THE EFFECTS OF ULTRAVIOLET-IRRADIATION ON P680(-II CORE COMPLEXES MEASURED FOR INDIVIDUAL S-STATES AND DURING REPETITIVE CYCLING OF THE OXYGEN-EVOLVING COMPLEX() REDUCTION IN PS)

Flash-induced absorbance measurements at 830 nm on both nanosecond and microsecond timescales have been used to characterise the effect of u ltraviolet light on Photosystem II core particles. A combination of UV -A and UV-B, closely simulating the spectrum of sunlight below 350 nm, was found to have a primary effect on the donor side of P680. Repetit ive measurements indicated reductions in the nanosecond components of the absorbance decay with a concomitant appearance and increase in the amplitude of a component with a 10 mu s time constant attributed to s low reduction of P680(+) by Tyr(z) when the function of the oxygen evo lving complex is inhibited. Single-flash measurements show that the na nosecond components have amplitudes which vary with S-state. Increasin g UV irradiation inhibited the amplitude of these components without c hanging their S-state dependence. In addition, UV irradiation resulted in a reduction in the total amplitude, with no change in the proporti on of the 10 mu s contribution.