Estimation of phytoplankton photosynthesis using a fluorescence induction technique

An improved method for estimation of phytoplankton photosynthesiswas developed using in-vivo chlorophyll a fluorescence, andwas tested with cultured phytoplankton. The method is basedon a kinetic analysis of the fluorescence induction with andwithout DCMU. Photosynthetic rates were derived from, (i) measurementof fluorescence induction due to the reduction of Q, the primaryelectron acceptor for the photoreaction of photosystem II, and(ii) estimation of Q present in the water sample and of therate of Q reduction. ^*This paper is the result of a study made at the Group for AquaticPrimary Productivity (GAP), Second International Workshop heldat the National Oceanographic Institute, Haifa, Israel in April–May1984.     

Chlorophyll content and fluorescence responses cannot be used to gauge reliably phytoplankton biomass, nutrient status or growth rate

To consider the relationship between chlorophyll a (Chl a) content and phytoplankton growth and nutrient status, four phytoplankton species were grown in nitrogen (N)-limited [and, for one species, phosphorus (P)-limited] culture and measurements were made of CNP biomass, in vivo and in vitro Chl a content, the ratio of variable to maximum fluorescence (FV/FM) and the performance index for photosynthesis, PIABS (a derivative of the O-J-I-P analysis of photosystem II functionality). Interspecies differences plus the development of intraspecies differences during nutrient stress produced c. 10-fold variations in Chl : C. Estimates of C from in vivo Chl content were better than those from extracted Chl content, as the decline in Chl : C during nutrient stress was offset in part by increased Chl fluorescence. FV/FM was not a robust indicator of nutrient status or relative growth rate. Responses of FV/FM in cells re-fed the limiting nutrient showed no consistent pattern with which to gauge nutrient status. PIABS showed some promise as an indicator of nutrient status and relative growth rate. Chl a content and fluorescence parameters do not deserve the unquestioned status they usually enjoy as indicators of biomass and physiological status.     

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.     

Chlorophyll a fluorescence and light-scattering kinetics displayed by leaves during induction of photosynthesis

Light-scattering, which can be taken as an indicator of the transthylakoid proton-gradient, and chlorophyll a fluorescence, have been followed simultaneously during re-illumination of spinach leaves at different energy fluence rates and carbon dioxide concentrations. The slow fluorescence transient (M peak), which has been associated with photosynthetic induction, was observed in air only at the lower fluence rates used. Data are presented that indicate that M peaks in chlorophyll fluorescence kinetics can only be observed if there is also a simultaneous transient in light-scattering and that these transients are observed when the dark period is relatively long, fluence rate relatively low, and CO₂ concentration relatively high.The results are discussed in relation to the varying demands on ATP by carbon assimilation during induction of photosynthesis at different carbon dioxide concentrations and the manner in which these variations influence the quenching of chlorophyll a fluorescence.Abbreviation Chl chlorophyll     

Chlorophyll f-driven photosynthesis in a cavernous cyanobacterium

Chlorophyll (Chl) f is the most recently discovered chlorophyll and has only been found in cyanobacteria from wet environments. Although its structure and biophysical properties are resolved, the importance of Chl f as an accessory pigment in photosynthesis remains unresolved. We found Chl f in a cyanobacterium enriched from a cavernous environment and report the first example of Chl f-supported oxygenic photosynthesis in cyanobacteria from such habitats. Pigment extraction, hyperspectral microscopy and transmission electron microscopy demonstrated the presence of Chl a and f in unicellular cyanobacteria found in enrichment cultures. Amplicon sequencing indicated that all oxygenic phototrophs were related to KC1, a Chl f-containing cyanobacterium previously isolated from an aquatic environment. Microsensor measurements on aggregates demonstrated oxygenic photosynthesis at 742 nm and less efficient photosynthesis under 768- and 777-nm light probably because of diminished overlap with the absorption spectrum of Chl f and other far-red absorbing pigments. Our findings suggest the importance of Chl f-containing cyanobacteria in terrestrial habitats.The textbook concept that oxygenic phototrophs primarily use radiation in the visible range (400–700 nm) has been challenged by several findings of unique cyanobacteria and chlorophylls (Chl) over the past two decades (Miyashita et al., 1996; Chen et al., 2010; Croce and van Amerongen, 2014) Unicellular cyanobacteria in the genus Acaryochloris primarily employ Chl d for oxygenic photosynthesis at 700–720 nm (Miyashita et al., 1996) and thrive in shaded habitats with low levels of visible light but replete of near-infrared radiation (NIR, >700 nm, Kühl et al., 2005; Behrendt et al., 2011, 2012). Furthermore, Chl f was recently discovered in filamentous (Chen et al., 2010; Airs et al., 2014; Gan et al., 2014) and unicellular cyanobacteria (Miyashita et al., 2014), enabling light harvesting even further into the NIR region up to ∼740 nm, often aided by employing additional far-red light-absorbing pigments such as Chl d and phycobiliproteins (Gan et al., 2014). Whereas the biochemical structure (Willows et al., 2013) and biophysical properties (Li et al., 2013; Tomo et al., 2014) of Chl f have been studied in detail, the actual importance of this new chlorophyll for photosynthesis is hardly explored (Li et al., 2014).Chlorophyll f has been found in cyanobacteria originating from aquatic/wet environments: the filamentous Halomicronema hongdechloris from stromatolites in Australia (Chen et al., 2012), a unicellar morphotype (Strain KC1) from Lake Biwa in Japan (Akutsu et al., 2011; Miyashita et al., 2014) and a filamentous Leptolyngbya sp. strain (JSC-1, Gan et al., 2014) from a hot-spring and in a unicellular Chlorogloeopsis fritschii strain from rice paddies (Airs et al., 2014). In this study, we report on a unicellular Chl f-containing cyanobacterium originating from a wet cavernous habitat and demonstrate its capability of NIR-driven oxygenic photosynthesis. Enrichments of the new cyanobacterium were obtained from a dense dark green-blackish biofilm dominated by globular morphotypes of Nostocaceae growing on moist limestone outside Jenolan Caves, NSW, Australia. The sampling site was heavily shaded even during mid-day with low irradiance levels of 400- to 700-nm light varying from 0.5 to 5 μmol photons m⁻² s⁻¹. Biofilms were carefully scraped off the substratum and kept in shaded zip-lock bags in a moist atmosphere until further processing. Samples were then incubated at 28 °C in a f/2 medium under NIR at 720 nm (∼10 μmol photons m⁻² s⁻¹) yielding conspicuous green cell aggregates after several months of incubation. Repeated transfer of the aggregates into fresh medium resulted in a culture predominated by green cell clusters (Figure 1a), exhibiting orange-red fluorescence upon excitation with blue light (Figure 1b). Transmission electron microscopy revealed that the green clusters consisted of slightly elongated unicellular cyanobacteria (∼1- to 2-μm wide and ∼2- to 3-μm long), with stacked thylakoids and embedded in a joint polymer matrix (Figure 1c). Hyperspectral microscopy (Kühl and Polerecky, 2008) of the clusters revealed distinct troughs in the transmission spectra at absorption maxima indicative of Chl a (675–680 nm) and Chl f (∼720 nm; Figure 1d, red line). In situ spectral irradiance measurements at the sampling site showed strong depletion of visible wavelengths in the 480- to 710-nm range (Figure 1d, gray line), whereas highest light levels were found in the near-infrared region of the solar spectrum at 710–900 nm. The presence of Chl a and f was further confirmed in enrichment cultures using high-performance liquid chromatography-based pigment analysis (Figure 1e, Supplementary Figure S1), while no Chl d was detected. In addition, weak spectral signatures of carotenoids and phycobilins, with absorption occurring at ∼495 and 665 nm, were evident in the hyperspectral data. Cyanobacteria, including those producing Chl d/f, are known to actively remodel their pigment content in response to the available light spectrum (Stomp et al., 2007; Chen and Scheer, 2013; Gan et al., 2014) and Chl d/f has almost exclusively been found in cyanobacteria grown under far-red light and not under visible light (Kühl et al., 2005; Chen et al., 2010; Airs et al., 2014; Gan et al., 2014; Li et al., 2014; Miyashita et al., 2014). Recent work describes this acclimation response as ‘Far-Red Light photoacclimation'' (FaRLiP), which, in strain JSC-1, comprises a global change in gene expression and structural remodeling of the PSII/PSI core proteins and phycobilisome constituents (Gan et al., 2014). The extent to which this arrangement results in optimized photosynthetic performance is only known for the NIR (=710 nm)-acclimated strain JSC-1, where exposure to wavelengths >695 nm resulted in 40% higher O₂ evolution rates as compared with cells that were previously adapted to red light (645 nm; Gan et al., 2014). Yet the discrimination of actinic wavelengths and their relative effect on gross photosynthesis in Chl f-containing cells needs further investigation. Using an O₂ microsensor and the light–dark shift method (Revsbech et al., 1983) on embedded Chl f-containing aggregates, we found maximal gross photosynthesis rates (∼1.06 μmol O₂ cm⁻³ s⁻¹) to occur at irradiances of ∼250 μmol photons m⁻² s⁻¹ of 742 nm (half-bandwidth, HBW, 25 nm, Figures 2a and b) with light saturation to occur very early at ∼35 μmol photons m⁻² s⁻¹. Further red-shifted actinic light, that is, 768 nm (HBW 28 nm) and 777 nm (HBW 30 nm), yielded lower O₂ evolution rates, which, in all likelihood, are an effect of the diminished overlap with far-red light-absorbing pigments, including Chl f (Figures 2a and b). As O₂ evolution rates were measured on non-axenic cell aggregates, 16S rDNA amplicon sequencing was employed to determine the microbial diversity found within the enrichment culture. This revealed the presence of a variety of bacterial types, including anoxygenic phototrophs, yet all sequences for known oxygenic phototrophs in the data set (∼9.3% of all reads on the order level, Supplementary Figure S2) formed a single operational taxonomic unit (OTU) closely affiliated with the Chl f-containing strain KC1 (Miyashita et al., 2014, Figure 2c).Open in a separate windowFigure 1Imaging and pigment analysis of Chl f-containing cyanobacteria isolated from a cavernous low-light environment. (a) Representative bright field microscope image of cultured cells grown under 720 nm NIR. (b) Fluorescence image of the same cells as in a, excited at 450–490 nm, with emission being detected at >510 nm. (c) Transmission electron microscopy of a Chl f-containing cyanobacterium with densely stacked thylakoid membranes. (d) Transmittance spectrum of cell aggregate determined by hyperspectral imaging (red line). Ambient light conditions at the site of isolation (gray line), as measured by a spectroradiometer. Note the Chl f-specific in vivo absorption at ∼720 nm in the transmittance spectrum (dotted line). Small insert picture denotes the cells and area of interest (black arrow) from which the spectrum was taken. (e) In vitro absorption spectrum of Chl f extracted from enrichment cultures and analyzed via high-performance liquid chromatography. The two Chl f-specific absorption peaks (404 and 704 nm in acetone:MeOH solvent) are indicated.Open in a separate windowFigure 2Taxonomic affiliation and O₂ evolution of Chl f-containing cells as determined by O₂ microelectrode measurements and 16 S rDNA amplicon sequencing. (a) Emission spectra of narrow-band light-emitting diodes (LEDs) used in this study, with peak emissions at 742, 768 and 777 nm indicated by a–c, respectively. (b) Gross photosynthesis measured via an O₂ microsensor placed in a clump of agarose-embedded Chl f-containing cells. Different NIR irradiance was administered by the LEDs in a and by altering the distance of the LEDs to the embedded cells. (c) Phylogenetic affiliation of known Chl f and/or Chl d-containing cyanobacteria (highlighted in gray) and their respective habitat/place of isolation. Taxonomy was determined by clustering all known oxygenic phototrophs found in enrichment cultures from this study (at order level) into a single OTU (=292 bp length, see Supplementary Materials for details). Phylogeny was inferred using Maximum-likelihood in conjunction with the GTR +I +G nucleotide substitution model, tree stability was tested using bootstrapping with 100 replicates. The analysis involved 39 nucleotide sequences each truncated to a length of 292 bp. Here, the green-sulphur bacterium Chlorobium tepidum TLS was chosen as the outgroup.This advocates that cells from our enrichment culture are related to KC1 cells and supports, in conjunction with further morphological-, physiological- and ultrastructural evidence, that Chl f is extending the usable light spectrum for oxygenic photosynthesis in a cavernous low-light environment. Given the lifestyle and known habitats of recognized Chl d/f-producing cyanobacteria (Figure 2c), we propose that many, if not all, surface-associated cyanobacteria are intrinsically capable of producing far-red light-absorbing pigments and to actively employ them in oxygenic photosynthesis as a result of FaRLiP or similar, yet unknown, mechanisms.     

Modeling chlorophyll a fluorescence transient: Relation to photosynthesis

To honor Academician Alexander Abramovitch Krasnovsky, we present here an educational review on the relation of chlorophyll a fluorescence transient to various processes in photosynthesis. The initial event in oxygenic photosynthesis is light absorption by chlorophylls (Chls), carotenoids, and, in some cases, phycobilins; these pigments form the antenna. Most of the energy is transferred to reaction centers where it is used for charge separation. The small part of energy that is not used in photochemistry is dissipated as heat or re-emitted as fluorescence. When a photosynthetic sample is transferred from dark to light, Chl a fluorescence (ChlF) intensity shows characteristic changes in time called fluorescence transient, the OJIPSMT transient, where O (the origin) is for the first measured minimum fluorescence level; J and I for intermediate inflections; P for peak; S for semi-steady state level; M for maximum; and T for terminal steady state level. This transient is a real signature of photosynthesis, since diverse events can be related to it, such as: changes in redox states of components of the linear electron transport flow, involvement of alternative electron routes, the build-up of a transmembrane pH gradient and membrane potential, activation of different nonphotochemical quenching processes, activation of the Calvin-Benson cycle, and other processes. In this review, we present our views on how different segments of the OJIPSMT transient are influenced by various photosynthetic processes, and discuss a number of studies involving mathematical modeling and simulation of the ChlF transient. A special emphasis is given to the slower PSMT phase, for which many studies have been recently published, but they are less known than on the faster OJIP phase.     

The relationship of environmental variability to the spatial patterns of phytoplankton biomass in Lake Tahoe

Horizontal transects measuring phytoplankton biomass and temperaturewere made in Lake Tahoe in the nearshore epilimnion in 1976and 1977, near the deep chlorophyll maximum in midlake in 1977,and in the deep thermocline in 1976. Variance spectra from thesetransects indicate that large length scale patchiness dependson large-scale habitat variability, manifested as nitrate patchiness,caused by stream inflow in the nearshore epilimnion and by differentialtransport processes in the deeper water. When large-scale sourcesof habitat variability are absent, phytoplankton spatial patternsare not dominated by large-scale patches and are consistentwith a random distribution. No significant small-scale patchinesswas seen under any environmental conditions. ^*Current address: Jet Propulsion Laboratory, 183/501, 4800 OakGrove Drive, Pasadena, CA 91109, and Scripps Institution ofOceanography, A-002, University of California, La Jolla, CA92093 USA     

Chlorophyll fluorescence measured using the Fraunhofer line-depth principle and relationship to photosynthetic rate in the field

Abstract. A field study was conducted to determine the relationship of solar-excited chlorophyll a fluorescence to net CO₂ assimilation rate in attached leaves. The Fraunhofer line-depth principle was used to measure fluorescence at 656.3 nm wavelength while leaves remained exposed to full sunlight and normal atmospheric pressures of CO₂ and O₂. Fluorescence induction kinetics were observed when leaves were exposed to sunlight after 10 min in darkness. Subsequently, fluorescence varied inversely with assimilation rate. In the C₄ Zea mays , fluorescence decreased from 2.5 to 0.8 mW m^-2 nm^-1 as CO₂ assimilation rate increased from 1 to 8 μmol m^-2 s^-1 (r²= 0.520). In the C₃ Liquidambar styraciflua and Pinus taeda , fluorescence decreased from 6 to 2 mW m^-2 nm^-1 as assimilation rate increased from 2 to 5 or 0 to 2 μmol m^-2 s^-1 (r²= 0.44 and 0.45. respectively). The Fraunhofer line-depth principle enables the simultaneous measurement of solar-excited fluorescence and CO₂ assimilation rate in individual leaves, but also at larger scales. Thus, it may contribute significantly to field studies of the relationship of fluorescence to photosynthesis.     

Temporal changes in biomass specific photosynthesis during the summer: regulation by environmental factors and the importance of phytoplankton succession

Measurements of phytoplankton photosynthesis vs. irradiance relationships have been made at 3–7 day intervals in Lake Erken (central Sweden) for three years during summer stratification. Both the rate of light-limited (^B) and light-saturated (P_max ^B) photosynthesis per unit chlorophyll a showed distinct and similar temporal trends in each year. Seasonal trends were especially evident for P_max ^B, which increased in value for several weeks following the onset of thermal stratification, and then declined in the presence of the large colonial blue-green alga, Gloeotrichia echinulata. By late summer, when the biomass of G. echinulata had decreased, P_max ^B again rose to its early summer value. The covariation of biomass-specific photosynthesis with the blooming of G. echinulata was the one clear seasonal (week-month) pattern which emerged in each of 3 years. Over short (day-week) time scales, changes in ^B were related to changes in irradiance exposure on the day of sampling. However, the relationship between these two parameters was variable in time, since it was superimposed upon longer term trends controlled by changes in phytoplankton species composition. Increases in G. echinulata biomass corresponded with a deepening of the thermocline, which both increased internal phosphorus loading and the transport of resting G. echinulata colonies into the epilimnion. The timing and magnitude of the yearly G. echinulata bloom was as a result related to the seasonal development of thermal stratification. These results illustrate the importance of seasonal changes in the phytoplankton community as a factor regulating rates of biomass specific photosynthesis, particularly when the successional changes involve species with very different life strategies.     

Chlorophyll fluorescence as a selection tool for cold tolerance of photosynthesis in maize (Zea mays L.)

The possibility of using quenching analysis of chlorophyll a fluorescence as a selection tool for improving the cold tolerance of maize was investigated in six genotypes differing greatly in the ability to develop a competent photosynthetic apparatus at low temperature. Upon gradual cooling measurements of the quantum yield of electron transport (PSII) indicated that leaves of tolerant genotypes, that developed at suboptimal temperature (15C), maintained higher rates of electron transport than leaves of sensitive genotypes. This difference was largely due to the ability of the tolerant plants to keep higher efficiency of excitation energy capture by open photosystem II reaction centres (F'v/F'm). The absence of genotypic differences in leaves that developed at optimal temperature indicates that the trait is not expressed constitutively, but relies on adaptation mechanisms. Furthermore, the genotypic difference was not expressed under increasing illumination at 15C and 25°C suggesting that the trait is also low-temperature-specific and is not expressed solely in response to increasing excess light energy. Applying the method to flint and dent breeding population led to a substantial increase (up to 31%) in the photosynthetic capacity of hybrids between selected F3 inbreeding families grown at suboptimal temperature, demonstrating that the method is an efficient selection tool for improving the cold tolerance of maize through breeding.     

Corrected fluorescence excitation and emission spectra of phytoplankton: toward a more uniform approach to fluorescence measurements

Techniques for correction of fluorescence emission and excitationspectra of phytoplankton are described, which can be appliedin any commercially available spectrophotometer. The correctionof the emission spectrum is based on the measurement of a calibratedlight source. The excitation spectra are corrected by meansof a quantum counter solution that measures the spectral intensityof the excitation system and separate correction for wavelength-dependenteffects of the excitation optics. The correction proceduresgive technically corrected spectra, i.e. spectra that are freefrom wavelength dependent bias, but do not give absolute intensityvalues. Spectra that have been properly corrected for instrumentalwavelength dependencies are suitable for intercomparison, bothintra- and interlaboratory. Another application is the derivationof spectral data that will be obtained by other techniques thatmake use of fluorescence measurements, such as flow cytometry,remote sensing and in situ instruments. A necessary conditionis that the spectral response functions of these instrumentsmust be known. ¹Present address: AKZO, Arla-CRL, PO Box 9300, NL-6800 SB Arnhem,The Netherlands     

Chlorophyll a fluorescence transients: a fast data acquisition system to facilitate in vivo measurements

Characteristics of primary phases in chlorophyll a fluorescence transients based on room temperature in vivo measurement with a Plant Productivity Fluorometer (Brancker Model SF-10) can be greatly facilitated by coupling the instrument to a fast data acquisition system. The SF-10 was linked to a Multitech Industrial Corporation Microprofessor Microcomputer and further modified to ensure simultaneous onset of light activation and signal capture. Circuit diagrams and program listings are given in detail. This microprocessor system is capable of capturing signal changes over a minimum period of 200 milliseconds to a maximum of 6 seconds. Accuracy of recorded data is dependent on rate of change of the input signal and the recording time period. Acquisition and storage of 5000 points from zero to 300 milliseconds ensured clear resolution of F_o, I and D when played back over 120 seconds on a chart recorder. For routine use, the primary transient can be captured over 0–2 seconds and then replayed as an accompaniment to standard slower presentation of primary plus secondary transients. Coincidence of signal amplitude for F_p on both systems can then be ascertained while retaining adequate resolution of F_o and I.     

Chlorophyll a fluorescence transients: a fast data acquisition system to facilitate in vivo measurements

Characteristics of primary phases in chlorophyll a fluorescence transients based on room temperature in vivo measurement with a Plant Productivity Fluorometer (Brancker Model SF-10) can be greatly facilitated by coupling the instrument to a fast data acquisition system. The SF-10 was linked to a Multitech Industrial Corporation Microprofessor Microcomputer and further modified to ensure simultaneous onset of light activation and signal capture. Circuit diagrams and program listings are given in detail. This microprocessor system is capable of capturing signal changes over a minimum period of 200 milliseconds to a maximum of 6 seconds. Accuracy of recorded data is dependent on rate of change of the input signal and the recording time period. Acquisition and storage of 5000 points from zero to 300 milliseconds ensured clear resolution of F_o, I and D when played back over 120 seconds on a chart recorder. For routine use, the primary transient can be captured over 0–2 seconds and then replayed as an accompaniment to standard slower presentation of primary plus secondary transients. Coincidence of signal amplitude for F_p on both systems can then be ascertained while retaining adequate resolution of F_o and I.     

Delayed fluorescence in photosynthesis

Photosynthesis is a very efficient photochemical process. Nevertheless, plants emit some of the absorbed energy as light quanta. This luminescence is emitted, predominantly, by excited chlorophyll a molecules in the light-harvesting antenna, associated with Photosystem II (PS II) reaction centers. The emission that occurs before the utilization of the excitation energy in the primary photochemical reaction is called prompt fluorescence. Light emission can also be observed from repopulated excited chlorophylls as a result of recombination of the charge pairs. In this case, some time-dependent redox reactions occur before the excitation of the chlorophyll. This delays the light emission and provides the name for this phenomenon—delayed fluorescence (DF), or delayed light emission (DLE). The DF intensity is a decreasing polyphasic function of the time after illumination, which reflects the kinetics of electron transport reactions both on the (electron) donor and the (electron) acceptor sides of PS II. Two main experimental approaches are used for DF measurements: (a) recording of the DF decay in the dark after a single turnover flash or after continuous light excitation and (b) recording of the DF intensity during light adaptation of the photosynthesizing samples (induction curves), following a period of darkness. In this paper we review historical data on DF research and recent advances in the understanding of the relation between the delayed fluorescence and specific reactions in PS II. An experimental method for simultaneous recording of the induction transients of prompt and delayed chlorophyll fluorescence and decay curves of DF in the millisecond time domain is discussed.     

Chlorophyll fluorescence and photoinhibition in a tropical rainforest understory plant

The data presented here deal with the effects of high-light exposure on the 77 K fluorescence characteristics of Elatostema repens. It is shown that the decrease of the variable fluorescence during the treatment is biphasic. The reactions responsible for the first phase of fluorescence quenching are saturated under 700 mol photon m^-2 s^-1 and insensitive to streptomycin, whereas those responsible for the second phase are not yet saturated under 700 mol photon m^-2 s^-1 and sensitive to streptomycin. It is concluded that only the second phase of fluorescence quenching is associated with photoinhibitory processes. Rate and amplitude of recovery from photoinhibition are maximum under very low light (3.5 mol photon m^-2 s^-1), and very small at a moderate light (160 mol photon m^-2 s^-1) which does not cause photoinhibition. It is concluded that recovery processes are inhibited during photoinhibition. It is suggested that they could be associated with damage occuring on the oxidizing side of PSII.Abbreviations F_o, F_v, F_m initial, variable and maximum fluorescence, respectively - PFD photon flux density - PS II photosystem II