Oxygen per flash from leaf disks quantifies Photosystem II

Abstract

The quantity of functional Photosystem II (PS II) centres in vivo has been determined by the oxygen yield from leaf disks, exposed to 1% CO2 and repetitive flashes at 4 Hz (xenon flash lamp, 3 μs), and compared with the diuron-binding capacity of thylakoids isolated from the corresponding tissue of several herbaceous species. With continuous background far-red light present during the repetitive flashing of leaves, a good correlation was obtained between the two assays of PS II, the concentration of diuron-binding sites being only slightly higher. Omitting the background far-red light reduced the estimate of functional PS II centres by only about 10%, even when the PS II/PS I ratio was nearly 2. It is suggested that PS I is able to turn over more than once per flash, thus preventing a backlog of electrons in the plastoquinone pool. The oxygen yield per flash provides a convenient, direct assay of PS II in vivo when conditions are selected so as to avoid limitation by PS I.