95
Miscanthus Production
so well. Only a few publications have evaluated changes in the phytoremediation parameters of the second-generation biomass crop Miscanthus when
it is growing on contaminated soil receiving different soil amendments
(Alasmary, 2020; Ameen et al., 2018; Kucharski et al., 2005; Mamirova et al.,
2020). See Chapter 3 for some specific examples.
5.5.1 Impact of Soil Amendments on the Phytoremediation
of Soil Contaminated by Organic Substances
Low molecular weight compounds. Plant roots secrete a wide range of chemical
compounds: multicarboxylic organic acids including aconitic, citric, malic,
malonic, oxalic, succinic, and tartaric acids; sugars and sugar conjugates;
amino acids and peptides; phenolics, some of which are allelopathic; and
diverse enzymes. Often the exudation results from complex interactions with
the root microbiome (Korenblum et al., 2020). Exudation is sometimes initiated by lack of nutrients, pollutant toxicity, or anoxia (Dakora & Phillips,
2002; Zeng et al., 2008). Root exudates thus serve to interact directly with contaminants, or indirectly by their influence on the microbiome. See Chapter 2
for more details.
In 1995, Hülster & Marschner proposed a hypothesis that root exudates
can bind with persistent organic pollutants in soil and form a more hydrophilic complex which can be more easily absorbed by roots and translocated to aboveground biomass. Campanella & Paul (2000) supported this
hypothesis, finding that Cucurbita pepo and melon (Cucumis melo) root exudates bind dioxins and furan molecules facilitating their translocation to
aboveground biomass. They suggested that at least some part of this mix
of carrier molecules was proteinaceous. The impact of organic acids, citrate,
and EDTA (ethylenediaminetetraacetic acid) on the p,p′-DDE (p,p’-dichlorodiphenyl-1,1-dichloroethene) uptake by Cucurbita pepo, Trifolium incarnatum, Brassica juncea, Vicia villosa, and Lolium multiflorum was investigated by
White et al. (2003) and White and Kottler (2002). They observed significant
increases in uptake of p,p′-DDE for Cucurbita pepo (succinic acid – 19%; tartaric acid – 27%; malic acid – 31%; malonic acid – 36%; oxalic acid – 45%;
citric acid – 58%; EDTA – 80%) and for Trifolium incarnatum, Brassica juncea,
and Vicia villosa (citrate – 39%). Citrate also chelated metals, altering their
bioavailability. More recently a 17 kDa protein of the major latex protein
class was identified in the xylem sap of C. pepo and shown by genetic means
to be correlated with enhanced translocation of broad classes of POPs (Inui
et al., 2013). So, it may be that there are multiple facilitators of their uptake
at multiple steps.
Surface-active compounds. Surfactants are chemical compounds that
decrease surface tension. Surfactants can reduce the hydrophobicity of
organic compounds, for example, nonionic surfactants decreased the hydrophobicity of polychlorinated biphenyls in a soil-water system (Park & Boyd,
1999). There are surfactants of chemical (Tweens, Polysorbate, Triton) and