ON THE MECHANISM OF PHOTOSYSTEM II DETERIORATION BY UV‐B IRRADIATION

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Abstract
The influence of UV‐B irradiation on photosystem II activities has been investigated using isolated photosystem II membrane fragments from spinach. It was found: (a) The average amount of DCIP reduced per flash declined drastically with increasing irradiation time in the absence of DPC but remained almost unaffected in its presence, (b) After UV‐B irradiation, the maximum amplitude of laser flash induced 830 nm absorption changes decreases only slightly; whereas the relaxation kinetics exhibit marked effects: the (JLS components dominate the decay at the expense of ns components. The γ.s kinetics already arise after illumination with a single flash of dark adapted samples, (c) The manganese content decreases only partly at irradiation times where the oxygen evolution capacity is almost completely lost, (d) The polypeptide pattern is hardly affected; the number of atrazine binding sites markedly decreases. Based on the results of this study, UV‐B irradiation is inferred to deteriorate primarily the function of water oxidation. The action spectrum of the UV‐B effect does not reveal a specific target molecule. It is assumed that structural changes of the D‐l/D‐2 polypeptide matrix are responsible for the modification by UV‐B irradiation of the capacity of water oxidation and atrazine binding.