Photosynthetic Measurements
Carbon dioxide exchange by a single flag leaf blade or by an ear was examined
by enclosing them in a Perspex chamber, 2 by 15·5 cm in cross-section, or glass
assimilation tube, 2·5 cm in diameter, respectively. The differential in CO2 concentration
of an air stream before and after passing over the plant organ was
determined with a Grubb Parsons infrared gas analyser (model SB2). Air flow rates
were such that the maximum difference in C02 concentration was no greater than
50 p.p.m. by volume. Typically a flow rate of 1·5 litres per minute was used for a
leaf area of 20 cm 2.
All photosynthetic measurements were carried out in an artificially lit (L.B.)
cabinet (Morse and Evans 1962) with air temperature controlled at 21°C. Light
intensity could be varied up to 4000 f.c. by altering the number of high intensity
fluorescent tubes and was measured by a flat EEL selenium photoelectric light-meter
with a X 10 neutral filter.
The CO2 concentration passing over the leaf was altered, in several experiments,
by the addition of either CO2-free air or CO2-enriched air (2% CO2 by volume in air)
to the normal air stream. The response of the gas analyser was checked from known
differentials of CO2 for each concentration of CO2 used.