Conformational changes of chloroplasts induced by illumination of leaves in vivo.

Abstract 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 N 2 , 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 N 2 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 CO 2 -free air, O 2 was an electron acceptor and caused shrinkage at higher intensities of red but not at low intensity far-red light. Shrinkage promoted in N 2 by far-red or low intensity red light was strongly suppressed by O 2 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 N 2 was a Photosystem I reaction, while in the presence of O 2 Photosystems I and II cooperated to produce shrinkage. 4. 4. While shrinkage was greatly stimulated by low concentrations of O 2 , when both photosystems were sufficiently excited, increasing O 2 concentrations suppressed shrinkage increasingly in the investigated plant species possibly by inhibiting electron flow. 5. 5. The affinity of the shrinkage reaction for O 2 was high. Half maximal stimulation or inhibition of shrinkage was obtained at an O 2 concn. of about 1.4 μM in the tissue or 1200 ppm in the gas phase. Half maximal fluorescence quenching by O 2 occurred at a similar concentration. The kinetics of shrinkage and of fluorescence during a change in the gas atmosphere from N 2 to CO 2 -free air or vice versa were similar indicating that the effects of O 2 on the redox state of the quencher and on the shrinkage were indirect and were both mediated by the same reaction of O 2 with a component of the electron transport chain beyond Photosystem I. 6. 6. Shrinkage caused by electron flow to O 2 in air or by the cyclic electron flow in N 2 was effectively suppressed by CO 2 . However, CO 2 relieved the inhibition of shrinkage caused by high intensity red light under N 2 probably by stimulating electron flow. The affinity of the system for CO 2 as judged by the ability of CO 2 to act as a fluorescence quencher in N 2 in blue light or as an inhibitor of shrinkage in far-red light was higher than that for O 2 . Half maximal response was obtained in leaves capable of high rates of photosynthesis at a CO 2 concn. in the gas phase of 10–20 ppm. 7. 7. The results indicate that CO 2 and O 2 in vivo both act as electron acceptors of photosynthesis; O 2 reduction probably supplies ATP in a pseudocyclic type of photo-phosphorylation. Cyclic electron transfer in the presence of O 2 is unlikely to occur except under conditions where the reaction reducing O 2 is saturated. The observations support the series model of photosynthesis and do not seem to fit into a model with separate and independent photoreactions.