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Arabidopsis plants were grown from seeds at different photon flux densities (PFDs) of white light ranging from 65 to 800 µmol photons m−2 s−1. Increasing PFD brought about a marked accumulation of plastoquinone (PQ) in leaves. However, the thylakoid photoactive PQ pool, estimated to about 700 pmol mg−1 leaf dry weight, was independent of PFD; PQ accumulation in high light mostly occurred in the photochemically non-active pool (plastoglobules, chloroplast envelopes) which represented up to 75% of total PQ. The amounts of PSII reaction center (on a leaf dry weight basis) also were little affected by PFD during growth, leading to a constant PQ/PSII ratio at all PFDs. Boosting PQ biosynthesis by overexpression of a solanesyl diphosphate-synthesizing enzyme strongly enhanced the PQ levels, particularly at high PFDs. Again, this accumulation occurred exclusively in the non-photoactive PQ pool. Mutational suppression of the plastoglobular ABC1K1 kinase led to a selective reduction of the thylakoid PQ pool size to ca. 400 pmol mg−1 in a large range of PFDs, which was associated with a restriction of the photosynthetic electron flow. Our results show that photosynthetic acclimation to light intensity does not involve modulation of the thylakoid PQ pool size or the amounts of PSII reaction centers. There appears to be a fixed amount of PQ molecules for optimal interaction with PSII and efficient photosynthesis, with the extra PQ molecules being stored outside the thylakoid membranes, implying a tight regulation of PQ distribution within the chloroplasts. [ABSTRACT FROM AUTHOR]

Maize ( Zea mays L.) inbred lines of contrasting chilling sensitivity (three tolerant, three sensitive lines) were acclimated to 280 μmol photons m−2 s−1 white light at a 17°C sub-optimal temperature. They showed no symptoms of photoinhibition, despite slight changes in photosystem II (PSII) fluorescence and thermoluminescence properties in two tolerant lines. A luminescence “afterglow” emission [Bertsch and Azzi (1965) Biochim Biophys Acta 94:15–26], inducible by a far-red (FR) illumination of unfrozen leaf discs, was detected either as a bounce in decay kinetics at constant temperatures or as a sharp thermoluminescence afterglow band at about 45°C, in dark-adapted leaves. This band reflects the induction by warming of an electron pathway from stromal reductants to plastoquinones and to the QB secondary acceptor of PSII, resulting in a luminescence-emitting charge recombination in the fraction of centres that were initially in the S2/3QB non-luminescent state. A 5-h exposure of plants to growth chamber light shifted this luminescence emission towards shorter times and lower temperatures for several hours in the three chilling-tolerant lines. This downshift was not observed, or only transiently, in the three sensitive lines. In darkness, the downshifted afterglow band relaxed within hours to resume its dark-adapted location, similar for all maize lines. A faster dark re-reduction of P700+ oxidized by FR light (monitored by 820-nm absorbance) and an increase of photochemical energy storage under FR excitation (determined by photoacoustic spectroscopy) confirmed that a cyclic pathway induced by white actinic light remained activated for several hours in the tolerant maize lines. [ABSTRACT FROM AUTHOR]

Brassica napus (L.) was grown from seeds on a reconstituted soil contaminated with 100 mg Cd kg-1. Compared with roots and stems, leaves accumulated high amounts of Cd. Although the Cd concentration in the leaves remained high throughout plant growth and no appreciable change was noticed in the total, extractable or soluble Cd in the soil adhering to the roots, the symptoms of Cd toxicity (leaf chlorosis, growth retardation) decreased with time. Cd induced a noticeable accumulation of phytochelatins in young plants (aged 22 days), which decreased in parallel to the disappearance of the symptoms of Cd intoxication. The subcellular distribution of Cd in leaves of Cd-acclimated plants was determined using biochemical, microscopic and metal-imaging techniques. Leaf fractionation by differential centrifugations showed that Cd was present predominantly in the 'soluble' fraction corresponding to the vacuoles and the cytoplasm. Transmission electron microscopic analyses revealed that those cell compartments contained electron-dense granules associated with needle-like structures. Cd, and also high amounts of sulfur, was detected in those structures by electron-spectroscopic imaging. This technique also showed Cd binding to cell walls by a mechanism that does not involve sulfur atoms. In contrast, very little Cd was found in chloroplasts, and this is consistent with the preservation of photosynthesis in plants grown on Cd-polluted soil. The microanalytical results presented here confirm that long-term growth of B. napus on Cd-contaminated soil is accompanied by preferential storage of Cd in the vacuoles and the cell walls. This phenomenon diverted Cd ions from metabolically active compartments (cytosol, chloroplasts, mitochondria), resulting in a reduction of Cd toxicity in the leaves. [ABSTRACT FROM AUTHOR]

The npq1 mutant of Arabidopsis thaliana (L.) Heynh. has no xanthophyll cycle due to a lack of functional violaxanthin de-epoxidase. Short-term exposure (<2 days) of detached leaves or whole plants to the combination of high photon flux density (1,000 µmol m–2 s–1) and low temperature (10 °C) resulted in PSII photoinhibition which was more acute in npq1 than in the wild type. This increased photosensitivity of npq1 at chilling temperature was attributable to the inhibition of nonphotochemical energy quenching (NPQ) and not to the absence of zeaxanthin itself. In contrast to PSII, PSI was found to be phototolerant to chilling stress in the light in both genotypes. In the long term (10–12 days), PSII activity recovered in both npq1 and wild type, indicating that A. thaliana is able to acclimate to chilling stress in the light independently of the xanthophyll cycle. In npq1, photoacclimation involved a substantial reduction of the light-harvesting pigment antenna of PSII and an improvement of photosynthetic electron transport. Chilling stress also induced synthesis of early light-induced proteins (ELIPs) which, in the long term, disappeared in npq1 and remained stable in the wild type. In both genotypes, photoacclimation at low temperature induced the accumulation of various antioxidants including carotenoids (except β-carotene), vitamin E (α- and γ-tocopherol) and non-photosynthetic pigments (anthocyanins and other flavonoids). Analysis of flavonoid-deficient tt mutants revealed that UV/blue-light-absorbing flavonols have a strong protective function against excess visible radiations. In contrast to the defect in npq1, the absence of flavonoids could not be overcome in the long term by compensatory mechanisms, leading to extensive photooxidative and photoinhibitory damage to the chloroplasts. Depth profiling of the leaf pigments by phase-resolved photoacoustic spectroscopy showed that the flavonoid-related photoprotection was due to light trapping, which decreased chlorophyll excitation by blue light. In contrast to flavonoids, the xanthophyll cycle and the associated NPQ seem to be mainly relevant to the protection of photosynthesis against sudden increases in light intensity. [ABSTRACT FROM AUTHOR]

Brassica napus L. (oilseed rape) was grown from seeds on a reconstituted soil contaminated with cadmium (100 mg Cd kg-1 dry soil), resulting in a marked chlorosis of the leaves which was investigated using a combination of biochemical, biophysical and physiological methods. Spectroscopic and chromatographic analyses of the photosynthetic pigments indicated that chlorosis was not due to a direct interaction of Cd with the chlorophyll biosynthesis pathway. In addition, mineral deficiency and oxidative stress were apparently not involved in the pigment loss. Leaf chlorosis was attributable to a marked decrease in the chloroplast density caused by a reduction in the number of chloroplasts per cell and a change in cell size, suggesting that Cd interfered with chloroplast replication and cell division. Relatively little Cd was found in the chloroplasts and the properties of the photosynthetic apparatus (electron transport, protein composition, chlorophyll antenna size, chloroplast ultrastructure) were not affected appreciably in plants grown on Cd-polluted soil. Depth profiling of photosynthetic pigments by phase-resolved photoacoustic spectroscopy revealed that the Cd-induced decrease in pigment content was very pronounced at the leaf surface (stomatal guard cells) compared to the leaf interior (mesophyll). This observation was consistent with light transmission and fluorescence microscopy analyses, which revealed that stomata density in the epidermis was noticeably reduced in Cd-exposed leaves. Concomitantly, the stomatal conductance estimated from gas-exchange measurements was strongly reduced with Cd. When plants were grown in a high-CO2 atmosphere (4,000 μl CO2 l-1), the inhibitory effect of Cd on growth was not cancelled, suggesting that the reduced availability of CO2 at the chloroplast level associated with the low stomatal conductance was not the main component of Cd toxicity in oilseed rape. [ABSTRACT FROM AUTHOR]

Photoacoustic signals were measured in expanded tobacco leaves, exposed to a controlled atmosphere by being only partly enclosed within the photoacoustic cell. It was aimed to corroborate the conjecture of Reising and Schreiber (Photosynthesis Research 42: 65-73, 1994) that under exceptionally high CO2 levels (ca. 1–5%) the photobaric uptake contribution reflects CO2 uptake induced by light dependent stromal alkalinization. This is shown here by: (1) the shallower damping of the uptake signal vs. the modulation frequency, compared to a normal oxygen evolution signal; (2) the partial inhibition of the uptake signal under 5% CO2 by nigericin; (3) the complete absence of uptake signals under 5% CO2 in a carbonic-anhydrase-deficient mutant, which gave rather a normal oxygen evolution signal. The photoacoustic signals from the wild type and the transgenic tobacco in air could not be distinguished, indicating that the CO2 uptake signal is negligible under this condition. Uptake photobaric signals were also measured in modulated far-red light (ca. 715–750 nm), following addition of white background light (in light limiting intensity). In normal tobacco under 5% CO2, the background light induced an uptake transient, lasting about a minute, then declining to a low steady level. Significantly smaller transients were obtained under normal air, and in the carbonic-anhydrase deficient mutant also under 5% CO2. Extrapolation to zero frequency of the signal damping vs. modulation frequency, in both tobacco genotypes, suggests however similar magnitudes of the uptake transients. On the other hand, no proportional steady-state uptake was observed for the last two cases. Presumably, the steady uptake under 5% CO2 in modulated far-red light reflects CO2 solubilization, while it is an open question whether the transient could be partly contributed also by oxygen photoreduction by PS I (Mehler reaction). It is reasoned that, under conditions of low light, the respiratory activity results in accumulation of CO2 in the photoacoustic cell, which is sufficient to induce an uptake phenomenon, giving a more satisfactory interpretation for the so-called 'low light state' [Cananni and Malkin (1984) Biochim Biophys Acta 766: 525–532]. [ABSTRACT FROM AUTHOR]

When 23°C-grown potato leaves ( Solanum tuberosum L.) were exposed for 15 min to elevated temperatures in weak light, a dramatic and preferential inactivation of Photosystem (PS) II was observed at temperatures higher than about 38°C. In vivo photoacoustic measurements indicated that, concomitantly with the loss of PS II activity, heat stress induced a marked gas-uptake activity both in far-red light (>715 nm) exciting only PS I and in broadband light (350-600 nm) exciting PS I and PS II. In view of its suppression by nitrogen gas and oxygen and its stimulation by high carbon-dioxide concentrations, the bulk of the photoacoustically measured gas uptake by heat-stressed leaves was ascribed to rapid carbon-dioxide solubilization in response to light-modulated stroma alkalization coupled to PS I-driven electron transport. Heat-induced gas uptake was observed to be insensitive to the PS II inhibitor diuron, sensitive to the plastocyanin inhibitor HgCl and saturated at a rather high photon flux density of around 1200 μE m s. Upon transition from far-red light to darkness, the oxidized reaction center P700 of PS I was re-reduced very slowly in control leaves (with a half time t higher than 500 ms), as measured by leaf absorbance changes at around 820 nm. Heat stress caused a spectacular acceleration of the postillumination P700 reduction, with t falling to a value lower than 50 ms (after leaf exposure to 48°C). The decreased t was sensitive to HgCl and insensitive to diuron, methyl viologen (an electron acceptor of PS I competing with the endogenous acceptor ferredoxin) and anaerobiosis. This acceleration of the P700 reduction was very rapidly induced by heat treatment (within less than 5 min) and persisted even after prolonged irradiation of the leaves with far-red light. After heat stress, the plastoquinone pool exhibited reduction in darkness as indicated by the increase in the apparent Fo level of chlorophyll fluorescence which could be quenched by far-red light. Application (for 1 min) of far-red light to heat-pretreated leaves also induced a reversible quenching of the maximal fluorescence level Fm, suggesting formation of a pH gradient in far-red light. Taken together, the presented data indicate that PS I responded to the heat-induced loss of PS II photochemical activity by catalyzing an electron flow from stromal reductants. Heat-stress-induced PS I electron transport independent of PS II seems to constitute a protective mechanism since block of this electron pathway in anaerobiosis was observed to result in a dramatic photoinactivation of PS I. [ABSTRACT FROM AUTHOR]

When 23 °C-grown potato leaves ( Solanum tuberosum L.) were irradiated at 23 °C with a strong white light, photosynthetic electron transport and Photosystem-II (PS II) activity were inhibited in parallel. When the light treatment was given at a low temperature of 3 °C, the photoinhibition of photosynthesis was considerably enhanced, as expected. Surprisingly, no such stimulation of photoinhibition was observed with respect to the PS II function. A detailed functional analysis of the photosynthetic apparatus, using in-vivo fluorescence, absorbance, oxygen and photoacoustic measurements, and artificial electron donors/acceptors, showed a pronounced alteration of PS I activity during light stress at low temperature. More precisely, it was observed that both the pool of photooxidizeable reaction center pigment (P) of PS I and the efficiency of PS I to oxidize P were dramatically reduced. Loss of P activity was shown to be essentially dependent on atmospheric O and to require a continued flow of electrons from PS II, suggesting the involvement of the superoxide anion radical which is produced by the interaction of O and the photosynthetic electron-transfer chain through the Mehler reaction. Mass spectrometric measurements of O exchange by potato leaves under strong illumination did not reveal, however, any stimulation of the Mehler reaction at low temperature, thus leading to the conclusion that O toxicity mainly resulted from a chilling-induced inhibition of the scavenging system for O-radicals. Support for this interpretation was provided by the light response of potato leaves infiltrated with an inhibitor (diethyldithiocarbamate) of the chloroplastic Cu-Zn superoxide dismutase. It was indeed possible to simulate the differential inhibition of the PS II photochemical activity and the linear electron transport observed during light stress at low temperature by illuminating at 23 °C diethyldithiocarbamate-poisoned leaves. The experimental data presented here suggests that (i) the previously reported resistance of PS I to photoinhibition damage in-vivo is not an intrinsic property of PS I but results from efficient protective systems against O toxicity, (ii) PS I is photoinhibited in chilled potato leaf due to the inactivation of this PS I defence system and (iii) PS I is more sensitive to superoxide anion radicals than PS II. [ABSTRACT FROM AUTHOR]

The dynamics of light-induced closure of the PS II reaction centers was studied in intact, dark-adapted leaves by measuring the light-irradiance ( I) dependence of the relative variable chlorophyll fluorescence V which is the ratio between the amplitude of the variable fluorescence induced by a pulse of actinic light and the maximal variable fluorescence amplitude obtained with an intense, supersaturating light pulse. It is shown that the light-saturation curve of V is a hyperbola of order n. The experimental values of n ranged from around 0.75 to around 2, depending on the plant material and the environmental conditions. A simple theoretical analysis confirmed this hyperbolic relationship between V and I and suggested that n could represent the apparent number of photons necessary to close one reaction center. Thus, experimental conditions leading to n values higher than 1 could indicate that, from a macroscopic viewpoint, more than one photon is necessary to close one PS II center, possibly due to changes in the relative concentrations of the different redox states of the PS II reaction center complexes at the quasi-steady state induced by the actinic light. On the other hand, the existence of environmental conditions resulting in n noticeably lower than 1 suggests the possibility of an electron flow between PS II reaction center complexes. [ABSTRACT FROM AUTHOR]

The initial (F), maximal (F) and steady-state (F) levels of chlorophyll fluorescence emitted by intact pea leaves exposed to various light intensities and environmental conditions, were measured with a modulated fluorescence technique and were analysed in the context of a theory for the energy fluxes within the photochemical apparatus of photosynthesis. The theoretically derived expressions of the fluorescence signals contain only three terms, X=Jp/(1−G), Y=T/(1−G) and V, where V is the relative variable fluorescence, J is the light absorption flux in PS II, p is the probability of fluorescence from PS II, G and T are, respectively, the probabilities for energy transfer between PS II units and for energy cycling between the reaction center and the chlorophyll pool: F=X, F=X/(1−Y) and F=X(1+(YV/(1−Y))). It is demonstrated that the amplitudes of the previously defined coefficients of chlorophyll fluorescence quenching, q and q, reflect, not just photochemical (q) or nonphotochemical (q) events as implied in the definitions, but both photochemical and nonphotochemical processes of PS II deactivation. The coefficient q is a measure of the ratio between the actual macroscopic quantum yield of photochemistry in PS II (41-1) in a given light state and its maximal value measured when all PS II traps are open (41-2) in that state, with 41-3 and 41-4. When the partial connection between PS II units is taken into consideration, 1-q is nonlinearily related to the fraction of closed reaction centers and is dependent on the rate constants of all (photochemical as well as nonphotochemical) exciton-consuming processes in PS II. On the other hand, 1-q equals the (normalized) ratio of the rate constant of photochemistry (k) to the combined rate constant (k) of all the nonphotochemical deactivation processes excluding the rate constant k of energy transfer between PS II units. It is demonstrated that additional (qualitative) information on the individual rate constants, k-k and k, is provided by the fluorescence ratios 1/F and (1/F)−(1/F), respectively. Although, in theory, 41-5 is determined by the value of both k and k-k, experimental results presented in this paper show that, under various environmental conditions, 41-6 is modulated largely through changes in k, confirming the idea that PS II quantum efficiency is dynamically regulated in vivo by nonphotochemical energy dissipation. [ABSTRACT FROM AUTHOR]

A novel photosynthetic technique, photothermal deflection spectroscopy, is presented which is based on the 'mirage effect' and allows the rapid measurement of thermal deactivation of excited pigments in leaf samples placed in an open cell. Modulated heat emission from leaves illuminated with intensity-modulated light was measured via the detection of the periodic deflection of a laser beam parallel to the sample surface. Photothermal deflection signals can be monitored in vivo in leaves placed in various, liquid or gaseous, environments with a satisfactory signal-to-noise ratio close to 60-80 (in distilled water) at low modulation frequencies (below 50 Hz). Using this new and simple photothermal method, it was possible to easily obtain useful information on the leaf photochemical activity and its light-saturation characteristics under normal or stress conditions, suggesting that in vivo deflection signals could be used for assaying the photosynthetic state of health of crop plants. The beam deflection method presented in this paper appears to be a potentially useful photosynthetic tool complementary to the related photoacoustic technique. [ABSTRACT FROM AUTHOR]

The photosynthetic characteristics of leaves of atrazine-resistant and-susceptible biotypes of several weed species ( Solanum nigrum, Senecio vulgaris, Epilobium ciliatum and Chenopodium album) were compared using the photoacoustic method. Analysis of the dependence of the photoacoustic signal of the modulation frequency indicated that, in Solanum, Epilobium and Senecio, the relative quantum yield of O evolution ϕ (estimated by the ratio of the amplitude of the O signal, A, to that of the photothermal signal, A) was substantially reduced in the atrazine-resistant mutant, without any changes in the O diffusion characteristics of the leaves. In contrast, in Chenopodium, atrazine-resistance was associated with a concomitant change in ϕ and in the leaf diffusion parameters. This latter change suggests that the leaf internal anatomy was modified in the resistant Chenopodium. Measurements of the Emerson enhancement indicated that the reduction of ϕ observed in the atrazine-resistant mutants was caused by a marked decrease in the photochemical potential of PS II (β). The study of the light intensity dependence of the A/A ratio showed that saturation of O evolution occurred at the same light level (around 2000 μmol m s) in both types of plants. However, the relative maximal rate of O evolution was slightly lower (-10%) in the atrazine-resistant biotype as compared to the wild type. Reduced ϕ and light-saturated rate of O evolution were also measured in atrazine-resistant weed biotypes using a conventional Clark-type O electrode. [ABSTRACT FROM AUTHOR]

Initial (F), maximum (F) and steady-state (F) levels of modulated chlorophyll fluorescence were measured in intact avocado leaves ( Persea americana Mill.) during state 1-state 2 transitions using a combination of modulated and non-modulated lights with synchronized detection. Under normal temperature conditions (20°C), transition from state 2 to state 1 was associated with a substantial increase (about 20%) in F and F whereas the F/F ratio remained constant, reflecting increased absorption cross-section of PS II. On the contrary, at moderately elevated temperature (35°C), these fluorescence changes were very limited, indicating marked inhibition of the state regulation. The fraction of light distributed to PS II (β) was calculated from the F, F and F levels for both types of leaves. In control leaves, β varied from 48% (in state 2) to values as high as 58% (in state 1). In contrast, mild heat treatment resulted in β values close to 50% in both states, indicating the inability of heated leaves to reach extreme state 1. The results suggested that avocado leaves under moderately elevated temperature conditions are blocked in a state close to state 2. This effect was shown to occur in a non-injurious temperature range (as shown by the preservation of the (photoacoustically monitored) oxygen evolution activity) and to be rapidly reversed upon lowering of the temperature. Thermally induced development of state 2 (independent on the light spectral quality) could possibly be a protective mechanism to avoid photodamage of the heat-labile PS II by high light intensities which usually accompany heat stress in the field. [ABSTRACT FROM AUTHOR]

The temperature dependence of the yield of in vivo prompt and delayed chlorophyll fluorescence was investigated in maize and barley leaves. In the chilling-sensitive maize, delayed fluorescence at steady-state level showed a maximum near the temperature at which thylakoid membrane lipids undergo a phase transition as revealed by differential scanning calorimetry measurements. In the chilling-resistant barley, no phase transition was detected above 0°C and the delayed light emission varied in a monotonic fashion. It was shown that measurements of delayed luminescence intensity in vivo can provide a rapid and sensitive method for detecting the phase change of membrane lipids in intact leaves of chilling-sensitive plant species such as tomato, cotton, cucumber, castor bean or avocado. In contrast, the use of steady-state prompt chlorophyll fluorescence as an indicator of membrane fluidity change was not successful. [ABSTRACT FROM AUTHOR]

The temperature dependence of the yield of in vivo prompt and delayed chiorophyll fluorescence was investigated in maize and barley leaves. In the chilling-sensitive maize, delayed fluorescence at steady-state level showed a maximum near the temperature at which thylakoid membrane lipids undergo a phase transition as revealed by differential scanning calorimetry measurements. In the chilling-resistant barley, no phase transition was detected above 0°C and the delayed light emission varied in a monotonic fashion. It was shown that measurements of delayed luminescence intensity in vivo can provide a rapid and sensitive method for detecting the phase change of membrane lipids in intact leaves of chilling-sensitive plant species such as tomato, cotton, cucumber, castor bean or avocado. In contrast, the use of steady-state prompt chlorophyll fluorescence as an indicator of membrane fluidity change was not successful. [ABSTRACT FROM AUTHOR]

Exposure of barley ( Hordeum vulgare L.) leaves to strong white light (1500 μmol photons · m−2 · s−1) decreased the quantum yield of photosynthetic oxygen evolution in green light preferentially absorbed by carotenoids (Φ-510) but not in red light exclusively absorbed by chlorophylls (Φ-650). This phenomenon was observed to be (i) rapidly induced (within a few minutes), (ii) slowly reversible in darkness (within about 1 h), (iii) insensitive to dithiothreitol and (iv) maximally induced by photon flux densities higher than about 1000 μmol · m−2 · s−1. Determination of the carotenoid composition of the major light-harvesting complex of PSII (LHCII) and analysis of the thylakoid membrane lipid fluidity before and after strong illumination of barley leaves in the presence or the absence of dithiothreitol showed that the light-induced decrease in the Φ-510/Φ-650 ratio did not require the physical detachment of carotenoids from the pigment antennae. Compared to barley plants grown under moderate light and temperature conditions, plants grown in sustained high irradiance at elevated temperature exhibited (i) a lower Φ-510/Φ-650 ratio, (ii) a reduced size of the functional PSII pigment antenna in green light (but not in red light) and (iii) a marked increase in the amount of free carotenoids found in non-denaturing Deriphat-containing electrophoretic gels of thylakoid membranes. Similarly, the Φ-510/Φ-650 ratio of the LHCII-deficient chlorina-f2 barley mutant was very low compared to the wild type. Separation and quantification of the cis/trans carotenoid isomers of barley leaves revealed that strong illumination did not induce pronounced cis-trans isomerization of xanthophylls. Taken together, the data suggest that the efficiency of energy transfer from carotenoids to chlorophylls varies with the light environment both in the short term and in the long term, with excess light energy noticeably inhibiting the photosynthetic light-harvesting function of carotenoids. The photoprotective significance of this carotenoid decoupling from the chlorophyll antennae is discussed. [ABSTRACT FROM AUTHOR]

Moderately elevated temperatures induce a rapid increase in the heat and light resistance of photosystem II (PSII) in higher-plant leaves. This phenomenon was studied in intact potato leaves exposed to 35 °C for 2 h, using chlorophyll fluorometry, kinetic and difference spectrophotometry and photoacoustics. The 35 °C treatment was observed to cause energetic uncoupling between carotenoids and chlorophylls: (i) the steady-state chlorophyll fluorescence emission excited by a blue light beam (490 nm) was noticeably reduced as compared to fluorescence elicited by orange light (590 nm) and (ii) the quantum yield for photosynthetic oxygen evolution in blue light (400-500 nm) was preferentially reduced relative to the quantum yield measured in red light (590-710 nm). Analysis of the chlorophyll-fluorescence and light-absorption characteristics of the heated leaves showed numerous analogies with the fluorescence and absorption changes associated with the light-induced xanthophyll cycle activity, indicating that the carotenoid species involved in the heat-induced pigment uncoupling could be the xanthophyll violaxanthin. More precisely, the 35 °C treatment was observed to accelerate and amplify the non-photochemical quenching of chlorophyll fluorescence (in both moderate red light and strong white light) and to cause an increase in leaf absorbance in the blue-green spectral region near 520 nm, as do strong light treatments which induce the massive conversion of violaxanthin to zeaxanthin. Interestingly, short exposure of potato leaves to strong light also provoked a significant increase in the stability of PSII to heat stress. It was also observed that photosynthetic electron transport was considerably more inhibited by chilling temperatures in 35 °C-treated leaves than in untreated leaves. Further, pre-exposure of potato leaves to 35 °C markedly increased the amplitude and the rate of light-induced changes in leaf absorbance at 505 nm (indicative of xanthophyll cycle activity), suggesting the possibility that moderately elevated temperature increased the accessibility of violaxanthin to the membrane-located de-epoxidase. This was supported by the quantitative analysis of the xanthophyll-cycle pigments before and after the 35 °C treatment, showing light-independent accumulation of zeaxanthin during mild heat stress. Based on these results, we propose that the rapid adjustment of the heat resistance of PSII may involve a modification of the interaction between violaxanthin and the light-harvesting complexes of PSII. As a consequence, the thermoresistance of PSII could be enhanced either directly through a conformational change of PSII or indirectly via a carotenoid-dependent modulation of membrane lipid fluidity. [ABSTRACT FROM AUTHOR]

The photoacoustic technique was used to measure energy storage by cyclic electron transfer around photosystem I in intact Chlamydomonas reinhardtii cells illuminated with far-red light (>715 nm). The in-vivo cyclic pathway was characterized by investigating the effects of various chemicals on energy storage. Participation of plastoquinone and ferredoxin in the cyclic electron flow was confirmed by the complete suppression of energy storage in the presence of the plastoquinol antagonist 2,5-dibromo-3-methyl-6-isopropyl- p-benzoquinone (DBMIB) and the ferredoxin inhibitors/competitors methylviologen, phenylmercuric acetate and p-benzoquinone. Two alternative electron cycles are demonstrated to operate in vivo. One cycle is sensitive to antimycin A, myxothiazol and 2-( n-heptyl)-4-hydroxyquinoline N-oxide (HQNO) and is catalyzed by ferredoxin which reduces plastoquinone through a route involving cytochrome b and its protonmotive Q-cycle. The other cycle is unaffected by the above-mentioned inhibitors but is sensitive to N-ethylmaleimide (NEM), an inhibitor of the ferredoxin-NADP reductase, and 2′-monophosphoadenosine-5′-diphosphoribose (PADR), an analogue of NADP, showing that the electron recycling was mediated by NADPH. Possibly, electrons enter the plastoquinone pool through the action of a NAD(P)H dehydrogenase, which is insensitive to classical inhibitors of the mitochondrial NADH dehydrogenase. Loss of energy storage by photosystem-I-driven cyclic electron transfer in farred light was observed only when antimycin A, myxothiazol or HQNO was used in combination with NEM or PADR. Analysis of the light-intensity dependence and the rate of in-vivo cyclic electron transfer in the presence of various inhibitors indicates that the NADPH-dependent electron-cycle is the preferential cyclic pathway in Chlamydomonas cells illuminated with far-red light. [ABSTRACT FROM AUTHOR]

Fluorimetric, photoacoustic, polarographic and absorbance techniques were used to measure in situ various functional aspects of the photochemical apparatus of photosynthesis in intact pea leaves ( Pisum sativum L.) after short exposures to a high temperature of 40 ° C. The results indicated (i) that the in-vivo responses of the two photosystems to high-temperature pretreatments were markedly different and in some respects opposite, with photosystem (PS) II activity being inhibited (or down-regulated) and PSI function being stimulated; and (ii) that light strongly interacts with the response of the photosystems, acting as an efficient protector of the photochemical activity against its inactivation by heat. When imposed in the dark, heat provoked a drastic inhibition of photosynthetic oxygen evolution and photochemical energy storage, correlated with a marked loss of variable PSII-chlorophyll fluorescence emission. None of the above changes were observed in leaves which were illuminated during heating. This photoprotection was saturated at rather low light fluence rates (around 10 W · m). Heat stress in darkness appeared to increase the capacity for cyclic electron flow around PSI, as indicated by the enhanced photochemical energy storage in far-red light and the faster decay of P(oxidized reaction center of PSI) monitored upon sudded interruption of the far-red light. The presence of light during heat stress reduced somewhat this PSI-driven cyclic electron transport. It was also observed that heat stress in darkness resulted in the progressive closure of the PSI reaction centers in leaves under steady illumination whereas PSII traps remained largely open, possibly reflecting the adjustment of the photochemical efficiency of undamaged PSI to the reduced rate of photochemistry in PSII. [ABSTRACT FROM AUTHOR]

Photosynthesis produces organic carbon via a light-driven electron flow from H2O to CO2 that passes through a pool of plastoquinone molecules. These molecules are either present in the photosynthetic thylakoid membranes, participating in photochemistry (photoactive pool), or stored (non-photoactive pool) in thylakoid-attached lipid droplets, the plastoglobules. The photoactive pool acts also as a signal of photosynthetic activity allowing the adaptation to changes in light condition. Here we show that, in Arabidopsis thaliana, proton gradient regulation 6 (PGR6), a predicted atypical kinase located at plastoglobules, is required for plastoquinone homoeostasis, i.e. to maintain the photoactive plastoquinone pool. In a pgr6 mutant, the photoactive pool is depleted and becomes limiting under high light, affecting short-term acclimation and photosynthetic efficiency. In the long term, pgr6 seedlings fail to adapt to high light and develop a conditional variegated leaf phenotype. Therefore, PGR6 activity, by regulating plastoquinone homoeostasis, is required to cope with high light. Thibaut Pralon et al. show that a predicted atypical kinase located in plastoglobules (PGR6) is necessary for maintaining the pool of photoactive plastoquinone in the chloroplast. They find that the absence of this gene in mutant plants adversely affects photosynthetic efficiency. [ABSTRACT FROM AUTHOR]

In the last common enzymatic step of tetrapyrrole biosynthesis, prior to the branching point leading to the biosynthesis of heme and chlorophyll, protoporphyrinogen IX (Protogen) is oxidised to protoporphyrin IX (Proto) by protoporphyrinogen IX oxidase (PPX). The absence of thylakoid-localised plastid terminal oxidase 2 (PTOX2) and cytochrome b6f complex in the ptox2 petB mutant, results in almost complete reduction of the plastoquinone pool (PQ pool) in light. Here we show that the lack of oxidised PQ impairs PPX function, leading to accumulation and subsequently uncontrolled oxidation of Protogen to non-metabolised Proto. Addition of 3(3,4-Dichlorophenyl)-1,1-dimethylurea (DCMU) prevents the over-reduction of the PQ pool in ptox2 petB and decreases Proto accumulation. This observation strongly indicates the need of oxidised PQ as the electron acceptor for the PPX reaction in Chlamydomonas reinhardtii. The PPX-PQ pool interaction is proposed to function as a feedback loop between photosynthetic electron transport and chlorophyll biosynthesis. Pawel Brzezowski et al. demonstrated existence of a link between chlorophyll biosynthesis and photosynthetic electron transport in Chlamydomonas reinhardtii. The lack of oxidised plastoquinone pool impairs function of PROTOPORPHYRINOGEN IX OXIDASE, leading to accumulation of non-metabolised protoporphyrin IX. [ABSTRACT FROM AUTHOR]

Plastoquinone-9 is known as a photosynthetic electron carrier to which has also been attributed a role in the regulation of gene expression and enzyme activities via its redox state. Here, we show that it acts also as an antioxidant in plant leaves, playing a central photoprotective role. When Arabidopsis plants were suddenly exposed to excess light energy, a rapid consumption of plastoquinone-9 occurred, followed by a progressive increase in concentration during the acclimation phase. By overexpressing the plastoquinone-9 biosynthesis gene SPS1 (SOLANESYL DIPHOSPHATE SYNTHASE 1) in Arabidopsis, we succeeded in generating plants that specifically accumulate plastoquinone-9 and its derivative plastochromanol-8. The SPS1-overexpressing lines were much more resistant to photooxidative stress than the wild type, showing marked decreases in leaf bleaching, lipid peroxidation and PSII photoinhibition under excess light. Comparison of the SPS1 overexpressors with other prenyl quinone mutants indicated that the enhanced phototolerance of the former plants is directly related to their increased capacities for plastoquinone-9 biosynthesis. [ABSTRACT FROM AUTHOR]