Boron isotope variability related to boron speciation (change during uptake and transport) in bell pepper plants and SI traceable n(¹¹B)/n(¹⁰B) 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 ¹¹B isotope into the roots during low to high B concentrations while uptake of the light ¹⁰B isotope was promoted during B depletion, probably by active processes. A systematic enrichment of the heavy ¹¹B isotope in higher located plant parts was observed (average Δ¹¹B_leaf-roots = 20.3 ± 2.8‰ (1 SD)), possibly by a facilitated transport of the heavy ¹¹B 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.