Stimulation of photosynthesis by carbonyl compounds and chelators

A number of carbonyl compounds including bicarbonate, ethylene carbonate, dimethylcarbonate, propylene carbonate, bis-pentamethylene urea, and glycidol, and several chelators were tested for their effect on photosynthetic reactions in isolated spinach chloroplasts. It was found that carbonyl compounds inhibited the DCMU-insensitive silicomolybdate reduction by photosystem II but stimulated the O₂ evolution associated with ferricyanide reduction in presence of DBMIB and the H₂O→methylviologen reaction. Many chelators behaved in the same manner except 1,10-phenanthroline which shows the opposite effect. The carbonyl compounds did not uncouple because they stimulated the proton gradients associated with noncyclic photophosphorylation, whereas some chelators, such as bathocuproine or bathophenanthroline inhibited the proton gradients 100%. Electron transport in presence of ADP and inorganic phosphate showed a stimulation of rates beyond that obtained in presence of an uncoupler. The data are discussed in terms of inhibition of cyclic electron flow around PS II which leads to increased electron transport rates toward PS I.