Uranium accumulation and tolerance in Arabidopsis halleri under native versus hydroponic conditions

Abstract

Comparisons of uranium (U) accumulation and tolerance were conducted in terrestrial versus laboratory trials using an endemic, on a former U mining site growing Arabidopsis halleri. Sequential extractions of soil samples pointed out a correlation between the low bioavailability of the micronutrient iron and the uptake of the non-essential U. A prerequisite for the uptake of iron occurring mostly as Fe(III) oxides and hydroxides in this habitat is the reduction to Fe(II) at the root surface. This could be accompanied by a reduction of U because a similar reduction of U(VI) to U(IV) took place as it was shown by photoacoustic measurements. Arabidopsis plants growing in their native habitat accumulated ∼35 mg kg −1 (dry weight) U in roots and ∼17 mg kg −1 (dry weight) U in shoots. Concerning only the probable bioavailable U, the soil-to-plant transfer factor (TF) was around 1.2 for roots and 0.6 for shoots, respectively However, the uptake of hydroponically grown plants (laboratory trials) were 100-fold more in roots and 10-fold more in shoots compared with the native grown plants. This drastic increased U accumulation could be attributed to iron deficiency of hydroponically grown plants. To get more insights in mechanisms of U tolerance the tolerance index (TI) was calculated using root elongation measurements. In addition to this some impacts on basic photosynthetic traits, e.g. the chlorophyll content, were investigated. The chlorophyll a/ b ratio was around 7 in native grown plants whereas the ratio of hydroponically grown dropped during the growth cycle due to the above mentioned lack of iron. Fluorescence spectra of chlorophyll extracts from U containing leaves revealed an additional peak assignable to a flavonoid. Thus, the reported data are a further indication that a deficiency of an essential metal can facilitate the uptake of non-essential metal.