Impacts of Fluctuating CO2 On Leaf Gas Exchange and Plant Productivity.

The [CO₂] in FACE systems typically has pulses hundreds of μmol mol^-1 above the control set-point.  The importance of this fluctuation to plant function is unknown.  In our FACE system, peaks in [CO₂] variation had periods of about one minute. I used open-top chambers to expose cotton and wheat plants to either a constant elevated [CO₂] of 180 μmol mol^-1 above that of outside ambient air, or to the same mean [CO₂], but with the CO₂ enrichment cycling between about 30 and 330 μmol mol^-1 above the concentration of outside ambient air, with a period of one minute.  In both species, the fluctuating [CO₂] resulted in down-regulation of net photosynthesis (A) and stomatal conductance (g_s).  Vegetative cotton plants in the fluctuating [CO₂] treatment averaged 30% less shoot biomass than with constantly elevated [CO₂].  Wheat grain yields were 12% less in the fluctuating [CO₂] treatment than with constant elevated [CO₂].  In a second type of experiment, I exposed single leaves of wheat and rice plants to pulses of elevated [CO₂] similar to the frequency, duration, and magnitude of pulses observed in our and other FACE systems.  Leaves were exposed for 30 minutes to one pulse every 60 seconds, with pulse durations of 4, 8, or 12 s, and pulse magnitudes sufficient to increase the mean [CO₂] 45 μmol mol^-1 above the background concentration of 515 μmol mol^-1.  Both species had 20 to 40% lower g_s and 10 to 30% lower A for at least 30 minutes after the termination of the pulses, for all of the pulse durations tested.  Based on these results, it seems likely that plants in many FACE systems have lower A and g_s and slower growth than would plants exposed to the same mean carbon dioxide concentration but without pulses of higher concentration.