Aspalathus linearis(Rooibos tea) as potential phytoremediation agent: a review on tolerance mechanisms for aluminum uptake

Aspalathus linearis (Burm. F.) R. Dahlg., commonly referred to as Rooibos tea, grows naturally in nutrient-poor, sandy, acidic soils (pH 3–5.3) with high aluminum concentration ranging from 110 to 275 μg Al g−1in the Cederberg’s mountainous areas in South Africa. Earlier studies found significant differences in Al concentration in organs of A. linearis, with roots having higher amounts (1262–4078 μg Al g−1), suggesting that the plant is capable of accumulating excess Al in acidic soils. Identification of the mineralogical constituents of organs of A. linearis using X-ray diffraction (XRD) analysis revealed the presence of an Al–Si complex (aluminosilicate or hydroxyaluminosilicate (HAS) species) in the shoot and root, possibly to internally ameliorate Al toxicity. In addition, A. linearis has specialized cluster roots that exude Al-chelating organic acid ligands such as citric, malic, and malonic acids. Organic acids can bind strongly to Al in the plant and rhizosphere to reverse its phytotoxic effects to the plants. Field and glasshouse studies revealed significant differences in pH between rhizosphere and nonrhizosphere soils of A. linearis and also showed that roots of the plant release OH−and HCO3−anions to raise rhizosphere pH possibly to immobilize Al through complexation. Furthermore, A. linearis is easily infected by arbuscular mycorrhizae (AM) fungi, but mycorrhizal associations are known to inhibit transport of metallic cations into plant roots. These features of A. linearis are perceived as good indicators for bioremediation; and the plant could, therefore, be a suitable candidate for phytoremediation technologies such as phytoaccumulation, phytostabilization, and phytodegradation. The environmental and economic implications of the potential of A. linearis to bioremediate Al-contaminated soils are briefly discussed. Furthermore, this review briefly highlights future studies investigating the utilization of the shoot of A. linearis as adsorbent for the removal of trace and (or) heavy metal from aqueous solutions.