Detection of oxygen-evolving Photosystem II centers inactive in plastoquinone reduction

Quite different estimates of the number of Photosystem II centers present in thylakoid membranes are obtained depending on the technique used in making the determination. By using brief saturating light flashes and measuring the electron transport per flash, we have obtained two values for the number of functional centers. When the electrons produced reduce the intersystem plastoquinone pool, there are about 1.7 mmol of active Photosystem II centers per mol chlorophyll, whereas there are at least 3 mmol of active centers per mol chlorophyll when certain halogenated benzoquinones are being reduced. There are also at least 3 mmol of terbutryn binding sites per mol of chlorophyll when this tightly binding herbicide is employed as a specific inhibitor of Photosystem II. Thus only about 60% of the membrane's total complement of Photosystem II centers are able to transfer electrons to Photosystem I at appreciable rates. Many functional assays requiring significant rates of turnover sample only this more active pool, whereas herbicide-binding studies and measurements of changes in the Photosystem II electron donor Z and electron acceptor Q_A performed by other investigators reveal, in addition, a large population of Photosystem II reaction centers that normally have negligible turnover numbers. However, these normally inactive centers readily transfer electrons to the halogenated benzoquinones and are then counted among the active centers. Therefore, it can be concluded that all of herbicide-binding sites represent centers with operative water-oxidizing reactions. It can also be concluded that there are few, if any, centers capable of binding more than a single herbicide molecule.