites of Zn and Cd localisation and their contrasting (non uniform) distribution in the section of fast growing apical root tissue. To our knowledge this is the first time shown in very detail localisation of parts-per-million concentrations of Cd and Zn isotopes with high cellular resolution in the root apex of metal non-tolerant plant species exposed to week (2.2 µM) and short (24 h) metals treatment. It was confirmed that, although with different physiological functions and implications, both elements have strong ability to be taken up rapidly, even under low presence in the radish rhizosphere (Zn/Cd concentrations corresponded to those in uncontaminated environmental conditions), and that their localisation was non uniform across the root apex. From the root edge (epidermis), which was detected as Cd-/Zn-enriched hotspot, across the root cortex layer and towards to the root centre (stele) localisation of booth elements followed the similar patterns, and was markedly decreased, suggesting that observed elements share the same or quite similar routes in crossing the rhizosphere-root interfaces as well as simplistic/apopastic barriers within the root tissue. These observations are in accordance with some previous studies, and confirm that the epidermal root cells represent a site of preferential phyto-accumulation of toxic Cd and essential trace metal Zn. Employed high resolution secondary ion mass spectrometry technique has sufficient spatial resolution and analytical sensitivity to enable the distribution of observed elements in root specimen to be mapped at the cellular level. However, localisation and imaging with improved resolution at subcellular level of the elements of interest (Cd, Zn) is still remaining one of the highly technically-challenged in cell physiology, especially of metal(oid)s sensitive plant species from the perspective of NanoSIMS approach.

The expected final results and their potential impact and use
So far, the OMS project resulted with several new project proposals/experiments, e.g. intrigued and attracted several environmental scientists to further investigate some of the soil organic matter-metal-salt interactions. For instance, Maria Jose Poblaciones Suarezxbarcena (Assist/Prof) from the Universidad de Extremadura, Spain, came at UWA to continue new research with strawberries based on our results with Cd and Zn. Also, at UZFA we are planning to conduct controlled experiments with highly contaminated drainage channel sediments and the soil organics, and their influence on the phyto-accumulation of heavy metals in crops. Furthermore, it is expected that proposed collaboration with industrial sector could resulted in development of new product in the form of i) soil conditioners, ii) components of complex mineral fertilisers or iii) by application of aqueous solutions (e.g. humic acids) with the aim of reducing phyto-availability and phyto-accumulation of potentially toxic trace elements into the food crops grown on metal-contaminated and salt-degraded soils. It could contribute to enhancing food crop production over millions of ha of metal contaminated areas not only in European countries but worldwide. Also, in long-term it will help Europe to retain competitiveness of their economic sectors, like agriculture and related food industries. Finally, already published, as well as forthcoming scientific publication, and acquired knowledge/skills as a consequence of performed project will further ensure European Union to retain one of preeminent positions in environmental and life sciences in the world.