Isotopes analysis deepens understanding of plants’ responses to a changing climate

Our recent study expands our fundamental knowledge of the underlying mechanisms driving hydrogen isotope ratios (‘δ²H’, representing the relative abundance of deuterium (²H) and protium (¹H) in a sample) in plant compounds, particularly for carbohydrates. This research advances our understanding of how δ²H reflect not only climatic conditions but also non-climatic information in the form of biophysical and biochemical functional responses such as leaf gas exchange and the concentration of plant assimilates such as sugars and starch.

We conducted two growth chamber experiments on tobacco plants (Nicotiana sylvestris), generating large variations in biophysical and biochemical traits. In the first experiment, tobacco plants were treated with differing regimes of nitrogen fertilization solutions that varied in their nitrate to ammonium ratio. Plants under nitrate-dominated conditions grew better and were healthier compared to those under ammonium-dominated conditions. 

In the second experiment, we explored the effects of starch deficiency in mutant tobacco plants compared to wild type plants. Advances in mass spectrometry enabled us to measure δ²H values of carbohydrates such as plant assimilates (e.g. sugars and starch) and cellulose, and to compare the results with those of lipids in the form of plant waxes or n-alkanes. 

Our findings suggest that biochemical and biophysical traits changing with the nitrogen form and starch deficiency treatments significantly impacted δ²H values in plant compounds, particularly for carbohydrates. Particularly strong relationships were found between δ²H values of plant assimilates and the concentrations of sugars and starch in leaves. 

Overall, our study highlights the importance of considering non-climatic factors when interpreting δ²H values in plant compounds, particularly for carbohydrates, but it also provides a better understanding of the driving sources of plant lipids. This knowledge can now be applied to the interpretation of δ²H values of carbohydrate and lipid proxies derived from natural long-term archives such as such as tree rings, ice cores, and sediments. 

Considering the non-climatic information in δ²H values in plant compounds can lead to more accurate reconstructions of past climatic conditions and provide valuable insights into how plants have responded to environmental changes over time. Our study is a good example that disentangling the mechanism and drivers of isotopic signals in plants is complex endeavour, but once achieved, it opens new avenues for research focusing on plant-climatic interactions.