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

Viola tricolor subsp. macedonica and Viola arsenica are metallophytes from the abandoned Allchar mine site in the Republic of North Macedonia, an area extremely enriched in thallium and arsenic, with up to 5750 and 12 800 mg kg-1 in the soil, respectively. This study aimed to infer tolerance mechanisms evolved in the two Viola species by analysis of the tissue-level distribution of arsenic and thallium.

Seedlings of V. tricolor subsp. macedonica and V. arsenica were grown under different thallium and arsenic treatments in hydroponics. Synchrotron-based micro-X-ray fluorescence (µXRF) analysis was used to elucidate elemental distribution in hydrated plant organs and tissues.

Plants dosed with increasing concentrations of arsenic and thallium had higher accumulation of these elements, especially in the roots. In V. arsenica, thallium 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 thallium concentrations occured around the stomata.

Foliar accumulation of thallium is the main tolerance strategy in V. arsenica, whereas the limited translocation into the shoot and potentially excreting excess thallium through the stomata in V. tricolor subsp. macedonica appears to be an important mechanism for survival in the extremely toxic habitat at the Allchar site.