Simultaneous exogenous application of trans-zeatin riboside and EDDS: A powerful strategy to improve heavy metals phytoextraction efficiency by Miscanthus x giganteus L

To improve Miscanthus x giganteus phytoextraction efficiency, plants were grown for 16 weeks on an artificially spiked contaminated soil containing (in mg kg^-1) 20 Cd, 300 Zn and 1000 Pb and treated with ethylenediamine-N,N'-disuccinic acid (EDDS; 1 g kg^-1) and/or sprayed with cytokinin trans-zeatin riboside (CK; 100 μM). EDDS treatment increased heavy-metals bioavailability (by 76 % for Pb, 67 % for Cd and 60 % for Zn) and their accumulation in roots, rhizome, stems and leaves. It however drastically reduced plant growth in relation to a decrease in net photosynthesis (due to both stomatal and non-stomatal causes) and to hastening of the senescence processes (in relation to an increase in cell membrane relative leakage ratio and protease activity and a decrease in cell viability). EDDS increased root and leaf glutathione concentration and phytochelatin content. In contrast, CK treatment increased plant growth on contaminated soil, contributed to photosynthesis maintenance through an increase in stomatal conductance, and inhibited the leaf senescence. The low heavy-metal bioavailability in the soils limited heavy metal accumulation in CK-treated plants. CK reduced glutathione and decreased phytochelatin synthesis comparatively to EDDS treatment. The combined treatment (EDDS + CK) clearly increased tolerance to accumulated heavy-metals and maximized the bioconcentration factor (from 2.73, 1.17 and 0.035 in untreated plants to 3.99, 2.17 and 0.093 for Cd, Zn and Pb, respectively), the phytoextraction potential and the translocation factor. Simultaneous application of EDDS to the soils and trans-zeatin riboside spraying on the leaves thus allows to optimally conciliate heavy metal removal by phytoextraction and biomass production for energetical purposes.