Species and biosynthetic effects cause uncorrelated variation in oxygen and hydrogen isotope compositions of plant organic compounds

Strong variation in plant organic compound δ²H and δ¹⁸O values such as leaf waxes or cellulose among species is commonly observed; however, the extent to which this isotopic variation is driven by leaf water or biochemical isotope effects is relatively unknown. Therefore, we compared variation in leaf water and organic compound δ²H and δ¹⁸O values (cellulose - δ²H and δ¹⁸O, and n-alkanes – δ²H) across 192 species grown in a botanical garden to assess covariation of isotope values (1) between elements in a given compound, (2) between different (organic) compounds of a given element, and (3) across different growing seasons. Our results suggest that variation in leaf water δ²H values are likely not a strong driver for the observed variation in organic compound δ²H values across species, and that this may also be true for δ¹⁸O values. Furthermore, even though correlation between leaf water δ²H and δ¹⁸O values appears to be transferred to organic compounds, the explanatory power of this correlation is strongly diminished (R² < 0.04). This indicates that additional biochemical isotope fractionation leads to substantial variation in organic compound δ²H and possibly also δ¹⁸O values across species. Moreover, the low explanatory power of the correlation between cellulose and n-alkane δ²H values (R² = 0.06) suggests that the biochemical pathways associated with the different compounds are accompanied by different isotope effects. Lastly, cellulose δ²H and δ¹⁸O values appeared sensitive to environmental differences between growing seasons, while differences in model-predicted source water δ¹⁸O and δ²H values and also climate were negligible between years. By contrast, the species pattern in n-alkane δ²H values was highly conserved between the two years. This indicates that the environmental forcing effects on isotope values were not equal between compounds. Therefore, we conclude that variation in organic compound δ²H (and possibly also δ¹⁸O) values among species and growing seasons was more strongly driven by biochemical isotope fractionation rather than by isotope values of plant water. This should be considered in the application of organic compound δ²H and δ¹⁸O values to reconstruct past climate, where invariable biochemical isotope fractionation is often assumed. Alternatively, organic compound δ²H and δ¹⁸O values could be further developed into a tool to extract plant metabolic information.