Real-time monitoring of plant CO2 exchange using a direct absorption-based optical sensor

Understanding CO₂ plant exchange is essential for quantifying its role in the global carbon cycle, predicting ecosystem responses to environmental change, and evaluating long-term growth under varying environmental conditions across several types of photosynthesis^[1,2]. Plants exchange carbon dioxide with the atmosphere through three primary physiological processes: photosynthesis, which assimilates CO₂ during daylight to produce glucose and release O₂ as a byproduct; photorespiration, a light-dependent process that recycles harmful byproducts of photosynthesis while releasing excess energy and CO₂; mitochondrial respiration, which releases CO₂  and consume O₂ to produce energy, occurring both day and night. These CO₂ fluxes are coupled with transpiration that facilitates the loss of water vapor from leaves through stomata^[1]. These processes can be accurately quantified using gas-exchange techniques, in which a gas analyzer measures the exchange of CO₂ and H₂O between leaves and the atmosphere.
In this study, we employed a self-calibrated, optical sensor based on tunable diode laser spectroscopy to monitor plant CO₂ exchange in real time. The sensor consists of a quantum cascade laser emitting at 4.234 μm as the light source and a photodetector to measure CO₂ absorption along an open optical path of 10 cm. Measurements were performed using an amplitude modulation approach with first-harmonic detection at 10 kHz, employing a phase-sensitive lock-in amplifier. The optical sensor was placed inside a transparent plexiglass enclosure (525x375x300 mm³) containing a plant to monitor CO₂ exchange with the surrounding environment. A temperature and humidity sensor was also installed inside the enclosure, while a non-dispersive infrared CO₂ sensor (SEFRAM 9825) outside the enclosure was used to track ambient CO₂, temperature, and humidity. Continuous measurements were performed over approximately 20 days, covering both daytime and nighttime periods outside the laboratory, in a dedicated open area to minimize disturbances from nearby activity. Measured CO₂ concentrations inside the enclosure reflected both plant exchange and diffusive transport driven by the concentration gradient with the external environment. A differential equation model accounting for these processes was developed and applied to the experimental data to quantitatively determine the plant’s net CO₂ exchange rate.

References

• Niu, Z., Ye, Z. W. Y., Huang, Q., Peng, C. & Kang, H. Accuracy of photorespiration and mitochondrial respiration in the light fitted by CO2 response model for photosynthesis. Front. Plant Sci. 16, 1455533 (2025).
• Busch, F. A., Ainsworth, E. A., Amtmann, A., Cavanagh, A. P., Driever, S. M., et al. A guide to photosynthetic gas exchange measurements: Fundamental principles, best practice and potential pitfalls. Plant Cell Environ. 47, 3344–3364 (2024).