Inhibition of Photosynthetic Electron Transport by UV-A Radiation Targets the Photosystem II Complex

• Published in: Photochemistry and photobiology 2000-10, Vol.72 (4), p.513-520
• Main Authors: Turcsányi, Enikö, Vass, Imre
• Format: Article
• Language: English
• Subjects: Electron Transport - radiation effects; Photosynthesis - radiation effects; PHOTOSYNTHESIS AND PHOTOCONVERSION; Photosynthetic Reaction Center Complex Proteins - antagonists & inhibitors; Photosynthetic Reaction Center Complex Proteins - metabolism; Photosystem I Protein Complex; Photosystem II Protein Complex; Spinacia oleracea - chemistry; Ultraviolet Rays - adverse effects
• ISSN: 0031-8655; 1751-1097
• DOI: 10.1562/0031-8655(2000)072<0513:IOPETB>2.0.CO;2

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 QA− and QB− quinone electron acceptors occur. Flash-induced oscillation of the B TL band, originating from the S2QB− 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 QB− relative to QB 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 QB quinone electron acceptor in a similar way to that caused by UV-B radiation.