Distinctions in heterotrophic and autotrophic-based metabolism as recorded in the hydrogen and carbon isotope ratios of normal alkanes.

Abstract

The hydrogen isotope values of n-alkanes (δ2Hn-alkane) reflect a plant's water source and water relations, while the carbon isotope values (δ13Cn-alkane) relate to a plant's carbon metabolism and response to environmental conditions. However, the isotopic dynamics of the transition from heterotrophic to autotrophic metabolism during foliar development on δ2Hn-alkane and δ13Cn-alkane remain unclear. Here, we monitored δ2Hn-alkane and δ13Cn-alkane across a growing season from Betula occidentalis, Populus angustifolia, and Acer negundo. In addition, we compiled δ2H values of atmospheric vapor, leaf water, xylem water, and stream water as well as δ13C values of bulk leaf tissue (δ13Cbulk). We found δ2Hn-alkane and δ13Cn-alkane varied with leaf development and indicated that the majority of wax development occurred during the initial growing season. The patterns in δ2Hn-alkane were broadly consistent between species and with previous studies; however, each species had a unique final δ2Hn-alkane value. The δ13Cbulk for all species demonstrated a characteristic 13C-enrichment during the initial growing season, followed by 13C-depletion, while δ13Cn-alkane did not exhibit a consistent trend between the species. These δ13C data suggested a decoupling of the isotopic inputs between n-alkanes and photosynthetic leaf tissue. When coupled with δ2Hn-alkane, these data suggested that the precursor compounds utilized in initial production of n-alkanes might be variable and possibly indicated that the stored precursors used for initial leaf tissue and wax production originated from different sources. Nonetheless, these data indicated that the isotopic signatures of n-alkanes relate to a mixture of precursors, but only during a distinct period of leaf ontogeny.