Upon illumination leaves showed both fast absorbance changes and a slow increase in their apparent absorbance (maximum at about 530 mμ) which appeared to be caused by a light-dependent shrinkage reaction of chloroplasts. The extent of shrinkage was strongly influenced by the quality and intensity of exciting light and by the presence or absence of electron acceptors. The following observations pertain to the control of electron flow within the electron transport chain of photosynthesis. 1. 1. In N2, shrinkage was promoted by either far-red light or low intensity red light which caused a mediated cyclic electron flow to occur in Photosystem I. Shrinkage was inhibited by illumination with high intensity red light. 2. 2. Addition of red to a beam of far-red light illuminating a leaf under N2 stimulated shrinkage if the intensities of both beams were sufficiently low and led to inhibition of shrinkage if the red beam was intense. A dark period was required to relieve inhibition. 3. 3. In CO2-free air, O2 was an electron acceptor and caused shrinkage at higher intensities of red but not at low intensity far-red light. Shrinkage promoted in N2 by far-red or low intensity red light was strongly suppressed by O2 which appeared to interrupt cyclic electron transfer by oxidizing an electron acceptor in the pathway between Photosystem I and NADP+. This view was supported by measurements of cytochrome f changes at 420 mμ. Action spectra indicated also that shrinkage in N2 was a Photosystem I reaction, while in the presence of O2 Photosystems I and II cooperated to produce shrinkage. 4. 4. While shrinkage was greatly stimulated by low concentrations of O2, when both photosystems were sufficiently excited, increasing O2 concentrations suppressed shrinkage increasingly in the investigated plant species possibly by inhibiting electron flow. 5. 5. The affinity of the shrinkage reaction for O2 was high. Half maximal stimulation or inhibition of shrinkage was obtained at an O2 concn. of about 1.4 μM in the tissue or 1200 ppm in the gas phase. Half maximal fluorescence quenching by O2 occurred at a similar concentration. The kinetics of shrinkage and of fluorescence during a change in the gas atmosphere from N2 to CO2-free air or vice versa were similar indicating that the effects of O2 on the redox state of the quencher and on the shrinkage were indirect and were both mediated by the same reaction of O2 with a component of the electron transport chain beyond Photosystem I. 6. 6. Shrinkage caused by electron flow to O2 in air or by the cyclic electron flow in N2 was effectively suppressed by CO2. However, CO2 relieved the inhibition of shrinkage caused by high intensity red light under N2 probably by stimulating electron flow. The affinity of the system for CO2 as judged by the ability of CO2 to act as a fluorescence quencher in N2 in blue light or as an inhibitor of shrinkage in far-red light was higher than that for O2. Half maximal response was obtained in leaves capable of high rates of photosynthesis at a CO2 concn. in the gas phase of 10-20 ppm. 7. 7. The results indicate that CO2 and O2 in vivo both act as electron acceptors of photosynthesis; O2 reduction probably supplies ATP in a pseudocyclic type of photo-phosphorylation. Cyclic electron transfer in the presence of O2 is unlikely to occur except under conditions where the reaction reducing O2 is saturated. The observations support the series model of photosynthesis and do not seem to fit into a model with separate and independent photoreactions. © 1969.
