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.

The electron transfer kinetics from Z to P680+ was analyzed as a function of temperature in the range of 248 < T< 295 K by measuring absorption changes induced at 830 nm by a laser flash train in dark adapted 0, evolving PS II membrane fragments from spinach. It was found: (i) that the kinetics of P680+ reduction and their dependence on the redox state S, of the catalytic site of water oxidation are only slightly affected by temperature within the physiological range of 270 < T< 295 K. (ii) In the dark relaxed state S, the electron transfer from Z to P680+ exhibits an activation energy of the order of 10 kJ/mol in 248 < T< 295 K. (iii) In the 2nd and subsequent flashes of the train the ability for a stable charge separation between P680+ and Qi markedly decreases below -10°C. This phenomenon is assumed to be due to a strong effect of temperature on the electron transfer from Qn to Qs. The results are briefly discussed in relation to possible effects of structural changes in the D-l/D-2 polypeptide complex on the reaction coordinate of electron transfer steps in PS II.

Photoinhibition was analyzed in O2-evolving and in Tris-treated PS II membrane fragments by measuring flash-induced absorption changes at 830 nm reflecting the transient P680(+) formation and oxygen evolution. Irradiation by visible light affects the PS II electron transfer at two different sites: a) photoinhibition of site I eliminates the capability to perform a 'stable' charge separation between P680(+) and QA (-) within the reaction center (RC) and b) photoinhibition of site II blocks the electron transfer from YZ to P680(+). The quantum yield of site I photoinhibition (2-3×10(-7) inhibited RC/quantum) is independent of the functional integrity of the water oxidizing system. In contrast, the quantum yield of photoinhibition at site II depends strongly on the oxygen evolution capacity. In O2-evolving samples, the quantum yield of site II photoinhibition is about 10(-7) inhibited RC/quantum. After selective elimination of the O2-evolving capacity by Tris-treatment, the quantum yield of photoinhibition at site II depends on the light intensity. At low intensity (<3 W/m(2)), the quantum yield is 10(-4) inhibited RC/quantum (about 1000 times higher than in oxygen evolving samples). Based on these results it is inferred that the dominating deleterious effect of photoinhibition cannot be ascribed to an unique target site or a single mechanism because it depends on different experimental conditions (e.g., light intensity) and the functional status of the PS II complex.

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The transients of normalized fluorescence yield induced by an actinic laser flash in dark adapted leaves of Arabidopsis thaliana plants were measured with new equipment, that was developed as part of this work and permits the covarage of a wide time domain of 8 decades from 100 ns to 10 s. The raw data obtained were processed and analyzed within the framework of the "3-quencher" model with Q(A) as photochemical and P680(+)(*) and (3)Car as nonphotochemical quenchers. Comparative measurements with hydroxylamine treated PS II membrane fragments from spinach revealed that the widely used "dogma"of virtually identical efficiency of photochemical (Q(A)) and nonphotochemical (P680(+)(*)) quenching has to be revised: the constant of the latter exceeds that of the former by a factor of about 2. As a consequence, the probability of recombination between P680(+)(*) and Q(A)(-) and its kinetics have to be explicitly taken into account for the interpretation of flash induced fluorescence yield transients. The analysis of the experimental data within this extended "3-quencher" model reveals that a fully consistent description is achieved for the data gathered from measurements with intact leaves from wild type plants excited with actinic laser flashes of different energies (number of photons per flash and unit area). On the basis of these results it is shown that, in dark adapted leaves excited with a single laser flash, P680(+)(*) is predominantly (about 80% of the total reaction) reduced by Y(Z) via nanosecond kinetics and Q(A)(-) reoxidation is dominated by a kinetics of about 150 mus that are ascribed to PS II complexes with the Q(B) site occupied by PQ. The excess of excited chlorophyll singlet states decays to a significant extent via the carotenoid "triplet valve"with transient population of (3)Car. The present data provide the basis for analyses of A. thaliana mutants with modified lipid content and composition. The results of these investigations are described in an accompanying report (Steffen, R., Kelly, A. A., Huyer, J., Dormann, P., and Renger, G. (2005) Investigations on the reaction pattern of photosystem II in leaves from Arabidopsis thaliana wild type plants and mutants with genetically modified lipid content, Biochemistry 44, 3134-3142).