Some effects of changes in gas phase on the steady-state chlorophyll a fluorescence exhibited by illuminated leaves

Excursions in chlorophyll fluorescence were obtained under continuous illumination by changing the composition of the atmosphere surrounding pieces of spinach leaf. Removal of CO₂ caused a transient rise and a subsequent fall to a new quasi-steady-state level. This response could be interrupted at any time by readmission of air, and similar but smaller excursions could be initiated by decreasing the CO₂ concentration. The effect was then proportional to the decrease in CO₂ whereas the steady-state value was related to the final CO₂ concentration. The excursions promoted by the decrease in CO₂ concentration were enhanced if O₂ was simultaneously decreased from 20 to 2%. Responses to changes in O₂
concentration in the absence of CO₂ , to increases in CO₂ concentration alone, and to pulses of CO₂ and O₂ in a N₂ atmosphere, are also described. The results are discussed in relation to changes in NADPH and ATP brought about by photosynthetic carbon assimilation and its regulatory mechanisms.