Effects of Salinity on Chlorophyll Fluorescence and Photosynthesis of Barley (Hordeum Vulgare L.) Grown Under a Triple-Line-Source Sprinkler System in the Field

In flag leaves of four cultivars of barley (Hordeum vulgare L.) grown in the field under a triple-line-source sprinkler system, that produces a linear soil salinity gradient, a decrease in net carbon dioxide assimilation rate (P_N) and stomatal conductance for water vapour (gs) was found. These changes were related to salinity tolerance at moderate salinity. With increasing salinity, P_N was saturated at low irradiances and stomatal frequencies increased. A decrease in photosystem 2 (PS2) efficiency was not found in the field after dark adaptation even at high salinity. Salinity induced only small decreases in the actual PS2 efficiency at midday steady-state photosynthesis, indicating that the photosynthetic electron transport was little affected by salinity. Therefore, using PS2 efficiency estimates in attached leaves is probably not a useful tool to screen barley genotypes grown under saline conditions in the field for salinity tolerance. In contrast, excised flag leaves from high salinity plots, once in the laboratory, exhibited a decrease in the variable to maximum chlorophyll fluorescence ratio as compared to excised leaves from control plants. On the other hand, the PN rate might allow for a good discrimination between tolerant and non-tolerant cultivars. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of Elevated Temperatures on the Activity of Alternative Pathways of Photosynthetic Electron Transport in Intact Barley and Maize Leaves

The light-induced rise in chlorophyll fluorescence and the subsequent decay of fluorescence in darkness were measured in barley and maize leaves exposed to heat treatment. The redox conversions of the photosystem I primary donor P700, induced by far-red light, were also monitored from the absorbance changes at 830 nm. After heating of leaves at temperatures above 40°C, the ratio of variable and maximum fluorescence decreased for leaves of both plant species, indicating the inhibition of photosystem II (PSII) activity. A twofold reduction of this ratio in barley and maize leaves was observed after heating at 45.3 and 48.1°C, respectively, which suggests the higher functional resistance of PSII in maize. The amplitude of the slow phase in the dark relaxation of variable fluorescence did not change after the treatment of barley and maize leaves at temperatures up to 48°C. In leaves treated at 42 and 46°C, the slow phase of dark relaxation deviated from an exponential curve. The relaxation kinetics included a temporary increase in fluorescence to a peak about 1 s after turning off the actinic light. Unlike the slow component, the fast and intermediate phases in the dark relaxation of variable fluorescence disappeared fully or partly after the treatment of leaves at 46°C. The photooxidation of P700 in heat-treated leaves was saturated at much higher irradiances of far-red light than in untreated leaves. At the same time, the dark reduction of P700⁺ was substantially accelerated after heat treatment. The data provide evidence that the heating of leaves stimulated the alternative pathways of electron transport, i.e., cyclic transport around photosystem I and/or the donation of electrons to the plastoquinone pool from the reduced compounds located in the chloroplast stroma. The rate of alternative electron transport after the heat treatment was higher in maize leaves than in barley leaves. It is supposed that the stimulation of alternative electron transport, associated with proton pumping into the thylakoid, represents a protective mechanism that prevents the photoinhibition of PSII in leaves upon a strong suppression of linear electron transport in chloroplasts exposed to heat treatment.     

Iron deficiency causes changes in chlorophyll fluorescence due to the reduction in the dark of the Photosystem II acceptor side

Iron deficiency was found to affect the redox state of the Photosystem II acceptor side in dark-adapted, attached leaves of sugar beet (Beta vulgaris L.). Dark-adapted iron-deficient leaves exhibited relatively high F_o and F_pl levels in the Kautsky chlorophyll fluorescence induction curve when compared to the iron-sufficient controls. However, far-red illumination led to marked decreases in the apparent F_o and F_pl levels. Modulated fluorescence showed that far-red light decreased the fluorescence yield to the true F_o levels by increasing photochemical quenching, without inducing changes in the level of non-photochemical quenching. In dark-adapted, iron-deficient leaves, far-red illumination induced a faster fluorescence decay in the µs-ms time domain, indicating an improvement in the electron transport after the primary quinone acceptor in the reducing side of Photosystem II. All these data indicate that in iron-deficient leaves the plastoquinone pool was reduced in the dark. The extent of the plastoquinone reduction in sugar beet depended on the chlorophyll concentration of the leaf, on the time of preillumination and on the duration of dark adaptation. The dark reduction of plastoquinone was observed not only in sugar beet but also in other plant species affected by iron deficiency both in controlled conditions and in the field.     

A portable, microprocessor operated instrument for measuring chlorophyll fluorescence kinetics in stress physiology

A portable instrument for measuring chlorophyll fluorescence induction kinetics is described and examples of measurements are given. The instrument is centered around a statistically-mixed bifurcated optical fiber. One fiber branch guides the actinic light to the sample, whereas the other branch carries the emitted chlorophyll fluorescence to the photodetector. Scattered actinic light is cut out from the detector by a red interference filter. The instrument measures fast as well as slow fluorescence induction kinetics, but is particularly well designed for analyzing fast kinetics. The high time resolution and strong, variable actinic light mean that both F_o (non-variable fluorescence) and F_m (maximal fluorescence at the P-peak) are well defined. A built in microprocessor unit with attached memory stores the fluorescence induction curve and calculates key fluorescence parameters such as F_o, F_m, F_v (variable fluorescence equals F_m?F_o), F_v/F_m (the photochemical efficiency of photosystem II) and t_1/2 (half rise time from F_o, to F_m). These values are digitally displayed after each recording and they (or the whole induction curve) can be stored in a memory and later retrieved. Because of a flexible setting of the instrument it can be used with high accuracy both for optically thick leaves and for diluted suspensions of algae or chloroplasts. A simple, light weight clamp cuvette for dark adaptation of leaves has been developed. It is equipped with a gate allowing the optical fiber to be inserted without daylight reaching the dark adapted portion of the leaf. The instrument has been developed for rapid monitoring of changes in activities and organization of the photosynthetic apparatus in vivo when plants are exposed to environmental stress both in the field and in the laboratory. Examples of measurements are given for differently treated leaves of Pinus sylvestris, Salix sp., Betula verrucosa, Zea mays, Epilobium angustifo-lium and for chloroplast thylakoids isolated from Spinacia oleracea.     

Salt tolerance in crop plants monitored by chlorophyll fluorescence in vivo

The potential of measurements of chlorophyll fluorescence in vivo to detect cellular responses to salinity and degrees of salt stress in leaves was investigated for three crop plants. Sugar beet (Beta vulgaris L.) (salt tolerant), sunflower (Helianthus annuus L.) (moderately salt tolerant), and bean (Phaseolus Vulgaris L. cv Canadian Wonder) (salt intolerant) were grown in pots and watered with mineral nutrient solution containing 100 millimolar NaCl. The fast rise in variable chlorophyll fluorescence yield that is correlated with photoreduction of photosystem II acceptors increased in leaves of sugar beet plants treated with salt suggesting stimulation of photosystem II activity relative to photosystem I. In sunflower, this fast rise was depressed by approximately 25% and the subsequent slow rate of quenching of the chlorophyll fluorescence was stimulated. These differences were more marked in the older mature leaves indicating an increasing gradient of salt response down the plant. The salt effect in vivo was reversible since chloroplasts isolated from mature leaves of salt-treated and control sunflower plants gave similar photosystem II activities. Unlike in sugar beet and sunflower, leaves of salt-treated bean progressively lost chlorophyll. The rate of slow quenching of chlorophyll fluorescence decreased indicating development of a partial block after photosystem II and possible initial stimulation of photosystem II activity. With further loss of chlorophyll photosystem II activity declined. It was concluded that measurements of chlorophyll fluorescence in vivo can provide a rapid means of detecting salt stress in leaves, including instances where photosynthesis is reduced in the absence of visible symptoms. The possible application to screening for salt tolerance is discussed.     

Effects of combined NaCl and CaCl2 salinity on photosynthetic parameters of barley grown in nutrient solution

The changes caused by NaCl− and CaCl₂-induced salinity on several leaf parameters have been measured in two cultivars of barley ( Hordeum vulgare L.) growing in a growth chamber in nutrient solution. Salinity was induced by adding to the nutrient solution equal weights of NaCl and CaCl₂, to obtain conductivities of 2, 6, 12, 19 and 26 dS m⁻¹. Salinity induced decreases in the leaf water potential and in the osmotic potential. Salinity did not induce significant changes in the relative photosynthetic pigment composition of barley leaves, the photosynthetic pigment stoichiometry for neoxanthin:violaxanthin cycle pigments:lutein:β-carotene:Chl b :Chl a being close to 3:6:14:12:25:100 (mol:mol). Salinity per se did not induce interconversions in the carotenoids within the violaxanthin cycle in most barley leaves. The PSII photochemistry of most barley leaves was unchanged by salinity. However, some apparently healthy leaves growing in high salinity exhibited sudden decreases in PSII photochemistry and increases in zeaxanthin (at the expense of violaxanthin), that preceded rapid leaf drying. Salinity induced significant changes in the slow part of the chlorophyll fluorescence induction curve from barley leaves.     

Apparent absence of viruses in most symptomless field-grown sweet potato in Uganda

Heat tolerance of groundnut (Arachis hypogaea L.) genotypes was evaluated by solute leakage and chlorophyll fluorescence techniques in heat-hardened and non-hardened plants. To determine the appropriate hardening treatment, 1-month-old plants of two groundnut genotypes, ICGV 86707 and Chico were conditioned at five combinations of hardening (37°C) and non-hardening (30°C) air temperatures over a 5-day period. Heat injury, was assessed through measurements of electrolyte leakage after stressing leaf discs to 55°C for 15 min. The relative injury was significantly influenced by the conditioning temperatures and by the temperature during 24 h prior to measurement if those involved non-hardening conditions. Relative injury and chlorophyll fluorescence were measured after stressing leaves of six genotypes at a range of temperatures between 49°C and 55°C. Significant genotype × hardening treatment interactions were observed in relative injury and chlorophyll fluorescence. Chico was susceptible to heat stress, the relative injury test identified ICGV 86707 as tolerant, and the chlorophyll fluorescence test identified ICGV 86707 as tolerant under hardened conditions and ICGV 87358 as tolerant when non-hardened. When expressed as percentage of control values, the relative injury and chlorophyll fluorescence measurements over the 49–53°C stress temperature range were strongly correlated. Chlorophyll concentrations were increased by hardening in all genotypes except Chico. In Chico, chl_b concentration was decreased and the chl_a/b ratio increased by hardening, and chlorophyll concentrations were correlated with chlorophyll fluorescence parameters. Chlorophyll concentration may therefore provide an alternative means of screening for heat tolerance.     

Effect of light and gibberellic acid on photosynthesis during leaf senescence of alstroemeria cut flowers.

The functioning of the photosynthetic apparatus during leaf senescence was investigated in alstroemeria cut flowers by a combination of gas-exchange measurements and analysis of in vivo chlorophyll fluorescence. Chlorophyll loss in leaves of alstroemeria cut flowers is delayed by light and by a treatment of the cut flowers with gibberellic acid (GA₃). The maximal photosynthesis of the leaves was approximately 6 μmol CO₂ m⁻² s⁻¹ at I 350 μmol m⁻² s⁻¹ (PAR) which is relatively low for intact C₃ leaves. Qualitatively the gas-exchange rates followed the decline in chlorophyll content for the various treatments, i.e. light and GA₃-treatment delayed the decline in photosynthetic rates. However, when chlorophyll loss could not yet be observed in the leaves, photosynthetic rates were already strongly decreased. In vivo fluorescence measurements revealed that the decrease in CO₂ uptake is (partly) due to a decreased electron flow through photosystem II. Furthermore, analysis of the fluorescence data showed a high nonphotochemical quenching under all experimental conditions, indicating that the consumption of reducing power in the Calvin cycle is very low. The chlorophyll, remaining after 9 days incubation of leaves with GA₃ in the dark should be considered as a 'cosmetic' pigment without any function in the supply of assimilates to the flowers.     

Stomatal conductance is a key parameter to assess limitations to photosynthesis and growth potential in barley genotypes

Fourteen genotypes of barley were compared for response to salinity by monitoring the parameters gas exchange and chlorophyll fluorescence. We present relationships between stomatal conductance (gs) gas exchange chlorophyll fluorescence parameters and aboveground dry matter (AGDM). We found that genetic variability provided a continuum of data for gs across control and saline conditions. We used this continuum of gs values to test the overall relationships between gs and net photosynthesis (A), leaf internal CO2 concentration (Ci), actual quantum yield of PSII electron transport (PhiPSII), relative electron yield over net CO2 assimilation rate (ETR/A), and AGDM. The relationship between gs and A was highly significant (P < 0.0001) for both control and saline treatments, while correlations between gs and Ci, and Ci and A were significant only under control conditions. Unexpectedly, we found positive correlations between gs and PhiPSII (P < 0.0001) for both conditions. A comparison between relationships of gs and A, and gs and PhiPSII seemed to indicate a possible acclimation to salinity at the chloroplastic level. Finally, the relationships between gs and ETR/A were exceptionally strong for both growing conditions (P < 0.0001) indicating that, as gs values were negatively affected in barley by genetics and salinity as main or interactive effects, there was a progressive increase in photorespiration in barley. Overall, we found that stomatal conductance was a key parameter in the study of barley responses to limiting situations for photosynthesis. We also found a strong relationship between AGDM and gs regardless of growing conditions and genotypes. For breeding evaluations to select barley genotypes for salinity tolerance, it may be possible to replace all measurements of gas exchange and chlorophyll fluorescence by the simple use of a porometer.     

Chlorophyll fluorescence changes at high temperatures induced by linear heating of greening barley leaves

A relative decrease of the high temperature part (above 60°C) of the chlorophyll fluorescence temperature curve during 3 h to 10 h greening period of barley (Hordeum vulgare L.) leaves was found to be concomitant to a decrease of Chl alb ratio and to a gradual increase of LHCP/core ratio found by electrophoresis and the ratio of granal to total length of thylakoid membranes. It is suggested that the high temperature part of the fluorescence temperature curve depends inversely on the relative amount of LHC II in thylakoid membranes.Abbreviations Chl a(b) chlorophyll a(b) - CPa chlorophyll a protein complex of PS II - CP1 P700 chlorophyll a protein complex of PS I - FP free pigments - FTC fluorescence temperature curve - F(T30) fluorescence intensity at 30°C - LHC II light harvesting complex II - LHCP light harvesting chlorophyll protein - LHCP3 (LHCPm) monomeric form of LHC II - LHCPo oligomeric form of LHC II complex - M1 first maximum of FTC - M2 second maximum (region) of FTC - PAA polyacrylamide - PAR photosynthetically active radiation - PS I(II) Photosystem I(II) - SDS-PAGE sodium dodecyl sulfate polyacrylamide gel electrophoresis     

Iron deficiency interrupts energy transfer from a disconnected part of the antenna to the rest of Photosystem II

Iron deficiency changed markedly the shape of the leaf chlorophyll fluorescence induction kinetics during a dark-light transition, the so-called Kautsky effect. Changes in chlorophyll fluorescence lifetime and yield were observed, increasing largely the minimal and the intermediate chlorophyll fluorescence levels, with a marked dip between the intermediate and the maximum levels and loss of the secondary peak after the maximum. During the slow changes, the lifetime-yield relationship was found to be linear and curvilinear (towards positive lifetime values) in control and Fe-deficient leaves, respectively. These results suggested that part of the Photosystem II antenna in Fe-deficient leaves emits fluorescence with a long lifetime. In dark-adapted Fe-deficient leaves, measurements in the picosecond-nanosecond time domain confirmed the presence of a 3.3-ns component, contributing to 15% of the total fluorescence. Computer simulations revealed that upon illumination such contribution is also present and remains constant, indicating that energy transfer is partially interrupted in Fe-deficient leaves. Photosystem II-enriched membrane fractions containing different pigment-protein complexes were isolated from control and Fe-deficient leaves and characterized spectrophotometrically. The photosynthetic pigment composition of the fractions was also determined. Data revealed the presence of a novel pigment-protein complex induced by Fe deficiency and an enrichment of internal relative to peripheral antenna complexes. The data suggest a partial disconnection between internal Photosystem II antenna complexes and the reaction center, which could lead to an underestimation of the Photosystem II efficiency in dark-adapted, low chlorophyll Fe-deficient leaves, using chlorophyll fluorescence. This revised version was published online in August 2006 with corrections to the Cover Date.     

Analysis of the slow phases of the in vivo chlorophyll fluorescence induction curve. Changes in the redox state of Photosystem II electron acceptors and fluorescence emission from Photosystems I and II

An analysis of the photo-induced decline in the in vivo chlorophyll a fluorescence emission (Kautsky phenomenon) from the bean leaf is presented. The redox state of PS II electron acceptors and the fluorescence emission from PS I and PS II were monitored during quenching of fluorescence from the maximum level at P to the steady state level at T. Simultaneous measurement of the kinetics of fluorescence emission associated with PS I and PS II indicated that the ratio of PS I/PS II emission changed in an antiparallel fashion to PS II emission throughout the induction curve. Estimation of the redox state of PS II electron acceptors at given points during P to T quenching was made by exposing the leaf to additional excitation irradiation and determining the amount of variable PS II fluorescence generated. An inverse relationship was found between the proportion of PS II electron acceptors in the oxidised state and PS II fluorescence emission. The interrelationships between the redox state of PS II electron acceptors and fluorescence emission from PS I and PS II remained similar when the shape of the induction curve from P to T was modified by increasing the excitation photon flux density. The contributions of photochemical and non-photochemical quenching to the in vivo fluorescence decline from P to T are discussed.     

Photosynthetic Responses of Leaves to Water Stress, Expressed by Photoacoustics and Related Methods : II. The Effect of Rapid Drought on the Electron Transport and the Relative Activities of the Two Photosystems

The effect of rapid dehydration of detached tobacco leaves (Nicotiana tabacum L.) on the photochemical apparatus of photosynthesis was studied in vivo by a combination of methods: photoacoustics, chlorophyll a fluorescence, and cytochrome f difference spectroscopy. It was shown that the inhibition of gross O₂ evolution was mainly caused by inactivation of PSII: (a) The saturation curve of cytochrome-f photooxidation by farred (>710 nanometers) light was resistant to the stress, leading to the conclusion that photosystem I (PSI) was largely unaffected by the stress. (b) The extent of the chlorophyll a variable fluorescence arising from photosystem II (PSII) decreased with the progression of the stress, but was largely unaffected when the leaf was preincubated with electron donors to PSII, such as hydroxylamine. It is concluded that the drought damage to PSII occurred on the photooxidative side. Despite the extensive inhibition of PSII and the relative preservation of PSI, the apparent PSII/PSI activity balance was somewhat larger in stressed leaves than in the control, as indicated by photoacoustic measurements of Emerson enhancement. These measurements were performed continuously under conditions which favor transitions to either state 1 or 2, showing that the transition to state 2 was considerably inhibited. Simultaneous measurements of chlorophyll fluorescence induction at 680 and 730 mm at room temperature were also used to probe changes in energy distribution between PSII and PSI and indicated that the transition from a dark adapted state to state 2 was also affected in water-stressed leaves. The saturation curve of the far-red light effect in Emerson enhancement was not changed by the stress, giving another independent evidence for the drought resistance of PSI activity. This apparent preservation of the imbalance in photochemical activities in favor of PSII, despite the fact that PSII is strongly inhibited, and PSI is not, supports a previous suggestion that the electron transfer between the two photosystems is not random but that a large extent of PSII and PSI units are specifically linked.     

Photosynthesis and photoinhibition in leaves of chlorophyll b-less barley in relation to absorbed light

The response of photosynthesis to absorbed light by intact leaves of wild-type ( Hordeum vulgare L. cv. Gunilla) and chlorophyll b -less barley ( H. vulgare L. cv. Dornaria, chlorina-f2²⁸⁰⁰) was measured in a light integrating sphere. Up to the section where the light response curve bends most sharply the responses of the b -less and wild-type barley were similar but not identical. Average quantum yield and convexity for the mutant light response curves were 0.89 and 0.90, respectively, times those of the wild-type barley. The maximum quantum yield for PSII photochemistry was also 10% lower as indicated by fluorescence induction kinetics (F_v/F_m). Just above the region where the light curve bends most sharply, photosynthesis decreased with time in the mutant but not in the wild-type barley. This decrease was associated with a decrease in F_v/F_m indicating photoinhibition of PSII. This photoinhibition occurred in the same region of the light response curve where zeaxanthin formation occurs. Zeaxanthin formation occurred in both the chlorophyll b -less and wild-type leaves. However, the epoxidation state was lower in the mutant than in the wild-type barley. The results indicate that chlorophyll b -less mutants will have reduced photosynthetic production as a result of an increased sensitivity to photoinhibition and possibly a lowered quantum yield and convexity in the absence of photoinhibition.     

蕾后期和花前期切花菊品种'神马'不同部位叶片光合作用和叶绿素荧光特性的比较

对蕾后期和花前期切花菊(Chrysanthemum morifolium Ramat.)品种'神马'('Jinba')不同部位叶片光合作用参数日变化、叶绿素荧光参数、光响应曲线及参数进行了研究.结果表明:蕾后期和花前期品种'神马'叶片蒸腾速率(Tr)、气孔导度(Gs)和净光合速率(Pn)的日变化均为单峰曲线,峰值出现在10:00或12:00;胞间CO2浓度(Ci)的日变化则先降低后升高,谷值出现在12:00.蕾后期和花前期品种'神马'叶片Tr、Ci和Gs值的平均值总体上随叶片位置降低而逐渐升高;蕾后期不同部位叶片Pn值的平均值差异较小,花前期叶片Pn值的平均值则随叶片位置降低而逐渐降低.随着叶片位置降低,蕾后期和花前期品种'神马'叶片的初始荧光(Fo)、最大荧光(Fm)、可变荧光(Fv)、表观量子效率(AQE)和最大净光合速率(Pmax)以及蕾后期的暗呼吸速率(Rd)均逐渐降低,而花前期的Rd值以及蕾后期和花前期的光补偿点(LCP)均逐渐升高.随着光合有效辐射(PAR)升高,蕾后期不同部位叶片以及花前期中部叶和下部叶的Pn值呈先急剧升高后趋于平稳的变化趋势,而花前期上部叶的Pn值则呈先急剧升高后逐渐下降的变化趋势.研究结果显示:在切花菊设施栽培过程中适当补充光照可提高切花菊品质.     

Chlorophyll Fluorescence and Photon Yield of Oxygen Evolution in Iron-Deficient Sugar Beet (Beta vulgaris L.) Leaves

The response of sugar beet (Beta vulgaris L.) leaves to iron deficiency can be described as consisting of two phases. In the first phase, leaves may lose a large part of their chlorophyll while maintaining a roughly constant efficiency of photosystem II photochemistry; ratios of variable to maximum fluorescence decreased by only 6%, and photon yields of oxygen evolution decreased by 30% when chlorophyll decreased by 70%. In the second phase, when chlorophyll decreased below a threshold level, iron deficiency caused major decreases in the efficiency of photosystem II photochemistry and in the photon yield of oxygen evolution. These decreases in photosystem II photochemical efficiency were found both in plants dark-adapted for 30 minutes and in plants dark-adapted overnight, indicating that photochemical efficiency cannot be repaired in that time scale. Decreases in photosystem II photochemical efficiency and in the photon yield of oxygen evolution were similar when measurements were made (a) with light absorbed by carotenoids and chlorophylls and (b) with light absorbed only by chlorophylls. Leaves of iron-deficient plants exhibited a room temperature fluorescence induction curve with a characteristic intermediate peak I that increases with deficiency symptoms.     

The efficiency of electron transfer from QA − to the donor side of Photosystem II decreases during induction of photosynthesis: Evidences from chlorophyll fluorescence and photoacoustic techniques

The amplitudes ratio of the fast and slow phases (A_fast/A_slow) in the kinetics of the dark relaxation of variable chlorophyll fluorescence (F_V) was studied after various periods of illumination of dark-adapted primary barley leaves. Simultaneously, photosynthetic activity was monitored using the photoacoustic technique and the photochemical and non-photochemical fluorescence quenching parameters. The ratio A_fast/A_slow changed with the preceding illumination time in a two-step manner. During the first stage of photosynthetic induction (0–20 s of illumination), characterized by a drop in O₂-dependent photoacoustic signal following an initial spike and by a relatively stable small value of photochemical F_V quenching, the ratio A_fast/A_slow remained practically unaltered. During the second stage (20–60 s of illumination), when both the rate of O₂ evolution and the photochemical F_V quenching were found to be sharply developed, a marked increase in the above ratio was also observed. A linear correlation was found between the value of the photochemical quenching and the ratio A_fast/A_slow during the second phase of photosynthetic induction. It is concluded that the slow phase appearing in the kinetics of F_V dark relaxation is not due to the existence of Photosystem II reaction centres lacking the ability to reduce P700⁺ with high rates, but is instead related to the limitation of electron release from Photosystem I during the initial stage of the induction period of photosynthesis. This limitation keeps the intersystem electron carriers in the reduced state and thus increases the probability of back electron transfer from Q_A ^– to the donor side of Photosystem II.Abbreviations A_fast/A_slow the ratio of magnitudes between the fast and slow phases of dark relaxation of variable fluorescence - F_O initial level of chlorophyll fluorescence - F_V variable chlorophyll fluorescence (F-F_O) - (F_V)_S the yield of variable chlorophyll fluorescence under saturating pulse in illuminated leaves - (F_V)_M the yield of variable chlorophyll fluorescence under saturating pulse in dark-adapted leaves - PA photoacoustic - PSI Photosystem I - PS II Photosystem II - qN non-photochemical quenching - qQ photochemical quenching     

On the quantitative relation between dark kinetics of NPQ-induced changes in variable fluorescence and the activation state of the CF<Subscript>0</Subscript>·CF<Subscript>1</Subscript>·ATPase in leaves

The variable fluorescence at the maximum F_m of the fluorescence induction (Kautsky) curve is known to be substantially suppressed shortly after light adaption due to nonphotochemical q_E quenching. The kinetic pattern of the dark decay at F_m consists of three components with rates ~20, ~1, and ~0.1 s^–1, respectively. Light adaptation has no or little effect on these rate constants. It causes a decrease in the ratio between the amplitudes of the slow and fast one with negligible change in the small amplitude of the ultra-slow component. Results add to evidence for the hypothesis that the dark-reversible decrease in variable fluorescence accompanying light adaptation during the P–S phase of the fluorescence induction curve is due to an alteration in nonphotochemical q_E quenching caused by changes in the trans-thylakoid proton motive force in response to changes in the proton conductance g_H+ of the CF₀-channel of the CF₀·CF₁·ATPase.     

Cell wall glycanases and their activity against the hemicelluloses from pine hypocotyls

O₂ evolution and chlorophyll a fluorescence emission have been monitored in intact cells of the cyanobacterium Anacystic nidulans 1402–1 to stdy the influence of carbon and nitrogen assimilation on the operation of the photosynthetic apparatus. The pattern of fluorescence induction in dark-adapted cyanobacterial cells was different from that of higher plants. Cyanobacteria undergo large, rapid state transitions upon illumination, which lead to marked changes in the fluorescence yield, complicating the estimation of quenching coefficients. The Kautsky effect was not evident, although it could be masked by a state II–state I transition, upon illumination with actinic light. The use of inhibitors of carbon assimilation such as D,L-glyceraldehyde or iodoacetamide allowed us to relate changes in variable fluorescence to active CO₂ fixation. Ammonium, but not nitrate, induced non-photochemical fluorescence quenching, in agreement with a previous report on green algae, indicative of an ammonium-induced state I transition.     

Inhibition of photosynthetic activity by UV-B radiation in radish seedlings

Inhibition of primary photosynthetic reactions by UV-B radiation (280 nm-320 nm) was demonstrated in radish leaves ( Raphanus sativus cv. Saxa Treib). Detached radish cotyledons from 10-day-old seedlings were irradiated with continuous white light and increasing UV-B irradiances using cut-off filters with increasing transmission for shorter wavelengths (WG 360, WG 345, WG 320, WG 305, WG 295, WG 280). Photosynthetic activity measured in terms of chlorophyll fluorescence induction (Kautsky effect) after 2, 4, 6, 8 and 24 h irradiation decreased in a wavelength dependent way with increasing UV-B irradiance and irradiation time.
Radish seedlings grown for 10 days from the time of germination under the same UV-B irradiation conditions exhibited similar reductions of the variable fluorescence as detached cotyledons irradiated for short time periods. They additionally had lower initial fluorescence at high UV-B radiation levels, although the chlorophyll content per leaf area increased. In contrast to short term experiments, the plastoquinone and flavonoid content increased with increasing UV-B irradiance when based on leaf area.