The curious case of selenium hyperaccumulation in Coelospermum decipiens from the Cape York Peninsula (Queensland, Australia).

Background and aims: The tropical shrub Coelospermum decipiens (Rubiaceae) is an extreme selenium (Se) hyperaccumulator, reported to accumulate up to 1140 µg Se g-1 when found growing on soils with levels of Se below the limit of detection (i.e. <0.01 mg Se kg-1) leading to a bioconcentration factor of >100 000.

Methods: Coelospermum decipiens plants were sampled from different populations in far north Queensland and analysed for Se concentrations. Plant material was subjected to synchrotron X-ray fluorescence microscopy (XFM) and synchrotron X-ray absorption spectroscopy (XAS) investigations to gain insights into the elemental distribution and chemical speciation of Se.

Results: The foliar Se concentrations ranged from 100 to 1000 µg Se g-1, except for the seeds, which had up to 28 000 µg Se g-1. The soils from the Hope Vale area were locally Se-enriched up to 48 mg Se kg-1, but there was no relationship between soil and plant Se concentrations. Synchrotron XFM analysis revealed that Se was localized in the blade margin tissue of the younger leaves, whilst the XAS analysis determined that Se occurs as an organo-Se compound.

Conclusions: We report the occurrence of seleniferous soils in the Cape York Peninsula soils for the first time, which may partly explain the evolution of Se hyperaccumulation in C. decipiens. The extremely high concentrations of Se in the seeds is suggestive of a herbivory protection function. The capacity of this species to accumulate and hyperaccumulate Se from non-seleniferous soils is akin to that of other 'seed'-based accumulators, such as some members of the Lecythidaceae family.