Leaf Gas Exchange and Photosystem II Fluorescence Responses to CO₂ Cycling

Experimental systems to simulate future elevated CO₂ conditions in the field often have large, rapid fluctuations in CO₂. 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 CO₂ 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 CO₂ treatments, the steady-state responses of leaf gas exchange and fluorescence to CO₂ were determined. In four of the five species, in which stomatal conductance decreased with increasing CO₂, the cyclic CO₂ treatments reduced stomatal conductance. In those species, both photosynthesis and the photochemical efficiency of PSII were reduced at limiting internal CO₂ levels, but not at saturating CO₂. In the fifth species, there was no change in stomatal conductance with CO₂ and no change in either photosynthesis or PSII efficiency at any CO₂ level with CO₂ cycling. It is concluded that in many, but not all, species, fluctuations in CO₂ may reduce photosynthesis at low CO₂, partly by decreasing the photochemical efficiency of photosystem II as well as by decreasing stomatal conductance.