EGU2020-19361
https://doi.org/10.5194/egusphere-egu2020-19361
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Drought does not affect hydrogen isotope fractionation during lipid biosynthesis by the tropical plant Pachira aquatica

S. Nemiah Ladd1,2, Daniel B. Nelson3, Ines Bamberger1, Erik Daber1, Ansgar Kahmen3, Carsten J. Schubert2, and Christiane Werner1
S. Nemiah Ladd et al.
  • 1University of Freiburg, Ecosystem Physiology, Freiburg, Germany (nemiah.ladd@cep.uni-freiburg.de)
  • 2Eawag, Department of Surface Waters -- Research and Management
  • 3University of Basel, Department of Environmental Sciences, Basel, Switzerland

Hydrogen isotope ratios (2H/1H) of plant waxes and other lipids preserved in sediments are increasingly used as a paleohydrologic proxy for past water isotopes. The relationship between precipitation 2H/1H ratios and those of plant waxes in surface sediments is linearly correlated at a global scale. However, there are large residuals in this relationship, and the offsets in 2H/1H ratios for the same compound produced by different species growing at the same site, as well as for different compounds produced within the same plant, can approach the magnitude of continental scale variability in precipitation isotopes. This indicates that lipid 2H/1H ratios are influenced by significant factors besides the 2H/1H ratios of local precipitation. One possibility is that plant metabolic responses to stresses such as drought cause changes in 2H/1H fractionation during lipid synthesis.

 

In order to assess the effects of drought on 2H/1H fractionation during plant lipid synthesis, we grew Pachira aquatica seedlings in controlled growth chamber conditions, with half of the individual plants experiencing drought conditions (soil moisture content reduced to ~10%) and half serving as well-watered controls (soil moisture content ~25%). We used position-specific 13C-pyruvate labeling to assess if there were changes in lipid production under drought, and focused on a diverse range of compounds including palmitic acid, n-C29 and n-C31-alkanes, phytol, squalene, and sitosterol. We also measured natural abundance 2H/1H ratios from the same compounds and from cryogenically extracted leaf water to quantify biosynthetic H isotope fractionation (εBio).

 

Biosynthetic 2H/1H fractionation spanned a 150‰ range among compounds, with palmitic acid being the least 2H-depleted compound (εBio = -140 ± 10‰) and phytol being the most 2H-depleted compound (εBio = -317 ± 7‰). These fractionation factors did not change under drought, although 13C-pyruvate labeling indicated that the compounds were being actively produced. There was no change in the production rate of any compound under drought, however. Differential incorporation of 13C depending on whether the 1st or 2nd carbon in pyruvate was labeled showed clear distinctions among compound classes, with the acetogenic compounds only becoming enriched from the C2 label, and isoprenoids using roughly equal proportions of carbon from each position. These results suggest that under this level of drought stress, Pachira aquatica did not make any changes to its lipid metabolism, and lipid 2H/1H ratios were therefore unperturbed. If replicated in additional plants types and under more severe drought, this result is encouraging for the use of plant lipid 2H/1H ratios as robust paleohydroclimate tracers.  

How to cite: Ladd, S. N., Nelson, D. B., Bamberger, I., Daber, E., Kahmen, A., Schubert, C. J., and Werner, C.: Drought does not affect hydrogen isotope fractionation during lipid biosynthesis by the tropical plant Pachira aquatica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19361, https://doi.org/10.5194/egusphere-egu2020-19361, 2020

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