Blue light-modulation of chlorophyll a fluorescence transients in guard cell chloroplasts

Chlorophyll a fluorescence transients from mesophyll and single guard cell pairs of Vicia faba were measured by microspectrofluorometry. In both chloroplast types, fluorescence induction (O to P) was similar under actinic blue and green light. In slow transients from mesophyll cell chloroplasts, blue and green light induced identical, typical rapid quenching from P to S, and the M peak. In contrast, the P to S transient from guard cell (GC) chloroplasts irradiated with blue light showed a much slower quenching rate, and the P to T transition showed no M peak. Actinic green light induced mesophyll-like transients in GC chloroplasts, including rapid quenching from P to S and the M peak. Detection of these transients in single pairs of GC and isolated protoplasts ruled out mesophyll contamination as a signal source. Green light induced a rapid quenching and the M peak in GC chloroplasts from several species. The effect of CO₂ concentration on the fluorescence transients was investigated in the presence of HCO₃⁻ at pH 6.8 and 10.0. In transients induced by green light in both chloroplast types, a pH increase concomitant with a reduction in CO₂ concentration caused an increase in the initial rate of quenching and the elimination of the M peak. Actinic blue light induced mesophyll-like transients from GC chloroplasts in the presence of 10 micromolar KCN, a concentration at which the blue light-induced stomatal opening is inhibited. Addition of 100 to 200 micromolar phosphate also caused large increases in fluorescence quenching rates and a M peak. These results indicate that blue light modulates photosynthetic activity in GC chloroplasts. This blue light effect is not observed in the absence of transduction events connected with the blue light response and in the presence of high phosphate concentrations.     

SINGLE‐CELL PULSE AMPLITUDE MODULATION FLUORESCENCE MEASUREMENTS OF THE GIANT DIATOM ETHMODISCUS (BACILLARIOPHYCEAE)1

Diurnal changes in effective yield (ΔF:F_m′), rapid light curves (RLCs), and induction/dark recovery time series were measured on individual cells of the giant diatom Ethmodiscus Castracane using active fluorescence (pulse amplitude modulation fluorometry). Unlike the co‐occurring diatom Hemiaulus and bulk phytoplankton, there was no observable diurnal down‐regulation of yield or relative electron transport rates in Ethmodiscus. Yields were constant at or near maximum values (0.7–0.8). Increases in ΔF:F_m′ during the initial actinic levels are consistent with dark nonphotochemical quenching mechanisms. Sustained actinic illumination (660 μmol photon·m^?2·s^?1) resulted in a ΔF:F_m′ of 0.2–0.3, but rapid recovery to near‐maximum values occurred in subsequent dark periods. Such recovery occurred even after exposure to full sunlight for 28 min, but not at 60 min. Thus, the lack of diurnal down‐regulation in Ethmodiscus is apparent, not real, and is an artifact of the time scale of sample extraction from net tows. These positively buoyant cells showed no evidence of routine photodamage, probably due to mixing and reduction in the average light exposure. The general patterns seen in RLCs from light‐and dark‐adapted higher plants are significantly different from those observed in Ethmodiscus. These results suggest that active fluorescence characteristics require careful examination to differentiate habitat‐ and taxon‐specific characteristics from light‐history effects. It is unclear whether the rapid recovery seen in Ethmodiscus is unique. The differences seen between Hemiaulus and Ethmodiscus from the same samples suggest that changes in community yield values measured in countertop systems could be the result of species replacement in addition to experimental or environmental perturbations.     

Co-regulation of photosynthetic processes under potassium deficiency across CO<Subscript>2</Subscript> levels in soybean: mechanisms of limitations and adaptations

The slow kinetic phases of the chlorophyll a fluorescence transient (induction) are valuable tools in studying dynamic regulation of light harvesting, light energy distribution between photosystems, and heat dissipation in photosynthetic organisms. However, the origin of these phases are not yet fully understood. This is especially true in the case of prokaryotic oxygenic photoautotrophs, the cyanobacteria. To understand the origin of the slowest (tens of minutes) kinetic phase, the M–T fluorescence decline, in the context of light acclimation of these globally important microorganisms, we have compared spectrally resolved fluorescence induction data from the wild type Synechocystis sp. PCC 6803 cells, using orange (λ?=?593 nm) actinic light, with those of mutants, ΔapcD and ΔOCP, that are unable to perform either state transition or fluorescence quenching by orange carotenoid protein (OCP), respectively. Our results suggest a multiple origin of the M–T decline and reveal a complex interplay of various known regulatory processes in maintaining the redox homeostasis of a cyanobacterial cell. In addition, they lead us to suggest that a new type of regulatory process, operating on the timescale of minutes to hours, is involved in dissipating excess light energy in cyanobacteria.     

Interaction of exogenous quinones with membranes of higher plant chloroplasts: modulation of quinone capacities as photochemical and non-photochemical quenchers of energy in Photosystem II during light-dark transitions

Light modulation of the ability of three artificial quinones, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), 2,6-dichloro-p-benzoquinone (DCBQ), and tetramethyl-p-benzoquinone (duroquinone), to quench chlorophyll (Chl) fluorescence photochemically or non-photochemically was studied to simulate the functions of endogenous plastoquinones during the thermal phase of fast Chl fluorescence induction kinetics. DBMIB was found to suppress by severalfold the basal level of Chl fluorescence (F_o) and to markedly retard the light-induced rise of variable fluorescence (F_v). After irradiation with actinic light, Chl fluorescence rapidly dropped down to the level corresponding to F_o level in untreated thylakoids and then slowly declined to the initial level. DBMIB was found to be an efficient photochemical quencher of energy in Photosystem II (PSII) in the dark, but not after prolonged irradiation. Those events were owing to DBMIB reduction under light and its oxidation in the dark. At high concentrations, DCBQ exhibited quenching behaviours similar to those of DBMIB. In contrast, duroquinone demonstrated the ability to quench F_v at low concentration, while F_o was declined only at high concentrations of this artificial quinone. Unlike for DBMIB and DCBQ, quenched F_o level was attained rapidly after actinic light had been turned off in the presence of high duroquinone concentrations. That finding evidenced that the capacity of duroquinone to non-photochemically quench excitation energy in PSII was maintained during irradiation, which is likely owing to the rapid electron transfer from duroquinol to Photosystem I (PSI). It was suggested that DBMIB and DCBQ at high concentration, on the one hand, and duroquinone, on the other hand, mimic the properties of plastoquinones as photochemical and non-photochemical quenchers of energy in PSII under different conditions. The first model corresponds to the conditions under which the plastoquinone pool can be largely reduced (weak electron release from PSII to PSI compared to PSII-driven electron flow from water under strong light and weak PSI photochemical capacity because of inactive electron transport on its reducing side), while the second one mimics the behaviour of the plastoquinone pool when it cannot be filled up with electrons (weak or moderate light and high photochemical competence of PSI).     

Effect of light intensity on chlorophyll fluorescence in wheat leaves: Application of PAM-fluorometry

Application of pulse-amplitude-modulation (PAM) fluorometers for measuring slow stages of chlorophyll fluorescence induction (CFI) is considered. With an example of Triticum aestuvum L. plants grown under continuous illumination at a photon flux density of 600 μmol/(m² s) photosynthetically active radiation (PAR), the CFI curves were analyzed with leaves of various ages as a function of actinic light intensity. The fluorometer PAM-2100 was applied for measurements of CFI curves. The characteristic peaks of CFI curves in wheat leaves were most conspicuous and had the largest amplitudes at 600–800 μmol/(m² s) PAR, which corresponds to the middle range of actinic light intensities employed in PAM-2100 fluorometers. In plants exposed to favorable and stressful conditions, the developmental stages may proceed at different rates; thus, the comparison of fluorescence parameters for leaves of equal calendar age but having different physiological states may provide ambiguous data. Therefore, the feasibility of recording CFI curves of different types is quite important for rapid diagnostics of the age and state of plant leaves, as well as for adequate physiological conclusions.     

Interaction of exogenous quinones with membranes of higher plant chloroplasts: modulation of quinone capacities as photochemical and non-photochemical quenchers of energy in Photosystem II during light-dark transitions

Light modulation of the ability of three artificial quinones, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), 2,6-dichloro-p-benzoquinone (DCBQ), and tetramethyl-p-benzoquinone (duroquinone), to quench chlorophyll (Chl) fluorescence photochemically or non-photochemically was studied to simulate the functions of endogenous plastoquinones during the thermal phase of fast Chl fluorescence induction kinetics. DBMIB was found to suppress by severalfold the basal level of Chl fluorescence (F(o)) and to markedly retard the light-induced rise of variable fluorescence (F(v)). After irradiation with actinic light, Chl fluorescence rapidly dropped down to the level corresponding to F(o) level in untreated thylakoids and then slowly declined to the initial level. DBMIB was found to be an efficient photochemical quencher of energy in Photosystem II (PSII) in the dark, but not after prolonged irradiation. Those events were owing to DBMIB reduction under light and its oxidation in the dark. At high concentrations, DCBQ exhibited quenching behaviours similar to those of DBMIB. In contrast, duroquinone demonstrated the ability to quench F(v) at low concentration, while F(o) was declined only at high concentrations of this artificial quinone. Unlike for DBMIB and DCBQ, quenched F(o) level was attained rapidly after actinic light had been turned off in the presence of high duroquinone concentrations. That finding evidenced that the capacity of duroquinone to non-photochemically quench excitation energy in PSII was maintained during irradiation, which is likely owing to the rapid electron transfer from duroquinol to Photosystem I (PSI). It was suggested that DBMIB and DCBQ at high concentration, on the one hand, and duroquinone, on the other hand, mimic the properties of plastoquinones as photochemical and non-photochemical quenchers of energy in PSII under different conditions. The first model corresponds to the conditions under which the plastoquinone pool can be largely reduced (weak electron release from PSII to PSI compared to PSII-driven electron flow from water under strong light and weak PSI photochemical capacity because of inactive electron transport on its reducing side), while the second one mimics the behaviour of the plastoquinone pool when it cannot be filled up with electrons (weak or moderate light and high photochemical competence of PSI).     

Analysis of the Heat Stress Induced Quenching of Variable Chlorophyll a Fluorescence in Beet Spinach Leaves: Possible Accumulation of Oxidized Quencher P680+ under High Illumination

Effect of preheating of beet spinach leaves on chlorophyll a fluorescence yield was analyzed with the help of additional high intensity illumination pulses using a pulse modulated fluorometer. Preheating at mildly elevated temperature (35–45°C) causes a shift in the redox state of secondary donor of photosystem II, possibly due to uncoupling of phosphorylation because of thermal induced membrane disorganization and associated alkalinization of intra thylakoid space. Also, at these preheating temperatures, a rise in photosystem I catalyzed electron transfer has been shown to occur. These two effects induce rapid quenching of Chi a fluorescence, which drops even in the presence of actinic light, below the level of initial fluorescence (F_o′ monitored by the weak modulated probing light. Preheating of leaf segments induces an increase in fluorescence in the presence of dluron, which blocks electron flow between two photosystems, and thus this increases in fluorescence yield (F_o′ as monitored by weak modulated light, is not solely due to disorganization of light harvesting Chi-protein complex but also due to a shift in the redox equilibrium of the donor at the oxidizing side of photosystem II resulting in rapid reduction of Q_A the stable primary acceptor of photosystem II. In 50°C preheated DCMU treated samples, the fluorescence yield increases in weak modulated light and it approaches that of maximal steady state (F_max) level. At preheating temperature of 48°–50°C, the inactivation of enzymes in the reducing side of photosystem I, causes an impairment of the reoxidation of QA and under this condition, a strong illumination causes quenching of Chi a fluorescence. This quenching seems to arise because of accumulation of the P680⁺, the oxidized physiological donor of photosystem which is a quencher of Chi a fluorescence. This quenching depended on the pulse intensity and duration which saturates P680⁺ accumulation and is greatly manifested when water oxidation complex is damaged.     

Parallel inductive kinetics of fluorescence and photoacoustic signal in dark-adapted thalli of Bryopsis maxima

The inductive kinetics of fluorescence and photoacoustic signal were measured simultaneously in dark-adapted thalli of the green coenocytic alga Bryopsis maxima. Under illumination with weak red light modulated at 60 Hz, the fluorescence yield varied, showing three maxima P, M₁ and M₂ almost immediately, 10 s and 6 min after the onset of the illumination, respectively (Yamagishi, A., Satoh, K. and Katoh, S. (1978) Plant Cell Physiol. 19, 17–25). The photoacoustic signal also showed inductive transients which parallel well those of the fluorescence up to the M₂ stage. After M₂, the photoacoustic signal remained at a constant level, while the emission yield gradually decreased. The first peak of the fluorescence induction and a corresponding peak of the photoacoustic transients were selectively eliminated by prior illumination or methyl viologen treatment of the dark-adapted thalli. The second peaks of the two induction curves were abolished by carbonylcyanide-m-chlorophenylhydrazone, whereas dicyclohexylcarbodiimide enhanced their peak heights and suppressed the subsequent decreases. The results indicate that the fluorescence yield is mainly determined by the redox state of the Photosystem II reaction center throughout the induction period except the last phase. Mechanisms underlying inductive transients of fluorescence are discussed in the light of the present findings.     

Fluorescence decline as a function of redox potential and actinic light intensity in spinach thylakoids

Excitation of isolated thylakoids with sufficiently strong actinic light increases the fluorescence quantum yield up to a maximum level, F_max, followed by a slower decline under certain experimental conditions. In this study the latter effect was analyzed as a function of the ambient redox potential and the actinic light intensity. Two different types of fluorescence decrease were found. (a) In the presence of specific quinones widely used as redox mediators a fast and comparatively small decrease (30% of F_max), referred to as ΔF_SQ, was observed at moderate redox potentials (−300 <E_m < + 200 mV). ΔF_SQ disappears at positive values with E_{m, 7.5} = + 110 mV, whereas the decrease at negative redox potential depends on the midpoint potential of the quinone. (b) A more pronounced fluorescence decline was observed at redox potentials below −300 mV, which comprises 65–70% of the maximum fluorescence. The full expression of this effect, referred to as ΔF^max_LP, requires markedly higher actinic light intensities than ΔF^max_SQ. The extent of ΔF^max_LP as a function of the redox potential is dependent on the presence of redox mediators. In their absence the full expression of ΔF^max_LP can be only observed below −400 mV. Based on the hypothesis of Pheo⁻ photoaccumulation being responsible for the fluorescence decline at low redox potentials (Klimov, V.V., Klevanik, A.V. and Shuvalov, V.A. (1977) FEBS Lett. 82, 182–186), a reaction scheme is presented that qualitatively describes the time course of ΔF_LP at different actinic light intensities and redox potentials. Based on this analysis, the rate of Pheo^⨪ reoxidation is inferred to be limited by the reaction center apoprotein acting as a barrier to redox equilibration. The implications for the interpretations of redox titration curves are briefly discussed.     

A portable, microprocessor operated instrument for measuring chlorophyll fluorescence kinetics in stress physiology

A portable instrument for measuring chlorophyll fluorescence induction kinetics is described and examples of measurements are given. The instrument is centered around a statistically-mixed bifurcated optical fiber. One fiber branch guides the actinic light to the sample, whereas the other branch carries the emitted chlorophyll fluorescence to the photodetector. Scattered actinic light is cut out from the detector by a red interference filter. The instrument measures fast as well as slow fluorescence induction kinetics, but is particularly well designed for analyzing fast kinetics. The high time resolution and strong, variable actinic light mean that both F_o (non-variable fluorescence) and F_m (maximal fluorescence at the P-peak) are well defined. A built in microprocessor unit with attached memory stores the fluorescence induction curve and calculates key fluorescence parameters such as F_o, F_m, F_v (variable fluorescence equals F_m?F_o), F_v/F_m (the photochemical efficiency of photosystem II) and t_1/2 (half rise time from F_o, to F_m). These values are digitally displayed after each recording and they (or the whole induction curve) can be stored in a memory and later retrieved. Because of a flexible setting of the instrument it can be used with high accuracy both for optically thick leaves and for diluted suspensions of algae or chloroplasts. A simple, light weight clamp cuvette for dark adaptation of leaves has been developed. It is equipped with a gate allowing the optical fiber to be inserted without daylight reaching the dark adapted portion of the leaf. The instrument has been developed for rapid monitoring of changes in activities and organization of the photosynthetic apparatus in vivo when plants are exposed to environmental stress both in the field and in the laboratory. Examples of measurements are given for differently treated leaves of Pinus sylvestris, Salix sp., Betula verrucosa, Zea mays, Epilobium angustifo-lium and for chloroplast thylakoids isolated from Spinacia oleracea.     

A System Approach to Fluorescence Induction in Green Leaves

Intact green oat leaves (Avena saliva) were irradiated with modulated exciting light. Ai the application of initial square pulses of exciting light, the fluorescence induction pattern will change, but after some repeated pulses the fluorescence response pattern stabilizes and becomes identical. This periodic stimulation gave a better resolution of the two peaks constituting the fast fluorescence transient (the DPS region) compared to a single light pulse. The first peak could be depressed by weak light of wavelengths up to about 740 nm, given between the intense light phases of the pulse, while this treatment did not alter the second peak. The intensity of the higher light level of the pulse as well as the frequency were systematically varied. The results are tentatively rationalized in the form of sequential relaxation processes within the frame of the Hill-Bendull scheme, leading to the possibility of two substances, one belonging to Photosystem II and the other to Photosystem I. underlying the two peaks in the DPS region of the fluorescence induction curve.     

Evidence for the operation of alternative electron transport routes through photosystem I in intact barley leaves under weak and moderate white light

The functioning of alternative routes of photosynthetic electron transport was analyzed from the kinetics of dark reduction of P700⁺ , an oxidized primary donor of PSI, in barley (Hordeum vulgare L.) leaves irradiated by white light of various intensities. Redox changes of P700 were monitored as absorbance changes at 830 nm using PAM 101 specialized device. Irradiation of dark-adapted leaves caused a gradual P700⁺ accumulation, and the steady-state level of oxidized P700 increased with intensity of actinic light. The kinetics of P700⁺ dark reduction after a pulse of strong actinic light, assayed from the absorbance changes at 830 nm, was fitted by a single exponential term with a halftime of 10–12 ms. Two slower components were observed in the kinetics of P700⁺ dark reduction after leaf irradiation by attenuated actinic light. The contribution of slow components to P700⁺ reduction increased with the decrease in actinic light intensity. Two slow components characterized by halftimes similar to those observed after leaf irradiation by weak white light were found in the kinetics of dark reduction of P700⁺ oxidized in leaves with far-red light specifically absorbed by PSI. The treatment of leaves with methyl viologen, an artificial PSI electron acceptor, significantly accelerated the accumulation of P700⁺ under light. At the same time, the presence of methyl viologen, which inhibits ferredoxin-dependent electron transport around PSI, did not affect three components of the kinetics of P700⁺ dark reduction obtained after irradiations with various actinic light intensities. It was concluded that some part of PSI reaction centers was not reduced by electron transfer from PSII under weak or moderate intensities of actinic light. In this population of PSI centers, P700⁺ was reduced via alternative electron transport routes. Insensitivity of the kinetics of P700⁺ dark reduction to methyl viologen evidences that the input of electrons to PSI from the reductants (NADPH or NADH) localized in the chloroplast stroma was effective under those light conditions.Translated from Fiziologiya Rastenii, Vol. 52, No. 1, 2005, pp. 5–11.Original Russian Text Copyright © 2005 by Bukhov, Egorova.     

Effect of Elevated Temperatures on the Activity of Alternative Pathways of Photosynthetic Electron Transport in Intact Barley and Maize Leaves

The light-induced rise in chlorophyll fluorescence and the subsequent decay of fluorescence in darkness were measured in barley and maize leaves exposed to heat treatment. The redox conversions of the photosystem I primary donor P700, induced by far-red light, were also monitored from the absorbance changes at 830 nm. After heating of leaves at temperatures above 40°C, the ratio of variable and maximum fluorescence decreased for leaves of both plant species, indicating the inhibition of photosystem II (PSII) activity. A twofold reduction of this ratio in barley and maize leaves was observed after heating at 45.3 and 48.1°C, respectively, which suggests the higher functional resistance of PSII in maize. The amplitude of the slow phase in the dark relaxation of variable fluorescence did not change after the treatment of barley and maize leaves at temperatures up to 48°C. In leaves treated at 42 and 46°C, the slow phase of dark relaxation deviated from an exponential curve. The relaxation kinetics included a temporary increase in fluorescence to a peak about 1 s after turning off the actinic light. Unlike the slow component, the fast and intermediate phases in the dark relaxation of variable fluorescence disappeared fully or partly after the treatment of leaves at 46°C. The photooxidation of P700 in heat-treated leaves was saturated at much higher irradiances of far-red light than in untreated leaves. At the same time, the dark reduction of P700⁺ was substantially accelerated after heat treatment. The data provide evidence that the heating of leaves stimulated the alternative pathways of electron transport, i.e., cyclic transport around photosystem I and/or the donation of electrons to the plastoquinone pool from the reduced compounds located in the chloroplast stroma. The rate of alternative electron transport after the heat treatment was higher in maize leaves than in barley leaves. It is supposed that the stimulation of alternative electron transport, associated with proton pumping into the thylakoid, represents a protective mechanism that prevents the photoinhibition of PSII in leaves upon a strong suppression of linear electron transport in chloroplasts exposed to heat treatment.     

On the polyphasic quenching kinetics of chlorophyll a fluorescence in algae after light pulses of variable length

This study reports on kinetics of the fluorescence decay in a suspension of the alga Scenedesmus quadricauda after actinic illumination. These are monitored as the variable fluorescence signal in the dark following light pulses of variable intensity and duration. The decay reflects the restoration of chlorophyll fluorescence quenching of the photosystem II (PSII) antennas and shows a polyphasic pattern which suggests the involvement of different processes. The overall quenching curve after a fluorescence-saturating pulse (SP) of 250-ms duration, commonly used in pulse amplitude modulation applications as the tool for estimating the maximal fluorescence (F _m), has been termed P–O, in which P and O have the same meaning as used in the OJIP induction curve in the light. Deconvolution of this signal shows at least three distinguishable exponential phases with reciprocal rate constants of the order of 10, 10², and 10³ ms. The size of the long (>10³ ms) and moderate (~10² ms) lasting components relative to the complete quenching signal after an SP increases with the duration of the actinic pulse concomitantly with an increase in the reciprocal rate constants of the fast (~10 ms) and moderate quenching phases. Fluorescence responses upon single turnover flashes of 30-μs duration (STFs) given at discrete times during the P–O quenching were used as tools for identifying the quencher involved in the P–O quenching phase preceding the STF excitation. Results are difficult to interpret in terms of a single-hit two-state trapping mechanism with distinguishable quenching properties of open and closed reaction centers only. They give support for an earlier hypothesis on a double-hit three-state trapping mechanism in which the so-called semi-closed reaction centers of PSII are considered. In these trapping-competent centers the single reduced acceptor pair [PheQ _A]^1?, depending on the size of photoelectrochemically induced pH effects on the Q _B-binding site, functions as an efficient fluorescence quencher.     

Influence of UV radiation and visible light on Porphyra umbilicalis: Photoinhibition and MAA concentration

Porphyra umbilicalis was cultured under constant light conditions but showed a diurnal pattern in chlorophyll fluorescence. Photoinhibition after light treatment was determined by PAM fluorescence measurements. Treatment with only UV irradiation caused a slow but steady decline in the effective photosynthetic quantum yield from which there was no recovery. Solar simulated irradiation led to a large decrease in quantum yield after short periods of irradiation; partial recovery occurred after shading the samples. No significant difference was found between samples exposed to PAR only or to PAR + UV-A and/or UV-B irradiation. Determination of mycosporine-like amino acids (MAAs)before and during exposure to solar simulated irradiation showed a high initial concentration of MAAs but no increase due to the irradiance treatment. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of High Temperature on Chlorophyll Fluorescence Induction and the Kinetics of Far Red Radiation-Induced relaxation of apparent F0 in maize leaves

Temperature dependence (25–50 °C) of chlorophyll (Chl) fluorescence induction, far-red radiation (FR)-induced relaxation of the post-irradiation transient increase in apparent F₀, and the trans-thylakoid proton gradients (pH) was examined in maize leaves. Temperatures above 30 °C caused an elevation of F₀ level and an enhancement of F₀ quenching during actinic irradiation. Millisecond delayed light emission (ms-DLE), which reflects the magnitude of pH, decreased strikingly above 35 °C, and almost disappeared at 50 °C. It indicates that the heat-enhanced quenching of F₀ under actinic irradiation could not be attributed mainly to the mechanism of pH-dependent quenching. The relaxation of the post-irradiation transient increase in apparent F₀ upon FR irradiation could be decomposed into two exponential components (₁ = 0.7–1.8 s, ₂ = 2.0–9.9 s). Decay times of both components increased with temperature increasing from 25 to 40–45 °C. The bi-phasic kinetics of FR-induced relaxation of the post-irradiation transient increase in apparent F₀ and its temperature dependence may be related to plastoquinone (PQ) compartmentation in the thylakoid membranes and its re-organisation at elevated temperature.     

Temperature and light dependent modifications of chlorophyll fluorescence kinetics in spruce needles during winter

Prompt chlorophyll a fluorescence kinetics at room temperature were measured from intact spruce needles. The fluorescence signal was recorded after varying light pretreatments. During the winter, induction curves showed characteristic changes in both the initial peak of fluorescence FV/FP (FP-FO/FP) and the steady state level F_dr (FP-FT/FP). Winter stress induced decreases in both values which showed close correlation to the light and temperature pre-history of the plants. In February changes in fluorescence induction indicative of a restoration of photosynthesis were detected and these corresponded to a rise of temperature above zero in combination with low light levels. In March increasing light intensity combined with chilling temperatures induced again decreases of both values of chlorophyll fluorescence induction suggesting the occurrence of photoinhibition.     

Analysis of photoprotection and apparent non-photochemical quenching of chlorophyll fluorescence in <Emphasis Type="Italic">Tradescantia</Emphasis> leaves based on the rate of irradiance-induced changes in optical transparence

The kinetics of irradiation-induced changes in leaf optical transparence (ΔT) and non-photochemical quenching (NPQ) of chlorophyll fluorescence in Tradescantia fluminensis and T. sillamontana leaves adapted to different irradiance in nature was analyzed. Characteristic times of a photoinduced increase and a dark decline of ΔT in these species were 12 and 20 min, respectively. The ΔT was not confirmed to be the main contributor to the observed middle phase of NPQ relaxation kinetics (τ = 10-28 min). Comparison of rate of photoinduced increase in ΔT and photosystem II quantum yield recovery showed that the former did not affect the tolerance of the photosynthetic apparatus (PSA) to irradiances up to 150 μmol PAR·m^–2·s^–1. Irradiance tolerance correlated with the rate of “apparent NPQ” induction. Considering that the induction of apparent NPQ involves processes significantly faster than ΔT, we suggest that the photoprotective mechanism induction rate is crucial for tolerance of the PSA to moderate irradiance during the initial stage of light acclimation (first several minutes upon the onset of illumination).     

Structural and functional aspects of the Nuphar lutea (L.) Smith heterophylly: Ultrastructure and photosynthesis

The ultrastructure and functional characteristics of the photosynthetic apparatus of floating and submersed leaves of the heterophyllous plant Nuphar lutea (L.) Smith have been examined. Differences have been revealed in mesophyll cell chloroplasts, content of pigments, and chlorophyll fluorescence parameters between floating and submersed leaves and submersed leaves at different depths. A sharp decline in the PSII (photosystem II) efficiency of submersed leaves when exposed to an actinic light intensity of more than 60 ??mol m^?2 s^?1 has been noted. The described differences may be considered as an adaptation mechanism of submersed leaves to life in an aquatic environment with a reduced light intensity and changed light spectral composition.