Boron isotope variability related to boron speciation (change during uptake and transport) in bell pepper plants and SI traceable n ( 11 B)/ n ( 10 B) ratios for plant reference materials

Rationale Boron (B) is an essential micronutrient in plants and its isotope variations are used to gain insights into plant metabolism, which is important for crop plant cultivation. B isotope variations were used to trace intra‐plant fractionation mechanisms in response to the B concentration in the irrigation water spanning the range from B depletion to toxic levels. Methods A fully validated analytical procedure based on multi‐collector inductively coupled plasma mass spectrometry (MC‐ICP‐MS), sample decomposition and B matrix separation was applied to study B isotope fractionation. The validation was accomplished by establishing a complete uncertainty budget and by applying reference materials, yielding expanded measurement uncertainties of 0.8‰ for pure boric acid solutions and ≤1.5‰ for processed samples. With this validated procedure SI traceable B isotope amount ratios were determined in plant reference materials for the first time. Results The B isotope compositions of irrigation water and bell pepper samples suggest passive diffusion of the heavy 11 B isotope into the roots during low to high B concentrations while uptake of the light 10 B isotope was promoted during B depletion, probably by active processes. A systematic enrichment of the heavy 11 B isotope in higher located plant parts was observed (average Δ 11 B leaf‐roots  = 20.3 ± 2.8‰ (1 SD)), possibly by a facilitated transport of the heavy 11 B isotope to growing meristems by B transporters. Conclusions The B isotopes can be used to identify plant metabolism in response to the B concentration in the irrigation water and during intra‐plant B transfer. The large B isotope fractionation within the plants demonstrates the importance of biological B cycling for the global B cycle.