Two types of kinetics of membrane potential of water plant leaves illuminated by ultraviolet light

The effects of ultraviolet-B (UV-B) radiation on plasma membrane of water plant (Elodea canadensis, Vallisneria spiralis) cells were investigated by using microelectrode methods. A fast and reversible depolarization of membrane potential occurs initially during exposure of leaf cells to UV on a white light background, after which a slow phase of depolarization sets in. On action series, UV is pulsed for 15 s, with dark interval of 3 min, no monotonous response of systems on the UV excitation is observed. The action spectrum of the fast UV response lies in the interval of 300–330 nm and that of the slow phase—in the interval of 280–300 nm. The input impedance of membranes remains unchanged during the period of exposure. It is concluded that the H+-extruding complex of plant cell plasma membranes really consists of two types of interrelated electronic H+-pumps: an H+-pump of redox-active nature and the H+-pump of the H+-ATPase enzyme complex. Clearly, during the exposure of leaf cells to UV light, initially, the H+-pump of redox-active nature and then H+-ATPase are inhibited. It is proposed that the initial chromophore of UV-B light on plasma membrane can be one of the components of H+-pump of redox-active nature. It is probably the molecular of quinone.