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 (δ²H_n-alkane) reflect a plant's water source and water relations, while the carbon isotope values (δ¹³C_n-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 δ²H_n-alkane and δ¹³C_n-alkane remain unclear. Here, we monitored δ²H_n-alkane and δ¹³C_n-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 (δ¹³C_bulk). We found δ²H_n-alkane and δ¹³C_n-alkane varied with leaf development and indicated that the majority of wax development occurred during the initial growing season. The patterns in δ²H_n-alkane were broadly consistent between species and with previous studies; however, each species had a unique final δ²H_n-alkane value. The δ¹³C_bulk for all species demonstrated a characteristic ¹³C-enrichment during the initial growing season, followed by ¹³C-depletion, while δ¹³C_n-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 δ²H_n-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.

Keywords: Compound-specific isotope analysis; Cuticle; Ecophysiology; Leaf wax; Stable isotope.