Raman Spectroscopic Online Investigation of Respiratory Quotients in Pinus Sylvestris and Picea Abies during Drought and Shading

Drought and heat waves have been linked to forest mortality event across the globe. The underlying physiological processes are still not elucidated but both tree carbon and water relations have been identified as the driving forces. While studies on tree hydraulics are straightforward, studies on the tree carbon balance are not. For example, the use of different carbon compounds for maintenance respiration during drought cannot be assessed with measurements of carbon pools but requires real-time analyses of respiration stoichiometry. However, so far there were no technical solutions for such applications.

Here we introduce cavity-enhanced Raman spectrometry (CERS) for simultaneous real-time monitoring of O₂ and CO₂ and rapid and continuous quantification of dark respiration rates and the respiratory quotient (RQ), i.e. the ratio of CO₂ produced over O₂ consumed during respiration. This ratio indicates the proportions of different substrates (carbohydrates [COH], lipids, proteins) used during respiration and allows fundamental insights into tree physiology. CERS combines high temporal resolution with a high dynamic concentration range for all important gases, ranging from few ppm to 100 vol. % with a single measurement every few seconds. The respiration analysis of tree branches was performed in a closed chamber for two species of different drought tolerance, Pinus sylvestris and Picea abies. We applied not only drought but also a shading treatment because both cause reductions in carbon assimilation rates but have different effects on tree hydraulics.

Declines in RQ during shading in both species indicate a switch from pure COH metabolism to a mixture of COH, lipids and proteins. During drought such declines occurred only in the drought-tolerant pine but not in spruce and the underlying more dynamic carbon use strategy in pine may provide a physiological basis for its drought tolerance, more detailed investigation still pending.

Our study highlights the suitability of CERS for applications in plant ecophysiology. This technology can contribute to significant advances in research on the causes and mechanisms of tree and forest mortality.