Designing an Oscillating CO2 Concentration Experiment for Field Chambers.

Questions have arisen about photosynthetic response to fluctuating carbon dioxide (CO2), which might affect yield in free-air CO2 enrichment (FACE) systems and in open top chambers.  A few studies have been conducted based on CO2 controlled to cycles of fixed time-periods and fixed, large amplitude.  In wheat leaves, electron transport through Photosystem II did not change with square-wave alternating 350-800 (mean 575) ppm CO2 exposure periods of 0.2 to 8 sec, but decreased linearly with periods up to 120 sec, and constant to a 240-sec period.   In another study, when exposed to constant CO2 of 600 compared to 370 ppm, CO2 uptake rates in seedling leaves of teak and barrigon increased by 28 and 52%, respectively. However, when exposed to 420-770 ppm saw-tooth oscillations with a period of 40 sec, stimulation in CO2 uptake rates of the two species was less, 19 and 36%, respectively. Similar responses were observed in teak exposed to 80-sec periods of CO2 oscillations.  Even less enhancement in cotton and winter wheat whole-plant responses to CO2 cycling at 60-sec periods have been found in open top chambers.  However, there is a problem with relating these studies directly to plant responses in FACE studies.  Within FACE, CO2 fluctuations are chaotic showing variable ranges of both period and amplitude.  This presentation examines the challenges of designing oscillating CO2 concentration experiments for whole-plant field chambers and also leaf chambers.  Methods include “FACE-tracking” whereby actual FACE CO2 oscillations (historical or real-time) would be fed instantly to a mass flow controller supplying CO2 to a field chamber, and development of an algorithm that would feed a continuously varying set of fluctuating periods and amplitudes of CO2 concentrations to an injection controller. Small chambers could be used for determining photosynthetic responses to oscillating CO2 over a wide range of cycle time periods.