Photosynthetic quantum yield dynamics: from photosystems to leaves

The mechanisms underlying the wavelength dependence of the quantum yield for CO(2) fixation (α) and its acclimation to the growth-light spectrum are quantitatively addressed, combining in vivo physiological and in vitro molecular methods. Cucumber (Cucumis sativus) was grown under an artificial sunlight spectrum, shade light spectrum, and blue light, and the quantum yield for photosystem I (PSI) and photosystem II (PSII) electron transport and α were simultaneously measured in vivo at 20 different wavelengths. The wavelength dependence of the photosystem excitation balance was calculated from both these in vivo data and in vitro from the photosystem composition and spectroscopic properties. Measuring wavelengths overexciting PSI produced a higher α for leaves grown under the shade light spectrum (i.e., PSI light), whereas wavelengths overexciting PSII produced a higher α for the sun and blue leaves. The shade spectrum produced the lowest PSI:PSII ratio. The photosystem excitation balance calculated from both in vivo and in vitro data was substantially similar and was shown to determine α at those wavelengths where absorption by carotenoids and nonphotosynthetic pigments is insignificant (i.e., >580 nm). We show quantitatively that leaves acclimate their photosystem composition to their growth light spectrum and how this changes the wavelength dependence of the photosystem excitation balance and quantum yield for CO(2) fixation. This also proves that combining different wavelengths can enhance quantum yields substantially.     

An alternate photosynthetic electron donor system for PSI supports light dependent nitrogen fixation in a non-heterocystous cyanobacterium,Plectonema boryanum

Plectonema boryanum exhibits temporal separation of photosynthesis and nitrogen fixation under diazotrophic conditions. During nitrogen fixation, the photosynthetic electron transport chain becomes impaired, which leads to the uncoupling of the PSII and PSI activities. A 30-40% increase in PSI activity and continuous generation of ATP through light-dependent processes seem to support the nitrogen fixation. The use of an artificial electron carrier that shuttles electrons between the plastoquinone pool and plastocyanin, bypassing cytochrome b/f complex, enhanced the photosynthetic electron transport activity five to six fold during nitrogen fixation. Measuring of full photosynthetic electron transport activity using methyl voilogen as a terminal acceptor revealed that the photosynthetic electron transport components beyond plastocyanin might be functional. Further, glycolate can act as a source of electrons for PSI for the nitrogen fixing cells, which have residual PSII activity. Under conditions when PSI becomes largely independent of PSII and glycolate provides electrons for PSI activity, the light-dependent nitrogen fixation also was stimulated by glycolate. These results suggest that during nitrogen fixation, when the photosynthetic electron transport from PSII is inhibited at the level of cytochrome b/f complex, an alternate electron donor system for PSI may be required for the cells to carry out light dependent nitrogen fixation.     

Adaptation to shade of the light-harvesting apparatus in Silene dioica

Abstract. The physiological characteristics and photo-system composition of the photosynthetic apparatus of Silene dioica , a woodland plant, grown in sun and natural shade are examined. As expected, shade leaves exhibited lower chlorophyll a/b ratios, light saturated rates of CO₂ assimilation (A_sat), dark respiration (R_d,) and light compensation points ( Г ), with both sun and shade leaves having similar absorptances and quantum yields of CO₂ assimilation (φ). Shade leaves were able to utilize far-red light for electron transport and carbon assimilation and reach the compensation point. Sun leaves in far-red light had a rate of carbon assimilation equivalent to their dark respiration rate. Chlorophyll fluorescence kinetics from leaves at 77 K together with analyses of thylakoid contents of photosystems (PS) I and II and the light-harvesting cholorphyll a/b protein complex associated with PSII (LHCII) demonstrated that the antenna size of PSII was similar in thylakoids of sun and shade leaves, but shade leaves contained ca. 20% more PSII and ca. 12% less PSI complexes. The increased PSII/PSI ratio in shade leaves accounted for their ability to achieve the compensation point in far-red light. An important feature of photosynethic shade adaptation in S. dioica is an increase in the PSII/PSI ratio and not an increase in the antenna size of PSII. The adaptive response of sun leaves when placed in a shade environment was rapid and had a half-time of ca. 18h.     

Growth under Red Light Enhances Photosystem II Relative to Photosystem I and Phycobilisomes in the Red Alga Porphyridium cruentum

Acclimation of the photosynthetic apparatus to light absorbed primarily by photosystem I (PSI) or by photosystem II (PSII) was studied in the unicellular red alga Porphyridium cruentum (ATCC 50161). Cultures grown under green light of 15 microeinsteins per square meter per second (PSII light; absorbed predominantly by the phycobilisomes) exhibited a PSII/PSI ratio of 0.26 ± 0.05. Under red light (PSI light; absorbed primarily by chlorophyll) of comparable quantum flux, cells contained nearly five times as many PSII per PSI (1.21 ± 0.10), and three times as many PSII per cell. About 12% of the chlorophyll was attributed to PSII in green light, 22% in white light, and 39% in red light-grown cultures. Chlorophyll antenna sizes appeared to remain constant at about 75 chlorophyll per PSII and 140 per PSI. Spectral quality had little effect on cell content or composition of the phycobilisomes, thus the number of PSII per phycobilisome was substantially greater in red light-grown cultures (4.2 ± 0.6) than in those grown under green (1.6 ± 0.3) or white light (2.9 ± 0.1). Total photosystems (PSI + PSII) per phycobilisome remained at about eight in each case. Carotenoid content and composition was little affected by the spectral composition of the growth light. Zeaxanthin comprised more than 50% (mole/mole), β-carotene about 40%, and cryptoxanthin about 4% of the carotenoid pigment. Despite marked changes in the light-harvesting apparatus, red and green light-grown cultures have generation times equal to that of cultures grown under white light of only one-third the quantum flux.     

Changes in the photosynthetic characteristics and photosystem stoichiometries in wild-type and Chl <Emphasis Type="Italic">b</Emphasis>-deficient mutant rice seedlings under various irradiances

By using a wild-type rice (Oryza sativa L. cv. Norin No. 8) and the chlorophyll (Chl) b-deficient mutant derived from Norin No. 8 (chlorina 11), the present study monitored the oxygen evolution, contents of Chl a and b, β-carotene, and lutein in leaf and the contents of cytochrome f, and the reaction centres of photosystem I (PSI) and photosystem II (PSII) in thylakoids. The oxygen evolution, maximal quantum yield of PSII (F_v/F_m) and Chl concentration remained constant in both Norin No. 8 and chlorina 11 under 5 and 2% of full sunlight for six days. On the other hand, on the thylakoid level, the PSII reaction centre of chlorina 11 was more stable even under high irradiance, while approximately 40% decrease in levels of the PSII reaction centre occurred under 2% of full sunlight for six days. However, under such conditions, by regulating the stoichiometry of active PSII and PSI centres, the light absorption balance in both rice types was adjusted between the two photosystems. The present study attempted to examine whether the light absorption balance between PSII and PSI is altered to effectively conduct photosynthesis in the wild-type and Chl b-deficient mutant rice seedlings.     

Expressing an RbcS Antisense Gene in Transgenic Flaveria bidentis Leads to an Increased Quantum Requirement for CO2 Fixed in Photosystems I and II

It was previously shown with concurrent measurements of gas exchange and carbon isotope discrimination that the reduction of ribulose-1,5-bisphosphate carboxylase/oxygenase by an antisense gene construct in transgenic Flaveria bidentis (a C4 species) leads to reduced CO2 assimilation rates, increased bundle-sheath CO2 concentration, and leakiness (defined as the ratio of CO2 leakage to the rate of C4 acid decarboxylation; S. von Caemmerer, A. Millegate, G.D. Farquhar, R.T. Furbank [1997] Plant Physiol 113: 469-477). Increased leakiness in the transformants should result in an increased ATP requirement per mole of CO2 fixed and a change in the ATP-to-NADPH demand. To investigate this, we compared measurements of the quantum yield of photosystem I and II ([phi]PSI and [phi]PSII) with the quantum yield of CO2 fixation ([phi]CO2) in control and transgenic F. bidentis plants in various conditions. Both [phi]PSI/[phi]CO2 and [phi]PSII/[phi]CO2 increased with a decrease in ribulose-1,5-bisphosphate carboxylase/oxygenase content, confirming an increase in leakiness. In the wild type the ratio of [phi]PSI to [phi]PSII was constant at different irradiances but increased with irradiance in the transformants, suggesting that cyclic electron transport may be higher in the transformants. To evaluate the relative contribution of cyclic or linear electron transport to extra ATP generation, we developed a model that links leakiness, ATP/NADP requirements, and quantum yields. Despite some uncertainties in the light distribution between photosystem I and II, we conclude from the increase of [phi]PSII/[phi]CO2 in the transformants that cyclic electron transport is not solely responsible for ATP generation without NADPH production.     

Changes in Stoichiometry among PSI, PSII and Cyt b6-f Complexes in Response to Chromatic Light for Cell Growth Observed with the Red Alga Porphyra yezoensis

Stoichiometry among 3 thylakoid components, PSI and PSII andCyt b6-f complexes, was determined with the red alga Porphyrayezoensis with special reference to the regulation of PSI/PSIIstoichiometry in response to light regime. The ratio of PSIto PSII abundance was four times greater in thalli grown underorange light which excites mainly phycobilisome, thus PSII,than that under red light which excites preferentially Chl a,thus PSI. Cyt b₆-f abundance remained almost constant. The PSIand PSII content was regulated separately under the two growthlight conditions as was also observed with the red alga Porphyridiumcruentum by Cunningham et al. [(1990) Plant Physiol. 93: 888].This differs from the cyanophyte Synechocystis PCC 6714 whereadjustment occurs only in the PSI content [(1987) Plant CellPhysiol. 28: 1547]. However, results on the marine cyanophyteSynechococcus NIBB 1071 indicate that changes in the PSI/PSIIsoichiometry is similar to red algae. In this species, as inthe red algae, more than one PSII is associated with each phycobilisome.The light regime also induced changes in the phycobiliproteincomposition in Porphyra yezoensis. Under PSII light, phycoerythrinincreased, and phycocyanin decreased, while under PSI lightthe response was reversed. The change suggests an occurrenceof complementary chromatic adaptation. (Received April 8, 1994; Accepted June 1, 1994)     

Light-Harvesting Function in the Diatom Phaeodactylum tricornutum: II. Distribution of Excitation Energy between the Photosystems

The distribution of excitation energy between photosystems I and II (PSI and PSII) was investigated in the marine diatom Phaeodactylum tricornutum (Bohlin) using light-induced changes in fluorescence yield and rate of modulated O₂ evolution. The intensity dependence of the fast fluorescence rise in dark adapted cells (±DCMU) suggests that light absorbed by the major antenna complex was not delivered preferentially to PSII but is more equally distributed between the photosystems. Reversible, slow fluorescence yield changes measured in the absence of DCMU were correlated with decreased initial fluorescence and rate constants for PSII photochemistry, increased variable fluorescence, alteration of the fluorescence excitation and emission spectra, and could be effected by either 510 nm (PSII) or 704 nm (PSI) light. Slow, reversible fluorescence yield changes were also observed in the presence of DCMU, but were characterized by a loss of both initial and variable fluorescence and could not be induced by PSI light. The absence of slow changes in the yield of fluorescence and rate of modulated O₂ evolution, following addition or removal of PSI background light to modulated PSII excitation, does not support regulation of excitation energy density in PSI at the expense of PSII. The results suggest that adjustments are made at the level of excitation energy transfer to the PSII reaction center which prevent prolonged loss of photosynthetic capacity. Energy distribution is regulated by ionic distributions independently of the plastoquinone pool redox state. These differences in light-harvesting function are probably a response to the aquatic light field and may account for the success of diatoms in low and variable light environments.     

Inhibition of photosystems I and II and enhanced back flow of photosystem I electrons in cucumber leaf discs chilled in the light

Pre-illumination of cucumber leaf discs at a chilling temperature in low-irradiance white light resulted in accelerated re-reduction of P700(+) [the special Chl pair in the photosystem I (PSI) reaction centre] when the far-red measuring light was turned off. Measurements (in +/- methyl viologen or +/- DCMU conditions) of the re-reduction half time suggest that accelerated re-reduction of P700(+) appeared to be predominantly due to charge recombination and only partly due to reductants sustained by previous cyclic electron flow around PSI. Apparently, charge recombination in PSI was greatly enhanced by inhibition of forward, linear electron flow. Inhibition of PSII electron transport was observed to occur to a lesser extent than that of PSI, but only if the measurement of PSII functionality was free from complications due to downstream accumulation of electrons in pools. We suggest that promotion of controlled charge recombination and cyclic electron flow round PSI during chilling of leaves in the light may partly prevent further damage to both photosystems.     

Action spectra of photosystems II and I and quantum yield of photosynthesis in leaves in State 1

The spectral global quantum yield (Y_II, electrons/photons absorbed) of photosystem II (PSII) was measured in sunflower leaves in State 1 using monochromatic light. The global quantum yield of PSI (Y_I) was measured using low-intensity monochromatic light flashes and the associated transmittance change at 810 nm. The 810-nm signal change was calibrated based on the number of electrons generated by PSII during the flash (4 · O₂ evolution) which arrived at the PSI donor side after a delay of 2 ms. The intrinsic quantum yield of PSI (y_I, electrons per photon absorbed by PSI) was measured at 712 nm, where photon absorption by PSII was small. The results were used to resolve the individual spectra of the excitation partitioning coefficients between PSI (a_I) and PSII (a_II) in leaves. For comparison, pigment–protein complexes for PSII and PSI were isolated, separated by sucrose density ultracentrifugation, and their optical density was measured. A good correlation was obtained for the spectral excitation partitioning coefficients measured by these different methods. The intrinsic yield of PSI was high (y_I = 0.88), but it absorbed only about 1/3 of quanta; consequently, about 2/3 of quanta were absorbed by PSII, but processed with the low intrinsic yield y_II = 0.63. In PSII, the quantum yield of charge separation was 0.89 as detected by variable fluorescence F_v/F_m, but 29% of separated charges recombined (Laisk A, Eichelmann H and Oja V, Photosynth. Res. 113, 145–155). At wavelengths less than 580 nm about 30% of excitation is absorbed by pigments poorly connected to either photosystem, most likely carotenoids bound in pigment–protein complexes.     

Photon- and carbon-use efficiency in<Emphasis Type="Italic"> Ulva rigida</Emphasis> at different CO<Subscript>2</Subscript> and N levels

The seaweed Ulva rigida C. Agardh (Chlorophyta) was cultured under two CO₂ conditions supplied through the air bubbling system: non-manipulated air and 1% CO₂-enriched aeration. These were also combined with N sufficiency and N limitation, using nitrate as the only N source. High CO₂ in U. rigida led to higher growth rates without increasing the C fixed through photosynthesis under N sufficiency. Quantum yields for charge separation at photosystem II (PSII) reaction centres (_PSII) and for oxygen evolution (_O2) decreased at high CO₂ even in N-sufficient thalli. Cyclic electron flow around PSII as part of a photoprotection strategy accompanied by decreased antennae size was suspected. The new re-arrangement of the photosynthetic energy at high CO₂ included reduced investment in processes other than C fixation, as well as in carbon diverted to respiration. As a result, quantum yield for new biomass-C production (^growth) increased. The calculation of the individual quantum yields for the different processes involved allowed the completion of the energy flow scheme through the cell from incident light to biomass production for each of the CO₂ and N-supply conditions studied.Abbreviations A total thallus absorptance - A_pig absorptance due to pigments - A_str Absorptance due to non-pigmented structures - a* spectrally averaged in vivo absorption cross-section of chlorophyll a - CCM carbon-concentrating mechanism - Chl chlorophyll - DOC dissolved organic carbon - ETR electron transport rate - F_v/F_m optimum quantum yield for PSII charge separation - GP gross O₂ evolution rate - k_pig specific light absorption coefficient for pigments - k_str specific light absorption coefficient for non-pigmented structures - OP optimum O₂ evolution rate - PFR photon fluence rate - POC particulate organic carbon - PS photosystem - q_N non-photochemical quenching - q_P photochemical quenching - ^growth quantum yield for new biomass-C production - _O2 quantum yield for gross O₂ evolution - _PSII quantum yield for PSII charge separation     

Comparative analysis of photosensitivity in photosystem I donor and acceptor side mutants of Chlamydomonas reinhardtii

Electron input from plastocyanin into photosystem I (PSI) is slowed down in the Chlamydomonas reinhardtii mutants affected at the donor side (PsaF or PsaB, lumenal loop j) of PSI. In contrast, electron exit from PSI to ferredoxin is diminished in the PSI acceptor side PsaC mutants K35E and FB₁. Although, the electron transfer reactions are diminished to a similar extent in both type of mutants, the PsaC mutants K35E and FB₁ are more light‐sensitive than the PsaF‐deficient strain 3bF or the PsaB mutants E613N and W627F. To assess the differential photosensitivity of donor and acceptor side mutants fluorescence transients, gross oxygen evolution and uptake, PSII photo‐inhibition and rate of recovery were measured as well as NADP⁺ photoreduction. The NADP⁺ photoreduction measurements indicated that the donor side is limiting the reduction rate. In contrast, measurements of gross oxygen evolution and uptake showed that the reducing side limits linear electron transfer. However, under high light, donor and acceptor side mutations lead to PSII photo‐inhibition and to a diminished rate of PSII recovery, cause lipid peroxidation and result in a decrease in the levels of PSI and PSII. The wild type is not affected under the same conditions. These responses are most pronounced in the PsaC‐K35E and PsaB‐W627F mutants, and they correlate with the light sensitivity of these strains. The correlation between limitation of electron transfer through PSI and the formation of reactive oxygen species as a cause for the light‐sensitivity is discussed.     

Heterogeneity of photosystem II reaction centers as influenced by heat treatment of barley leaves

Three functionally distinct populations of PSII reaction centers differing in the ability to keep the primary acceptors in a reduced state and to transfer electrons to PSI were estimated using chlorophyll fluorescence measurements in primary barley leaves exposed to elevated temperatures in the range of 37–51°C. The capacity of the PSII reaction centers to perform at least one light-induced charge separation was not affected by a 5-min heat treatment at temperatures up to 51°C. The first population containing Q_B-non-reducing centers corresponded to 15–20% of the total PSII activity up to 45°C. In a second population, PSII reaction centers maintained Q_A reduction under light in the presence of oxygen, but not in the presence of a strong artificial PSI electron acceptor, methyl viologen. In a third population that gradually increases from zero at 37°C to about 60% at 45°C, the PSII centers were not able to keep Q_A in the reduced state even in the presence of oxygen as the sole electron acceptor. Three electron transport pathways, the pseudocyclic one involving both PSII and PSI, the NAD(P)H-dependent pathway mediated by PSI alone after the loss of activity in some PSII centers, and the PSI-driven ferredoxin-dependent route enhanced by weakly efficient PSII centers that are able to provide only catalytic amounts of electrons, are suggested to create a proton gradient in chloroplasts of heat-stressed leaves thus protecting PSII reaction centers from photodamage.     

Light quality during growth of Tradescantia albiflora regulates photosystem stoichiometry, photosynthetic function and susceptibility to photoinhibition

Tradescantia albiflora (Kunth) was grown under two different light quality regimes of comparable light quantity: in red + far-red light absorbed mainly by photosystem I (PSI light) and yellow light absorbed mainly by photosystem II (PSII light). The composition, function and ultrastructure of chloroplasts, and photoinhibition of photosynthesis in the two types of leaves were compared. In contrast to regulation by light quantity (Chow et al. 1991. Physiol. Plant. 81: 175–182), light quality exerted an effect on the composition of pigment complexes, function and structure of chloroplasts in Tradescantia: PSII light-grown leaves had higher Chl a/b ratios, higher PSI concentrations, lower PSII/PSI reaction centre ratios and less extensive thylakoid stacking than PSI light-grown leaves. Light quality triggered modulations of chloroplast components, leading to a variation of photosynthetic characteristics. A larger proportion of primary quinone acceptor (Q_A) in PSI light-grown leaves was chemically reduced at any given irradiance. It was also observed that the quantum yield of PSII photochemistry was lower in PSI light-grown leaves. PSI light-grown leaves were more sensitive to photoinihibition and recovery was slower compared to PSII light-grown leaves, showing that the PSII reaction centre in PSI light-grown leaves was more easily impaired by photoinhibition. The increase in susceptibility of leaves to photoinhibition following blockage of chloroplast-encoded protein synthesis was greater in PSII light-grown leaves, showing that these leaves normally have a greater capacity for PSII repair. Inhibition of zeaxanthin formation by dithiothreitol slightly increased sensitivity to photoinhibition in both PSI and PSII light-grown leaves.     

Temporal Pattern of Photosynthesis under Continuous Illumination of Radish Plants

Changes in photosynthetic activity, redox state of photosystem I (PSI) and photosystem II (PSII), as well as starch and sucrose content were studied on the source leaves of 18- to 20-day-old radish (Raphanus sativus L.) plants that were dark-adapted for 12 h and then exposed to continuous white light (170 mol quanta/(m² s)). The kinetic pattern of photosynthetic activity comprised three phases. Within the first 6 h of light adaptation (first phase), the maximum photosynthetic rate and the quantum yield of photosynthesis increased 1.6 times in the illuminated leaves compared to the leaves of plants placed in darkness. Further illumination led to the decrease of both photosynthetic indices by about 20% (12 h after the onset of light exposure, second phase) and finally increased them to the level observed after 6-h light exposure (72 h, third phase). The stationary photooxidation level of PSI primary donor was relatively low within the first 6 h of light adaptation, and then it steeply increased. The linear relationship between the amounts of photoreduced PSII primary acceptor and photooxidized PSI primary donor did not change during prolonged light adaptation, showing a highly coordinated functioning of both photosystems. The amount of sucrose in leaves attained its peak after 12 h of light adaptation and did not change further on. The starch content increased to its peak within 24 h of illumination and decreased gradually upon longer exposures. It is concluded that, despite active export of assimilates to the developing storage organ, the source leaves exhibit a nonmonotonic temporal course of endogenously regulated photosynthetic activity, which was related to changes in the effectiveness and, possibly, the number of the components of photosynthetic apparatus.     

The stoichiometry of the two photosystems in higher plants revisited

The stoichiometry of Photosystem II (PSII) to Photosystem I (PSI) reaction centres in spinach leaf segments was determined by two methods, each capable of being applied to monitor the presence of both photosystems in a given sample. One method was based on a fast electrochromic (EC) signal, which in the millisecond time scale represents a change in the delocalized electric potential difference across the thylakoid membrane resulting from charge separation in both photosystems. This method was applied to leaf segments, thus avoiding any potential artefacts associated with the isolation of thylakoid membranes. Two variations of this method, suppressing PSII activity by prior photoinactivation (in spinach and poplar leaf segments) or suppressing PSI by photo-oxidation of P700 (the chlorophyll dimer in PSI) with background far-red light (in spinach, poplar and cucumber leaf segments), each gave the separate contribution of each photosystem to the fast EC signal; the PSII/PSI stoichiometry obtained by this method was in the range 1.5-1.9 for the three plant species, and 1.5-1.8 for spinach in particular. A second method, based on electron paramagnetic resonance (EPR), gave values in a comparable range of 1.7-2.1 for spinach. A third method, which consisted of separately determining the content of functional PSII in leaf segments by the oxygen yield per single turnover-flash and that of PSI by photo-oxidation of P700 in thylakoids isolated from the corresponding leaves, gave a PSII/PSI stoichiometry (1.5-1.7) that was consistent with the above values. It is concluded that the ratio of PSII to PSI reaction centres is considerably higher than unity in typical higher plants, in contrast to a surprisingly low PSII/PSI ratio of 0.88, determined by EPR, that was reported for spinach grown in a cabinet under far-red-deficient light in Sweden [Danielsson et al. (2004) Biochim. Biophys. Acta 1608: 53-61]. We suggest that the low PSII/PSI ratio in the Swedish spinach, grown in far-red-deficient light with a lower PSII content, is not due to greater accuracy of the EPR method of measurement, as suggested by the authors, but is rather due to the growth conditions.     

The stoichiometry of the two photosystems in higher plants revisited

The stoichiometry of Photosystem II (PSII) to Photosystem I (PSI) reaction centres in spinach leaf segments was determined by two methods, each capable of being applied to monitor the presence of both photosystems in a given sample. One method was based on a fast electrochromic (EC) signal, which in the millisecond time scale represents a change in the delocalized electric potential difference across the thylakoid membrane resulting from charge separation in both photosystems. This method was applied to leaf segments, thus avoiding any potential artefacts associated with the isolation of thylakoid membranes. Two variations of this method, suppressing PSII activity by prior photoinactivation (in spinach and poplar leaf segments) or suppressing PSI by photo-oxidation of P700 (the chlorophyll dimer in PSI) with background far-red light (in spinach, poplar and cucumber leaf segments), each gave the separate contribution of each photosystem to the fast EC signal; the PSII/PSI stoichiometry obtained by this method was in the range 1.5-1.9 for the three plant species, and 1.5-1.8 for spinach in particular. A second method, based on electron paramagnetic resonance (EPR), gave values in a comparable range of 1.7-2.1 for spinach. A third method, which consisted of separately determining the content of functional PSII in leaf segments by the oxygen yield per single turnover-flash and that of PSI by photo-oxidation of P700 in thylakoids isolated from the corresponding leaves, gave a PSII/PSI stoichiometry (1.5-1.7) that was consistent with the above values. It is concluded that the ratio of PSII to PSI reaction centres is considerably higher than unity in typical higher plants, in contrast to a surprisingly low PSII/PSI ratio of 0.88, determined by EPR, that was reported for spinach grown in a cabinet under far-red-deficient light in Sweden [Danielsson et al. (2004) Biochim. Biophys. Acta 1608: 53-61]. We suggest that the low PSII/PSI ratio in the Swedish spinach, grown in far-red-deficient light with a lower PSII content, is not due to greater accuracy of the EPR method of measurement, as suggested by the authors, but is rather due to the growth conditions.     

Chromatic variation of the abundance of PSII complexes observed with the red alga Prophyridium cruentum.

Chromatic regulation of photosystem stoichiometry in cyanophytes, green algae and probably vascular plants is achieved by regulation of the abundance of PSI in response to thylakoid electron transport state at least under our experimental conditions [cf. Fujita (1997) Photosyn. Res. 53: 83]. However, variation of not only PSI but also PSII, in reverse of each other, is characteristic of the stoichiometry regulation in red algae and some of marine cyanophytes. Our previous study with the red alga Porphyridium cruentum has revealed that PSII is inactivated by 50% upon a light shift from the light absorbed by Chl a, PSI light, to that mainly absorbed by phycobilisomes (PBS), PSII light [Fujita (1999) Plant Cell Physiol. 40: 924]. To evaluate the contribution of the photoinactivation to the chromatic variation of PSII, variation of the abundance of PSI, PSII and PBS, together with the fluorescence parameter and the activity of PSII, was followed after a light shift from PSI light to PSII light. Upon a light shift to PSII light, PSII, determined as Cyt b(559) per PBS, decreased rapidly, following the photoinactivation, down to the level a half of that before the light shift, and remained constant. Since the increase in PBS was not significant during this period, a rapid decrease of PSII/PBS led us to tentatively conclude that the degradation of PSII is a main cause for variation of the abundance of PSII. Photoinactivation of PSII, and also decrease in Cyt b(559), was accelerated, but only slightly, by the addition of chloramphenicol (CAP) at a moderate concentration while CAP at the same concentration significantly suppressed the increment of PSI determined as P700. A selective effect of CAP supports the above conclusion.     

Effects of growth irradiance levels on the ratio of reaction centers in two species of marine phytoplankton

Cells of two species of single-celled marine algae, the diatom Skeletonema costatum (Greve), Cleve, and the chlorophyte Dunaliella tertiolecta Butcher, were cultured in white light of high (500-600 microeinsteins per square meter per second) and low (30 microeinsteins per square meter per second) intensity. For both algal species, cells grown at low light levels contained more chlorophyll a and had a lower ratio of chlorophyll a to chlorophylls b or c than did cells grown at high light levels. When photosynthetic unit sizes were measured on the basis of either oxygen flash yields or P₇₀₀ photooxidation, different results were obtained with the different species. In the chlorophyte, the cellular content of photosystem I (PSI) and photosystem II (PSII) reaction centers increased in tandem as chlorophyll a content increased so that photosynthetic unit sizes changed only slightly and the ratio PSI:PSII reaction centers remained constant at about 1.1. In the diatom, as the chlorophyll content of the cells increased, the number of PSI reaction centers decreased and the number of PSII reaction centers increased so that the ratio of PSI:PSII reaction centers decreased from about unity to 0.44. In neither organism did photosynthetic capacity correlate with changes in cellular content of PSI or PSII reaction centers. The results are discussed in relationship to the physical and biological significance of the photosynthetic unit concept.