Thallium accumulation and distribution in Silene latifolia (Caryophyllaceae) grown in hydroponics

Abstract

Abstract Purpose Thallium (Tl) is one of the most toxic elements known and its contamination is an emerging environmental issue associated with base metal (zinc-lead) mining wastes. This study investigated the nature of Tl tolerance and accumulation in Silene latifolia, which has so far only been reported from field-collected samples. Methods Silene latifolia was grown in hydroponics at different Tl concentrations (0, 2.5, 5, 30 and 60 μM Tl). Elemental analysis with Inductively coupled plasma atomic emission spectroscopy (ICP-AES) and laboratory-based micro-X-ray fluorescence spectroscopy (μ-XRF) were used to determine Tl accumulation and distribution in hydrated organs and tissues. Results This study revealed unusually high Tl concentrations in the shoots of S. latifolia, reaching up to 35,700 μg Tl g−1 in young leaves. The species proved to have exceptionally high levels of Tl tolerance and had a positive growth response when exposed to Tl dose rates of up to 5 μM. Laboratory-based μXRF analysis revealed that Tl is localized mainly at the base of the midrib and in the veins of leaves. This distribution differs greatly from that in other known Tl hyperaccumulators. Conclusions Our findings show that S. latifolia is among the strongest known Tl hyperaccumulators in the world. The species has ostensibly evolved mechanisms to survive excessive concentrations of Tl accumulated in its leaves, whilst maintaining lower Tl concentrations in the roots. This trait is of fundamental importance for developing future phytoextraction technologies using this species to remediate Tl-contaminated mine wastes.