Open-System chamber for measurements of gas exchanges at plant level

Gas exchanges of whole canopy can be studied bycovering entire plants with a chamber and using portableinfrared gas analyzers (IRGAs) to measure CO2 and H2Oexchanged with the air blown through the chamber enclosure.The control of temperature rise inside the chamber,which should be kept low, and the accurate measurementof the air flow are two crucial aspects for realistic andprecise estimation of photosynthesis and transpiration. Anautomated open-system plant chamber (clear flexibleballoon enclosure) for small plants was developed toameliorate such a technique. The temperature rise is herepredicted by heat balance analysis inside the chamber.The analysis shows that when as much as 500 W m2 of solarradiation is converted to sensible heat, a flow rate of0.98 mol s-1 (!20 L s-1) of air blown into a cylindershapedenclosure (0.8 m high, 0.5 m wide) is adequate tolimit temperature increase to 2 K. An improved calibrationfor the measurement of the chamber airflow was obtainedby combining the use of a Pitot tube anemometer withthe classical CO2 injection approach. The concentrationincrease due to the injection of CO2 at a known rate intothe chamber was predicted by the air flow calculated fromthe "Pitot" air velocity. The turbulent regime of airassured that a single-point Pitot measurement was enoughfor a good estimation (slope ) 0.99; R2) 0.999) of theactual air flow. The open-system chamber was tested onpotted sunflower (Helianthus annuus, L.) and maize(Zea mays, L.) plants under variable solar radiation,temperature, and air humidity during the daytime. Asexpected, similar rates of maximal leaf-area basedphotosynthesis (about 40 ?mol m-2 s-1) were observed inthe two species confirming the reliability of our system.The consistency of data also resulted from the typicalrelationships observed between photosynthetic rate andlight.