Chlorophyll fluorescence in the leaves of Tradescantia species of different ecological groups: Induction events at different intensities of actinic light

Chlorophyll fluorescence analysis is one of the most convenient and widespread techniques used to monitor photosynthesis performance in plants. In this work, after a brief overview of the mechanisms of regulation of photosynthetic electron transport and protection of photosynthetic apparatus against photodamage, we describe results of our study of the effects of actinic light intensity on photosynthetic performance in Tradescantia species of different ecological groups. Using the chlorophyll fluorescence as a probe of photosynthetic activity, we have found that the shade-tolerant species Tradescantia fluminensis shows a higher sensitivity to short-term illumination (≤20 min) with low and moderate light (≤200 μE m⁻² s⁻¹) as compared with the light-resistant species Tradescantia sillamontana. In T. fluminensis, non-photochemical quenching of chlorophyll fluorescence (NPQ) and photosystem II operational efficiency (parameter Φ_PSII) saturate as soon as actinic light reaches ≈200 μE m⁻² s⁻¹. Otherwise, T. sillamontana revealed a higher capacity for NPQ at strong light (≥800 μE m⁻² s⁻¹). The post-illumination adaptation of shade-tolerant plants occurs slower than in the light-resistant species. The data obtained are discussed in terms of reactivity of photosynthetic apparatus to short-term variations of the environment light.     

Biophysical Methods of Ecological Monitoring. Photosynthetic characteristics of tree plants growing in Moscow city

In this work, we studied the influence of ecological factors (distance from thoroughfares) on photosynthetic characteristics of leaves of four tree species growing in Moscow city. Photosynthetic activity of leaves was assayed by instrumental methods of probing the functional state of the photosynthetic apparatus, using electron paramagnetic resonance for measuring the kinetics of photooxidation of P₇₀₀ centers, thermoluminescence, and slow induction of chlorophyll fluorescence. It has been shown that the kinetic parameters of the induction curves, as determined from the kinetics of P₇₀₀ photooxidation and slow fluorescence induction in dark-adapted leaves, are sensitive to variations of plant growth conditions. These parameters can be used as informative characteristics in ecological monitoring.     

Tolerance of the photosynthetic apparatus to acidification of the growth medium as a possible determinant of CO<Subscript>2</Subscript>-tolerance of the symbiotic microalga <Emphasis Type="Italic">Desmodesmus</Emphasis> sp. IPPAS-2014

The symbiotic unicellular chlorophyte Desmodesmus sp. IPPAS-2014 capable of growth at extremely high CO₂ levels prohibitive for most other microalgae is an interesting model for studies of CO₂ tolerance mechanisms and a promising organism for CO₂ biocapture. We studied the initial (0-60 min) phase of acclimation of this microalga to an abrupt decrease in pH of the medium sparged with air/20% CO₂ mixture. Acclimation of the culture to these conditions was accompanied by a sharp decrease in photochemical activity of the chloroplast followed by its recovery with a characteristic time of 10-50 min. We hypothesize that acidification of the cultivation medium by dissolving CO₂ plays a key role in the observed decrease in the photochemical activity. The possible role of photosynthetic apparatus tolerance to abrupt acidification in overall high tolerance of symbiotic microalgae to extremely high CO₂ levels is discussed.     

An exceptional irradiance-induced decrease of light trapping in two Tradescantia species: an unexpected relationship with the leaf architecture and zeaxanthin-mediated photoprotection

Leaf anatomy and irradiance-dependent leaf transmittance changes serving as irradiance acclimation mechanisms in leaves were studied in two ecologically contrasting Tradescantia species, a shade plant T. fluminensis Vell. and a sun plant T. sillamontana Matuda, grown at different irradiances. A dramatic increase in leaf thickness (2 to 4-fold) under a high growth irradiance (800 μmol m^?2 s^?1) compared with a low growth irradiance (60 μmol m^?2 s^?1), achieved mainly by expansion of the epidermis, was recorded in both species. The effect took place on the background of modest changes in mesophyll thickness (1.8-fold in T. fluminensis and 1.15-fold in T. sillamontana) and chloroplast size (0.8-fold in T. fluminensis and an insignificant change in T. sillamontana). Mesophyll structure and growth irradiance response did not seem to facilitate significantly light-dependent chloroplast (avoidance) movement in these species. Nevertheless, an exceptionally large (2 to 4-fold) irradiance-induced increase in light transmittance attributable to chloroplast avoidance movement was revealed. This increase by far exceeded that in other higher plants according to available literature. The magnitude of the irradiance-dependent transmittance changes positively correlated both with the rate of photosystem II recovery and with the extent of xanthophyll deepoxidation in the leaves. This was opposite to a negative correlation observed between the same parameters in different plant species. We hypothesize that, at the evolutionary timescale, chloroplast avoidance movement might adjust independently from other photoprotective mechanisms, e.g., non-photochemical quenching, whereas, on the ontogenetic timescale, adjustment of these mechanisms inevitably follows the same trend.     

Semi-continuum electrostatic calculations of redox potentials in photosystem I

The midpoint redox potentials (E(m)) of all cofactors in photosystem I from Synechococcus elongatus as well as of the iron-sulfur (Fe(4)S(4)) clusters in two soluble ferredoxins from Azotobacter vinelandii and Clostridium acidiurici were calculated within the framework of a semi-continuum dielectric approach. The widely used treatment of proteins as uniform media with single dielectric permittivity is oversimplified, particularly, because permanent charges are considered both as a source for intraprotein electric field and as a part of dielectric polarizability. Our approach overcomes this inconsistency by using two dielectric constants: optical epsilon(o)=2.5 for permanent charges pre-existing in crystal structure, and static epsilon(s) for newly formed charges. We also take into account a substantial dielectric heterogeneity of photosystem I revealed by photoelectric measurements and a liquid junction potential correction for E(m) values of relevant redox cofactors measured in aprotic solvents. We show that calculations based on a single permittivity have the discrepancy with experimental data larger than 0.7 V, whereas E(m) values calculated within our approach fall in the range of experimental estimates. The electrostatic analysis combined with quantum chemistry calculations shows that (i) the energy decrease upon chlorophyll dimerization is essential for the downhill mode of primary charge separation between the special pair P(700) and the primary acceptor A(0); (ii) the primary donor is apparently P(700) but not a pair of accessory chlorophylls; (iii) the electron transfer from the A branch quinone Q(A) to the iron-sulfur cluster F(X) is most probably downhill, whereas that from the B branch quinone Q(B) to F(X) is essentially downhill.     

Oxygen as an alternative electron acceptor in the photosynthetic electron transport chain of C3 plants

This study deals with effects of oxygen on the kinetics of P(700) photoinduced redox transitions and on induction transients of chlorophyll fluorescence in leaves of C(3) plants Hibiscus rosa-sinensis and Vicia faba. It is shown that the removal of oxygen from the leaf environment has a conspicuous effect on photosynthetic electron transport. Under anaerobic conditions, the concentration of oxidized P700 centers in continuous white light was substantially lower than under aerobic conditions. The deficiency of oxygen released non-photochemical quenching of chlorophyll fluorescence, thus indicating a decrease in the trans-thylakoid pH gradient (DeltapH). Quantitative analysis of experimental data within the framework of an original mathematical model has shown that the steady-state electron flux toward oxygen in Chinese hibiscus leaves makes up to approximately 40% of the total electron flow passing through photosystem 1 (PS1). The decrease in P700+ content under anaerobic conditions can be due to two causes: i) the retardation of electron outflow from PS1, and ii) the release of photosynthetic control (acceleration of electron flow from PS2 to P700+) owing to lower acidification of the intra-thylakoid space. At the same time, cyclic electron transport around PS1 was not stimulated in the oxygen-free medium, although such stimulation seemed likely in view of possible rearrangement of electron flows on the acceptor side of PS1. This conclusion stems from observations that the rates of P700+ reduction in DCMU-poisoned samples, both under aerobic and anaerobic conditions, were negligibly small compared to rates of electron flow from PS2 toward P700+ in untreated samples.     

Slow fluorescence induction and CO<Subscript>2</Subscript> exchange in bean plants treated with <Emphasis Type="Italic">Reynoutria sachalinensis</Emphasis> extract

Treatment of bean seedlings with a water extract of Reynoutria sachalinensis (giant knotweed) increases visible photosynthesis and the dark respiration of plants. The increase in CO₂ uptake under intense illumination (10 000 lx) correlates with an increase in the fluorescence parameter (F _M − F _T)/F _T.     

Regulation of electron transport in C(3) plant chloroplasts in situ and in silico: short-term effects of atmospheric CO(2) and O(2)

In this work, we have investigated the effects of atmospheric CO(2) and O(2) on induction events in Hibiscus rosa-sinensis leaves. These effects manifest themselves as multiphase kinetics of P(700) redox transitions and non-monotonous changes in chlorophyll fluorescence. Depletion of CO(2) and O(2) in air causes a decrease in linear electron flux (LEF) and dramatic lowering of P(700)(+) level. This is explained by the impediment to electron efflux from photosystem 1 (PS1) at low acceptor capacity. With the release of the acceptor deficit, the rate of LEF significantly increases. We have found that oxygen promotes the outflow of electrons from PS1, providing the rise of P(700)(+) level. The effect of oxygen as an alternative electron acceptor becomes apparent at low and ambient concentrations of atmospheric CO(2) < or = 0.06-0.07%). A decrease in LEF at low CO(2) is accompanied by a significant (about 3-fold) rise of non-photochemical quenching (NPQ) of chlorophyll fluorescence. Such an increase in NPQ can be explained by more significant acidification of the thylakoid lumen. This occurs due to lessening the proton flux through the ATP synthases caused by a decrease in the ATP consumption in the Bassham-Benson-Calvin (BBC) cycle. pH-dependent mechanisms of electron transport control have been described within the frames of our mathematical model. The model describes the reciprocal changes in LEF and NPQ and predicts the redistribution of electron fluxes on the acceptor side of PS1. In particular, the contribution of cyclic electron flow around PS1 (CEF1) and water-water cycle gradually decays during the induction phase. This result is consistent with experimental data indicating that under the steady-state conditions the contribution of CEF1 to photosynthetic electron transport in Hibiscus rosa-sinensis is insignificant (< or = 10%).     

Cationic penetrating antioxidants switch off Mn cluster of photosystem II in situ

Photosynthesis Research - Photosystem I (PSI) generates the most negative redox potential found in nature, and the performance of solar energy conversion into alternative energy sources in...     

Spin-probes designed for measuring the intrathylakoid pH in chloroplasts

Nitroxide radicals are widely used as molecular probes in different fields of chemistry and biology. In this work, we describe pH-sensitive imidazoline- and imidazolidine-based nitroxides with pK values in the range 4.7-7.6 (2,2,3,4,5,5-hexamethylperhydroimidazol-1-oxyl, 4-amino-2,2,5,5-tetramethyl-2,5-dihydro-1H-imidazol-1-oxyl, 4-dimethylamino-2,2-diethyl-5,5-dimethyl-2,5-dihydro-1H-imidazol-1-oxyl, and 2,2-diethyl-5,5-dimethyl-4-pyrrolidyline-1-yl-2,5-dihydro-1H-imidazol-1-oxyl), which allow the pH-monitoring inside chloroplasts. We have demonstrated that EPR spectra of these spin-probes localized in the thylakoid lumen markedly change with the light-induced acidification of the thylakoid lumen in chloroplasts. Comparing EPR spectrum parameters of intrathylakoid spin-probes with relevant calibrating curves, we could estimate steady-state values of lumen pHin established during illumination of chloroplasts with continuous light. For isolated bean (Vicia faba) chloroplasts suspended in a medium with pHout=7.8, we found that pHin approximately 5.4-5.7 in the state of photosynthetic control, and pHin approximately 5.7-6.0 under photophosphorylation conditions. Thus, ATP synthesis occurs at a moderate acidification of the thylakoid lumen, corresponding to transthylakoid pH difference DeltapH approximately 1.8-2.1. These values of DeltapH are consistent with a point of view that under steady-state conditions the proton gradient DeltapH is the main contributor to the proton motive force driving the operation of ATP synthesis, provided that stoichiometric ratio H+/ATP is n> or =4-4.7.