engleski

The responses of salt-affected plants to cadmium

Salinisation and contamination by trace elements are expected to be some of the most critical environmental and sustainability issues in the forthcoming era of global warming and human population growth. Agro-ecosystems could be increasingly influenced by salinity, given that exploitation of saline pedo/hydroresources (>20 mM in water or soil saturation extract) will have to increase across many irrigated as well as rain-fed areas, despite poor yield and low foodstuff quality. Using experimental and computational (modelling) approaches it has been well established that one of crucial plant responses under salinity is increased Cd phytoextraction. In salt-affected rhizosphere environments excessive concentrations of dissolved ions can impact Cd biogeochemistry through complexation and/or competition reactions with inorganic and organic ligands. For ecologically relevant conditions (e.g. Cd-contaminated and organically-depleted salinised soil) it was estimated that under low salinity (<30 mM) free Cd2+ ion predominates across a wide range of pHs (3.5-9), whereas in moderate-to-strong salinity (135-270 mM) Cd-chloro/sulphate complexes prevail (mostly comprising Cd-Cls). Although Cd-Cl interactions are still under intensive investigation and not fully understood, complexation is likely to be one of the main mechanisms for increasing Cd transfer to plants from the salt-affected rhizosphere. Also, there is evidence that Cd uptake by plants is underpinned by additive effects of salt and Cd stresses. Great efforts have been made in elucidating Cd biochemistry after uptake, (re)translocation and its deposition in plants differing in salt/metal tolerance ; it is highly likely that the role of Cd-Cl complexation in plants is of negligible importance vs. that in the rhizosphere. Sharing similar or the same routes for crossing plant membranes with essential elements (Zn, Cu, Fe, Ca), Cd-organo complexes (with S, O and N radicals) relatively easily reach transpirational tissues via the xylem, and thereafter the depositing tissues dominantly via the phloem. Genetic engineering is a promising strategy in i) increasing plant resistance to excessive soil salinity, and ii) producing genotypes for enhanced phytoremediation of areas overloaded by metals. However, under ecologically-relevant conditions (e.g. rhizosphere soil with poor metal-buffering capacity) Cd soil-plant transfer can be enhanced by soil salinity, simultaneously interfering with nutrient (Cu, Zn, Fe) extraction. So, if salinity resistance (as a multi-gene trait) is closely associated with gene loci responsible for Cd extraction, care needs to be taken that genetically improved (e.g. salt-resistant) genotypes do not impair crops foodstuff safety/security via increased Cd accumulation and/or micronutrient deficiency.

salinity, metal contamination, Cd complexation, Cd soil-plant transfer

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