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

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.