On the possible site of inhibition of photosynthetic electron transport by ultraviolet-B (UV-B) radiation

In Amaranthus chloroplasts that are exposed to ultraviolet-B (UV-B) radiation, the electron flow from water to dichlorophenol indophenol (DCPIP) was inhibited, but the electron flow from reduced DCPIP to methyl viologen remains unaffected. Diphenylcarbazide was ineffective in restoring the activity of DCPIP Hill reaction in UV-B irradiated chloroplasts. Electron flow from water to ferricyanide or dichloro-dimethoxy- p -benzoquinone was inhibited to a degree similar to that of the DCPIP Hill reaction.
The rate of carotenoid photobleaching in the presence of carbonyl cyanide- m -chlorophenylhydrazone, an indicator of the photochemical reaction near the vicinity of reaction centre of photosystem II, was suppressed and paralleled with the inhibition of the DCPIP Hill reaction.
In the UV-B treated chloroplasts, the variable part of the fluorescence transient was diminished. Though the fluorescence yield was lowered by the UV-B radiation, addition of 3-(3,4-dichlorophenyl)-l, l-dimethylurea (DCMU) and/or sodium dithionite increased the emission markedly. With the increase in the dosage of UV-B irradiation, the time required to reach the steady state fluorescence level became longer in the absence of DCMU and shorter in the presence of DCMU. The kinetics of 520 nm absorbance change was markedly unaltered by the UV-B irradiation but its dark decay was prolonged. It is concluded that UV-B irradiation inactivates the photosystem II reaction centre.