| Summary: | Experimental systems to simulate future elevated CO<sub>2</sub> conditions in the field often have large, rapid fluctuations in CO<sub>2</sub>. To examine possible impacts of such fluctuations on photosynthesis, the intact leaves of the field-grown plants of five species were exposed to two-minute cycles of CO<sub>2</sub> between 400 and 800 μmol mol<sup>−1</sup>, lasting a total of 10 min, with photosynthesis, stomatal conductance and PSII fluorescence measured at the end of each half-cycle and also 10 min after the end of the cycling. Prior to the cyclic CO<sub>2</sub> treatments, the steady-state responses of leaf gas exchange and fluorescence to CO<sub>2</sub> were determined. In four of the five species, in which stomatal conductance decreased with increasing CO<sub>2</sub>, the cyclic CO<sub>2</sub> treatments reduced stomatal conductance. In those species, both photosynthesis and the photochemical efficiency of PSII were reduced at limiting internal CO<sub>2</sub> levels, but not at saturating CO<sub>2</sub>. In the fifth species, there was no change in stomatal conductance with CO<sub>2</sub> and no change in either photosynthesis or PSII efficiency at any CO<sub>2</sub> level with CO<sub>2</sub> cycling. It is concluded that in many, but not all, species, fluctuations in CO<sub>2</sub> may reduce photosynthesis at low CO<sub>2</sub>, partly by decreasing the photochemical efficiency of photosystem II as well as by decreasing stomatal conductance.
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