Information about gas-exchange processes between vegetation and the atmosphere, including variation under different conditions, is important to solving problems of ecology, plant physiology, and plant growth. Gas-exchange processes are also key to raising crop yields and improving plant resistance to climatic and anthropogenic impacts.1 The significance of gas-exchange studies has grown in recent years because of global climate change.
Photosynthesis and plant respiration in the dark are the main components of the gas exchange cycle. The principal evolving gas in the process is CO2, which is also central to the greenhouse effect. In addition, recent data on methane emissions by different plants indicates that it is the second most important greenhouse gas.2 These estimates of methane emissions suggest that a reappraisal of atmospheric methane dynamics and their impact on plant biochemistry is necessary. To study the kinetics of gas exchange between vegetation and the atmosphere, we use two types of laser gas analyzers with frequency-tunable IR lasers.
Several methods can be used to measure small variations in the concentrations of gases such as CO2, C2H4, and CH4 during gas-exchange processes. Gas chromatography and mass spectrometry are the most popular.2 Our research shows, however, that laser absorptional spectroscopy, based on the use of frequency-tunable CO2 or diode lasers, is very efficient for studying the kinetics of gas emissions from plants.
We developed and applied two types of highly sensitive gas analyzers for studying CO2, C2H4, and CH4 emissions by plant tissue. The photoacoustic (PA) gas analyzer is based on the photoacoustic effect that results from the absorption of CO2 laser radiation by gases having absorption bands in the spectral range of 9.2–10.8µm. The gas analyzer is composed of a waveguide CO2 laser and a differential resonant Helmholtz photoacoustic detector.3 The detector was specially designed for continuous flow measurements of a gas probe. It provides real-time measurements of CO2 (0.4ppm) and C2H4 (0.03ppm) concentrations in the ambient air.
To study methane emissions, we used an improved gas analyzer designed at the General Physics Institute of the Russian Academy of Sciences. Located on the base of a near-IR diode laser (DL) and a multipass analytical cell,4,5 the detector is sensitive to CH4 concentrations as low as 0.04ppm. The mean ground concentration of CH4 in the atmosphere is ~2.00ppm.
We used the PA gas analyzer with the waveguide CO2 laser to study dark respiration of pine needles.3 The needles (20g) were separated from their branches and placed in a shadowed, sealed 5l chamber. After 3h, an air sample was taken from the chamber by blowing it through the PA detector. Figure 1 presents the absorption spectra of the air in the chamber prior to the experiment, the obtained gas sample, and a mixture of CO2 (5000ppm) and N2. Comparison of the spectra shows that needle dark respiration under the experimental conditions is accompanied by emission of CO2—10(20) line, 944.194cm-1—and ethylene, which has a characteristic absorption peak at the 10P(14) line, 949.479cm-1.
Figure 1. Spectra of gas absorption in the wavelength range of CO2 laser radiation (P branch of the 10µm band). 1: Room air. 2: Needle emission. 3: Testing mixture.
We used the methane detector with a 1.65µm diode laser to measure CH4 emission by various plants. Fresh green samples (leaves or needles) were placed in a 5l closed volume filled with room air (at a background methane concentration of 1.9ppm), where they were kept during the exposure time (10–300h). Before taking measurements, the analytical cell was blown through with pure nitrogen (for zero-level definition) and the control air. The incubation chamber was then blown through with room air and again with nitrogen. The variation of the methane concentration in the incubation chamber was estimated from the amplitude of the characteristic peak in the time dependence.
Experiments6 with Siberian stone pine needles, torn off at a temperature of -40°C, showed a clearly pronounced decrease in the methane mixture ratio (see Figure 2), as did those with leaves of the indoor plant Dieffenbachia. Further measurements with different samples and longer exposure times also produced decreases rather than increases in the methane mixture ratio when room air blew through the incubation chamber. The rates of methane emission we observed are significantly smaller than those found by other researchers.2 This may be connected to differences in the gas chromatography and absorption gas analysis methods.
Figure 2. Methane mixture ratio measured after blowing room air through a 5l volume filled with a green mass of 85g.
Absorptional laser spectroscopy and multi-wavelength gas analysis are efficient for studying the kinetics of gas exchange between vegetation and the atmosphere under natural and anthropogenic stresses.
The author thanks his permanent co-workers in the realization of this experimental program: B. G. Ageev, V. A. Kapitanov, A. I. Karapuzikov, and I. V. Sherstov.