Light-harvesting properties of zinc complex of chlorophyll-<Emphasis Type="Italic">a</Emphasis> from <Emphasis Type="Italic">spirulina</Emphasis> in surfactant micellar media

Zn chlorophyll-a was prepareted from Mg chlorophyll-a from spirulina and the optical properties of the ground state and the photoexited state of Zn chlorophyll-a in aqueous surfactant micellar media were studied using UV-vis absorption, fluorescence emission spectra, electrochemical and fluorescence lifetime measurements. In comparison of the UV-vis absorption and fluorescence emission spectra of Zn chlorophyll-a and Mg chlorophyll-a, the blue-shift in the absorption bands and emission peak of Zn chlorophyll-a was observed. The energies of the first excited singlet state of Zn chlorophyll-a was 1.87eV. The first oxidation and reduction potentials of the photoexcited singlet state of Zn chlorophyll-a were −0.67 and 0.60V, respectively. Fluorescence lifetime of Zn chlorophyll-a was 9.0 ns in CTAB micellar solution. The fluorescence lifetime of Zn chlorophyll-a is shorter than that of Mg chlorophyll-a (9.8 ns). The photositability of Zn chlorophyll-a was superior to that of Mg chlorophyll-a in various pH conditions. Published online December 2004     

The spatial pattern of bioluminescent flashes in the polychaete <Emphasis Type="Italic">Eusyllis blomstrandi</Emphasis> (Annelida)

Each of the trunk segments of the polychaete Eusyllis blomstrandi is equipped with paired epidermal luminescent domains. They luminesce upon mechanical or electrical stimulation. Light emission can be rapidly turned on and off, appears intracellular and is highly coordinated among the trunk segments. Luminescent light is typically emitted in series of flashes. Light emission in a flash starts locally in a group of segments and recruits adjacent segments at a rate as fast as ≤1 ms/segment. The collapse of light emission at the end of a flash is almost simultaneous in all of the segments involved. In the intact worm, the luminescent reaction usually involves only a posterior group of segments. Facilitation becomes manifest as the consecutive flashes in a series increase in brightness and duration and recruit additional anterior segments that were not active in earlier flashes. The flash series stops abruptly instead of decreasing asymptotically in brightness. In posterior fragments, all the segments participate in flashing luminescence, indicating the loss of an inhibitory effect exerted by the anterior end in the case of whole animals. Posterior fragments survive and are still capable of luminescence weeks after fragmentation although they do not regenerate a head. Immediately upon fragmentation of the worm, the posterior fragment luminesces continuously for some seconds while the anterior part quickly stops light emission. This suggests a decoy and/or a predator-alerting function of prolonged, strong luminescence by the moribund posterior fragment to the benefit of the survival of the anterior fragment.     

Ultraweak emissions of developing <Emphasis Type="Italic">Xenopus laevis</Emphasis> eggs and embryos

We measured ultraweak emissions of the Xenopus laevis eggs and embryos during normal development and under the influence of stress factors in a spectral range of 250 to 800 nm using a photomultiplier. The registered emissions were analyzed by several basic characteristics: mean intensity, histograms, kurtosis, linear trends, and Fourier spectra. We followed relationships between these parameters and developmental stage, as well as the number of individuals in optic contact with each other. The ultraweak emissions did not differ from the background at all developmental stages according to the mean intensity. But Fourier analysis revealed the reliable presence of a number of spectral lines of ultraweak emission, predominantly in the range of 10^?2–50 Hz, in the embryos at developmental stages 2 to 11. The intensity of ultraweak emissions reliably decreased within the first 10 min after egg activation and fertilization, as well as in the case of optic interaction between groups of embryos. Sharp cooling, increase in osmotic medium pressure, and transfer in a Ca²⁺ and Mg²⁺-free medium induced a short term (～1–5 min) increase in the mean intensity of ultraweak emission. We studied specific features of ultraweak emissions from different parts of the embryo. The intensity of emission from the animal part of early blastula exceeded those from the vegetal area and entire embryo. Separated fragments of the lateral ectoderm at the neurula stage had higher mean intensities of ultraweak emission than intact embryos at the same developmental stages.     

Chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence induction kinetics in leaves predicted from a model describing each discrete step of excitation energy and electron transfer associated with Photosystem II

Chlorophyll a fluorescence induction (FI) is widely used as a probe for studying photosynthesis. On illumination, fluorescence emission rises from an initial level O to a maximum P through transient steps, termed J and I. FI kinetics reflect the overall performance of photosystem II (PSII). Although FI kinetics are commonly and easily measured, there is a lack of consensus as to what controls the characteristic series of transients, partially because most of the current models of FI focus on subsets of reactions of PSII, but not the whole. Here we present a model of fluorescence induction, which includes all discrete energy and electron transfer steps in and around PSII, avoiding any assumptions about what is critical to obtaining O J I P kinetics. This model successfully simulates the observed kinetics of fluorescence induction including O J I P transients. The fluorescence emission in this model was calculated directly from the amount of excited singlet-state chlorophyll in the core and peripheral antennae of PSII. Electron and energy transfer were simulated by a series of linked differential equations. A variable step numerical integration procedure (ode15s) from MATLAB provided a computationally efficient method of solving these linked equations. This in silico representation of the complete molecular system provides an experimental workbench for testing hypotheses as to the underlying mechanism controlling the O J I P kinetics and fluorescence emission at these points. Simulations based on this model showed that J corresponds to the peak concentrations of Q _A ⁻ Q_B (Q_A and Q_B are the first and second quinone electron acceptor of PSII respectively) and Q _A ⁻ Q _B ⁻ and I to the first shoulder in the increase in concentration of Q _A ⁻ Q _B ²⁻ . The P peak coincides with maximum concentrations of both Q _A ⁻ Q _B ²⁻ and PQH₂. In addition, simulations using this model suggest that different ratios of the peripheral antenna and core antenna lead to differences in fluorescence emission at O without affecting fluorescence emission at J, I and P. An increase in the concentration of Q_B-nonreducing PSII centers leads to higher fluorescence emission at O and correspondingly decreases the variable to maximum fluorescence ratio (F _v/F _m).     

Functional analysis of genes involved in the biosynthesis of isoprene in <Emphasis Type="Italic">Bacillus subtilis</Emphasis>

In comparison to other bacteria Bacillus subtilis emits the volatile compound isoprene in high concentrations. Isoprene is the smallest representative of the natural product group of terpenoids. A search in the genome of B. subtilis resulted in a set of genes with yet unknown function, but putatively involved in the methylerythritol phosphate (MEP) pathway to isoprene. Further identification of these genes would give the possibility to engineer B. subtilis as a host cell for the production of terpenoids like the valuable plant-produced drugs artemisinin and paclitaxel. Conditional knock-out strains of putative genes were analyzed for the amount of isoprene emitted. Differences in isoprene emission were used to identify the function of the enzymes and of the corresponding selected genes in the MEP pathway. We give proof on a biochemical level that several of these selected genes from this species are involved in isoprene biosynthesis. This opens the possibilities to investigate the physiological function of isoprene emission and to increase the endogenous flux to the terpenoid precursors, isopentenyl diphosphate and dimethylallyl diphosphate, for the heterologous production of more complex terpenoids in B. subtilis.     

Molecular Cloning and Expression of a Pearl Oyster (<Emphasis Type="Italic">Pinctada fucata</Emphasis>) Homologue of Mammalian Putative Tumor Suppressor QM

The QM gene was originally identified as a putative tumor suppressor gene from a Wilms tumor cell line by subtractive hybridization assay. Later studies showed that the QM protein is multifunctional, involved in cell growth and differentiation, energy metabolism, respiration, and cytoskeletal function. In this report a full-length complementary DNA encoding a QM counterpart in pearl oyster (Pinctada fucata) was isolated. Phylogenetic analysis shows that oyster QM is more closely related to its insect homologues than to the mammalian homologues. Analysis of the tissue expression pattern of the oyster QM gene showed that oyster QM messenger RNA is expressed in all tissues tested, with highest levels in the digestive gland and mantle. Furthermore, we expressed the QM protein in Escherichia coli; Western blotting showed that the antibody of human QM is immunoreactive to the expressed oyster QM protein. Incubation of the oyster QM with Zn²⁺ resulted in the reduction of intrinsic emission fluorescence and a red-shift in the _max emission, indicating the occurrence of Zn²⁺-induced conformational changes. This evidence presents a possible mechanism for the critical function of zinc ion in the interaction of QM with Jun.     

Induction of systemic acquired resistance in Zea mays also enhances the plant’s attractiveness to parasitoids

Plants under attack by caterpillars emit volatile compounds that attract the herbivore’s natural enemies. In maize, the caterpillar-induced production of volatiles involves the phytohormone jasmonic acid (JA). In contrast, pathogen attack usually up-regulates the salicylic acid (SA)-pathway and results in systemic acquired resistance (SAR) against plant diseases. Activation of the SA-pathway has often been found to repress JA-dependent direct defenses, but little is known about the effects of SAR induction on indirect defenses such as volatile emission and parasitoid attraction. We examined if induction of SAR in maize, by chemical elicitation with the SA-mimic benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester (BTH), attenuates the emission of volatiles induced by Spodoptera littoralis or exogenously applied JA. In addition, we determined how these treatments affected the attractiveness of the plants to the parasitoid Microplitis rufiventris in a six-arm-olfactometer. BTH treatment alone resulted in significant systemic resistance of maize seedlings against the pathogen Setosphaeria turcica, but had no detectable effect on volatile emissions. Induction of SAR significantly reduced the emission rates of two compounds (indole and (E)-β-caryophyllene) in JA-treated plants, whereas no such negative cross-talk was found in caterpillar-damaged plants. Surprisingly, however, BTH treatment prior to caterpillar-feeding made the plants far more attractive to the parasitoid than plants that were only damaged by the herbivore. Control experiments showed that this response was due to plant-mediated effects rather than attractiveness of BTH itself. We conclude that in the studied system, plant protection by SAR activation is compatible with and can even enhance indirect defense against herbivores.     

Evidence for the probable oil body association of a thiol-protease, leading to oleosin degradation in sunflower seedling cotyledons

The activity of a 65 kDa, cytosolic protease from sunflower seedling cotyledons coincides with the degradation of oleosins during seed germination. Further investigations carried out in this laboratory have demonstrated the probable association of a thiol-protease with oil bodies, leading to gradual degradation of oleosins during seedling growth. Evidence to this effect have been brought out through zymographic detection of protease activity from oil bodies, degradation of oleosins by electrophoretically eluted protease from the seedling cotyledons and inhibition of protease activity by thiol-protease inhibitor, such as N-ethylmaleimide (NEM). In addition to these biochemical evidence, visualization of thiol-protease activity has also been achieved by a novel fluorescence microscopic method and confocal imaging. It involves the uptake and binding of a fluorogenic thiol-protease inhibitor (fluorescein mercuric acetate, FMA) at the intracellular thiol-protease activity sites in protoplasts, leading to fluorescence emission at 523 nm following excitation at 499 nm. Maximum protease activity is observed in 4-d-old seedling cotyledons, coinciding with the phase of active triacylglycerol (TAGs) hydrolysis. All these observations provide evidence for the expression of the said thiol-protease activity on the oil body surface, leading to gradual proteolysis of oleosins during seed germination.     

A method for the destruction and analysis of biogenic silicon in two Antarctic diatom species:<Emphasis Type="Italic"> Thalassiosira</Emphasis> sp. and<Emphasis Type="Italic"> Chaetoceros brevis</Emphasis>

Diatoms in the Southern Ocean are limited by iron and light, and therefore produce little biomass. Sufficient biomass for analysis under these conditions requires large sample volumes, and diatom samples are therefore often pre-concentrated on a filter. A method for the digestion of diatom cells on polycarbonate filters, that is also suitable for trace metal analysis, is described here. Additional analysis by inductively coupled plasma-optical emission spectroscopy (ICP-OES) is used for the determination of biogenic silicon. Although several procedures were tested, the method of Hauptkorn et al., which uses tetramethylammonium hydroxide for the destruction of silicon is adapted here [Hauptkorn et al. (2001) Fres J Anal Chem 370:246–250]. Additional nitric acid is added to destroy the polycarbonate filters. The described method results in clear digests and a good correlation between cell numbers and silicon content. Using this procedure, the cellular silicon content for Chaetoceros brevis was determined as 86 ± 4 fmol cell⁻¹. For Thalassiosira sp. a sensitivity effect was observed, and silicon content was determined as . The obtained cellular silicon contents are in good agreement with values presented in the literature.     

Characteristics of the relative maximum in the decay of delayed light emission from Pothos leaf following far-red excitation

Following illumination with wavelengths longer than 700 nm, the intensity of light emission from Pothos aurea leaf falls for 1 min and then increases to a maximum after 2 min in the dark. The spectrum of this minute-range liminescence matches that of prompt fluorescence excited at the same wavelength, but differs from that of prompt or minute-range delayed emission excited by wavelengths shorter than 700 nm. This emission is less sensitive to heat damage than millisecond delayed emission, and may originate from photosystem I.     

Methane emission from tidal freshwater marshes

In two tidal freshwater marshes, methane emission,production and accumulation in the pore-water have beenstudied. The two sites differ in their dominantvegetation, i.e., reed and bulrush, and in theirheights above sea level. The reed site was elevated inrelation to the bulrush site and had higher rates ofmethane emission and production. It is argued thatthis difference in methane emission between sites wasprimarily due to a different effect of reed andbulrush plants on methane dynamics rather than methaneoxidation related to tidal elevation. Methane emissionshowed strong seasonality related primarily to plantphysiology and only secondarily to temperature. Twocontrol sites at which vegetation was removedsystematically had lower emission rates indicating anoverall stimulating effect of plants on methaneemission from tidal marshes. Flooding reduced methaneemission, probably by blocking the primary sites ofmethane release in the lower part of the plantstems.     

The composition and timing of flower odour emission by wild <Emphasis Type="Italic">Petunia axillaris</Emphasis> coincide with the antennal perception and nocturnal activity of the pollinator <Emphasis Type="Italic">Manduca sexta</Emphasis>

In the genus Petunia, distinct pollination syndromes may have evolved in association with bee-visitation (P. integrifolia spp.) or hawk moth-visitation (P. axillaris spp). We investigated the extent of congruence between floral fragrance and olfactory perception of the hawk moth Manduca sexta. Hawk moth pollinated P. axillaris releases high levels of several compounds compared to the bee-pollinated P. integrifolia that releases benzaldehyde almost exclusively. The three dominating compounds in P. axillaris were benzaldehyde, benzyl alcohol and methyl benzoate. In P. axillaris, benzenoids showed a circadian rhythm with an emission peak at night, which was absent from P. integrifolia. These characters were highly conserved among different P. axillaris subspecies and P. axillaris accessions, with some differences in fragrance composition. Electroantennogram (EAG) recordings using flower-blends of different wild Petunia species on female M. sexta antennae showed that P. axillaris odours elicited stronger responses than P. integrifolia odours. EAG responses were highest to the three dominating compounds in the P. axillaris flower odours. Further, EAG responses to odour-samples collected from P. axillaris flowers confirmed that odours collected at night evoked stronger responses from M. sexta than odours collected during the day. These results show that timing of odour emissions by P. axillaris is in tune with nocturnal hawk moth activity and that flower-volatile composition is adapted to the antennal perception of these pollinators.     

On the relationship between isoprene emission and photosynthetic metabolites under different environmental conditions

Isoprene emission is related to photosynthesis but the nature of the relationship is not yet known. To explore this relationship we have examined the rate of isoprene emission, photosynthesis, and the contents of photosynthetic metabolites in leaves of velvet bean (Mucuna deeringeniana L.) and red oak (Quercus rubra L.) in response to a light-to-dark transition and to changes in air composition. Isoprene emission fell when darkness was imposed and the drop was associated with reduced amounts of ribulose-1,5-bisphosphate and ATP. The rate of isoprene emission and ATP content were reduced to the same extent by exposure to low O₂ or high CO₂ partial pressures. Only when O₂ and CO₂ were simultaneously removed from the air did the rate of isoprene emission drop without a corresponding change in ATP. The results demonstrate that when carbon is not limiting, isoprene emission is highly correlated with ATP content. When synthesis of phosphoglyceric acid is inhibited, however, carbon availability may control isoprene production. Mr. Peter Vanderveer assisted with the measurements of enzymatic metabolites. Mr. Xavier Socias is gratefully acknowledged for Rubisco preparation. This research was supported by the U.S. National Science Foundation, grant no. IBN 9105274.     

Photon emissions from rice cells elicited by N-acetylchitooligosaccharide are generated through phospholipid signaling in close association with the production of reactive oxygen species

Biophotons are ultraweak light emissions from biochemical reactions in a living body. They increase in suspension-cultured rice (Oryza sativa L.) cells when elicited by N-acetylchitooligosaccharide. Biochemical analyses were undertaken to investigate the relationship between disease response and biophotons in order to clarify the emission mechanism of biophotons caused by this elicitor. Photon emissions induced by N-acetylchitohexaose were suppressed when cells were pretreated with the reactive oxygen species (ROS)-generating inhibitors: pyrocatechol-3,5-disulfonic acid disodium salt (Tiron); diphenylene iodonium (DPI); and salicylhydroxamic acid (SHAM). Conversely, exogenously applied ROS (superoxide and hydrogen peroxide) were able to induce photon emissions. The effects of protein phosphorylation (K-252a) and the Ca(2+) signaling inhibitors, ethylene glycol-bis(beta-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA) and LaCl(3), caused photon emissions to decrease. It is clear that photon emissions from rice cells elicited by N-acetylchitohexaose are closely associated with the ROS-generating system, and are regulated by Ca(2+) signaling and protein phosphorylation. Exogenously applied phosphatidic acid (PA), the second messenger in the signal transduction of disease response, raised photon emissions in rice cells. Comparisons of photon emissions from PA and N-acetylchitohexaose regarding time courses, spectral compositions, and the inhibition ratios of several inhibitors, as well as a loss- and gain-of-function assay using the protein synthesis inhibitor cycloheximide (CHX) and PA, showed the possibility that photon emissions from rice cells elicited by N-acetylchitooligosaccharide were generated through PA, an intermediate of phospholipid signaling.     

Short-chained oxygenated VOC emissions in <Emphasis Type="Italic">Pinus halepensis</Emphasis> in response to changes in water availability

Short-chained oxygenated VOC (oxVOCs) emissions from Pinus halepensis saplings were monitored in response to changes in water availability. Online measurements were made with a proton transfer reaction—mass spectrometer under controlled conditions, together with CO₂ and H₂O exchange measurements. Masses corresponding to methanol and acetone were the most emitted oxVOCs. All the oxVOC exchanges, except that of acetone (M59), were significantly related to stomatal conductance and transpiration. Acetaldehyde (M45) emission showed, moreover, a strong dependence on the concentration of acetaldehyde in the ambient: stomatal opening (stomatal conductance above 75 mmol m⁻² s⁻¹) only allowed increased emissions when external concentration were below 6 ppb. Acetone (M59) presented an important peak of emission following light and stomatal opening in the morning when plants were water stressed. Thus, the alterations in oxVOC emissions in P. halepensis caused by the water deficit seem to be mainly driven by water stress effect on stomatal closure and oxVOC air concentrations.     

Serum Levels of Zinc,Copper, Iron,Cobalt, Magnesium,and Selenium Elements in Children Diagnosed with <Emphasis Type="Italic">Giardia Intestinalis</Emphasis> and <Emphasis Type="Italic">Enterobiosis Vermicularis</Emphasis> in Hatay,Turkey

The intestinal parasites are noted to be an important health problem in Turkey as similarly reported in the globe. The aim of this study was to investigate the changes in total content of essential elements, namely, zinc, iron, copper, cobalt, magnesium, and selenium, in children infected with intestinal parasites aged between 6 and 12 years inhabiting in Hatay Province, Turkey. These essential elements were measured in the children/patient who was positive for intestinal parasites, Giardia intestinalis and Enterobius vermicularis. Scores were obtained from the positive study group (SG), and their age matched the healthy children control group (CG). Serological levels of zinc, iron, copper, cobalt, magnesium, and selenium were analyzed by Varian Liberty Series II inductively coupled plasma atomic emission spectrometer (ICP-AES). The mean magnesium concentrations were found to be statistically different at 95% confidence interval level between study groups. As a result of this study, selenium was found to be uncorrelated with all other elements examined; whereas, copper was observed to have statistically significant correlations with cobalt, magnesium, and zinc. In addition, cobalt-magnesium, cobalt-zinc, and magnesium-zinc metal pairs were found to have statistically significant correlations based on study findings.     

Chlorophyll fluorescence at 680 and 730 nm and leaf photosynthesis

Chlorophyll fluorescence constitutes a simple, rapid, and non-invasive means to assess light utilization in Photosystem II (PS II). This study examines aspects relating to the accuracy and applicability of fluorescence for measurement of PS II photochemical quantum yield in intact leaves. A known source of error is fluorescence emission at 730 nm that arises from Photosystem I (PS I). We measured this PS I offset using a dual channel detection system that allows measurement of fluorescence yield in the red (660 nm < F < 710 nm) or far red (F > 710 nm) region of the fluorescence emission spectrum. The magnitude of the PS I offset was equivalent to 30% and 48% of the dark level fluorescence F₀ in the far red region for Helianthus annuus and Sorghum bicolor, respectively. The PS I offset was therefore subtracted from fluorescence yields measured in the far red spectral window prior to calculation of PS II quantum yield. Resulting values of PS II quantum yield were consistently higher than corresponding values based on emission in the red region. The basis for this discrepancy lies in the finite optical thickness of the leaf that leads to selective reabsorption by chlorophyll of red fluorescence emission originating in deeper cell layers. Consequently, red fluorescence measurements preferentially sense emission from chloroplasts in the uppermost layer of the leaf where levels of photoprotective nonphotochemical quenching are higher due to increased photon density. It is suggested that far red fluorescence, corrected for the PS I offset, provides the most reliable quantitative basis for calculation of PS II quantum yield because of reduced sensitivity of these measurements to gradients in leaf transmittance and quenching capacity. This revised version was published online in August 2006 with corrections to the Cover Date.     

Light-induced fluorescence changes in<Emphasis Type="Italic"> Phycomyces</Emphasis>: evidence for blue light-receptor associated flavo-semiquinones

Light-induced fluorescence changes (LIFCs) were detected in sporangiophores of the blue-light-sensitive fungus Phycomyces blakesleeanus (Burgeff). The LIFCs can be utilized as a spectrophotometric assay for blue-light photoreceptors and for the in vivo characterization of their photochemical primary reactions. Blue-light irradiation of sporangiophores elicited a transient decrease and subsequent regeneration of flavin-like fluorescence emission at 525 nm. The signals recovered in darkness in about 120 min. In contrast to blue light, near-UV (370 nm) caused an increase in the fluorescence emission at 525 nm. Because the LIFCs were altered in a light-insensitive madC mutant with a defective photoreceptor, the fluorescence changes must be associated with early photochemical events of the transduction chain. Action spectra for the fluorescence changes at 525 nm showed major peaks near 470 and 600 nm. Double-pulse experiments involving two consecutive pulses of either blue and near-UV, blue and red, or near-UV and red showed that the responses depended on the sequence in which the different wavelengths were applied. The results indicate a blue-light receptor with intermediates in the near-UV, blue and red spectral regions. We explain the results in the framework of a general model, in which the three redox states of the flavin photoreceptor, the oxidized flavin (Fl), the flavo-semiquinone (FlH^·), and the flavo-hydroquinone (FlH₂) are each acting as chromophores with their own characteristic photochemical primary reactions. These consist of the photoreduction of the oxidized flavin generating semiquinone, the photoreduction of the semiquinone generating hydroquinone, and the photooxidation of the flavo-hydroquinone regenerating the pool of oxidized flavins. The proposed mechanism represents a photocycle in which two antagonistic photoreceptor forms, Fl and FlH₂, determine the pool size of the biological effector molecule, the flavo-semiquinone. The redox changes that are associated with the photocycle are maintained by redox partners, pterins, that function in the near-UV as secondary chromophores.Abbreviations FAD flavin adenine dinucleotide - Fl oxidized flavin - FlH flavo-semiquinone radical - FlH₂ flavo-hydroquinone - LIAC light-induced absorbance change - LIFC light-induced fluorescence change - Pt oxidized pterin - PtH₂ dihydro-pterin - PtH₄ tetrahydro-pterin     

Deriving room temperature excitation spectra for photosystem I and photosystem II fluorescence in intact leaves from the dependence of <Emphasis Type="Italic">F</Emphasis><Subscript>V</Subscript>/<Emphasis Type="Italic">F</Emphasis><Subscript>M</Subscript> on excitation wavelength

The F ₀ and F _M level fluorescence from a wild-type barley, a Chl b-less mutant barley, and a maize leaf was determined from 430 to 685 nm at 10 nm intervals using pulse amplitude-modulated (PAM) fluorimetry. Variable wavelengths of the pulsed excitation light were achieved by passing the broadband emission of a Xe flash lamp through a birefringent tunable optical filter. For the three leaf types, spectra of F _V/F _M (=(F _M − F ₀)/F _M) have been derived: within each of the three spectra of F _V/F _M, statistically meaningful variations were detected. Also, at distinct wavelength regions, the F _V/F _M differed significantly between leaf types. From spectra of F _V/F _M, excitation spectra of PS I and PS II fluorescence were calculated using a model that considers PS I fluorescence to be constant but variable PS II fluorescence. The photosystem spectra suggest that LHC II absorption results in high values of F _V/F _M between 470 and 490 nm in the two wild-type leaves but the absence of LHC II in the Chl b-less mutant barley leaf decreases the F _V/F _M at these wavelengths. All three leaves exhibited low values of F _V/F _M around 520 nm which was tentatively ascribed to light absorption by PS I-associated carotenoids. In the 550–650 nm region, the F _V/F _M in the maize leaf was lower than in the barley wild-type leaf which is explained with higher light absorption by PS I in maize, which is a NADP-ME C₄ species, than in barley, a C₃ species. Finally, low values of F _V/F _M at 685 in maize leaf and in the Chl b-less mutant barley leaf are in agreement with preferential PS I absorption at this wavelength. The potential use of spectra of the F _V/F _M ratio to derive information on spectral absorption properties of PS I and PS II is discussed.