Functional aspects of the photosynthetic light reactions in heat stressed Arabidopsis deficient in digalactosyl-diacylglycerol

Plants are often submitted, in their natural environment, to various abiotic stresses such as heat stress. However, elevated temperature has a detrimental impact on overall plant growth and development. We have examined the physiological response of the dgd1-2 and dgd1-3 Arabidopsis mutants lacking 30-40% of digalactosyl-diacylglycerol (DGDG) exposed to heat constraint. These mutants, which grow similarly to wild type under normal conditions, were previously reported to be defective in basal thermotolerance as measured by cotyledon development. However their functional properties were not described. Chlorophyll fluorescence measurements and absorbance changes at 820 nm were used to monitor photosystem II (PSII) and PSI activity, respectively. It was observed that both mutants have similar photosystem activities with some differences. The mutants were less able to use near saturation light energy and elicited higher rates of cyclic PSI electron flow compare to wild type. Arabidopsis leaves exposed to short-term (5 min) mild (40 °C) or strong (44 °C) heat treatment have shown a decline in the operating effective quantum yield of PSII and in the proportion of active PSI reaction centers. However, cyclic PSI electron flow was enhanced. The establishment of the energy-dependent non-photochemical quenching of chlorophyll fluorescence was accelerated but its decline under illumination was inhibited. Furthermore, heat stress affected the process implicated in the redistribution of light excitation energy between the photosystems known as the light state transitions. All the effects of heat stress mentioned above were more intense in the mutant leaves with dgd1-3 being even more susceptible. The decreased DGDG content of the thylakoid membranes together with other lipid changes are proposed to influence the thermo-sensitivity of the light reactions of photosynthesis towards heat stress.     

Digalactosyldiacylglycerol synthesis in chloroplasts of the Arabidopsis dgd1 mutant

Galactolipid biosynthesis in plants is highly complex. It involves multiple pathways giving rise to different molecular species. To assess the contribution of different routes of galactolipid synthesis and the role of molecular species for growth and photosynthesis, we initiated a genetic approach of analyzing double mutants of the digalactosyldiacylglycerol (DGDG) synthase mutant dgd1 with the acyltransferase mutant, act1, and the two desaturase mutants, fad2 and fad3. The double mutants showed different degrees of growth retardation: act1,dgd1 was most severely affected and growth of fad2,dgd1 was slightly reduced, whereas fad3,dgd1 plants were very similar to dgd1. In act1,dgd1, lipid and chlorophyll content were reduced and photosynthetic capacity was affected. Molecular analysis of galactolipid content, fatty acid composition, and positional distribution suggested that the growth deficiency is not caused by changes in galactolipid composition per se. Chloroplasts of dgd1 were capable of synthesizing monogalactosyldiacylglycerol, DGDG, and tri- and tetragalactosyldiacylglycerol. Therefore, the reduced growth of act1,dgd1 and fad2,dgd1 cannot be explained by the absence of DGDG synthase activity from chloroplasts. Molecular analysis of DGDG accumulating in the mutants during phosphate deprivation suggested that similarly to the residual DGDG of dgd1, this additional lipid is synthesized in association with chloroplast membranes through a pathway independent of the mutations, act1, dgd1, fad2, and fad3. Our data imply that the severe growth defect of act1,dgd1 is caused by a reduced metabolic flux of chloroplast lipid synthesis through the eukaryotic and prokaryotic pathway as well as by the reduction of photosynthetic capacity caused by the destabilization of photosynthetic complexes.     

Effect of moderate and high light on photosystem II function in Arabidopsis thaliana depleted in digalactosyl-diacylglycerol

The response of the heat-sensitive dgd1-2 and dgd1-3 Arabidopsis mutants depleted in the galactolipid DGDG to photoinhibition of chloroplasts photosystem II was studied to verify if there is a relationship between heat stress vulnerability due to depletion in DGDG and the susceptibility to photoinhibitory damage. Non-photochemical quenching (NPQ) is known to dissipate excessive absorbed light energy as heat to protect plants against photodamage. The main component of NPQ is dependent of the transthylakoid pH gradient and is modulated by zeaxanthin (Zx) synthesis. These processes together with chlorophyll fluorescence induction were used to characterize the response of the genotypes. The mutants were more sensitive to photoinhibition to a small extent but this was more severe for dgd1-3 especially at high light intensity. It was deduced that DGDG was not a main factor to influence photoinhibition but other lipid components could affect PSII sensitivity towards photoinhibition in relation to the physical properties of the thylakoid membrane. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.     

Characterization of the Arabidopsis thermosensitive mutant atts02 reveals an important role for galactolipids in thermotolerance

Plants are constantly challenged with various abiotic stresses in their natural environment. Elevated temperatures have a detrimental impact on overall plant growth and productivity. Many plants increase their tolerance to high temperatures through an adaptation response known as acquired thermotolerance. To identify the various mechanisms that plants have evolved to cope with high temperature stress, we have isolated a series of Arabidopsis mutants that are defective in the acquisition of thermotolerance after an exposure to 38 degrees C, a treatment that induces acquired thermotolerance in wild-type plants. One of these mutants, atts02, was not only defective in acquiring thermotolerance after the treatment, but also displayed a reduced level of basal thermotolerance in a 30 degrees C growth assay. The affected gene in atts02 was identified by positional cloning and encodes digalactosyldiacylglycerol synthase 1 (DGD1) (the atts02 mutant was, at that point, renamed dgd1-2). An additional dgd1 allele, dgd1-3, was identified in two other mutant lines displaying altered acquired thermotolerance, atts100 and atts104. Expression patterns of several heat shock proteins (HSPs) in heat-treated dgd1-2 homozygous plants were similar to those from identically treated wild-type plants, suggesting that the thermosensitivity in the dgd1-2 mutant was not caused by a defect in HSP induction. Lipid analysis of wild-type and mutant plants indicated a close correlation between the ability to acquire thermotolerance and the increases in digalactosyldiacylglycerol (DGDG) level and in the ratio of DGDG to monogalactosyldiacylglycerol (MGDG). Thermosensitivity in dgd1-2 and dgd1-3 was associated with (1) a decreased DGDG level and (2) an inability to increase the ratio of DGDG to MGDG upon exposure to a 38 degrees C sublethal temperature treatment. Our results suggest that the DGDG level and/or the ratio of DGDG to MGDG may play an important role in basal as well as acquired thermotolerance in Arabidopsis.     

Changes in the composition of the photosynthetic apparatus in the galactolipid-deficient dgd1 mutant of Arabidopsis thaliana.

The glycerolipid digalactosyl diacylglycerol (DGDG) is exclusively associated with photosynthetic membranes and thus may play a role in the proper assembly and maintenance of the photosynthetic apparatus. Here we employ a genetic approach based on the dgd1 mutant of Arabidopsis thaliana to investigate the function of DGDG in thylakoid membranes. The primary defect in the genetically well-characterized dgd1 mutant resulted in a 90% reduction of the DGDG content. The mutant showed a decreased photosystem II (PSII) to photosystem I ratio. In vivo room- and low-temperature (77 K) chlorophyll fluorescence measurements with thylakoid preparations are in agreement with a drastically altered excitation energy allocation to the reaction centers. Quantification of pigment-binding apoproteins and pigments supports an altered stoichiometry of individual pigment-protein complexes in the mutant. Most strikingly, an increase in the amount of peripheral light-harvesting complexes of PSII relative to the inner antenna complexes and the PSII reaction center/core complexes was observed. Regardless of the severe alterations in thylakoid organization, photosynthetic oxygen evolution was virtually not compromised in dgd1 mutant leaves.     

Cold acclimation of the Arabidopsis dgd1 mutant results in recovery from photosystem I-limited photosynthesis

We compared the thylakoid membrane composition and photosynthetic properties of non- and cold-acclimated leaves from the dgd1 mutant (lacking >90% of digalactosyl-diacylglycerol; DGDG) and wild type (WT) Arabidopsis thaliana. In contrast to warm grown plants, cold-acclimated dgd1 leaves recovered pigment-protein pools and photosynthetic function equivalent to WT. Surprisingly, this recovery was not correlated with an increase in DGDG. When returned to warm temperatures the severe dgd1 mutant phenotype reappeared. We conclude that the relative recovery of photosynthetic activity at 5 degrees C resulted from a temperature/lipid interaction enabling the stable assembly of PSI complexes in the thylakoid.     

Disruption of the two digalactosyldiacylglycerol synthase genes DGD1 and DGD2 in Arabidopsis reveals the existence of an additional enzyme of galactolipid synthesis

Two genes (DGD1 and DGD2) are involved in the synthesis of the chloroplast lipid digalactosyldiacylglycerol (DGDG). The role of DGD2 for galactolipid synthesis was studied by isolating Arabidopsis T-DNA insertional mutant alleles (dgd2-1 and dgd2-2) and generating the double mutant line dgd1 dgd2. Whereas the growth and lipid composition of dgd2 were not affected, only trace amounts of DGDG were found in dgd1 dgd2. The growth and photosynthesis of dgd1 dgd2 were affected more severely compared with those of dgd1, indicating that the residual amount of DGDG in dgd1 is crucial for normal plant development. DGDG synthesis was increased after phosphate deprivation in the wild type, dgd1, and dgd2 but not in dgd1 dgd2. Therefore, DGD1 and DGD2 are involved in DGDG synthesis during phosphate deprivation. DGD2 was localized to the outer side of chloroplast envelope membranes. Like DGD2, heterologously expressed DGD1 uses UDP-galactose for galactosylation. Galactolipid synthesis activity for monogalactosyldiacylglycerol (MGDG), DGDG, and the unusual oligogalactolipids tri- and tetragalactosyldiacylglycerol was detected in isolated chloroplasts of all mutant lines, including dgd1 dgd2. Because dgd1 and dgd2 carry null mutations, an additional, processive galactolipid synthesis activity independent from DGD1 and DGD2 exists in Arabidopsis. This third activity, which is related to the Arabidopsis galactolipid:galactolipid galactosyltransferase, is localized to chloroplast envelope membranes and is capable of synthesizing DGDG from MGDG in the absence of UDP-galactose in vitro, but it does not contribute to net galactolipid synthesis in planta.     

A novel P700 redox kinetics probe for rapid,non‐intrusive and whole‐tissue determination of photosystem II functionality,and the stoichiometry of the two photosystems in vivo

We sought a rapid, non‐intrusive, whole‐tissue measure of the functional photosystem II (PS II) content in leaves. Summation of electrons, delivered by a single‐turnover flash to P700⁺ (oxidized PS I primary donor) in continuous background far‐red light, gave a parameter S in absorbance units after taking into account an experimentally determined basal electron flux that affects P700 redox kinetics. S was linearly correlated with the functional PS II content measured by the O₂ yield per single‐turnover repetitive flash in Arabidopsis thaliana expressing an antisense construct to the PsbO (manganese‐stabilizing protein in PS II) proteins of PS II (PsbO mutants). The ratio of S to z_max (total PS I content in absorbance units) was comparable to the PS II/PS I reaction‐center ratio in wild‐type A. thaliana and in control Spinacea oleracea. Both S and S/z_max decreased in photoinhibited spinach leaf discs. The whole‐tissue functional PS II content and the PS II/photosystem I (PS I) ratio can be non‐intrusively monitored by S and S/z_max, respectively, using a quick P700 absorbance protocol compatible with modern P700 instruments.     

PS II model based analysis of transient fluorescence yield measured on whole leaves of Arabidopsis thaliana after excitation with light flashes of different energies

Our recently presented PS II model (Belyaeva et al., 2008) was improved in order to permit a consistent simulation of Single Flash Induced Transient Fluorescence Yield (SFITFY) traces that were earlier measured by Steffen et al. (2005) on whole leaves of Arabidopsis (A.) thaliana at four different energies of the actinic flash. As the essential modification, the shape of the actinic flash was explicitly taken into account assuming that an exponentially decaying rate simulates the time dependent excitation of PS II by the 10 ns actinic flash. The maximum amplitude of this excitation exceeds that of the measuring light by 9 orders of magnitude. A very good fit of the SFITFY data was achieved in the time domain from 100 ns to 10 s for all actinic flash energies (the maximum energy of 7.5 × 10¹⁶ photons/(cm² flash) is set to 100%, the relative energies of weaker actinic flashes were of ∼8%, 4%, ∼1%). Our model allows the calculation and visualization of the transient PS II redox state populations ranging from the dark adapted state, via excitation energy and electron transfer steps induced by pulse excitation, followed by final relaxation into the stationary state eventually attained under the measuring light. It turned out that the rate constants of electron transfer steps are invariant to intensity of the actinic laser flash. In marked contrast, an increase of the actinic flash energy by more than two orders of magnitude from 5.4 × 10¹⁴ photons/(cm² flash) to 7.5 × 10¹⁶ photons/(cm² flash), leads to an increase of the extent of fluorescence quenching due to carotenoid triplet (³Car) formation by a factor of 14 and of the recombination reaction between reduced primary pheophytin (Phe⁻) and P680⁺ by a factor of 3 while the heat dissipation in the antenna complex remains virtually constant.The modified PS II model offers new opportunities to compare electron transfer and dissipative parameters for different species (e.g. for the green algae and the higher plant) under varying illumination conditions.     

A portable multi-flash kinetic fluorimeter for measurement of donor and acceptor reactions of Photosystem 2 in leaves of intact plants under field conditions

A newly-developed field-portable multi-flash kinetic fluorimeter for measuring the kinetics of the microsecond to millisecond reactions of the oxidizing and reducing sides of photosystem 2 in leaves of intact plants is described and demonstrated. The instrumental technique is a refinement of that employed in the double-flash kinetic fluorimeter (Joliot 1974 Biochim Biophys Acta 357: 439–448) where a low-intensity short-duration light pulse is used to measure the fluorescence yield changes following saturating single-turnover light pulses. The present instrument uses a rapid series of short-duration (2 s) pulses to resolve a complete microsecond to millisecond time-scale kinetic trace of fluorescence yield changes after each actinic flash. Differential optics, using a matrix of optical fibers, allow very high sensitivity (noise levels about 0.05% F_max) thus eliminating the need for signal averaging, and greatly reducing the intensity of light required to make a measurement. Consequently, the measuring pulses have much less actinic effect and an entire multi-point trace (seven points) excites less than 1% of the reaction centers in a leaf. In addition, bu combining the actinic and measuring pulse light in the optical fiber network, the tail of the actinic flash can be compensated for, allowing measurements of events as rapidly as 20 s after the actinic flash. This resolution makes practical the routine measurement of the microsecond turnover kinetics of the oxygen evolving complex in leaves of intact plants in the field. The instrument is demonstrated by observing flash number dependency and inhibitor sensitivity of the induction and decay kinetics of flash-induced fluorescence transients in leaves of intact plants. From these traces the period-two oscillations associated with the turnover of the two-electron gate and the period-four oscillations associated with the turnover of the oxygen evolving complex can be observed. Applications of the instrument to extending our knowledge of chloroplast function to the whole plant, the effects on plants of environmental stress, herbicides, etc, and possible applications to screening of mutants are discussed.Abbreviations DCMU 3-(3,4-Dichlorophenol)-1,1-dimethylurea - PS 2 photosystem 2 - PS 1 photosystem 1 - P₆₈₀ primary electron donor of the PS 2 reaction center - Q_A primary acceptor quinone of PS 2 - Q_B secondary acceptor quinone of PS 2 - CCCP carbonyl cyanide-m-chlorophenylhydrazone - Y_z donor to P₆₈₀ ⁺ - F₀ level of fluorescence with all PS 2 centers open - F_max maximum level of fluorescence with all PS 2 centers closed - P₆₈₀Q_A Open reaction centers with P₆₈₀ reduced and Q_A oxidized (low fluorescence) - P₆₈₀Q_A ^- Closed reaction centers, in which P₆₈₀ is reduced (high fluorescence) - P₆₈₀ ⁺Q_A ^- Closed reaction centers, in which P₆₈₀ is oxidized (low fluorescence)     

Can digalactosyldiacylglycerol substitute for phosphatidylcholine upon phosphate deprivation in leaves and roots of Arabidopsis?

To explore the role of digalactosyldiacylglycerol (DGDG) in plants the dgd1 mutant of Arabidopsis thaliana was grown in the presence and absence of inorganic phosphate. Phosphate deficiency in the dgd1 mutant causes a strong decrease in all phospholipids accompanied by an increase in DGDG and sulpholipid. Moreover, a significant DGDG accumulation was found in roots upon phosphate deprivation as well. Our data indicate that DGDG accumulation upon phosphate deprivation is due to the activation of a specific eukaryotic dgd1-independent biosynthetic pathway. We propose that DGDG may substitute for phosphatidylcholine upon phosphate deprivation.     

Quantifying and monitoring functional photosystem II and the stoichiometry of the two photosystems in leaf segments: approaches and approximations

Given its unique function in light-induced water oxidation and its susceptibility to photoinactivation during photosynthesis, photosystem II (PS II) is often the focus of studies of photosynthetic structure and function, particularly in environmental stress conditions. Here we review four approaches for quantifying or monitoring PS II functionality or the stoichiometry of the two photosystems in leaf segments, scrutinizing the approximations in each approach. (1) Chlorophyll fluorescence parameters are convenient to derive, but the information-rich signal suffers from the localized nature of its detection in leaf tissue. (2) The gross O(2) yield per single-turnover flash in CO(2)-enriched air is a more direct measurement of the functional content, assuming that each functional PS II evolves one O(2) molecule after four flashes. However, the gross O(2) yield per single-turnover flash (multiplied by four) could over-estimate the content of functional PS II if mitochondrial respiration is lower in flash illumination than in darkness. (3) The cumulative delivery of electrons from PS II to P700(+) (oxidized primary donor in PS I) after a flash is added to steady background far-red light is a whole-tissue measurement, such that a single linear correlation with functional PS II applies to leaves of all plant species investigated so far. However, the magnitude obtained in a simple analysis (with the signal normalized to the maximum photo-oxidizable P700 signal), which should equal the ratio of PS II to PS I centers, was too small to match the independently-obtained photosystem stoichiometry. Further, an under-estimation of functional PS II content could occur if some electrons were intercepted before reaching PS I. (4) The electrochromic signal from leaf segments appears to reliably quantify the photosystem stoichiometry, either by progressively photoinactivating PS II or suppressing PS I via photo-oxidation of a known fraction of the P700 with steady far-red light. Together, these approaches have the potential for quantitatively probing PS II in vivo in leaf segments, with prospects for application of the latter two approaches in the field.     

Evolution of PS IIα and PS IIβ centers during the greening of Euglena gracilis Z: Correlations with changes in lipid content

The building up of the two types of reaction centers, PS II and PS II, was investigated during the greening of Euglena gracilis Z cells in resting medium. The maximal values in the proportion of PS II centers (55%) and in the oxygen evolved per chlorophyll were reached at the outbreak of greening, when accumulation of galactolipids (MGDG and DGDG) rich in unsaturated fatty acids occurred, and when anionic lipids (SQDG and PG) emerged. As the greening progressed, the chlorophyll accumulation corresponded to a secondary enrichment in PS II centers, which built up more rapidly than PS II centers; correlatively, a general saturation of the fatty acids constitutive of all lipid classes took place.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DGDG digalactosyldiacylglycerol - FAME Tatty acid methyl esters - HEPES acide (N-[2-hydroxyethyl]piperazine-N-[2-ethane sulfonic] - MGDG monogalactosyldiacylglycerol - PC phosphatidylcholine - PE phosphatidylethanolamine - PG phosphatidylglycerol - PQ plastoquinone - PS I Photosystem I - PS II Photosystem II - Q_A primary quinone electron acceptor of PS II - Q_B secondary quinone electron acceptor of PS II - SQDG sulfoquinovosyldiacylglycerol     

Light-harvesting chlorophyll a-b complex requirement for regulation of Photosystem II photochemistry by non-photochemical quenching

Recently, it has been suggested (Horton et al. 1992) that aggregation of the light-harvesting a-b complex (LHC II) in vitro reflects the processes which occur in vivo during fluorescence induction and related to the major non-photochemical quenching (qE). Therefore the requirement of this chlorophyll a-b containing protein complex to produce qN was investigated by comparison of two barley mutants either lacking (chlorina f2) or depressed (chlorina¹⁰⁴) in LHC II to the wild-type and pea leaves submitted to intermittent light (IL) and during their greening in continuous light. It was observed that qN was photoinduced in the absence of LHC II, i.e. in IL grown pea leaves and the barley mutants. Nevertheless, in these leaves qN had no (IL, peas) or little (barley mutants) inhibitory effect on the photochemical efficiency of Q_A reduction measured by flash dosage response curves of the chlorophyll fluorescence yield increase induced by a single turn-over flash During greening in continuous light of IL pea leaves, an inhibitory effect on Q_A photoreduction associated to qN developed as Photosystem II antenna size increased with LHC II synthesis. Utilizing data from the literature on connectivity between PS II units versus antenna size, the following hypothesis is put forward to explain the results summarized above. qN can occur in the core antenna or Reaction Center of a fraction of PS II units and these units will not exhibit variable fluorescence. Other PS II units are quenched indirectly through PS II-PS II exciton transfer which develops as the proportion of connected PS II units increases through LHC II synthesis.     

Evaluation of the role of State transitions in determining the efficiency of light utilisation for CO2 assimilation in leaves

Wheat leaves were exposed to light treatments that excite preferentially Photosystem I (PS I) or Photosystem II (PS II) and induce State 1 or State 2, respectively. Simultaneous measurements of CO₂ assimilation, chlorophyll fluorescence and absorbance at 820 nm were used to estimate the quantum efficiencies of CO₂ assimilation and PS II and PS I photochemistry during State transitions. State transitions were found to be associated with changes in the efficiency with which an absorbed photon is transferred to an open PS II reaction centre, but did not correlate with changes in the quantum efficiencies of PS II photochemistry or CO₂ assimilation. Studies of the phosphorylation status of the light harvesting chlorophyll protein complex associated with PS II (LHC II) in wheat leaves and using chlorina mutants of barley which are deficient in this complex demonstrate that the changes in the effective antennae size of Photosystem II occurring during State transitions require LHC II and correlate with the phosphorylation status of LHC II. However, such correlations were not found in maize leaves. It is concluded that State transitions in C₃ leaves are associated with phosphorylation-induced modifications of the PS II antennae, but these changes do not serve to optimise the use of light absorbed by the leaf for CO₂ assimilation.Abbreviations Fm, Fo, Fv maximal, minimal and variable fluorescence yields - Fm, Fv maximal and variable fluorescence yields in a light adapted state - LHC II light harvesting chlorophyll a/b protein complex associated with PS II - q_P photochemical quenching - A₈₂₀ light-induced absorbance change at 820 nm - _{PS I}, _{PS II} relative quantum efficiencies of PS I and PS II photochemistry - _{CO 2} quantum yield of CO₂ assimilation     

Decreased stability of photosystem I in dgd1 mutant of Arabidopsis thaliana

The dgd1 mutant of Arabidopsis thaliana provides us with a powerful tool for revealing the specific role of digalactosyldiacylglycerol (DGDG) in photosynthesis. Blue-native polyacrylamide gel electrophoresis analysis revealed that photosystem I (PSI) subunits are assembled into a PSI complex, and that a PSI subcomplex lacking stroma side subunits was also present. PSI subunits in the dgd1 mutant were decreased to a similar level compared with that in the wild type (WT) Arabidopsis. Further experiments showed that PSI subunits in the stroma side, PsaD and PsaE, in the dgd1 mutant were more susceptible to removal by chaotropic agents than those in the WT plant, indicating that the stability of PsaD and PsaE is impaired in the dgd1 mutant. These results provide evidence that DGDG is important for the stability of the PSI complex.     

Fluorometric equipment for monitoring P680+• reduction in PS II preparations and green leaves

Newly developed equipment is described that permits the monitoring of laser flash induced transients of the normalised chlorophyll-a fluorescence quantum yield in isolated PS II preparations and whole leaves with a high time resolution. The essential operational unit of the set-up is a rapidly gated photomultiplier. In this way, the fluorescence artefact, due to the high intensity excitation laser flash, is sufficiently suppressed and the dead time of the signal response is reduced to about 500 ns. It is shown that the fluorescence rise kinetics in the s time-domain, after flash excitation is strongly dependent on the redox state of the primary electron donor of PS II (P680). At high excitation energies, the decay of carotenoid triplets, which are very efficient quenchers of chlorophyll singlet states, dominates the rise kinetics of the flash induced fluorescence yield in the s time domain.     

Dynamics of electron transfer within and between PS II reaction center complexes indicated by the light-saturation curve of in vivo variable chlorophyll fluorescence emission

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.Abbreviations F₀ and F_m minimal and maximal levels of chlorophyll fluorescence emission, respectively - F_p peak fluorescence induced by a pulse of actinic light - I incident light irradiance (in W m^-2) - PS II Photosystem II - P₆₈₀ PS II reaction center - Q_A and Q_B primary and secondary (stable) electron acceptors of PS II - V relative variable chlorophyll fluorescence % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0Jf9crFfpeea0xh9v8qiW7rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaiikaiaadA% facqGH9aqpcaGGOaGaaeOramaaBaaaleaacaqGWbaabeaakiabgkHi% TiaabAeadaWgaaWcbaGaaeimaaqabaGccaGGPaGaai4laiaacIcaca% qGgbWaaSbaaSqaaiaab2gaaeqaaOGaeyOeI0IaaeOramaaBaaaleaa% caqGWaaabeaakiaacMcacaGGPaaaaa!47BD!\[(V = ({\text{F}}_{\text{p}} - {\text{F}}_{\text{0}} )/({\text{F}}_{\text{m}} - {\text{F}}_{\text{0}} ))\]     

Consequences of LHC II deficiency for photosynthetic regulation in chlorina mutants of barley

The light-harvesting chlorophyll a/b proteins associated with PS II (LHC II) are often considered to have a regulatory role in photosynthesis. The photosynthetic responses of four chlorina mutants of barley, which are deficient in LHC II to varying degrees, are examined to evaluate whether LHC II plays a regulatory role in photosynthesis. The efficiencies of light use for PS I and PS II photochemistry and for CO₂ assimilation in leaves of the mutants were monitored simultaneously over a wide range of photon flux densities of white light in the presence and absence of supplementary red light. It is demonstrated that the depletions of LHC II in these mutants results in a severe imbalance in the relative rates of excitation of PS I and PS II in favour of PS I, which cannot be alleviated by preferential excitation of PS II. Analyses of xanthophyll cycle pigments and fluorescence quenching in leaves of the mutants indicated that the major LHC II components are not required to facilitate the light-induced quenching associated with zeaxanthin formation. It is concluded that LHC II is important to balance the distribution of excitation energy between PS I and PS II populations over a wide range of photon flux densities. It appears that LHC II may also be important in determining the quantum efficiency of PS II photochemistry by reducing the rate of quenching of excitation energy in the PS II primary antennae.Abbreviations Fm, Fv maximal and variable fluorescence yields in a light adapted state - LHC II light harvesting chlorophyll a/b protein complex associated with PS II - q_p photochemical quenching - A₈₂₀ light-induced absorbance change at 820 nm - ø_PSI, ø_PSII relative quantum efficiencies of PS I and PS II photochemistry - øCO₂ quantum yield of CO₂ assimilation