Can Stress-Induced Biochemical Differences drive Variation in the Hydrogen Isotope Composition of Leaf Wax n-Alkanes from Terrestrial Higher Plants?

Abstract:

Recent research has identified that interspecies variation in leaf wax n-alkane ²H/¹H from plants growing at the same geographical location can exceed 100‰. These differences cannot easily be explained by mechanisms that influence the isotopic composition of leaf water. Biochemical processes are therefore likely to drive some of this variability. Currently, however, little is known about the relative importance of different biochemical processes in shaping n-alkane hydrogen isotope composition.

To explore this issue, we combined n-alkane δ²H analysis with measurements of: (i) the percentage content of leaf C and N; and (ii) foliar δ¹⁵N, from seven plants growing at Stiffkey salt marsh, Norfolk, UK. These species differ biochemically in respect of the protective compounds they produce under salt or water stressed conditions, with monocots generally producing more carbohydrates, and dicots producing more nitrogenous compounds. We found that monocots had higher %C, while dicots had higher %N and ¹⁵N-enriched leaf tissue. We identified a systematic relationship between the nature of the dominant protective compound produced (carbohydrate vs. nitrogenous) and n-alkane ²H/¹H: species with a greater proportion of carbohydrates have more negative δ²H values.

These findings might imply that shifts in the relative contribution of H to pyruvate from NADPH (²H-depleted) and recycled carbohydrates (²H-enriched) can influence n-alkane δ²H. The ²H-depletion of monocot n-alkanes relative to dicots may therefore be due to a greater proportion of NADPH-derived H incorporated into pyruvate because of their enhanced demand for carbohydrates. The production of protective compounds in plant species is a common response to a range of abiotic stresses (e.g. high UV irradiation, drought, salinity, high/low temperature). Species-specific biochemical responses to stress could therefore influence n-alkane ²H/¹H across a range of habitats. This study highlights the importance of detailed investigation of interrelated metabolic networks in a range of plants, to further constrain the isotope effects associated with the cycling of H in plant secondary compounds. Such research will be critical to further develop quantitative interpretations of leaf wax biomarker δ²H records in both modern and ancient contexts.