D6 Gas Exchange and Leaf Atmosphere

Topic

Various gas fluxes occur at the leaf-atmosphere interface between the leaf and the surrounding air, and each flux reveals a distinct aspect of leaf function: the carbon dioxide flux indicates the rates of photosynthesis and respiration; the water vapor flux indicates the rate of transpiration; the ozone flux reveals the amount of airborne pollutant entering the leaf; and the flux of volatile organic compounds reveals what the leaf is emitting into the atmosphere. A suite of measurement tools allows for the separate quantification of each of these fluxes crossing the leaf-atmosphere interface.

Carbon dioxide and water vapor fluxes are measured simultaneously using an infrared gas analyzer equipped with a leaf chamber; this device encloses a portion of the leaf in a sealed volume and records the amount of carbon dioxide entering or leaving the leaf, as well as the amount of water released into the enclosed air. With these two simultaneous measurements, the infrared gas analyzer with a leaf chamber enables an assessment of the leaf's photosynthetic status with high mechanistic resolution—distinguishing between the individual components of the photosynthetic process rather than merely measuring the net outcome.

In contrast, ozone flux is measured using specialized ozone detectors; this flux allows for the quantification of pollutant-induced damage caused as the gas enters through the leaf stomata: the greater the ozone flux crossing the leaf-atmosphere interface and entering the leaf, the greater the pollutant-induced damage expected within the leaf tissue. The flux of volatile organic compounds is measured using detectors that identify and quantify each compound individually—either via proton-transfer-reaction mass spectrometry, which detects the compounds in near real-time, or via gas chromatography-mass spectrometry, which separates and identifies each compound present in the flow in greater detail. These measurements characterize volatile organic compound emissions as a leaf's response to stress, given that the leaf emits different compounds in varying quantities depending on the specific type of stress it is experiencing.

In addition to fluxes crossing the leaf-atmosphere interface, open-flow chambers measure similar gas fluxes at the soil scale rather than the leaf scale; they enclose a portion of soil and record emissions of carbon dioxide, water vapor, or other gases into the surrounding atmosphere.

Collectively, the infrared gas analyzer with a leaf chamber, ozone detectors, volatile organic compound detectors, and open-flow chambers allow for the quantification—with high mechanistic resolution—of fluxes of carbon dioxide, water vapor, ozone, and volatile organic compounds across the leaf-atmosphere interface and the soil. Together, these tools diagnose the leaf's photosynthetic status, quantify damage caused by pollutants entering through that same leaf-atmosphere interface, and characterize the volatile organic compound emissions produced by the leaf in response to stress.

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