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Abstract:
Photosynthesis and respiration are major components of the plant carbon balance. During stress, like drought, carbohydrate supply from photosynthesis is reduced and the Krebs cycle respiration must be
fueled with other stored carbon compounds. However, the dynamics of storage use are still unknown.
The respiratory quotient (RQ, CO2 released per O2 consumed during respiration) is an excellent indicator
of the nature of the respiration substrate. In plant science, however, online RQ measurements have been
challenging or even impossible so far due to very small gas exchange fluxes during respiration. Here we
apply cavity-enhanced multi-gas Raman spectrometry (CERS) for online in situ RQ measurements in
drought-tolerant pine (Pinus sylvestris [L.]) and drought-intolerant spruce (Picea abies [L. H. Karst]). Two
different treatments, drought and shading, were applied to reduce photosynthesis and force dependency
on stored substrates. Changes in respiration rates and RQ values were continuously monitored over
periods of several days with low levels of variance. The results show that both species switched from
COH-dominated respiration (RQ = 1.0) to a mixture of substrates during shading (RQ = 0.77–0.81), while
during drought only pine did so (RQ = 0.75). The gas phase measurements were complemented by concentration
measurements of non-structural carbohydrates and lipids. These first results suggest a physiological
explanation for greater drought tolerance in pine. CERS was proven as powerful technique for
non-consumptive and precise real-time monitoring of respiration rates and respirational quotients for the
investigation of plant metabolism under drought stress conditions that are predicted to increase with future climate change.
