Effect of Mn2+ and Ca2+ on O2 evolution and on the variable fluorescence yield associated with Photosystem II in preparations of Anacystis nidulans

Extraction with EDTA of lyophilized and lysozyme treated preparations of the blue-green algae Anacystis nidulans resulted in loss of the capacity for photoevolution of O₂. Reactivation was achieved by the addition of both cations: Mn²⁺ and Ca²⁺ (or to a smaller extent by Mn²⁺ and Sr²⁺). The dual requirement for Mn²⁺ and Ca²⁺ could be demonstrated when the O₂ evolution under short saturating light flashes and the variable chlorophyll fluorescence associated with the reduction of the primary acceptor of Photosystem II was examined. The fluorescence experiments in addition showed that incorporation of the cations was a light dependent step, since the fluorescence rise only started after a lag period.     

Anacystis nidulans Demonstrates a Photosystem II Cation Requirement Satisfied Only by Ca or Na

Anacystis nidulans exhibits a total loss of photosystem II (PSII) activity upon incubation in a nutrient medium deficient in Ca²⁺ and Na⁺ and containing a divalent cation chelator. This loss of activity is light-dependent, which corresponds to an energy requirement. Likewise, Ca²⁺ efflux takes place only in cells incubated in light. The loss of PSII activity is reversible by addition of submillimolar amounts of either Ca²⁺ or Na⁺ to the external medium but not by the addition of any other cation. Restoration of lost PSII activity also requires light. Light saturation curves for partially depleted cells demonstrate both lower maximum O₂ evolution rates and decreased relative quantum yields when compared to control cells. Partial electron transport reactions isolate the site of the Ca²⁺/Na⁺ effect to the reaction center itself or immediately on its oxidizing side and exclude the water-splitting complex. O₂ flash yields decline during cation depletion, indicating a decrease in the number of functional PSII reaction centers, but the maximum turnover rate for still functional reaction centers does not decline. Thus, PSII of A. nidulans exhibits an all-or-none cation requirement, satisfied only by Ca²⁺ or Na⁺.     

Calcium depletion alters energy transfer and prevents state changes in intact Anacystis

A time-dependent loss of Photosystem II (PS II) activity seen in Anacystis nidulans grown without Ca²⁺ was paralleled by a loss in chlorophyll (Chl) a fluorescence of variable yield which reflects inhibition of Q reduction and of state changes. Both inhibitions were fully reversed by the addition of Ca²⁺ to the growth medium. The lack of state changes in Ca²⁺-depleted cells was confirmed in 77 K fluorescence difference spectra of light versus dark-adapted cells.Absorption spectra of control and of Ca²⁺-depleted cells were identical whether measured at room temperature or at 77 K. Fluorescence emission spectra measured at 39°C (cell growth temperature) demonstrated higher yields in Ca²⁺-depleted cells compared to controls. Fluorescence emission spectra at 77 K also produced higher yields in Ca²⁺-depleted cells but the increased fluorescence at this temperature occurred principally at 683 nm. The increased relative fluorescence yield in Ca²⁺-depleted samples results from light absorbed by phycocyanin (PC), but not from light absorbed almost exclusively by Chl. The 683 run fluorescence peak probably represents increased allophycocyanin (APC) emission as intact phycobilisomes become energetically disassociated from the photosynthetic apparatus. This inferred disassociation occurred only after PSII activity was mostly inhibited in Ca²⁺-depleted cells, and was not fully reversible.Abbreviations APC Allophycocyanin - Chl chlorophyll - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - EDTA ethylenediaminotetraacetic acid - PC phycocyanin - PS photosystem - Q primary quinone electron acceptor of Photosystem II also a quencher of Chl a fluorescence DPB-CIW Publ. No. 817     

The photochemical and fluorescence properties of whole cells,spheroplasts and spheroplast particles from the blue-green alga Phormidium luridum

The photochemical activities and fluorescence properties of cells, spheroplasts and spheroplast particles from the blue-green alga Phormidium luridum were compared. The photochemical activities were measured in a whole range of wavelengths and expressed as quantum yield spectra (quantum yield vs. wavelength). The following reactions were measured: Photosynthesis (O₂ evolution) in whole cells; Hill reaction (O₂ evolution) with Fe(CN)₆^3? and NADP as electron acceptors (Photosystem II and Photosystem II+Photosystem I reactions); electron transfer from reduced 2,6-dichlorophenolindophenol to diquat (Photosystem I reaction). The fluorescence properties were emission spectra, quantum yield spectra and the induction pattern.On the basis of comparison between the quantum yield spectra and the pigments compositions the relative contribution of each pigment to each photosystem was estimated. In normal cells and spheroplasts it was found that Photosystem I (Photosystem II) contains about 90 % (10 %) of the chlorophyll a, 90 % (10 %) of the carotenoids and 15 % (85 %) of the phycocyanin. In spheroplast particles there is a reorganization of the pigments: they loose a certain fraction (about half) of the phycocyanin but the remaining phycocyanin attaches itself exclusively to Photosystem I (!). This is reflected by the loss of Photosystem II activity, a flat quantum yield vs. wavelength dependence and a loss of the fluorescence induction.The fluorescence quantum yield spectra conform qualitatively to the above conclusion. More quantitative estimation shows that only a fraction (20–40 %) of the chlorophyll of Photosystem II is fluorescent. Total emission spectrum and the ratio of variable to constant fluorescence are in agreement with this conclusion.The fluorescence emission spectrum shows characteristic differences between the constant and variable components. The variable fluorescence comes exclusively from chlorophyll a; the constant fluorescence is contributed, in addition to chlorophyll a, by phycocyanine and an unidentified long wavelength component.The variable fluorescence does not change in the transition from whole cells to spheroplasts. However, the constant fluorescence increases considerably. This indicates the release of a small fraction of pigments from the photosynthetic photochemical apparatus which then become fluorescent.     

Inhibition of the oxygen-evolving complex of photosystem II and depletion of extrinsic polypeptides by nickel

The toxic effect of Ni²⁺ on photosynthetic electron transport was studied in a photosystem II submembrane fraction. It was shown that Ni²⁺ strongly inhibits oxygen evolution in the millimolar range of concentration. The inhibition was insensitive to NaCl but significantly decreased in the presence of CaCl₂. Maximal chlorophyll fluorescence, together with variable fluorescence, maximal quantum yield of photosystem II, and flash-induced fluorescence decays were all significantly declined by Ni²⁺. Further, the extrinsic polypeptides of 16 and 24 kDa associated with the oxygen-evolving complex of photosystem II were depleted following Ni²⁺ treatment. It was deduced that interaction of Ni²⁺ with these polypeptides caused a conformational change that induced their release together with Ca²⁺ from the oxygen-evolving complex of photosystem II with consequent inhibition of the electron transport activity.     

Effect of action potential on photosynthesis and spatially distributed H+ fluxes in cells and Chloroplasts of Chara corallina

When exposed to light, the cells of characean algae produce intermittent regions of H⁺ extrusion and H⁺ absorption, featuring different photosynthetic activities. Methods for local measurements of outer pH, O₂ content, and photochemical activity of photosystem II (PSII) were applied to examine microscopic regions of Chara coralline Klein ex Willd. internodes. The results show that the functional spatial heterogeneity of these excitable cells is controlled not only by light but also by electric excitation of the plasma membrane. Generation of a single action potential (AP) induced a reversible transition to the state with homogenous pH distribution and had different effects on photosynthesis in cell regions producing alkaline and acid zones. The effective quantum yield of PSII primary processes and the maximal chlorophyll fluorescence decreased after AP in the alkaline cell regions but were almost unaffected in the acidic cell regions. The suppression of photosynthesis after AP was also evident in the decrease of photosynthetic O₂ evolution. The results provide evidence that electric signals arising at the plasmalemma are transmitted to the level of thylakoid membranes. The effects of electric excitation on fluorescence and the quantum yield of PSII photochemistry were best pronounced at low light intensities and low level of nonphotochemical quenching. The sensitivity of chlorophyll fluorescence in resting and excited cells to light intensity and protonophores indicates that the AP-induced fluorescence changes derive from the increase in pH gradient at the thylakoid membrane. The temporal elimination of alkaline zones and inhibition of photosynthesis apparently arise from parallel operational sequences that have a common initial stage. A possible role of cytosolic Ca²⁺ rise in the mechanism of photosynthesis suppression after electric excitation of the plasma membrane is discussed.     

Effects of O(2) and CO(2) Concentrations on Quantum Yields of Photosystems I and II in Tobacco Leaf Tissue

The interactive effects of irradiance and O₂ and CO₂ levels on the quantum yields of photosystems I and II have been studied under steady-state conditions at 25°C in leaf tissue of tobacco (Nicotiana tabacum). Assessment of radiant energy utilization in photosystem II was based on changes in chlorophyll fluorescence yield excited by a weak measuring beam of modulated red light. Independent estimates of photosystem I quantum yield were based on the light-dark in vivo absorbance change at 830 nanometers, the absorption band of P700⁺. Normal (i.e. 20.5%, v/v) levels of O₂ generally enhanced photosystem II quantum yield relative to that measured under 1.6% O₂ as the irradiance approached saturation. Photorespiration is suspected to mediate such positive effects of O₂ through increases in the availability of CO₂ and recycling of orthophosphate. Conversely, at low intercellular CO₂ concentrations, 41.2% O₂ was associated with lower photosystem II quantum yield compared with that observed at 20.5% O₂. Inhibitory effects of 41.2% O₂ may occur in response to negative feedback on photosystem II arising from a build-up in the thylakoid proton gradient during electron transport to O₂. Covariation between quantum yields of photosystems I and II was not affected by concentrations of either O₂ or CO₂. The dependence of quantum yield of electron transport to CO₂ measured by gas exchange upon photosystem II quantum yield as determined by fluorescence was unaffected by CO₂ concentration.     

Evidence for Cyclic Electron Flow around Photosystem II in Chlorella pyrenoidosa

Electron flow around photosystem II was investigated in Chlorella pyrenoidosa. Using a bare platinum O₂ electrode, simultaneous measuremnts were made of steady-state photosynthesis in continuous light, the yield of oxygen (Y_o2) produced by a superimposed saturating xenon flash, and the change in fluorescence yield of a weak flash triggered before and 70 microseconds after the saturating flash. Throughout most of the continuous photosynthesis-irradiance curve, normalized O₂ flash yields (Y_o2/Y_o2max) and normalized variable fluorescence yields (Δ/Δ′) were linearly correlated with a slope of 1.0. As photosynthetic rates reached light saturation, however, the variable fluorescence yields remained relatively constant while O₂ flash yields decreased. These results strongly suggest that there is a cyclic electron flow around photosystem II in unpoisoned intact cells at light saturation and supraoptimal light intensities.     

Dark Ammonium Assimilation Reduces the Plastoquinone Pool of Photosystem II in the Green Alga Selenastrum minutum

The impact of dark NH₄⁺ and NO₃⁻ assimilation on photosynthetic light harvesting capability of the green alga Selenastrum minutum was monitored by chlorophyll a fluorescence analysis. When cells assimilated NH₄⁺, they exhibited a large decline in the variable fluorescence/maximum fluorescence ratio, the fluorescence yield of photosystem II relative to that of photosystem I at 77 kelvin, and O₂ evolution rate. NH₄⁺ assimilation therefore poised the cells in a less efficient state for photosystem II. The analysis of complementary area of fluorescence induction curve and the pattern of fluorescence decay upon microsecond saturating flash, indicators of redox state of plastoquinone (PQ) pool and dark reoxidation of primary quinone electron acceptor (Q_A), respectively, revealed that the PQ pool became reduced during dark NH₄⁺ assimilation. NH₄⁺ assimilation also caused an increase in the NADPH/NADP⁺ ratio due to the NH₄⁺ induced increase in respiratory carbon oxidation. The change in cellular reductant is suggested to be responsible for the reduction of the PQ pool and provide a mechanism by which the metabolic demands of NH₄⁺ assimilation may alter the efficiency of photosynthetic light harvesting. NO₃⁻ assimilation did not cause a reduction in PQ and did not affect the efficiency of light harvesting. These results illustrate the role of cellular metabolism in the modulating photosynthetic processes.     

Nuclear pea mutants deficient in chlorophyll b and the major polypeptide of the light-harvesting chlorophyll a/b protein of photosystem II

Eight chlorophyll b deficient nuclear mutants of pea (Pisum sativum L.) have been characterized by low temperature fluorescence emission spectra of their leaves and by the ultrastructure, photochemical activities and polypeptide compositions of the thylakoid membranes. The room temperature fluorescence induction kinetics of leaves and isolated thylakoids have also been recorded. In addition, the effects of Mg²⁺ on the fluorescence kinetics of the membranes have been investigated. The mutants are all deficient in the major polypeptide of the light-harvesting chlorophyll a/b protein of photosystem II. The low temperature fluorescence emission spectra of aurea-5106, xantha-5371 and –5820 show little or no fluorescence around 730 nm (photosystem I fluorescence), but possess maxima at 685 and 695 nm (photosystem II fluorescence). These three mutants have low photosystem II activities, but significant photosystem I activities. The long-wavelength fluorescence maximum is reduced for three other mutants. The Mg²⁺ effect on the variable component of the room temperature fluorescence (685 nm) induction kinetics is reduced in all mutants, and completely absent in aurea-5106 and xantha-5820. The thylakoid membranes of these 2 mutants are appressed pairwise in 2-disc grana of large diameter. Chlorotica-1-206A and–130A have significant long-wavelength maxima in the fluorescence spectra and show the largest Mg²⁺ enhancement of the variable part of the fluorescence kinetics. These two mutants have rather normally structured chloroplast membranes, though the stroma regions are reduced. The four remaining mutants are in several respects of an intermediate type.Abbreviations Chl chlorophyll - CPI Chi-protein complex I, F_o, F_v - F_m parameters of room temperature chlorophyll fluorescence induction kinetics - F685, F695 and F-1 components of low temperature chlorophyll emission with maximum at 685, 695 and ca 735 nm, respectively - PSI photosystem I - PSII photosystem II - LHCI and LHCII light-harvesting chlorophyll a/b complexes associated with PSI and PSII, respectively - SDS sodium dodecyl sulfate     

Mechanism of Interaction of Al3+ with the Proteins Composition of Photosystem II

The inhibitory effect of Al3+on photosystem II (PSII) electron transport was investigated using several biophysical and biochemical techniques such as oxygen evolution, chlorophyll fluorescence induction and emission, SDS-polyacrylamide and native green gel electrophoresis, and FTIR spectroscopy. In order to understand the mechanism of its inhibitory action, we have analyzed the interaction of this toxic cation with proteins subunits of PSII submembrane fractions isolated from spinach. Our results show that Al ³⁺, especially above 3 mM, strongly inhibits oxygen evolution and affects the advancement of the S states of the Mn₄O₅Ca cluster. This inhibition was due to the release of the extrinsic polypeptides and the disorganization of the Mn4O5Ca cluster associated with the oxygen evolving complex (OEC) of PSII. This fact was accompanied by a significant decline of maximum quantum yield of PSII (F_v/F_m) together with a strong damping of the chlorophyll a fluorescence induction. The energy transfer from light harvesting antenna to reaction centers of PSII was impaired following the alteration of the light harvesting complex of photosystem II (LHCII). The latter result was revealed by the drop of chlorophyll fluorescence emission spectra at low temperature (77 K), increase of F₀ and confirmed by the native green gel electrophoresis. FTIR measurements indicated that the interaction of Al ³⁺ with the intrinsic and extrinsic polypeptides of PSII induces major alterations of the protein secondary structure leading to conformational changes. This was reflected by a major reduction of α-helix with an increase of β-sheet and random coil structures in Al ³⁺-PSII complexes. These structural changes are closely related with the functional alteration of PSII activity revealed by the inhibition of the electron transport chain of PSII.     

Zinc-inhibited Electron Transport of Photosynthesis in Isolated Barley Chloroplasts

In isolated barley chloroplasts, the presence of 2 millimolar ZnSO₄ inhibits the electron transport activity of photosystem II, as measured by photoreduction of dichlorophenolindophenol, O₂ evolution, and chlorophyll a fluorescence. The inhibition of photosystem II activity can be restored by the addition of the electron donor hydroxylamine or diphenylcarbazide, but not by benzidine and MnCl₂. These observations suggest that Zn inhibits electron flow at the oxidizing side of photosystem II at a site prior to the electron donating site(s) of hydroxylamine and diphenylcarbazide. No inhibition of photosystem I-dependent electron transport by 3 millimolar ZnSO₄ is observed. However, with concentrations of ZnSO₄ above 5 millimolar, photosystem I activity is partially inactivated. Washing Zn²⁺-treated chloroplasts partially restores the O₂-evolving activity.     

Changes in Photosystem II Account for the Circadian Rhythm in Photosynthesis in Gonyaulax polyedra

Cell-free extracts that show activity in photosynthetic electron flow have been prepared from the unicellular dinoflagellate, Gonyaulax polyedra. Electron flow, as O₂ uptake, was measured through both photo-system I and II from water to methyl viologen, through photosystem I alone from reduced 2,6-dichlorophenol indophenol to methyl viologen which does not include the plastoquinone pool or from duroquinol to methyl viologen which includes the plastoquinone pool. Electron flow principally through photosystem II was measured from water to diaminodurene and ferricyanide, as O₂ evolution. Cultures of Gonyaulax were grown on a 12-hour light:12 hour dark cycle to late log phase, then transferred to constant light at the beginning of a light period. After 3 days, measurements of electron flow were made at the maximum and minimum of the photosynthetic rhythm, as determined from measurements of the rhythm of bioluminescence. Photosynthesis was also measured in whole cells, either as ¹⁴C fixation or O₂ evolution. Electron flow through both photosystems and through photosystem II alone were clearly rhythmic, while electron flow through photosystem I, including or excluding the plastoquinone pool, was constant with time in the circadian cycle. Thus, only changes in photosystem II account for the photosynthesis rhythm in Gonyaulax.     

Analysis of changes in minimal and maximal fluorescence yields with irradiance and o(2) level in tobacco leaf tissue

The responses of minimal and maximal fluorescence yields of chlorophyll a to irradiance of actinic white light were determined by pulse modulated fluorimetry in leaf discs from tobacco, Nicotiana tabacum, at 1.6, 20.5, and 42.0% (v/v) O₂. Steady-state maximal fluorescence yield (F_m′, measured during a saturating light pulse) declined with increasing irradiance at all O₂ levels. In contrast, the steady-state minimal fluorescence yield (F_o′, measured during a brief dark interval) increased with irradiance relative to that recorded for the fully dark-adapted leaf (F_o) or that observed after 5 minutes of darkness (F_o^*). The relative magnitude of this increase was somewhat greater and extended to higher irradiances at the elevated O₂ levels compared with 1.6% O₂. Suppression of F_o′ was only observed consistently at saturating irradiance. The results are interpreted in terms of the occurrence of photosystem II units possessing exceedingly slow turnover times (i.e. “inactive” units). Inactive units play an important role, along with thermal deactivation of excited chlorophyll, in determining the response of in vivo fluorescence yield to changes in irradiance. Also, a significant interactive effect of O₂ concentration and the presence or absence of far red light on oxidation of photosystem II acceptors in the dark was noted.     

The relationship between photosynthesis and a mastoparan-induced hypersensitive response in isolated mesophyll cells

The G-protein activator mastoparan (MP) was found to elicit the hypersensitive response (HR) in isolated Asparagus sprengeri mesophyll cells at micromolar concentrations. The HR was characterized by cell death, extracellular alkalinization, and an oxidative burst, indicated by the reduction of molecular O₂ to O₂⁻. To our knowledge, this study was the first to monitor photosynthesis during the HR. MP had rapid and dramatic effects on photosynthetic electron transport and excitation energy transfer as determined by variable chlorophyll a fluorescence measurements. A large increase in nonphotochemical quenching of chlorophyll a fluorescence accompanied the initial stages of the oxidative burst. The minimal level of fluorescence was also quenched, which suggests the origin of this nonphotochemical quenching to be a decrease in the antenna size of photosystem II. In contrast, photochemical quenching of fluorescence decreased dramatically during the latter stages of the oxidative burst, indicating a somewhat slower inhibition of photosystem II electron transport. The net consumption of O₂ and the initial rate of O₂ uptake, elicited by MP, were higher in the light than in the dark. These data indicate that light enhances the oxidative burst and suggest a complex relationship between photosynthesis and the HR.     

The Susceptibility of Photosynthesis to Photoinhibition and the Capacity of Recovery in High and Low Light Grown Cyanobacteria, Anacystis nidulans

The susceptibility of photosynthesis to photoinhibition and the rate of its recovery were studied in the cyanobacterium Anacystis nidulans grown at a low (10 micromoles per square meter per second) and a high (120 micromoles per square meter per second) photosynthetically active radiation. The rate of light limited photosynthetic O₂ evolution was measured to determine levels of photoinhibition and rates of recovery. Studies of photoinhibition and recovery with and without the translation inhibitor streptomycin demonstrated the importance of a recovery process for the susceptibility of photosynthesis to photoinhibition. We concluded that the approximately 3 times lower susceptibility to photoinhibition of high light than of low light grown cells, significantly depended on high light grown cells having an approximately 3 times higher recovery capacity than low light grown cells. It is suggested that these differences in susceptibility to photoinhibition and recovery depends on high light grown cells having a higher turnover rate of photosystem II protein(s) that is(are) the primary site(s) of photodamage, than have low light grown cells. Furthermore, we demonstrated that photoinhibition of A. nidulans may occur under physiological light conditions without visible harm to the growth of the cell culture. The results give support for the hypotheses that the net photoinhibitory damage of photosystem II results from the balance between the photoinhibitory process and the operation of a recovery process; the capacity of the latter determining significant differences in the susceptibility of photosynthesis to photoinhibition of high and low light grown A. nidulans.     

Similar Photosynthetic Performance in Low Light-Grown Isonuclear Triazine-Resistant and -Susceptible Brassica napus L

Triazine-resistant plants grown under moderate to high photon flux density (PFD) conditions exhibit decreased photon yield, decreased light-saturated O₂ evolution and slower growth than triazine-susceptible plants. In this study we tested the hypothesis that the comparable growth previously observed in resistant and susceptible Brassica napus L. lines grown under low PFD was accompanied by comparable photon yield and light-saturated O₂ evolution. We measured photon yield, O₂ flash yield, fluorescence decay kinetics, fluorescence transient kinetics, and quenching components, F_v/F_m and light saturated O₂ evolution in leaf disks of low PFD-grown triazine-resistant and susceptible B. napus isogenic lines. Results indicated that slow electron transfer from the primary to secondary quinone electron acceptors of photosystem II was still present in the resistant line but photon yield and light-saturated O₂ evolution were similar in the two B. napus lines. We conclude that the alteration in the D1 protein that confers resistance does not necessarily cause decreased photosynthetic performance. Decreased photon yield in resistant plants grown at high PFD is not a direct consequence of the alteration in D1, but represents secondary damage.     

Calcium and magnesium effect on divalent cation deficientHydrilla verticillata thylakoid electron transport activity

The role of divalent cations like magnesium (Mg²⁺) and calcium (Ca²⁺) was irrvestigated on energy distribution process ofHydrilla verticillata thylakoids. Effect of these cations was tested on relative quantum yield of photosystem (PS) II catalyzed electron transport activity, room and liquid nitrogen temperature fluorescence emission properties and thylakoid light scattering characteristics. The electron transport activity was found to be stimulated in the presence of these cations in a light intensity independent manner. The concentration of cation required for maximum stimulation was nearly 10–12 mM. Comparatively, Ca²⁺ was more effective than Mg²⁺. Cation induced stimulation in electron transport activity was not accompanied by increase in chlorophylla fluorescence intensity either at room (25°C) or liquid nitrogen (77°K) temperatures. Furthermore, 540 nm absorption and 90° light scattering properties of thylakoids remained insensitive towards divalent cations. These facts together suggest that divalent cations inHydrilla thylakoids are not effective in supporting the excitation distribution between the interacting photosystem complexes.     

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.     

Photoinhibition at chilling temperature

The effects of moderate light at chilling temperature on the photosynthesis of unhardened (acclimated to +18° C) and hardened (cold-acclimated) spinach (Spinacea oleracea L.) leaves were studied by means of fluorescence-induction measurements at 20° C and 77K and by determination of quantum yield of O₂ evolution. Exposure to 550 mol photons·m^-2·s^-1 at +4° C induced a strong photoinhibition in the unhardened leaves within a few hours. Photoinhibition manifested by a decline in quantum yield was characterized by an increase in initial fluorescence (F _o) and a decrease in variable fluorescence (F _v) and in the ratio of variable to maximum fluorescence (F _V/F _M), both at 77K and 20° C. The decline in quantum yield was more closely related to the decrease in the F _V/F _M ratio measured at 20° C, as compared with F _V/F _M at 77K. Quenching of the variable fluorescence of photosystem II was accompanied by a decline in photosystem-I fluorescence at 77K, indicating increased thermal de-excitation of pigments as the main consequence of the light treatment. All these changes detected in fluorescence parameters as well as in the quantum yield of O₂ evolution were fully reversible within 1–3 h at a higher temperature in low light. The fast recovery led us to the view that this photoinhibition represents a regulatory mechanism protecting the photosynthetic apparatus from the adverse effects of excess light by increasing thermal energy dissipation. Long-term cold acclimation probably enforces other protective mechanisms, as the hardened leaves were insensitive to the same light treatment that induced strong inhibition of photosynthesis in unhardened leaves.Abbreviations F ₀ initial fluorescence - F _M maximum fluorescence - F _V variable fluorescence (F _M-F ₀ - PFD photon flux density - PS photosystem