The effect of redox potential on the kinetics of fluorescence induction in Photosystem II particles from Phormidium laminosum. Sigmoidicity, energy transfer and the slow phase

Analysis of fluorescence induction in thylakoids with the method of moments reveals two different active Photosystem II centres

There is presently a debate concerning the number of phases in fluorescence induction and on the identification of the several possible heterogeneities in PS II centres. However, the usual methods of analysis present numerical problems, including a lack of 'robustness' (robustness being defined as the ability to give the correct answer in the presence of distortions or artefacts). We present here the adaptation of the method of moments, which was developed for robustness, to the analysis of fluorescence induction. We were thus able to identify three phases in the fluorescence induction in the presence of DCMU. The slowest phase was attributed to the centres inactive in plastoquinone reduction by using duroquinone as electron acceptor. In order to compare fluorescence with and without DCMU, we introduced the 'rate of photochemistry', defined as the product of the area times the rate constant of an exponential. This quantity is invariant for a given centre no matter what the size of the electron acceptor pool is. The two fastest phases in the presence of DCMU were attributed to active centres because their rate of photochemistry was the same as that of the plastoquinone-reducing phases in the absence of DCMU. Because their reduction of plastoquinone showed different kinetics, these two types of active centres were either separated by more than 250 nm or were associated with discrete plastoquinone pools having restricted diffusion domains.

Studies on the limitations to photosynthesis in leaves of the atrazine-resistant mutant ofSenecio vulgaris L

In leaves of an atrazine-resistant mutant ofSenecio vulgaris the quantum efficiency of CO2 assimilation was reduced by 21% compared to the atrazine-susceptible wild type, and at a light level twice that required to saturate photosynthesis in the wild type the CO2 fixation rate in the mutant was decreased by 15%. In leaves at steady-state photosynthesis there was a measurable increase in the reduction state of the photosystem II (PSII) primary quinone acceptor,Q A. Although this would lead to a decreased rate of PSII electron transport and may thus explain the decrease in quantum efficiency, this cannot account for the fall in the maximum rate of CO2 fixation. The atrazine-resistant mutant showed an appreciably longer photosynthetic induction time which indicates an effect on carbon metabolism; however, the response of CO2-fixation rate to intercellular CO2 concentration revealed no differences in carboxylation efficiency. There were also no differences in the ability to perform a State 1-State 2 transition between the atrazine-resistant and susceptible biotypes and no difference in the profiles of phosphorylated thylakoid polypeptides. It is concluded that the alteration of the redox equilibrium between PSII quinone electron acceptors in the atrazine-resistant biotype limits appreciably the photosynthetic efficiency in non-saturating light. Additionally, there is a further, as yet unidentified, limitation which decreases photosynthesis in the resistant mutant under light-saturating conditions.

Studies on thylakoid phosphorylation and noncyclic electron transport

The effect of thylakoid phosphorylation on noncyclic electron transport in spinach chloroplasts was investigated by measuring both the reduction of nicotinamide adenine dinucleotide phosphate (NADP) and the steady-state redox level of the primary electron acceptor quinone of photosystem II (Q) during electron flow to NADP. These data are compared with the theoretical predictions for an electron transport model which relates both the redox levels of Q and the photosystem II optical cross section to the overall velocity of noncyclic electron flow. It is demonstrated that transfer of 15-20% of the photosystem II antenna to photosystem I may stimulate electron flow to NADP only if Q is less than 60-70% oxidized (this condition exists with our thylakoids, even at extremely low absorption fluxes, when the illumination is not specifically enriched in photosystem I absorbed wavelengths); in phosphorylated thylakoids the steady-state redox level Q is substantially shifted to a more oxidized one (measurements of this parameter using light of different wavelengths quantitatively support the idea that thylakoid phosphorylation leads to increased photosystem I and decreased photosystem II cross sections); thylakoid phosphorylation leads to stimulated noncyclic electron flow to NADP only when the increased photosystem I antenna is able to bring about large increases in the steady-state level of oxidized Q.

Heterogeneity in chloroplast photosystem II

Photosystem-two (PSII) in the chloroplasts of higher plants and green algae is not homogeneous. A review of PSII heterogeneity is presented and a model is proposed which is consistent with much of the data presented in the literature. It is proposed that the non-quinone electron acceptor of PSII is preferentially associated with the sub-population of PSII known as PSIIß.

Characterization of photosystem ii electron acceptors in Phormidium laminosum

Chlorophyll a fluorescence has been used to monitor the redox state of the primary electron acceptor of photosystem II (PS II) in the blue-green alga Phormidium laminosum during equilibrium titrations. The shape of induction curves measured in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) have been analyzed. The induction curves were very similar in unfractionated thylakoid membranes and PS II particles. In both, the fast (alpha) phase was sigmoidal, and was followed by a slow (beta) exponential tail. Thus, the structural organization and complexity of the particles (J. M. Bowes and P. Horton, Biochim, Biophys. Acta 680, 127-133 (1982), as indicated by the occurrence of energy transfer between alpha centers and presence of beta centers, must preexist in the membranes. Redox titration of the initial level of fluorescence indicated the presence of a single quencher QH in the unfractionated thylakoids, midpoint potential: Em7.0 approximately -35 mV (n = 1). Thus, the occurrence of a single acceptor is characteristic for P. laminosum and the absence of a low potential acceptor in PS II particles (J.M. Bowes, P. Horton, and D.S. Bendall, FEBS Lett. 135, 261-264 (1981] was not the result of its removal during their preparation. The midpoint potential of Q varied by -60 mV/pH unit in PS II particles and membrane fragments, with a pK at pH greater than 8.5 (particles) and at pH 7.5 (fragments). In PS II particles, DCMU raised the pK by approximately 0.5 pH units. It is argued that the pH dependence of Q is conferred by protonation of a protein which accompanies its reduction rather than protonation of the semiquinone Q X itself.

Photosynthetic adaptation of pea plants grown at different light intensities: State 1 - State 2 transitions and associated chlorophyll fluorescence changes

A study of pea plants grown at different light intensities has been made. Using a leaf oxygen electrode, it was shown that plants grown under low light intensities had lower saturated rates of photosynthesis than high-light-grown plants however, at low light intensities the photosynthetic rates were similar for both types of plants. State 1- State 2 transitions have been monitored with attached leaves using a modulated fluorescence technique. It is shown that peas grown under low light intensities (20 W m(-2)) had a faster State 1 to State 2 transition when compared with medium-(50 W m(-2)) and high-(70 W m(-2)) light-grown plants. Measurement of fast-fluorescence-induction curves in the absence of 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU) have shown that low-light plants are, when in State 1, more effective at using Photosystem-two (PSII) light to reduce their plastoquinone pool than high-light plants. Transition from State 1 to State 2 for all plants led to a decrease in the reduction level of the plastoquinone pool inidcating that the transition had increased electron flow through Photosystem one (PSI) relative to PSII. Analyses of fast fluorescence induction in the presence of DCMU indicate that low-light-grown plants have a higher PSII-α/PSII-β ratio than high-light-grown plants. Such a difference is in line with the increase in the PSII/PSI ratio of low-light plants and is reflected in their high chlorophyll b/chlorophyll a ratio and their larger appressed to non-appressed thylakoid-membrane areas. It is suggested that these two latter factors give rise to the faster State 1 - State 2 transitions in low-light plants.