Abstract
The soil–root interface is the micro-ecosystem where roots uptake metals. However, less than 10% of hyperaccumulators’ rhizosphere has been examined. The present study evaluated the root and shoot response to nickel in hyperaccumulator and non-hyperaccumulator species, through the analysis of root surface and biomass and the ecophysiological response of the related aboveground biomass. Ni-hyperaccumulators Alyssoides utriculata (L.) Medik. and Noccaea caerulescens (J. Presl and C. Presl) F.K. Mey. and non-hyperaccumulators Alyssum montanum L. and Thlaspi arvense L. were grown in pot on Ni-spiked soil (0–1000 mg Ni kg−1, total). Development of root surfaces was analysed with ImageJ; fresh and dry root biomass was determined. Photosynthetic efficiency was performed by analysing the fluorescence of chlorophyll a to estimate the plants’ physiological conditions at the end of the treatment. Hyperaccumulators did not show a Ni-dependent decrease in root surfaces and biomass (except Ni 1000 mg kg−1 for N. caerulescens). The non-hyperaccumulator A. montanum suffers metal stress which threatens plant development, while the excluder T. arvense exhibits a positive ecophysiological response to Ni. The analysis of the root system, as a component of the rhizosphere, help to clarify the response to soil nickel and plant development under metal stress for bioremediation purposes.
Highlights
The root system provides the structural element of the rhizospheric microenvironment [1] determining a plant’s access to soil-borne elements [2]
The red colour intensity of the leaves shown by the 1% colorimetric dimethylglyoxime test (DMG) test enhances with increasing Ni concentration, suggesting a nickel accumulation in the leaf epidermis for both the hyperaccumulator species starting from 100 mg kg−1 of Ni concentration in pot
The analysis in non-accumulator T. arvense (Figures 1D and 2D) show an increase in terms of root surface area and root dry weight (p < 0.01) at increasing Ni concentrations, which is less evident for the aboveground organs (p < 0.05)
Summary
The root system provides the structural element of the rhizospheric microenvironment [1] determining a plant’s access to soil-borne elements [2]. The root surface plays a significant role in element uptake through membrane transporters and in some hyperaccumulators the root grows towards trace elements in soil [2]. This metallophilic behaviour allows plants to mainly develop roots towards metal-rich patches [3]. The induction of root proliferation (i.e., root foraging) in response to Ni, Cd and Zn in soils were reported in few hyperaccumulators, like Noccaea caerulescens Presl) F.K. Mey., Thlaspi goesingense Halácsy, Sedum alfredii Hance and Streptanthus polygaloides Gray. A recent study shows that the polymetallic hyperaccumulator of Zn, Cd and Pb Noccaea rotundifolia (L.) Moench ssp. cepaeifolia has a larger root and shoot biomass in soils where there is a heterogeneous distribution of the metal in the growth substrate, with a prevalence of the avoidance strategy against metal-rich patches in the soil [9]
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