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.

Biological weighting function for xanthophyll de‐epoxidation induced by ultraviolet radiation

“…We do not know if this occurs in Nannochloropsis under our experimental conditions, but it is possible that in a similar way, Z accumulation could be a response to a decrease in PSII activity because of direct UVR damage (Renger et al 1986, Trebst and Depka 1990, Turcsanyi and Vass 2000. In contrast, Pfü ndel et al (1992) argued that PSII inhibition by UVR could result in oxidation of electron carriers between photosystems, which reduces V availability, decreasing Z formation (Pfü ndel et al 1992, Siefermann andYamamoto 1975).…”

The vegetative arctic freshwater green alga Zygnema is insensitive to experimental UV exposure

“…Remarkably, a significantly lower DF=F 0 m in plants exposed to PAR supplemented with UV-A (PA) compared to plants exposed to the whole radiation spectra (PAB) and PAR-only treatment was observed. UV-A irradiation is reported to be damaging for PSII, affecting electron transport at the water-oxidizing complex and the binding site of the Q B quinine electron acceptor (Turcsá nyi and Vass, 2000). UV-A can also damage the catalytic manganese cluster of the wateroxidizing complex (Vass et al, 2002).…”

Effect of Exclusion of Solar UV radiation on Plants

“…He et al (1993) observed that decrease in the ratios of variable to maximum chlorophyll fluorescence yield and in the quantum yield of photosynthetic oxygen evolution by supplemental UV-B in pea and rice leaves. In contrast to UV-B, UV-A is not usually regarded as a potentially damaging stress factor, although Turcsanyi and Vass (2000) reported the UV-A induced impairment of photosystem II and concluded that UV-A radiation is highly damaging for PSII, and affected the electron transport at both 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. Hakala-Yatkin, Mäntysaari, Mattila, & Tyystjärvi (2010) studied the contribution of the UV part of sunlight in photoinhibition of PSII in leaves and also suggested that especially the UV-A part, are potentially highly important in photoinhibition of PSII by the means a UV-permeable or UV-blocking filters.…”