Abstract
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‰). Two simultaneous operating processes are a possible explanation for the observed systematic intra-plant δ11B variation: 1) B is fixed in cell walls in its tetrahedral form (borate), which preferentially incorporates the light B isotope and the remaining xylem sap gets enriched in the heavy B isotope and 2) certain transporter preferentially transport the trigonal 11B-enriched boric acid molecule and thereby the heavy 11B towards young plant compartments which were situated distal of the roots and typically high in the plant. Consequently, an enrichment of the heavy 11B isotope in the upper young plant parts located at the top of the plant could explain the observed isotope systematic. The identification and understanding of the processes generating systematic intra-plant δ11B variations will potentially enable the use of B isotope for plant metabolism studies.
Highlights
Stable isotope systems of major nutrients like oxygen or carbon have been successfully used to trace the provenance of plants and food products [1]
The boron (B) 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 [2]
Published δ11B values in crop plants and fruits range from about -12‰ to +40‰ [4,5,6] covering more than half of the natural B isotope variability
Summary
Stable isotope systems of major nutrients like oxygen or carbon have been successfully used to trace the provenance of plants and food products [1]. 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). B exists as uncharged trigonal boric acid B(OH) in acidic media or as tetrahedral borate ion B(OH)4- in alkaline media. It was already suggested that depending on the involvement of natural and/or anthropogenic boron sources site specific δ11B signatures occur in plants and food products [7]. Provenance studies using boron isotopes as a tracer might be hampered by a potential isotope fractionation at the plant-soil interface and/or in the plant itself
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