Response of plants to ectomycorrhizae in N-limited conditions: which factors determine its variation?

In the present work, the following hypotheses were tested: (1) the negative effects of mycorrhization over host plant productivity in N-limited conditions are due to N retention by the fungal partner and not due to excessive C drainage; (2) If mycorrhization results in decreased N uptake, the host plant decreases its C investment in fungal growth. The effects of mycorrhization over a wide range of combinations between N availability, N concentration in plant tissues, and degree of mycorrhizal colonization were studied in Pinus pinaster L. mycorrhizal with Pisolithus tinctorius. Several plant productivity parameters, the seedlings' N status, chl a fluorescence (JIP test), and mycorrhizal colonization were measured. N was always limiting. A gradient of mycorrhizal effects over the host plant's growth and vitality was successfully obtained. The mycorrhizal effects on plant growth and N uptake were very strongly and positively correlated, and no evidence was found of a C limitation to growth, confirming hypothesis 1. Indications were found that the plants continued to provide C to the fungus although the N supplied by it was increasingly lower, denying hypothesis 2. A new index, the mycorrhizal N demand-supply balance, was found to efficiently explain, and to have a curvilinear relation with, the variation in response to mycorrhization. The mycorrhizal effect on host plant growth was not related to a negative effect on its photosynthetic performance and, therefore, reflected changes in resource allocation between host plant and mycorrhizal fungus, not in plant vitality.

Kinetics of delayed chlorophyll a fluorescence registered in milliseconds time range

Delayed fluorescence dark decays in the time interval from 0.35 to 5.5 ms are measured during dark to light adaptation in whole barley leaves and isolated thylakoid membranes, using a disc phosphoroscope. The changes in delayed fluorescence features are compared with variable chlorophyll fluorescence simultaneously registered with the same apparatus as well as in parallel by Handy PEA (Hansatech Instruments Ltd.), and absorbance changes at 820 nm. The registered delayed fluorescence signal is a sum of three components - submillisecond with lifetime of about 0.6 ms, millisecond decayed 2-4 ms and slow component with lifetime > >5.5 ms. The submillisecond delayed fluorescence component is proposed to be a result of radiative charge recombination in Photosystem II reaction centers in the state Z+PQA -QB -, and its lifetime is determined by the rate of electron transfer from QA - to QB -. The millisecond delayed fluorescence component is associated with recombination in Z+PQA -QB = centers with a lifetime determined by the sum of the rate constants of electron transfer from the oxygen-evolving complex to Z+ and of the exchange between the reduced and oxidized plastoquinone pool in the QB-site. On the basis of these assumptions and of the different share of the three components in the integral delayed fluorescence during induction, an attempt has been made to interpret the changes in the delayed fluorescence intensity during the transition of the photosynthetic apparatus from dark to light adapted state.

Photosynthetic response of transgenic soybean plants, containing an Arabidopsis P5CR gene, during heat and drought stress

The biochemical basis of heat/drought tolerance was investigated by comparing the response of antisense and sense transgenic soybean plants (containing the L-delta1-pyrroline-5-carboxylate reductase gene) with non-transgenic wild-type plants. The plants were subjected to a simultaneous drought and heat stress of 2 days, whereafter they were rewatered at 25 degrees C. During this time the sense plants only showed mild symptoms of stress compared to the antisense plants which were severely stressed. Upon stress, nicotinamide adenine dinucleotide phosphate (NADP+) levels decreased in antisense while it increased in sense plants. Recovery with respect to NADP+ levels was best in sense plants. Sense plants had the highest ability to accumulate proline during stress and to metabolise proline after rewatering. Analyses of the fast phase chlorophyll-a fluorescence transients showed dissociation of the oxygen-evolving complex (OEC) upon stress in all plants tested. In the sense plants, which best resisted the stress, OEC dissociation was bypassed by proline feeding electrons into photosystem 2 (PSII), maintaining an acceptable nicotinamide adenine dinucleotide hydrogen phosphate (NADPH) level, preventing further damage. Upon recovery, NADPH is consumed during oxidation of accumulated proline providing high Levels of NADP+ to act as electron acceptor to PSII, which indirectly may ameliorate the inhibition and/or the effect of uncoupling of the OEC.

Multiple effects of chromate on the photosynthetic apparatus of Spirodela polyrhiza as probed by OJIP chlorophyll a fluorescence measurements

Chromate (Cr) decreases the growth of Spirodela polyrhiza. The fronds lost their pigments. The O2 evolution was also decreased. The Cr effect was found to be dose dependent. The toxic effects of Cr have further been studied on the photosynthetic activity of Spirodela polyrhiza by means of the chlorophyll a (Chl a) fluorescence transient O-J-I-P. The Chl a fluorescence transients were recorded in vivo with high time resolution and analyzed according to the JIP-test which can quantify the photosystem II behavior. Cr treated plants show a decrease in yield for primary photochemistry, phi Po. The performance index of PSII, PIABS, which is the combination of the indexes of three independent parameters, (1) the total number of active reaction centers per absorption (RC/ABS), (2) yield of primary photochemistry (phi Po) and (3) efficiency with which a trapped exciton can move an electron into the electron transport chain (psi 0), decreased due to Cr treatment. Chromate sensitivity varies within plant populations. In summary Cr affects several targets of PSII. More specifically, the main targets of Cr, according to the JIP-test, can be listed as a decrease in the number of active reaction centers and damage to the oxygen-evolving complex.

Beech (Fagus sylvatica) response to ozone exposure assessed with a chlorophyll a fluorescence performance index

This paper describes a relationship between ozone exposure, biomass, visual symptoms and a chlorophyll a fluorescence performance index for young beech trees (Fagus sylvatica). The plants were exposed to four levels of ozone in open-top fumigation chambers (50, 85, 100% of ambient, and 50% of ambient+30 nl l(-1) ozone) that fluctuated in parallel with ambient ozone during a single growing season. The trees were fumigated in the four treatments with ozone levels corresponding to an AOT40 (accumulated exposure above a threshold of 40 nl l(-1)) of 0.01, 3.35, 7.06 and 19.70 microl l(-1) h, respectively. Highly significant differences were found between the 50% of ambient+30 nl l(-1) ozone treatment and all other treatments, with a 70.5% reduction in primary photosynthetic performance, as measured with the PI index. The reduction of the PI values demonstrated a high correlation with visual symptom development (r(2)=0.98), and by the end of September with biomass loss (r(2)=0.99). A significant ozone exposure-response relationship was found between AOT40 and primary photochemistry (r(2)=0.97). Thus, analysis of PI provides an alternative method for regional monitoring of tree health within the context of the currently employed AOT40.

Action of the allelochemical, fischerellin A, on photosystem II

The cyanobacterium, Fischerella muscicola, produces a secondary metabolite named fischerellin A (FS) that strongly inhibits the growth of cyanobacteria and other photosynthetic organisms. The compound exhibits a unique structure and is composed of two cyclic amines and a C15 substituent that contains a double bond in the (Z) configuration and two triple bonds [L. Hagmann, F. Jüttner, Tetrahedron Lett., 37 (1996) 6539-6542]. The site of FS action is located in photosystem II (PSII). The chlorophyll fluorescence induction transient and O2 evolution methods have been used to determine the site of action of FS in PSII. FS affects the fluorescence transients, as well as O2 evolution by the cyanobacterium, Anabaena P9. The green alga, Chlamydomonas reinhardtii, and higher plants were also affected by FS in a concentration- and time-dependent fashion. FS acts at several sites which appear with increasing half-time of interaction in the following sequence: (1) effect on the rate constant of QA- reoxidation; (2) primary photochemistry trapping; (3) inactivation of PSII reaction center; and (4) segregation of individual units from grouped units. FS does not affect the photosynthetic activity of purple bacteria, Rhodospirillum rubrum.

Refolding of triosephosphate isomerase in low-water media investigated by fluorescence resonance energy transfer

The refolding and reassociation of rabbit muscle triosephosphate isomerase (TPI) monomers unfolded with guanidine hydrochloride (GdnHCl) was studied in aqueous media and in reverse micelles (RM) formed with hexadecyltrimethylammonium bromide and n-octane/hexanol. Fluorescence resonance energy transfer (FRET) studies were carried out using TPI labeled at Cys-217 with 5-((2-((iodoacetyl)-amino)ethyl)amino)naphthalene-1-sulfonic acid (1,5-IAEDANS) and TPI labeled at one of the free amino groups with fluorescein 5'-isothiocyanate (FITC). Efficient FRET between monomers of AEDANS-TPI and FITC-TPI in aqueous media was detected 2-3 min after denaturant dilution and remained constant for hours. The distance between AEDANS and FITC in a labeled, renatured hetero-TPI dimer calculated from FRET results was 48 A, in reasonable agreement with estimates based on the crystal structure of TPI. In RM, recovery of enzyme activity during renaturation correlates with the appearance of a high-intrinsic fluorescence intermediate believed to be a partially folded monomer (Fernández-Velasco et al., 1995). Nevertheless, when AEDANS- and FITC-labeled monomers were mixed in RM, FRET occurred as soon as GdnHCl was diluted (FRET efficiency = 0.36), preceding the changes in TPI intrinsic fluorescence and reactivation. Thereafter, the efficiency of FRET increased during the next hour up to approximately 0.50, where it remained after 24 h, when 80% of the enzyme activity was recovered. The high initial FRET seen in RM could indicate the formation of an inactive dimer within the first minutes after denaturant dilution. The further increase in FRET observed over the next hour could reflect conformational rearrangements of the protein and transition from the inactive to the active dimer.

Polyphasic rise of chlorophyll a fluorescence in herbicide-resistant D1 mutants of Chlamydomonas reinardtii

Chlorophyll (Chl) a fluorescence transient, a sensitive and non-invasive probe of the kinetics and heterogeneity of the filling up of the electron acceptor pool of Photosystem II (PS II), was used to characterize D1-mutants of Chlamydomonas reinhardtii. Using a shutter-less system (Plant Efficiency Analyzer, Hansatech, UK), which provides the first measured data point at 10 μs and allows data accumulation over several orders of magnitude of time, we have characterized, for the first time, complete Chl a fluorescence transients of wild type (WT), cell wall less (CW-15) C. reinhardtii and several herbicide-resistant mutants of the D1 proteins: D1-V219I The mutants are labeled as follows: the single letter code for the wild type amino acid, followed by the residue number, then the code for the mutated amino acid. A251V, F255Y, S264A G256D and L275F. In all cases, the Chl a fluorescence induction transients follow a pattern of O-J-I-P where J and I appear as two steps between the minimum Fo (O) and the maximum Fmax (Fm, P) levels. The differences among the mutants are in the kinetics of the filling up of the electron acceptor pool of PS II (this paper) in addition to those in the re-oxidation kinetics of QA (-) to QA, published elsewhere (Govindjee et al. (1992) Biochim Biophys. Acta: 1101: 353-358; Strasser et al. (1992) Archs. Sci. Genève 42: 207-224) and not in the ratio of the maximal fluorescence Fm to the initial fluorescence Fo. The value of this experimental ratio is Fm/Fo = 4.4±0.21 independent of the mutation. At 600 W m(-2) of 650 nm excitation, distinct hierarchy in the fraction of variable Chl a fluorescence at the J level is observed: S264A > A251V ∼ G256A > L275F ∼ V219I > F255Y ∼ CW-15 ∼ WT. At 300 and 60 W m(-2) excitation, a somewhat similar hierarchy among the mutants was observed for the intermediate levels J and I. Addition of bicarbonate-reversible inhibitor formate did not change the O to J phases, slowed the I to P rise, and in many cases, slowed the decay of fluorescence beyond the P level. These observations are interpreted in terms of formate effect being on the acceptor rather than on the donor side (S-states) of PS II. The formate effect was different in different mutants, with L275F being the most insensitive mutant followed by others (V219I, F255Y, WT, A251V and S264A). Further, in the presence of high concentrations of DCMU, identical transients were observed for all the mutants and the WT.The quantum yield of photochemistry of PS II, calculated from 1-(Fo/Fm), is in the range of 0.73 to 0.82 for the WT as well as for the mutants examined. Thus, in contrast to differences in the kinetics of the electron acceptor side of PS II, there were no significant differences in the maximum quantum yield of PS II, among the mutants tested. We suggest that earlier photochemistry yield values were much lower (0.4-0.6) than those reported here due to either higher measured values of Fo by instruments using camera shutters, or due to the use of cells grown in less than-optimal conditions.

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

The dynamics of light-induced closure of the PS II reaction centers was studied in intact, dark-adapted leaves by measuring the light-irradiance (I) dependence of the relative variable chlorophyll fluorescence V which is the ratio between the amplitude of the variable fluorescence induced by a pulse of actinic light and the maximal variable fluorescence amplitude obtained with an intense, supersaturating light pulse. It is shown that the light-saturation curve of V is a hyperbola of order n. The experimental values of n ranged from around 0.75 to around 2, depending on the plant material and the environmental conditions. A simple theoretical analysis confirmed this hyperbolic relationship between V and I and suggested that n could represent the apparent number of photons necessary to close one reaction center. Thus, experimental conditions leading to n values higher than 1 could indicate that, from a macroscopic viewpoint, more than one photon is necessary to close one PS II center, possibly due to changes in the relative concentrations of the different redox states of the PS II reaction center complexes at the quasi-steady state induced by the actinic light. On the other hand, the existence of environmental conditions resulting in n noticeably lower than 1 suggests the possibility of an electron flow between PS II reaction center complexes.

Functioning of photosystems I and II in pea leaves exposed to heat stress in the presence or absence of light : Analysis using in-vivo fluorescence, absorbance, oxygen and photoacoustic measurements

Fluorimetric, photoacoustic, polarographic and absorbance techniques were used to measure in situ various functional aspects of the photochemical apparatus of photosynthesis in intact pea leaves (Pisum sativum L.) after short exposures to a high temperature of 40 ° C. The results indicated (i) that the in-vivo responses of the two photosystems to high-temperature pretreatments were markedly different and in some respects opposite, with photosystem (PS) II activity being inhibited (or down-regulated) and PSI function being stimulated; and (ii) that light strongly interacts with the response of the photosystems, acting as an efficient protector of the photochemical activity against its inactivation by heat. When imposed in the dark, heat provoked a drastic inhibition of photosynthetic oxygen evolution and photochemical energy storage, correlated with a marked loss of variable PSII-chlorophyll fluorescence emission. None of the above changes were observed in leaves which were illuminated during heating. This photoprotection was saturated at rather low light fluence rates (around 10 W · m(-2)). Heat stress in darkness appeared to increase the capacity for cyclic electron flow around PSI, as indicated by the enhanced photochemical energy storage in far-red light and the faster decay of P 700 (+) (oxidized reaction center of PSI) monitored upon sudded interruption of the far-red light. The presence of light during heat stress reduced somewhat this PSI-driven cyclic electron transport. It was also observed that heat stress in darkness resulted in the progressive closure of the PSI reaction centers in leaves under steady illumination whereas PSII traps remained largely open, possibly reflecting the adjustment of the photochemical efficiency of undamaged PSI to the reduced rate of photochemistry in PSII.

The herbicide-resistant D1 mutant L275F of Chlamydomonas reinhardtii fails to show the bicarbonate-reversible formate effect on chlorophyll a fluorescence transients

Herbicide-resistant mutants of the eukaryotic green alga Chlamydomonas reinhardtii, that are altered in specific amino acids in their D-1 protein, show differential bicarbonate-reversible formate effects. These results suggest the involvement of D1 protein in the 'bicarbonate effect'. A 25 mM formate treatment of mixotrophically or photoautotrophically grown wild type cells results in a slower rise of chlorophyll a fluorescence transient followed by a dramatically slowed decline during measurements in continuous light. These effects are fully reversed upon addition of 10 mM bicarbonate. The mutant BR-202 [L275F] is, however, highly insensitive to 25 mM formate suggesting that a significant change in formate (bicarbonate) binding has occurred in helix V of the D1 protein near histidine involved in Fe binding. With the exception of DCMU-4 [S264A], which is considerably more sensitive to formate than the wild type, five other different [V219I, A25IV, F255Y, G256D and cell-wall deficient CW-15] mutants display a relatively similar response to formate as wild type. Absence of formate effect on a PS II-lacking [FuD-7] mutant confirms the sole involvement of PS II in the 'bicarbonate effect'.

A theoretical and experimental analysis of the qP and q N coefficients of chlorophyll fluorescence quenching and their relation to photochemical and nonphotochemical events

The initial (F0), maximal (FM) and steady-state (FS) levels of chlorophyll fluorescence emitted by intact pea leaves exposed to various light intensities and environmental conditions, were measured with a modulated fluorescence technique and were analysed in the context of a theory for the energy fluxes within the photochemical apparatus of photosynthesis. The theoretically derived expressions of the fluorescence signals contain only three terms, X=J2p2F/(1-G), Y=T/(1-G) and V, where V is the relative variable fluorescence, J2 is the light absorption flux in PS II, p2F is the probability of fluorescence from PS II, G and T are, respectively, the probabilities for energy transfer between PS II units and for energy cycling between the reaction center and the chlorophyll pool: F0=X, FM=X/(1-Y) and FS=X(1+(YV/(1-Y))). It is demonstrated that the amplitudes of the previously defined coefficients of chlorophyll fluorescence quenching, qP and qN, reflect, not just photochemical (qP) or nonphotochemical (qN) events as implied in the definitions, but both photochemical and nonphotochemical processes of PS II deactivation. The coefficient qP is a measure of the ratio between the actual macroscopic quantum yield of photochemistry in PS II (41-1) in a given light state and its maximal value measured when all PS II traps are open (41-2) in that state, with 41-3 and 41-4. When the partial connection between PS II units is taken into consideration, 1-qP is nonlinearily related to the fraction of closed reaction centers and is dependent on the rate constants of all (photochemical as well as nonphotochemical) exciton-consuming processes in PS II. On the other hand, 1-qN equals the (normalized) ratio of the rate constant of photochemistry (k2b) to the combined rate constant (kN) of all the nonphotochemical deactivation processes excluding the rate constant k22 of energy transfer between PS II units. It is demonstrated that additional (qualitative) information on the individual rate constants, kN-k22 and k2b, is provided by the fluorescence ratios 1/FM and (1/F0)-(1/FM), respectively. Although, in theory, 41-5 is determined by the value of both k2b and kN-k22, experimental results presented in this paper show that, under various environmental conditions, 41-6 is modulated largely through changes in k N, confirming the idea that PS II quantum efficiency is dynamically regulated in vivo by nonphotochemical energy dissipation.

Violaxanthin de-epoxidase in etiolated leaves

In etiolated leaves the occurrence of the enzymatic violaxanthin de-epoxidation to zeaxanthin is shown. The carotenoid transformation is provoked by the infiltration of whole leaves with ascorbate at pH 5 and is susceptible to DTT. Identification of the de-epoxidase activity is achieved by in vivo spectroscopy and pigment analysis (TLC).

Mono- bi- tri- and polypartite models in photosynthesis

It is shown how energy fluxes in mono-bi-tri- and polypartite photosystems can be described. The derivation of the energy distribution term α and the probability of spill over p21 as proposed by W.L. BUTLER are reviewed.

Comparative measurements of membrane potentials with microelectrodes and voltage-sensitive dyes

The usefulness of a new voltage-sensitive fluorescent dye, the membrane permeant negatively charged oxonol dye diBA-C4-(3)-, was evaluated by measuring the membrane potentials of BICR/M1R-k and L cells with glass microelectrodes and simultaneously recording the fluorescence of the stained cells. The membrane potential of BICR/M1R-k cells was varied between -25 mV and -90 mV by changing the bicarbonate concentration in the medium or by voltage clamping. To avoid any interference by the inserted electrodes with the fluorescence measurement of the cytoplasm, the cells were fused by polyethyleneglycol to form giant cells (homokaryons). These homokaryons also allowed penetration by two glass microelectrodes without causing a serious leakage of the plasma membrane. The slow responding dye diBA-C4-(3)- had a fluorescence response of about 1% per mV. Mathematical analysis of the fluorescence changes after voltage clamping revealed a first-order reaction with a rate constant between 0.1 min-1 and 0.8 min-1, depending on the cell size which was determined by the number of nuclei per homokaryon. A model for the mechanism of the fluorescence changes is proposed.

Alteration in the acyl lipid composition of thylakoids induced by aging and its effect on thylakoid structure

The alteration in both the lipid composition and chlorophyll proteins obtained by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of detergent solubilized thylakoids was investigated on differentially aged spinach (Spinacia oleracea L.) chloroplasts. Freshly isolated material demonstrated seven major bands upon electrophoretic fractionation. Membranes aged in vitro showed a diminution and/or a disappearance of some bands concomitant with changes in the acyl lipid composition of these membranes. The extent of these changes was influenced by the purity of the preparation. Low temperature fluorescence measurements (77K) showed that upon aging, the photochemical capacity of photosystem II decreased prior to alterations in the molecular organization of the photochemical apparatus as indicated by the energy distribution between the two photosystems.The results are discussed in terms of whether the series of changes in the acyl lipid composition upon aging is related to the variations in the electrophoretic pattern of the chlorophyll proteins.

Fluorescence emission spectra of photosystem I, photosystem II and the light-harvesting chlorophyll a/b complex of higher plants

Fluorescence emission spectra excited at 514 and 633 nm were measured at -196 degrees C on dark-grown bean leaves which had been partially greened by a repetitive series of brief xenon flashes. Excitation at 514 nm resulted in a greater relative enrichment of the 730 nm emission band of Photosystem I than was obtained with 633 nm excitation. The difference spectrum between the 514 nm excited fluorescence and the 633 nm excited fluorescence was taken to be representative of a pure Photosystem I emission spectrum at -196 degrees C. It was estimated from an extrapolation of low temperature emission spectra taken from a series of flashed leaves of different chlorophyll content that the emission from Photosystem II at 730 nm was 12% of the peak emission at 694 nm. Using this estimate, the pure Photosystem I emission spectrum was subtracted from the measured emission spectrum of a flashed leaf to give an emission spectrum representative of pure Photosystem II fluorescence at -196 degrees C. Emission spectra were also measured on flashed leaves which had been illuminated for several hours in continuous light. Appreciable amounts of the light-harvesting chlorophyll a/b protein, which has a low temperature fluorescence emission maximum at 682 nm, accumulate during greening in continuous light. The emission spectra of Photosystem I and Photosystem II were subtracted from the measured emission spectrum of such a leaf to obtain the emission spectrum of the light-harvesting chlorophyll a/b protein at -196 degrees C.

Does the rate of cooling affect fluorescence properties of chloroplasts at -196 degrees C?

The question addressed in the title was examined by measuring fluorescence emission spectra and light-induced fluorescence-yield changes of chloroplasts which had been frozen to -196 degrees C rapidly, as very thin samples adsorbed into substrates whick were plunged directly into liquid nitrogen, or slowly by the cooling action of liquid nitrogen through the wall of the cuvette. Contrary to previous reports, we found that the rate of cooling had no influence on the shape of the emission spectrum, the extent of the variable fluorescence or the fraction of the absorbed quanta which are delivered initially to Photosystem I.

Tripartite model for the photochemical apparatus of green plant photosynthesis

Equations for fluorescence and the rates of photochemistry of photosystem I and photosystem II are derived from a photochemical model for the photosynthetic apparatus that includes the various interactions of the light-harvesting chlorophyll a/b complex with photosystem I and photosystem II as specific photochemical rate constants. The degree of coupling between photosystem II and the chlorophyll a/b complex which is expressed as a product of two probability terms plays a central role in this three-pigment system. The cycling of excitation energy back and forth between photosystem II and the chlorophyll a/b complex increases the exciton density in both arrays of chlorophyll according to a simple analytical expression in the equations. These equations of the tripartite model provide new and credible insights into the photochemical apparatus of photosynthesis.

Energy transfer and the distribution of excitation energy in the photosynthetic apparatus of spinach chloroplasts

Equations are derived from our model of the photochemical apparatus of photosynthesis to show that the yield of energy transfer from Photosystem II to Photosystem I, phi T(II leads to I), can be obtained from measurements on an individual sample of chloroplasts frozen to -196 degrees C by comparing the sum of two specifically defined fluorescence excitation spectra with the absorption spectrum of the sample. Then, given that value of phiT(II leads to I), the fraction of the quanta absorbed by the photochemical apparatus which is distributed initially to Photosystem I, alpha, can be determined as a function of the wavelength of excitation from the same fluorescence excitation spectra. The results obtained in this study of individual samples of chloroplasts frozen to -196 degrees C in the absence of divalent cations, namely, that phi T(II leads to I)varies from a minimum value of 0.10 when the Photosystem II reaction centers are all open to a maximum value of 0.25 when the centers are all closed and that alpha has a value of about 0.30 which is almost independent of wavelength for wavelength shorter than 675 nm (alpha increases rapidly toward unity at wavelength longer than 675 nm), agrees quite well with results obtained previously from comparative measurements of chloroplasts frozen to -196 degrees C in the presence and absence of divalent cations.

Energy transfer in the photochemical apparatus of flashed bean leaves

Fluorescence and energy transfer properties of bean leaves greened by brief, repetitive xenon flashes were studied at -196 degrees C. The bleaching of P-700 has no influence on the yield of fluorescence at any wavelength of emission. The light-induced fluorescence yield changes which are observed in both the 690 and 730 nm emission bands in the low temperature fluorescence spectra are due to changes in the state of the Photosystem II reaction centers. The fluorescence yield changes in the 730 nm band are attributed to energy transfer from Photosystem II to Photosystem I. Such energy transfer was also confirmed by measurements of the rate of photooxidation of P-700 AT -196 degrees C in leaves in which the Photosystem II reaction centers were either all open or all closed. It is concluded that energy transfer from Photosystem II to Photosystem I occurs in the flashed bean leaves which lack the light-harvesting chlorophyll a/b protein.

Correlation of absorbance changes and thylakoid fusion with the induction of oxygen evolution in bean leaves greened by brief flashes

Dark-grown bean leaves (Phaseolus vulgaris) which had been greened for several days in a repetitive series of brief xenon flashes were studied during the initial induction period when O(2) evolution first appears. The induction of O(2) evolution requires actinic irradiation (e.g. 2 mw/cm(2) of red light) and goes to completion in about 8 minutes with a half-time just under 3 minutes. Absorbance measurements on the intact leaves showed that a change of a carotenoid pigment, monitored at 505 nm, was closely correlated with the rate of O(2) evolution during the induction period. Inhibitor studies, however, showed that the absorbance change persisted in the presence of a number of inhibitors which blocked O(2) evolution. Electron microscopy revealed that the primary thylakoids which were unfused in the flashed leaves before induction became fused in pairs or groups of three during the 8-minute induction period. It is postulated that the 505-nm absorbance change of the carotenoid pigment is correlated more directly with the fusion process than with O(2) evolution. Heat treatment (45 C for 5 min) or infiltration with 0.8 m tris, which prevented the fusion process, also prevented the absorbance change.If the leaves were preilluminated for 8 minutes with very weak red light (20 muw/cm(2)) which induced no O(2) evolution, absorbance change, or thylakoid fusion, there was an immediate burst of O(2) evolution at the onset of actinic irradiation and the induction period, as noted by O(2) evolution or by the 505-nm absorbance change, was reduced to 2 minutes (half-time of 40 seconds). It is concluded that the electron transport system in the flashed leaves is blocked at the Mn site between water and photosystem II and that the photoactivation of Mn into the thylakoid membranes occurs during the low light, photoactivation process. After the electron transport chain is thus repaired, ion-pumping mechanisms driven by actinic light may lead to steady-state photosynthesis as well as to thylakoid fusion.