Boron Isotope Fractionation in Bell Pepper

Various plant compartments of a single bell pepper plant were studied to verify the variability of boron isotope composition in plants and to identify possible intra-plant isotope fractionation.

Boron mass fractions varied from 9.8 mg/kg in the fruits to 70.0 mg/kg in the leaves. Boron (B) isotope ratios reported as δ11B ranged from -11.0‰ to +16.0‰ (U ≤ 1.9‰, k=2) and showed a distinct trend to heavier δ11B values the higher the plant compartments were located in the plant.

A fractionation of Δ11Bleaf-roots = 27‰ existed in the studied bell pepper plant, which represents about about 1/3 of the overall natural boron isotope variation (ca. 80‰).

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Boron Isotope Fractionation in Bell Pepper

Stable isotope systems of major nutrients like oxygen or carbon have been successfully used to trace the provenance of plants and food products. The boron  isotope system is of great interest in plants because B was found to be an essential micronutrient in plants occurring predominantly in the cell walls and acts as a strengthening component.

Boron has two stable isotopes, 11B (~80% abundance) and 10B (~20% abundance) and its isotope amount ratio n(11B)/n(10B) is reported as delta (δ)-values (eqn. 1), referring to the Standard Reference Material (SRM®) 951 from the National Institute for Standards and Technology (NIST; Gaithersburg, USA).

In aqueous solutions, B exists as uncharged trigonal boric acid B(OH)3 in acidic media or as tetrahedral borate ion B(OH)4 - in alkaline media. The lower vibrational energy of the trigonal coordinated boric acid species causes a preferential incorporation of the heavier B isotope compared to the tetrahedral species  leading to boron isotope fractionation in nature.

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