Leaf Gas Exchange and Photosystem II Fluorescence Responses to CO(2) Cycling

Experimental systems to simulate future elevated CO(2) conditions in the field often have large, rapid fluctuations in CO(2). 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(2) between 400 and 800 μmol mol(−1), 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(2) treatments, the steady-state responses of leaf gas exchange and fluorescence to CO(2) were determined. In four of the five species, in which stomatal conductance decreased with increasing CO(2), the cyclic CO(2) treatments reduced stomatal conductance. In those species, both photosynthesis and the photochemical efficiency of PSII were reduced at limiting internal CO(2) levels, but not at saturating CO(2). In the fifth species, there was no change in stomatal conductance with CO(2) and no change in either photosynthesis or PSII efficiency at any CO(2) level with CO(2) cycling. It is concluded that in many, but not all, species, fluctuations in CO(2) may reduce photosynthesis at low CO(2), partly by decreasing the photochemical efficiency of photosystem II as well as by decreasing stomatal conductance.