Stress is a major component of natural selection in soil ecosystems. Organisms can deal with different stress factors to different extents, which determines the limits of their ecological amplitudes (Roelofs et al. 2008). As the literature indicates, plants growing on metalliferous soils cannot prevent metal uptake, but only restrict it, and hence they accumulate metals in their tissues to varying degrees (Baker 1981). Functional genomic tools were used to reveal stress responses in different ecologically relevant soil organisms. Adaptation to stress factors appears to evolve through enhanced constitutive transcription of stress responsive genes in plants and animals (Roelofs et al. 2008). Three distinct categories of plant responses to metal(loid) levels have been defined: (i) metal hypotolerance, which is synonymous with hypersensitivity or sensitivity, and which describes mutants and transgenic plants that are more sensitive than their wild-type to one or several meta(loids) simultaneously; (ii) basal metal tolerance, which includes plant species or ecotypes that can regulate their distribution of metals at the cellular and whole-plant level in such a way as to survive and reproduce on non-metal-enriched soils or substrates; and (iii) metal hypertolerance, which is synonymous with metal resistant or metallicolous species or ecotypes, which can survive and reproduce on highly metal-enriched soils (Ernst and al. 2008). In reports of metal tolerance of plant species, populations and genotypes, the existence of such different degrees of metal tolerance is insufficiently recognized (Ernst 2006). Resistance to metals can be achieved by either of two strategies: avoidance, by which a plant is protected externally from the influences of the stress, and tolerance, by which a plant survives the effects of the internal stress. Hence, excluders, indicators and accumulators are frequently seen as plant survival strategies on metal-enriched soils (Baker 1987).
Tolerance is a phenomenon that involves the response of plants to external metals, and does not appear to be influenced by internal metals (Mcnair and al. 1999). In general, root growth responds more rapidly to metal exposure than shoot growth, and therefore the root tolerance index (TI = root growth in metal solution/ root growth in control solution) is frequently used as a measure of tolerance. However, short-term tests are insufficient for the evaluation of metal tolerances of a plant as a whole, because the survival of seedlings does not ensure the survival at later vegetative states and does not guarantee reproduction (Ernst and al. 1992; Ernst 2006). It has long been known that tolerance and hyperaccumulation are genetically independent characters (Macnair and al. 1999; Ernst 2006). Thus, metal tolerance as a plant trait should be considered before plant species can be used in any phytoremedial action.
We are frequently faced with multi-metal contaminations at specific sites, and therefore multi-metal tolerant plants are highly appreciated for phytoremedial efforts. Unfortunately, such plants are particularly rare. They are characterised by low growth rates and very restricted reproduction, as a consequence of the high metabolic costs of natural selection for the new beneficial alleles. They therefore produce less biomass compared to their non-metal-tolerant ancestors, which may also result from their adaptation to other environmental constraints, such as low nutrient and water supplies. An estimate of 20% integrated costs has been suggested relating to plant survival on metal-enriched soils that results in changes in the morphology and physiology among populations, independent of the genetics of metal tolerance (Brooks 2000; Raskin and Ensley 2000; Ernst 2006).
Metal accumulation of plants on metal-enriched soils combines two components: the intrinsic demand of the plant metabolism, and the impact of the external metal supply on metal uptake, with its consequences for storage in roots and for translocation from roots to shoots (Ernst 2006). Wide variations exist in the extent to which accumulated metals can be transported from the root system to the shoot. Therefore, transfer factors can be calculated (TF = shoot metal concentration/ root metal concentration), with a TF >1 being typical of an accumulator/ hyperaccumulator plant species; similarly, a TF <<1 is typical of an excluder species (Baker and al. 1994; Dahmani-Muller and al. 2000). Metal exclusion, however, cannot be regarded as an avoidance strategy at the whole plant level, since the uptake by the root has to meet the demand of a plant for its primary metabolic processes and its defence function (Ernst 2006). Efficient root-to-shoot translocation, on the other hand, is regarded as a key trait of hyperaccumulating plants as it is an important tolerance mechanism, which is potentially related to limited vacuolar sequestration of the roots (Xing and al. 2007).
Bioaccumulation factors (BAF = metal concentration in plant parts/ metal concentration in solution or soil) are used to allow for different metal concentrations in culture solutions/ soils (Baker and al. 1994). Some plants can accumulate exceptionally high concentrations of metals in their above-ground tissues. Thresholds for plant hyperaccumulation are set at 10,000 mg kg-1 for Zn and Mn, 1,000 mg kg-1 for Ni, Pb, Co, Cu and Se, and 100 mg kg-1 for Cd (Baker 1987; Brooks 2000; Raskin and Ensley 2000). Therefore, metal hyperaccumulating plants with particularly high BAFs are most appreciated for phytoextraction and phytomining (Brooks 2000; Raskin and Ensley 2000; McGrath and Zhao 2003; Audet and Charest 2007a). However, when testing the efficiency of phytoextraction, repeated studies are needed, along with a careful selection of sampling and metal analysis techniques, as the metals may be increasingly dissolved after their extraction due to equilibration processes (Keller and Hammer 2004). Some metal hyperaccumulating plant species can extract metals from the total metal pool, and therefore information on the bioavailable metal pools is not sufficient (Brooks 2000; Vogel-Mikus and al. 2005). Root-associated bacteria and fungi, including the mycorrhizal fungi, have been shown to contribute to biological weathering, in addition to physical disintegration and chemical decomposition of rocks (Hofland and al. 2004; Calvaruso and al. 2006). According to the element defence hypothesis, high BAFs, and therefore high metal concentrations in plants, can protect plants from different herbivores, which can also be beneficial for remediation activities. The selection of such plants, however, should be carefully considered in the framework of the environmental land use and the possible consequences of the intake of metals into the food chain, since human intake of metals generally has largely detrimental effects (Poschenreider and al. 2006; Jarup 2003).
As stress factors, metals cause a physiological strain on plants that is seen as reduced vigour, or in extreme cases, total inhibition of plant growth (Baker 1981). In an analysis designed to determine the relationships between Cd hyperaccumulation and plant vegetative and reproductive traits of natural Thlaspi caerulescens populations, it was shown that plants originating from populations with high Cd hyperaccumulation abilities had better growth, through developing more and bigger leaves, having taller stems, and producing more fruits and heavier seeds. The same study also demonstrated, that 75% of plant Cd concentrations can be accounted for by the concentrations in the soil, while all of the residual variance was covered by the concentrations of other metals in the plants. Hence, metal accumulation/ hyperaccumulation is also an important plant trait in metal-enriched soils that should be considered in phytoremedial actions (Basic and al. 2006).
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