Surviving Allchar: arsenic and thallium tolerance and distribution in Viola metallophytes

Background and Aims Viola tricolor subsp. macedonica and Viola arsenica are metallophytes from the abandoned Allchar Mine in the Republic of North Macedonia, an area extremely rich in thallium (Tl) and arsenic (As), with 5750 and 12 800 mg kg−1 in the soil, respectively. To infer these tolerance mechanisms, the distribution of As and Tl in two Viola species was analysed in this study.• Methods Seedlings of V. tricolor subsp. macedonica and V. arsenica were grown under different Tl and As treatments. Synchrotron micro-X-ray fluorescence analysis was performed to elucidate elemental distribution in hydrated plant organs and tissues.• Key Results Plant treatment with increasing concentrations of As and Tl promoted enrichment of these elements, especially in the roots. In V. arsenica, Tl was mainly accumulated in the shoots, with the mature leaves being the main site of deposition. In the leaves of V. tricolor subsp. macedonica, the highest Tl concentrations appeared to be distributed around the stomata.• Conclusions Foliar accumulation of Tl is the main tolerance strategy in V. arsenica, whereas the limited translocation into the shoot and release of the excess by transpiration in V. tricolor subsp. macedonica appears to be an important mechanism for survival in the extremely toxic habitat.