title: Inhibition of photosynthetic electron transport by UV-A radiation targets the photosystem II complex.
abstract: We have studied the inhibition of photosynthetic electron transport by UV-A (320-400 nm) radiation in isolated spinach thylakoids. Measurements of Photosystem II (PSII) and Photosystem I activity by Clark-type oxygen electrode demonstrated that electron flow is impaired primarily in PSII. The site and mechanism of UV-A induced damage within PSII was assessed by flash-induced oxygen and thermoluminescence (TL) measurements. The flash pattern of oxygen evolution showed an increased amount of the S0 state in the dark, which indicate a direct effect of UV-A in the water-oxidizing complex. TL measurements revealed the UV-A induced loss of PSII centers in which charge recombination between the S2 state of the water oxidizing complex and the semireduced Q(A)- and Q(B)- quinone electron acceptors occur. Flash-induced oscillation of the B TL band, originating from the S2Q(B)- recombination, showed a decreased amplitude after the second flash relative to that after the first one, which is consistent with a decrease in the amount of Q(B)- relative to Q(B) in dark adapted samples. The efficiency of UV-A light in inhibiting PSII electron transport exceeds that of visible light 45-fold on the basis of equal energy and 60-fold on the basis of equal photon number, respectively. In conclusion, our data show that UV-A radiation is highly damaging for PSII, whose electron transport is affected both at the water oxidizing complex, and the binding site of the Q(B) quinone electron acceptor in a similar way to that caused by UV-B radiation.
authors.affiliation: Institute of Plant Biology, Biological Research Center, Szeged, Hungary.
authors.forename: E
authors.initials: E
authors.lastname: Turcsányi
authors.forename: I
authors.initials: I
authors.lastname: Vass
mesh_terms: D004579:Electron Transport / Q000528:radiation effects; D010788:Photosynthesis / Q000528:radiation effects*; D045322:Photosynthetic Reaction Center Complex Proteins / Q000037:antagonists & inhibitors* / Q000378:metabolism; D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex; D018724:Spinacia oleracea / Q000737:chemistry; D014466:Ultraviolet Rays / Q000009:adverse effects*
chemical_list: D045322:Photosynthetic Reaction Center Complex Proteins; D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex