Root foraging and selenium uptake in the Australian hyperaccumulator Neptunia amplexicaulis and non‐accumulator Neptunia gracilis

Background and aims Neptunia amplexicaulis , endemic to Central Queensland (Australia), is one of the strongest selenium (Se) hyperaccumulators known globally, capable of accumulating up to 13 600 µg Se g − 1 in its leaves. This work aimed to elucidate root foraging in response to Se in N. amplexicaulis applied in two different chemical forms and concentrations compared to the sympatric non-accumulator N. gracilis . Methods Neptunia amplexicaulis and N. gracilis seeds were germinated and transplanted into rhizotrons filled with half control and half Se-dosed soils with low (5 µg Se g − 1 ) or high (30 µg Se g − 1 ) levels of Se in soluble (Na 2 SeO 4 ) or insoluble (CaSeO 3 ) form. After 3 weeks, the root density in the two areas of the rhizotrons was measured and plants were removed from the soil to determine biomass and for chemical analysis of Se and other elements. Results Major changes were observed in the low Se dosed side in Na 2 SeO 4 form, and in the high Se dosed side in CaSeO 3 form in N. amplexicaulis roots: a higher density, Se concentration, Se:S ratio, and a tendency to increase the biomass. In contrast, a reduction in the root density with 30 µg Se g − 1 in respose to the CaSeO 3 form was observed in N. gracilis . Conclusions Neptunia amplexicaulis preferentially foraged in Se soluble enriched soil, which may be beneficial for the plant given the increase in the root biomass at low Se dosed soil. In contrast, a reduction in the root density in N. gracilis indicated avoidance of soils enriched with high insoluble form of Se.