The wavelength dependence of UV-B photoinhibition of P680+ reduction in Photosystem II - a model for the plant UV-B photoinhibition action spectrum

Full text: We report the determination of the wavelength dependence of UV inhibition of the primary electron transfer processes in Photosystem II (PS II) of photosynthesis using a narrow linewidth tunable UV laser as the irradiating source and nanosecond transient absorption spectroscopy for evaluation of the extent of the UV damage to the sites involved in the primary electron transport processes. We have previously shown that the dominant UV damage mechanisms in PS II are (a) photodissociation of the manganese tetramer complex which forms the oxygen-evolving centre, and (b) photo damage of the primary donor Chl-aII (P680). The extent of damage at these sites can be determined directly by measuring the amplitudes of the 5 ns, 25 - 300 ns and 10 ms components of the P680+ reduction kinetics. Therefore, the dependence of the amplitudes of these decay components on the wavelength of the irradiating UV gives a measure of the action spectrum. The irradiation source used was an intracavity frequency-doubled tunable R6G dye laser and the nanosecond measurements of the P680+ reduction kinetics were made using equipment described previously. The shape of the action spectrum and the relative inhibition amplitudes over the range 285 - 315 nm agree well with previous broadband estimates. However, there is an enhanced inhibition of primary charge separation and of protein structural damage at wavelengths below 287 nm. Our UV action spectra should be reasonably representative of the whole plant system since the processes studied relate to the main UV damage sites in PS II, and PS II is the most UV-sensitive component in plants