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Phytomining
as already pointed out by Robinson et al. (2003), bears a number of serious environmental risks, such as persistence time of the chelators in the environment,
leaching of mobilized toxic trace elements to ground water, or toxicity to plants
and their associated microbes being used for phytomining (Hunt et al., 2014;
Robinson et al., 2009; Saxena et al., 2020; Wang et al., 2020; Wilson-Corral et al.,
2011). For that reason, more “natural” approaches such as soil management
via conventional agricultural technologies have been initiated already among
the first field trials, e.g., management of pH and fertilization (both inorganic,
e.g., NPK, and organic, e.g., composted sewage sludge) for enhancing plant
growth and phytoaccumulation of Ni (Chaney et al., 2007; Kidd et al., 2018; Li
et al., 2003). Later on, co-cropping has been adopted for enhancing phytoextraction (Tang et al., 2012). Due to the low availability of target elements often limiting phytomining success (Heilmeier & Wiche, 2020; Sheoran et al., 2009), the
stimulation of biological activity in the soil, particularly the rhizosphere, seems
to be a most promising approach (Robinson et al., 2009). Apart from enhancing biomass production by Plant Growth Promoting Rhizobacteria, the exudation of metabolites such as organic acids and metal-chelating siderophores by
soil microorganisms may change speciation of trace elements and thus greatly
increase their solubility and bioavailability in the rhizosphere (Kidd et al., 2018;
Koptsik, 2014; Saxena et al., 2020; Wiche et al., 2017).
Plant-targeted approaches include both traditional breeding for combining agronomic traits relevant for successful phytoaccumulation (Hunt et al.,
2014; Li et al., 2003; Nkrumah et al., 2016; Robinson et al., 2009) or genetic
approaches (Koptsik, 2014; Li et al., 2003), particularly for metal transporters
(Hunt et al., 2014; Robinson et al., 2009). The most important criteria when
selecting plant species for phytomining are as follows (Hunt et al., 2014;
Koptsik, 2014; Li et al., 2003; Nkrumah et al., 2016; Saxena et al., 2020):
• easy cultivation as an agricultural crop, e.g., high rates of germination and establishment, easy propagation via seeds or cuttings
• adaptation to site climatic and edaphic conditions
• resistance to diseases and pests
• rapid growth
• high biomass yield
• extensive root system (deep, highly branched)
• high tolerance of elevated concentrations of (toxic) trace elements
and extreme soil properties (pH, salinity)
• high specificity for target element(s)
• high uptake and translocation of target element(s) from roots to shoot
• high accumulation potential for target element(s) in aboveground
plant parts
• potential for use as energy crop (burning, fermentation)