The influence of magnesium on the chlorophyll fluorescence yield of isolated chloroplasts

Abstract

Isolated chloroplasts of Euglena show the slow increase of fluorescence yield upon magnesium addition frequently reported for higher plant chloroplasts. 1. 1. This response does not appear to be associated with either the increased Photosystem II electron transport induced by magnesium ions, or the magnesium-induced changes in light scattering, as both processes have a higher magnesium requirement than the fluorescence response. 2. 2. The full magnesium effect was observed in the presence of sufficient dithionite to reduce the electron transport components, indicating that electron transport is not necessary. 3. 3. 3-(3,4-Dichlorophenyl)-1,1-dimethylurea induced a slow decline in fluorescence which was stimulated by similar concentrations of magnesium as are required to induce the maximal fluorescence stimulation. 4. 4. Both trypsin and glutaraldehyde treatment prevented the magnesium effect, indicating that protein structural changes are involved. 5. 5. Removal of coupling factor from spinach chloroplasts did not influence the magnesium response. 6. 6. Fluorescence induction kinetics indicate a close relationship between the pool of reduced primary electron acceptor, Q, and the fluorescence response to magnesium. Only when Q was substantially reduced, either chemically or photochemically, was the fluorescence effect evident. It is concluded that the increased fluorescence yield is not due to magnesium interfering with the “spillover” between Photosystem II and Photosystem I, but is probably caused by an increase in Photosystem II fluorescing units, which is brought about by magnesium-induced protein structural changes when the primary electron acceptor pool is reduced.