Phytoremediation is a green technique used to restore polluted sites through plant-initiated biochemical processes. Its effectiveness, however, depends on the successful establishment of plants in the contaminated soil. Soils that are contaminated with polycyclic aromatic hydrocarbons (PAHs), especially low molecular weight, mobile PAHs such as naphthalene pose a significant challenge to this. Plant roots growing in these soils exhibit changes to their structure, physiology and growth patterns.
Tall fescue (Festuca arundinacea) roots grown in sand contaminated with either petroleum crude oil (10.8g total extractable hydrocarbons kg-1 sand dw) or naphthalene (0.8g kg-1 sand dw) exhibited a temporary inhibition in elongation with accelerated lateral growth (p<0.01), whilst also showing a deviation from the normal root orientation responses to gravity. Scanning electron micrographs (SEM) revealed that the stele in the contaminated roots was located much further away from the root epidermis, because the cortex was larger (p<0.001) due to the cells being more isodiametric in shape. Once past the initial acclimatisation period of 2.5-3.0 months, no visual differences were observed between control and treated plants, but the root ultrastructural modifications persisted.
The fluorescent hydrophobic probe „Nile red‟ was applied to the epidermis of a living root to mimic and visualise the uptake of naphthalene into the root through the transpiration stream. The root sections were also stained with 0.1% (w/v) berberine hemisulphate in order to stain Casparian bands. Overlaying images obtained with the use of Texas red HYQ filter (wavelength 589-615nm) and UV illumination (wavelength 345-458nm) revealed the presence of passage cells in the endodermis and uptake of Nile red into protoxylem vessels beyond the endodermis of control roots. On the other hand, the path of Nile red was blocked at the endodermis of naphthalene- treated roots. The cell walls in the endodermis of naphthalene-treated roots were prominently thickened (p<0.001) and lacked passage cells. The treated roots also possessed a well-formed exodermis (p<0.01). The results suggest that the well-formed endodermis lacking passage cells, the well-formed exodermis as well as the increased cortex zone provided an effective barrier to the flux of hydrophobic xenobiotics towards the inner core of the roots, if previously exposed to the contaminants.
The SEM images of naphthalene-treated as well as crude oil-treated roots showed partial collapse in the cortex zone, presumably due to water stress, but the treated plants withstood drought stress better than the control plants.
The underlying physiological changes responsible for the adaptive responses of tall fescue to the exposure to naphthalene contamination were studied through metabolic profiling of plant roots and shoots. The results indicated synergistic interactions between sugars or sugar- like compounds and phenolic compounds may assist to create an integrated redox system and contribute to stress tolerance in naphthalene-treated tall fescue. The signal for a compound speculated to be indole acetic acid (IAA) was either subdued or absent in the tissues of naphthalene-treated tall fescue, suggesting the existence of a detoxification mechanism/ defence pathway in the treated plants. The ultra-structural and molecular modifications, resulting from PAH stress enabled tall fescue to resist tougher challenges
