Catherine Keller

Root development and heavy metal phytoextraction efficiency: comparison of different plant species in the field

Heavy metal phytoextraction is a soil remediation technique which implies the optimal use of plants to remove contamination from soil. Plants must thus be tolerant to heavy metals, adapted to soil and climate characteristics and able to take up large amounts of heavy metals. Their roots must also fit the spatial distribution of pollution. Their different root systems allow plants to adapt to their environment and be more or less efficient in element uptake. To assess the impact of the root system on phytoextraction efficiency in the field, we have studied the uptake and root systems (root length and root size) of various high biomass plants (Brassica juncea, Nicotiana tabacum, Zea mays and Salix viminalis) and one hyperaccumulator (Thlaspi caerulescens) grown in a Zn, Cu and Cd contaminated soil and compared them with total heavy metal distribution in the soil. Changes from year to year have been studied for an annual (Zea mays) and a perennial plant (Salix viminalis) to assess the impact of the climate on root systems and the evolution of efficiency with time and growth. In spite of a small biomass, T. caerulescens was the most efficient plant for Cd and Zn removal because of very high concentrations in the shoots. The second most efficient were plants combining high metal concentrations and high biomass (willows for Cd and Zn and tobacco for Cu and Cd). A large cumulative root density/aboveground biomass ratio (LA/B), together with a relative larger proportion of fine roots compared to other plants seemed to be additional favourable characteristics for increased heavy metal uptake by T. caerulescens. In general, for all plants correlations were found between L A/B and heavy metal concentrations in shoots (r=0.758***, r=0.594***, r=0.798*** (P<0.001) for Cd, Cu and Zn concentrations resp.). Differences between years were significant because of variations in climatic conditions for annual plants or because of growth for perennial plants. The plants exhibited also different root distributions along the soil profile: T. caerulescens had a shallow root system and was thus best suited for shallow contamination (0.2 m) whereas maize and willows were the most efficient in colonising the soil at depth and thus more applicable for deep contamination (0.7 m). In the field situation, no plant was able to fit the contamination properly due to heterogeneity in soil contamination. This points out to the importance and the difficulty of choosing plant species according to depth and heterogeneity of localisation of the pollution.

Phytoextraction capacity of trees growing on a metal contaminated soil

Phytoremediation is an innovative biological technique to reclaim land contaminated by heavy metals or organic pollutants. In the present work, we studied the ability of five woody species to extract heavy metal (copper, zinc or cadmium) from a polluted soil to their above-ground tissues. Metal content in leaves and twigs was determined. Salix and Betula transferred zinc and cadmium to leaves and twigs, but Alnus, Fraxinus and Sorbus excluded them from their above-ground tissues. None of the species considered transferred copper to the shoots.

Benefits of plant silicon for crops: a review

Since the beginning of the nineteenth century, silicon (Si) has been found in significant concentrations in plants. Despite the abundant literature which demonstrates its benefits in agriculture, Si is generally not considered as an essential element. The integration of Si in agricultural practices is, however, effective in a few countries. Silicon fertilization by natural silicates has the potential to mitigate environmental stresses and soil nutrient depletion and as a consequence is an alternative to the extensive use of phytosanitary and NPK fertilizers for maintaining sustainable agriculture. This review focuses on recent advances on the mechanisms of Si accumulation in plants and its behavior in soil. Seven among the ten most important crops are considered to be Si accumulators, with concentration of Si above 1% dry weight. New approaches using isotopes and genetics have highlighted the mechanisms of uptake and transfer of Si in planta. There is a general agreement on an uptake of dissolved silica as H4SiO4 and precipitation as amorphous silica particles (the so-called phytoliths), but the mechanism, either active or passive, is still a matter of debate. The benefits of Si are well demonstrated when plants are exposed to abiotic and biotic stresses. The defense mechanisms provided by Si are far from being understood, but evidences for ex planta and in planta processes are given indicating multiple combined effects rather than one single effect. Phytoliths that are located mainly in shoots of monocots return to the soil through litterfall if the plants are not harvested and contribute to the biogeochemical cycle of Si. According to recent progress made on the understanding of the biogeochemical cycle of Si and the weathering process of silicate minerals, phytoliths may significantly contribute to the resupply of Si to plants. We suggest that straw of crops, which contains large amounts of phytoliths, should be recycled in order to limit the depletion of soil bioavailable Si.

Hyperaccumulation of Cadmium and Zinc in Thlaspi caerulescens and Arabidopsis halleri at the Leaf Cellular Level

Vacuolar compartmentalization or cell wall binding in leaves could play a major role in hyperaccumulation of heavy metals. However, little is known about the physiology of intracellular cadmium (Cd) sequestration in plants. We investigated the role of the leaf cells in allocating metal in hyperaccumulating plants by measuring short-term 109Cd and 65Zn uptake in mesophyll protoplasts of Thlaspi caerulescens “Ganges” and Arabidopsis halleri, both hyperaccumulators of zinc (Zn) and Cd, and T. caerulescens “Prayon,” accumulating Cd at a lower degree. The effects of low temperature, several divalent cations, and pre-exposure of the plants to metals were investigated. There was no significant difference between the Michaelis-Menten kinetic constants of the three plants. It indicates that differences in metal uptake cannot be explained by different constitutive transport capacities at the leaf protoplast level and that plasma and vacuole membranes of mesophyll cells are not responsible for the differences observed in heavy metal allocation. This suggests the existence of regulation mechanisms before the plasma membrane of leaf mesophyll protoplasts. However, pre-exposure of the plants to Cd induced an increase in Cd accumulation in protoplasts of “Ganges,” whereas it decreased Cd accumulation in A. halleri protoplasts, indicating that Cd-permeable transport proteins are differentially regulated. The experiment with competitors has shown that probably more than one single transport system is carrying Cd in parallel into the cell and that in T. caerulescens “Prayon,” Cd could be transported by a Zn and Ca pathway, whereas in “Ganges,” Cd could be transported mainly by other pathways.

Effect of silicon on reducing cadmium toxicity in durum wheat (Triticum turgidum L. cv. Claudio W.) grown in a soil with aged contamination.

Agricultural soil contamination and subsequently crops still require alternative solutions to reduce associated environmental risks. The effects of silica application on alleviating cadmium (Cd) phytotoxicity in wheat plants were investigated in a 71-day pot experiment conducted with a historically contaminated agricultural soil. We used amorphous silica (ASi) that had been extracted from a diatomite mine for Si distribution at 0, 1, 10 and 15 ton ASi ha(-1). ASi applications increased plant biomass and plant Si concentrations, reduced the available Cd in the soil and the Cd translocation to shoots, while Cd was more efficiently sequestrated in roots. But ASi is limiting for Si uptake by plants. We conclude that significant plant-available Si in soil contributes to decreased Cd concentrations in wheat shoots and could be implemented in a general scheme aiming at controlling Cd concentrations in wheat.

Clay formation and Podzol development from postglacial moraines in Switzerland

The fine silt (2 5 mm) and fine clay (<0.1 mm) fractions from four acid soils developed from moraines of increasing age (80, 400, 3,000 and 6,500 years old) in Switzerland, were studied by X-ray diffraction and chemical analyses. The soil parent material is homogeneous at the four sites and the soils can be considered as forming a chronosequence of soil development leading to the formation of Podzols. Mineralogical evolution of silt-sized phyllosilicates and fine clay fractions follows different pathways according to their composition and the soil horizon in which they are located. Dioctahedral and trioctahedral minerals in the soil parent material were both weathered in the Bw and Bs horizons but the trioctahedral phase more strongly and faster than the dioctahedral one. Weathering products are mica-vermiculite mixed-layers, vermiculite and finally gibbsite and Fe oxy-hydroxides. Weathering of the trioctahedral fraction was faster in the eluvial A or E horizons than in the B horizons, being almost complete after 3,000 years of soil development. Appreciable weathering of the dioctahedral fraction occurs only in the eluvial horizons leading to the formation of mica-smectite mixed-layers and smectite. Although smectite has been reported in the E horizon of Podzols in different environments, the significant finding in this work is the presence of this mineral in soils developed from the same parent material. This supports the fact that smectite is the endproduct of mica alteration in strongly leached and acidified E horizons of Podzols. The rate of clay formation in soils from temperate areas is of great interest in order to address the problem of soil resilience. This rate is required to study the possible effects of acid deposition on soils, the elemental cycling and retention and loss of nutrients in forest ecosystems. The rate of weathering and the sorption properties of weathered products also strongly influence the chemistry of surface and subsurface waters. Studies of weathering and clay formation rates require specific conditions to minimize the effect of soil-forming factors other than time (Jenny, 1941, 1980). Materials with identical characteristics exposed to weathering within various timespans must be selected in small areas to ensure that they were submitted to the same climatic events. Numerous weathering rate studies have been performed on volcanic ash deposits, which are generally well dated (Lowe, 1986), but there are fewer studies on glacial materials (Protz et al., 1984; Ugolini, 1986). We studied clay formation in soils developed from moraine bodies of increasing age which were deposited in a small valley of the Mont-Blanc area (Val d’Arpette, Switzerland) by a glacier which progressively retreated during the postand late-

Cadmium minimization in wheat: A critical review

Cadmium (Cd) accumulation in wheat (Triticum aestivum L.) and its subsequent transfer to food chain is a major environmental issue worldwide. Understanding wheat response to Cd stress and its management for aiming to reduce Cd uptake and accumulation in wheat may help to improve wheat growth and grain quality. This paper reviewed the toxic effects, tolerance mechanisms, and management of Cd stress in wheat. It was concluded that Cd decreased germination, growth, mineral nutrients, photosynthesis and grain yield of wheat and plant response to Cd toxicity varies with cultivars, growth conditions and duration of stress applied. Cadmium caused oxidative stress and genotoxicity in wheat plants. Stimulation of antioxidant defense system, osmoregulation, ion homeostasis and over production of signalling molecules are important adaptive strategies of wheat under Cd stress. Exogenous application of plant growth regulators, inorganic amendments, proper fertilization, silicon, and organic, manures and biochar, amendments are commonly used for the reduction of Cd uptake in wheat. Selection of low Cd-accumulating wheat cultivars, crop rotation, soil type, and exogenous application of microbes are among the other agronomic practices successfully employed in reducing Cd uptake by wheat. These management practices could enhance wheat tolerance to Cd stress and reduce the transfer of Cd to the food chain. However, their long-term sustainability in reducing Cd uptake by wheat needs further assessment.

Soluble and particulate transfers of Cu, Cd, Al, Fe and some major elements in gravitational waters of a Podzol

Abstract The particles moving in suspension through the soil can carry heavy metals with them. We have studied over two years the movement of Cu, Cd, Zn, Al, Fe, Ca, K, Mg, Na and Mn in this way with small open tension-free lysimeters installed in a Podzosol (French classification). Soil solutions were filtered through 0.45 μm membranes and analysed for the composition of the soluble fraction. The membranes were digested to obtain the elemental composition of the particulate matter. The particulate matter ( > 0.45 μm) was substantial and accounted for up to 40% of the total elemental mass-balance for Cu, Cd, Zn, Al and Fe, depending on the horizon. Copper occurred in the particulate fraction of all the horizons. Cadmium and Zn showed a pattern of distribution in the particulate fraction similar to those of Al and Fe. They may thus be associated with these elements, and possibly the organic matter also, forming sesquioxide-organic complexes similar to those found in the solid phase of the soil. Potassium, Ca, Mg and Na were not found in the particulate matter. Thus calculations made with the soluble-only fraction underestimated the total fluxes. The largest particulate and soluble fluxes were found under the E and BPh horizons. This suggests that the BPh horizon looses matter (soluble and particulate) which migrates deeper into the soil profile. The smallest particulate and soluble fluxes were always measured under the illuvial (BPs) horizon. Thus most elements tend to be immobilised in this horizon. Variation in the metal concentration of the soluble fraction was found through the time period. This variation was reduced by the addition of the metal concentrations found in the particulate fraction.

Changes in Hg fractionation in soil induced by willow

This study investigated the effect of willow (Salix viminalis ×  S. schwerinii) on soil characteristics, including changes in Hg fractionation in the soil solid phase, and Hg accumulation and distribution in pot-grown plants cultivated for 32 and 76 days in aged Hg-contaminated soil (30 mg Hg kg−1DW). Changes in soil pH and organic carbon content as well as in Hg fractionation were monitored in both rhizospheric soil and in soil without plants. Mercury fractionation was performed by a 5-step sequential soil extraction procedure. Organic carbon content increased while pH decreased in the rhizospheric soil. Both chemically defined exchangeable Hg (0.1) and Hg bound to humic and fulvic acids (1.1) decreased in the rhizospheric soil, whereas plant accumulation of Hg increased with cultivation time. The sum of the decrease of these two soil Hg fractions after 76 days of cultivation was approximately equal to the amount of Hg accumulated in plants. Furthermore, the major Hg fractions (Hg bound to residual organic matter (53), sulphides (43), and the residual fraction (2.5)) remained stable. Neither whole plant accumulation of Hg from the soil, approximately 0.2 of total Hg in soil after 76 days cultivation, nor the fraction of total plant Hg in the shoots, which accounted for about 3 of the total plant Hg pool regardless of the cultivation time, were high. The overall results suggest that plants might be suitable for phytostabilization of aged Hg-contaminated soil, where root systems trap bioavailable Hg and help to control both leaching of Hg and re-entrainment of Hg-containing particulates from a contaminated site.

Terrestrial plant-Si and environmental changes

Abstract The importance of silica in terrestrial land plants has been recognized since the middle of the 19th century with applications in agronomy and palaeovegetation reconstruction. In this presentation, we will review the latest advances in our understanding of phytolith formation and present a few examples of applications in the field of global environmental changes.