“Mining” the herbarium for hyperaccumulators: Discoveries of nickel and zinc (hyper)accumulation in the genus Noccaea (Brassicaceae) through X-ray fluorescence herbarium scanning

X-ray fluorescence spectroscopy (XRF) is a relatively new method for non-destructive elemental analysis of herbarium material that meets the scientific interest in being capable of discovering new hyperaccumulator plant species. Since the genus Noccaea (Brassicaceae family) is known to be one of the most numerous in term of the hyperaccumulator plant species it contains, especially those that hyperaccumulate Ni, the herbarium material available worldwide represents a great resource for expanding our knowledge of their elemental profiles. In this first systematic XRF scanning of herbarium specimens of the genus Noccaea, a total of 794 specimens from the collection of the National Museum of Natural History in Paris (MNHN) were analyzed, and the raw values obtained were corrected using regression formulas against inductively coupled plasma atomic emission spectroscopy data. Hyperaccumulation of Ni was detected in 90 specimens covering 21 taxa, with Ni concentrations reaching up to 48,700 mg kg⁻¹ in Noccaea cappadocica, an ultramaficophyte from Syria. Zinc concentrations above the hyperaccumulation threshold were found in 210 specimens covering 23 taxa, most of which belonged to different subspecies of Noccaea caerulescens, with the highest concentration reaching up to 56,200 mg kg⁻¹ in N. caerulescens subsp. caerulescens. Although the accumulation of Ni and Zn is contrasting in most of the specimens studied, in 10 specimens, predominantly belonging to N. caerulescens, simultaneous hyperaccumulation of Ni and Zn was found. This study also revealed previously unknown hyperaccumulation of Ni in several Noccaea taxa, as well as a simultaneous hyperaccumulation of Ni and Zn that needs to be confirmed by further experimental and field studies.