Vorübergehende pH-Änderungen im umgebenden Medium intakter grüner Zellen bei Beleuchtungswechsel

When light is switched on or off the pH of the medium of green cells of a large number of various plant species investigated (Elodea, Zea mays, Amaranthus caudatus, Oenothera hookeri x albicans, Mnium undulatum, Scenedesmus obliquus) increases or decreases respectively. This effect is linked with the non-cyclic photosynthetic electron flow: 1. the effect is independent of the gross ion content of the external medium, 2. green plant cells deficient in photosystem II do not show the effect, 3. addition of 2 · 10-6 M DCMU to the medium has a similar effect as switching off the light. The pH-effect is inhibited by FCCP, it is abolished in N2, but a normal effect is observed in CO2-free air. The transient changes of H⊕-net flux upon change of illumination are discussed in relation to both the proton pump of the thylakoid membranes of chloroplasts and to CO2 fixation after the uptake of HCO3 ⊖ from the external solution coupled with the release of OH⊖ ions. The results are also compared to other signals of similar kinetics observed in experiments with intact plant cells, especially with light-triggered transients of membrane potential and ATP-level.