Transient Phenomena in Leaves as Recorded by a 

 Gas Thermal Conductivity Meter 



WARREN L. BUTLER, University of Chicago {Pels Fund), Chicago, Illinois 



The work to be reported here deals with the measurement of gas 

 exchange rates and the determination of gas exchange quotients 

 during transient phenomena in photosynthesis. The method of gas 

 analysis introduced by Aufdemgarten and later used by Van der 

 Veen for the study of photosynthesis seemed well adapted to this 

 investigation. This method is based upon the change of heat con- 

 ductivity of a gas mixture which accompanies a change of gas com- 

 position. The advantage of this method is that the sensitivity to a 

 small change in concentration of a component is independent of the 

 concentration of that component already present. Thus photosyn- 

 thetic gas exchange can be determined in an atmosphere containing 

 4% COo and 20% O2 without loss of sensitivity due to these high back- 

 ground concentrations. In order to measure the O2 gas exchange, gas 

 mixtures containing He rather than No were used. In an atmosphere 

 of air, O2 changes are not manifest, since the heat conductivity of O2 

 is about the same as that of air. 



The gas mixture (generally 4% or 0.04% CO2 and 20% O2 in He) 

 passed over a leaf at a constant rate of flow and then through the 

 analyzing apparatus. Since a constant flow was maintained over the 

 leaf, the apparatus measured rates of gas exchange. The curve for 

 the rate of O2 exchange versus time was obtained by placing a CO2 

 trap in this gas line between the leaf and the analyzing chamber. 

 When this trap was bypassed, the combined effect of O2 and CO2 

 exchange was measured (called O2 + CO2 curve). Thus, in order to de- 

 termine the curve for the rate of CO2 exchange in the same experi- 

 mental atmosphere, two experiments had to be made under identical 

 conditions: one with and one without the CO2 trap. The difference 

 between the O2 and (h + CO2 curves gives the CO2 curve. A tyi^ica) 

 set of curves is shown in Fig. 1. 



The lag of about -HO seconds after the onset of illumination before 

 the photosynthetic gas exchange is recorded is due to the time re- 



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