Effects of N,N′-Dibutylurea on Photosynthetic Electron Transport Reactions in Isolated Chloroplasts

N,N′-dibutylurea, a secondary, accumulative degradation product of the fungicide benomyl, was investigated to determine its effects on the electron transport system in isolated chloroplasts. Dibutylurea was found to inhibit photosynthetic oxygen evolution, photo-induced reduction of ferricyanide, and reduction of NADP. It produced a sigmoidal dose response for inhibition of ferricyanide reduction with an I50of 167 μM. Other symmetric alkylureas were found to only weakly inhibit ferricyanide reduction regardless of the dose applied. Dibutylurea produced a dose response for electron transport inhibition when applied to KCN/Hg-inhibited chloroplasts and inhibited NADP reduction in the presence of 1 μM DBMIB and a TMPD bypass. These results suggest dibutylurea inhibits photosynthetic electron transport in a manner similar to known PSII inhibitors, with the possible site of action in PSII on the reducing side of the QBprotein complex.