Carbon Dioxide-Induced Oscillations in Fluorescence and Photosynthesis: Role of Thylakoid Membrane Energization in Regulation of Photosystem II Activity

The response of CO 2 fixation to a sudden increase in ambient CO 2 concentration has been investigated in intact leaf tissue from spinach ( Spinacia oleracea ) using a dual channel infrared gas analyzer. Simultaneous with these measurements, changes in fluorescence emission associated with a weak, modulated measuring beam were recorded. Application of brief (2-3 seconds) dark intervals enabled estimation of the dark fluorescence level ( F o ) under both steady state and transient conditions. The degree of suppression of F o level fluorescence in the light was strongly correlated with nonphotochemical quenching under all conditions. During CO 2 -induced oscillations in photosynthesis under 2% O 2 the changes in nonphotochemical quenching anticipate changes in the rate of uptake of CO 2 . At such low levels of O 2 and constant illumination, changes in the relative quantum efficiency of open photosystem II units were estimated as the ratio of the rate of CO 2 uptake and the photochemical quenching coefficient. Under the same conditions the relative quantum efficiency of photosystem II was found to vary inversely with the degree of nonphotochemical quenching. The relationship between changes in the rate of CO 2 uptake: photochemical quenching coefficient and nonphotochemical quenching was altered somewhat when the same experiment was conducted under 20% O 2 . The results suggest that electron transport coupled to reduction of O 2 occurs to varying degrees with time during oscillations, especially when ambient O 2 concentrations are high.