Bypass Humidity Control

Humidity Control for Parallel Measurements of CO2 Assimilation and 13C/12C isotope ratios

Measurements of ratios of the stable isotopes
carbon 13/carbon 12 (13C/12C) have long been established to gain information on the biochemical type of photosynthetic carbon acquisition. Also, conclusions on plant physiological parameters, like water use efficiency, can be derived from 13C/12C data. Usually, the 13C/12C ratio of plant matter is analyzed. Thus, the results yield insight into the long-term biochemistry of plants.

To understand the short-term response of photosynthesis to environmental variations, the effect of carbon assimilation on 13C/12C ratios of the air’s CO2 can be measured continuously together with CO2 gas exchange. To obtain meaningful 13C/12C data, photosynthetic CO2 uptake should consume the CO2 of the air markedly. This can be achieved by studying large leaf areas and keeping the net air flow through the leaf chamber low. Such conditions, however, increase air humidity and favor water condensation which interferes with 13C/12C measurements.


The problem of high humidity is overcome by a bypass humidity control, which removes water from air without affecting CO2. The unit is connected to a drier, three dew point mirrors and the 3010-GWK1 Gas Exchange Chamber which itself is coupled to a Portable Gas Exchange Fluorescence System GFS-3000 (see figure). In the present custom-made solution, the GFS-3000 keeps the CO2-concentration inside the GWK1 chamber constant. The complete range of VPD (vapor pressure deficit) can be provided to the leaf.

The bypass humidity control regulates a drying loop that compensates any humidity increase due to leaf transpiration (DPMin = DPMout). Leaf evaporation is calculated from the flow through the bypass and the humidity data gained from the highly precise dew point mirrors. The setup is targeted to establish the relationships between environmental factors (light, temperature, VPD, or CO2 concentration) and 13C/12C isotope ratio.