On the advantages of using green light to study fluorescence yield changes in leaves

In photosynthetic chains, the kinetics of fluorescence yield depends on the photochemical rates at the level of both Photosystem I and II and thus on the absorption cross section of the photosynthetic units as well as on the coupling between light harvesting complexes and photosynthetic traps. A new set-up is described which, at variance with the commonly used set-ups, uses of a weakly absorbed light source (light-emitting diodes with maximum output at 520 nm) to excite the photosynthetic electron chain and probe the resulting fluorescence yield changes and their time course. This approach optimizes the homogeneity of the exciting light throughout the leaf and we show that this homogeneity narrows the distribution of the photochemical rates. Although the exciting light is weakly absorbed, the possibility to tune the intensity of the light emitting diodes allows one to reach photochemical rates ranging from 10⁴ s^− 1 to 0.25 s^− 1 rendering experimentally accessible different functional regimes. The variations of the fluorescence yield induced by the photosynthetic activity are qualitatively and quantitatively discussed. When illuminating dark-adapted leaves by a weak light, the kinetics of fluorescence changes displays a pronounced plateau which precedes the fluorescence increase reflecting the full reduction of the plastoquinone pool. We ascribe this plateau to the time delay needed to reduce the photosystem I electron acceptors.     

Light-Induced Changes in the Fluorescence Yield of Chlorophyll a In Vivo: I. Anacystis nidulans

he fluorescence yield of chlorophyll a in dark adapted Anacystis nidulans undergoes a slow change with continuous illumination. After the completion of the initial fast transient, the fluorescence yield rises from the level S to a plateau M within a minute, declining only after prolonged illumination. Both normal and 1,1-dimethyl-3(3'4'-dichloro)-phenylurea (DCMU)-poisoned Anacystis are capable of these changes. In normal Anacystis, the slow increase in the fluorescence yield (S --> M) requires light absorbed in system II while light absorbed in system I is ineffective. In DCMU-poisoned Anacystis, however, these changes are also promoted by light absorbed in system I. Addition of carbonyl cyanide p-trifluoromethoxy phenylhydrazone (FCCP), a photophosphorylation uncoupler acting near the photosynthetic electron transport chain, abolishes the rise from S to M in normal but has no effect in the DCMU-poisoned system. Phlorizin, a phosphorylase inhibitor, has very little effect. These results suggest that the light-induced variation in the fluorescence yield is related to the conformational changes which accompany photophosphorylation. The fluorescence yield of the auxiliary pigment phycocyanin remains constant throughout the interval of the light-induced changes in the fluorescence yield of chlorophyll a. Consequently, the fluorescence spectrum of the alga is variable on continuous illumination.     

倪氏拟多甲藻叶绿素荧光活性对环境因子的响应

采用叶绿素荧光分析技术探讨了温度、pH、光强对水华优势种倪氏拟多甲藻光合活性的影响。结果表明, 倪氏拟多甲藻光系统Ⅱ的量子产量Y(Ⅱ)和最大光化学效率(Fv/Fm)随温度(7.5-20.0℃)升高而显著增加(P0.05), 在低温下电子传递速率未受阻, 细胞在7.5-20.0℃内均有高光合活性; 10.0 ℃下的光合活性随pH增大先升高后降低, 峰值出现在pH 7.3时, 光合活性顺序为: 弱碱性 中性 酸性; 快速叶绿素荧光动力学曲线分析显示pH 7.3下的光合活性为典型OJIP曲线, 其他pH下PSⅡ反应中心、电子受体库受损, 显示该藻适应较窄的pH范围, pH7.0-8.0内是其适宜的条件; 快速光响应曲线显示其半饱和光强Ek为385.52 mol photons/(m2s), 表明其具有高光饱和点, 耐受高光强。研究表明藻细胞光合活性对温度和光强变化有较强适应性, 对pH的变化敏感, 弱碱性条件是其光合作用的适宜条件; 低温时细胞通过环式电子链提高光化学效率, 降低高光强可能带来的光损伤; 弱酸性(pH 5.0)会同时损伤其光系统Ⅰ和光系统Ⅱ, 造成其光化学效率的显著下降;倪氏拟多甲藻在低温和高光强下的独特光合特性使其在春季淡水水体中占据竞争优势, 是其形成水华的内在原因。     

Light-Induced Changes in the Fluorescence Yield of Chlorophyll a In Vivo: IV. The Effect of Preillumination on the Fluorescence Transient of Chlorella pyrenoidosa

The fluorescence transient of Chlorella pyrenoidosa, excited by saturating blue light, has a base level O, hump I, dip D, peak P, and at 1.5 sec a quasi-steady level S (12). With 2 sec exciting exposures and 4 min dark periods, preillumination-1 (lambda >/= 690 nm, intensities 1-750 ergs/sec-cm(2) incident), replacing the dark periods, lowers I more effectively than preillumination-2 (650 nm 相似文献   

硝酸根胁迫对黄瓜幼苗叶片光合速率、PSⅡ光化学效率及光能分配的影响

研究了不同浓度NO₃^-胁迫对黄瓜幼苗叶片光合速率、PSⅡ光化学效率及光能分配的影响.结果表明,当NO₃^-浓度较低时（14～98 mmol·L^-1）,适当增加NO₃^-浓度,可增强黄瓜幼苗叶片对光的捕获能力,促进光合作用.随着NO₃^-浓度的进一步增加（140～182 mmol·L^-1）,PSⅡ光化学效率降低,电子传递受到抑制,净光合速率降低;吸收的光能中,通过天线色素的热耗散增加,用于光化学反应的能量降低,光化学效率下降.140和182 mmol·L^-1 NO₃^-处理黄瓜幼苗叶片6 d后净光合速率(P_n)极显著下降,分别比对照降低了35%和78%;PSⅡ最大光化学效率(F_v/F_m)、天线转化效率(F_v’/F_m’)、实际光化学效率(Φ_PSⅡ)、光化学猝灭系数(q_P)均低于对照,非光化学猝灭(NPQ)高于对照,激发能在两个光系统间的分配不平衡性(β/α-1)增大.高浓度NO₃^-处理的黄瓜幼苗叶片各荧光参数变化幅度比低浓度大.当光照增强时,高浓度NO₃^-胁迫下黄瓜幼苗叶片吸收的光能中应用于光化学反应的份额(P) 显著降低,天线热耗散的份额(D)显著增加. 天线热耗散是耗散过剩能量的主要途径.     

Fluorescence and oxygen evolution from Chlorella pyrenoidosa

The process of photosynthetic energy conversion in Chlorella pyrenoidosa was investigated by simultaneous measurement of transient and steady-state rates of O₂ evolution and fluorescence.

1. 1. Alternation or superimposition of light 1 and light 2 illumination induces both fast and slow changes in fluorescence and rate of O₂ evolution. The fast changes are ascribed to changes in conditions of the reaction centers in the context of the ¹ model and the kinetic analysis of ². The slow changes are interpreted as adaptations to the intensity and wavelength of illumination. The adaptive mechanism is described in terms of slow variation in fraction () of total absorbed quanta delivered to System 2. At low intensities, the calculated value of for cells adapted to light 2 illumination (light 2 state) is approx. 0.9 of for cells adapted to light 1 illumination (light 1 state).

2. 2. An increase in fluorescence yield was found to accompany the decrease in O₂ yield at the onset of light saturation with either light 1 or light 2 excitation. Variation in is proposed to account for the differences between the maximum fluorescence yield observed in steady-state conditions and the 1.5 times higher maximum yield observed in transient conditions or in cells inhibited by 3(3,4-dichlorophenyl)-1,1-dimethylurea. Variation in can also explain the observation of a higher rate of fluorescence emission with light 1 excitation than with light 2 excitation for a given steady-state rate of O₂ evolution.

3. 3. A model for energy conversion by System 2 is proposed to account for our observations. The model proposes competitive dissipation of absorbed energy by photochemical trapping at reaction centers and by fluorescence and radiationless de-excitation from both the pigment bed and reaction centers of System 2.

Diurnal cycle of chlorophyll fluorescence in <Emphasis Type="Italic">Phalaenopsis</Emphasis>

Chlorophyll (Chl) fluorescence of warm day/cool night temperature exposed Phalaenopsis plants was measured hourly during 48 h to study the simultaneous temperature and irradiance response of the photosynthetic physiology. The daily pattern of fluorescence kinetics showed abrupt changes of photochemical quenching (q_P), non-photochemical quenching (NPQ) and quantum yield of photosystem II electron transport (Φ_PSII) upon transition from day to night and vice versa. During the day, the course of Φ_PSII and NPQ was related to the air temperature pattern, while maximum quantum efficiency of PSII photochemistry (F_v/F_m) revealed a rather light dependent response. Information on these daily dynamics in fluorescence kinetics is important with respect to meaningful data collection and interpretation.     

追施氮肥时期对冬小麦旗叶叶绿素荧光特性的影响

在大田条件下,研究了不同追氮时期对小麦旗叶叶绿素荧光特性、光合速率及籽粒产量的影响.结果表明,拔节期追肥较起身期或挑旗期追肥,改善了小麦旗叶PSⅡ的活性(F_v/F_o)、光化学最大效率(F_v/F_m)、光化学猝灭系数(qP)、实际量子产量(ΦPSⅡ)及光合速率,降低了籽粒灌浆中前期非辐射能量耗散,有利于叶片所吸收的光能较充分地用于光合作用,提高了籽粒灌浆后期非辐射能量的耗散,减缓了叶片光抑制程度和衰老进程.拔节期追肥可显著增加穗粒数和千粒重,提高产量.     

外源水杨酸对黄瓜幼苗叶片PSⅡ活性和光能分配的影响

以黄瓜品种‘中农203号’幼苗为试材,采用水培法研究了根际施用0.05、0.10和0.50 mmol/L水杨酸对黄瓜幼苗叶片PSⅡ活性和光能分配的影响,以探讨水杨酸对光合作用的调节机制。结果显示:黄瓜幼苗叶片净光合速率(Pn)、荧光参数和光能分配对水杨酸的响应存在明显的浓度依赖性。0.05和0.10 mmol/L水杨酸处理提高了叶片PSⅡ最大光化学量子产量(Fv/Fm)、PSⅡ实际光化学效率(ΦPSⅡ)、PSⅡ潜在活性(Fv/F0)、电子传递速率(ETR)、光化学猝灭系数(qP),降低了非光化学猝灭系数(NPQ),使PSⅡ吸收光能中分配于光化学反应的能量增加,进而提高了Pn,并以0.10 mmol/L水杨酸施用效果最明显,差异达极显著水平(P<0.01);而0.50 mmol/L水杨酸处理降低了ΦPSⅡ、Fv/Fm等,使光能分配于热耗散和荧光耗散的比例升高,导致Pn下降。研究表明,水杨酸对黄瓜叶片光合的正负调节作用与浓度依存下的PSⅡ活性和光能分配改变有关。     

Sulfite inhibition of photochemical activity of intact pea leaves

Sulfite treatment of pea leaf disks in light caused a significant decrease in the relative quantum yield of photosynthetic oxygen evolution and energy storage (ES) as measured by photoacoustic (PA) spectroscopy. The inhibition was concentration dependent and was less in darkness than in light, indicating light-dependent inhibitory site(s) on the photosynthetic electron transport chain. Further, in darksulfite-treated leaves, the energy storage was more affected than the relative quantum yield of oxygen evolution, suggesting that photophosphorylation and/or cyclic electron transport around PS I are sites of sulfite action in darkness. The R_fd values, the ratio of fluorescence decrease (fd) to the steady-state fluorescence (fs), decreased significantly in leaves treated with sulfite in light but were not affected in dark-treated ones, confirming the photoacoustic observations. Similarly, the ratio of variable fluorescence (F_v) to maximum fluorescence (F_m), a measure of PS II photochemical efficiency, was affected by sulfite treatment in light and not changed by treatment in darkness. An attempt was made to explain the mechanism of sulfite action on photosynthetic electron transport in light and in darkness.Abbreviations A_PT amplitude of photothermal signal - A_ox amplitude of oxygen signal - ES energy storage - fd fluorescence decrease - fs steady-state fluorescence - F_m maximum fluorescence - F_v variable fluorescence - PA photoacoustic(s)     

The long-term response to fluctuating light quality is an important and distinct light acclimation mechanism that supports survival of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> under low light conditions

The long-term response (LTR) of higher plants to varying light qualities increases the photosynthetic yield; however, the benefit of this improvement for physiology and survival of plants is largely unknown, and its functional relation to other light acclimation responses has never been investigated. To unravel positive effects of the LTR we acclimated Arabidopsis thaliana for several days to light sources, which preferentially excite photosystem I (PSI) or photosystem II (PSII). After acclimation, plants revealed characteristic differences in chlorophyll fluorescence, thylakoid membrane stacking, phosphorylation state of PSII subunits and photosynthetic yield of PSII and PSI. These LTR-induced changes in the structure, function and efficiency of the photosynthetic machinery are true effects by light quality acclimation, which could not be induced by light intensity variations in the low light range. In addition, high light stress experiments indicated that the LTR is not involved in photoinhibition; however, it lowers non-photochemical quenching (NPQ) by directing more absorbed light energy into photochemical work. NPQ in turn is not essential for the LTR, since npq mutants performed a normal acclimation. We quantified the beneficial potential of the LTR by comparing wild-type plants with the LTR-deficient mutant stn7. The mutant exhibited a decreased effective quantum yield and produced only half of seeds when grown under fluctuating light quality conditions. Thus, the LTR represents a distinct acclimation response in addition to other already known responses that clearly improves plant physiology under low light conditions resulting in a pronounced positive effect on plant fitness.     

Effects of shading on photosynthetic characteristics and chlorophyll fluorescence parameters in leaves of Hydrangea macrophylla

Aims The objectives were to investigate the effects of different light intensities on photosynthetic characteristics and chlorophyll fluorescence parameters, to clarify the physiological responses and photo-protective mechanisms of Hydrangea macrophylla to changes in light regimes in view of the distribution of energy absorbed and photosynthetic characteristics.Methods Three light regimes including natural and shade (shading rate 50% and 75% of natural light) were applied to plants for 60 days. After the treatment, the gas-exchange, chlorophyll a fluorescence and photosynthesis-light curves were measured by a portable leaf gas exchange system (LI-6400).Important findings The results showed that the weak light intensity treatment reduced dark respiration rate, light compensation point and light saturation point of plant, but increased apparent quantum yield, suggesting that plants had the physiological strategy to utilize the weakening light by reducing respiration. The net photosynthetic rate, intercellular CO₂ concentration, transpiration rate and water use efficiency of plants grown below 50% of natural light showed significant difference compared with natural and shading rate 75% of natural light. There were significant difference between natural and shade treatments in the maximal quantum efficiency of PSII (F_v/F_m), as indicated that it was significantly less at full light than that at 50% of natural light. Initial fluorescence intensity (F_o) of plants was higher at full light than that at 50% of natural light, suggesting that photoinhibition occurred in natural light. The non-photochemical quenching (NQP) decreased with the aggravation of shade stress, indicating that shading decreased the efficiency of photochemical reaction by reducing the fraction of incident light in photochemical energy utilization and decreased thermal dissipation through regulating energy distribution in photosystem II (PSII) in the leaves of Hydrangea macrophylla. In general, the 70% of incident light in photochemical energy utilization was distributed to thermal dissipation, 20% was distributed to non-regulated energy dissipation and 4% was distributed to effective photochemical reaction. In conclusion, responses of plants to increased irradiance are governed by strategy: to utilize a high fraction of incident light in photochemistry and regulate energy dissipation in PSII and weaken the accumulation of excess excitation energy in PSII to protect the photosynthetic apparatus in the leaves of H. macrophylla under saturated radiation.     

早熟桃夏季叶色转红过程中的光化学响应特征

比较研究了‘早美’和‘春蕾’2个早熟桃品种夏季叶色转红对太阳光能的利用和光系统Ⅱ的叶绿素荧光特征的影响。结果表明：早熟桃叶片色素组成的变化会显著影响其光合和叶绿素荧光特性。叶色转红后,早熟桃净光合速率（Pn）日均值、PSII最大光化学效率（Fv/Fm）、PSII实际光化学效率（ФPSII）均上升,无显著光抑制,而绿叶对照‘红花碧桃’的电子传递速率（ETR）、Fv／Fm和ФPSII值均显著下降,7月光合明显受抑制。叶色转红程度较深的‘早美’在夏季高温强光下表现优于‘春蕾’和对照。淬灭分析表明：叶片花色素苷的积累能在短时间内增加PSII天线色素吸收的光能用于光化学反应的份额（P）与用于反应中心热耗散的相对份额（D）。转红后的叶片光化学淬灭系数（qp）显著高于绿叶,PSII光化学效率较高,但耗散过剩激发能的能力显著低于绿叶对照。     

USING A NONANALYTICAL APPROACH TO MODEL NONLINEAR DYNAMICS IN PHOTOSYNTHESIS AT THE PHOTOSYSTEM LEVEL1

Nonlinear dynamics in photon capture and uptake at the photosystem level may have a strong effect on photosynthetic yield. However, the magnitude of such effects is difficult to estimate theoretically because nonlinear systems often cannot be represented accurately using equations. A nonanalytical simulation was developed that used a simple decision tree and Monte Carlo methods, instead of equations, to model how a population of photosystems absorbs and utilizes photons from an ambient light field. This simulation replicated realistic kinetics in the closure and variable fluorescence yield of PSII on the single‐turnover timescale, as well as the saturating behavior in light‐driven electron flow that is observed in nature with increasing irradiance. This simulation indicated that the transfer of absorbed photon energy among PSII units can introduce strong nonlinear enhancement in light‐driven electron flow. However, this effect was seen only in populations with particular photosynthetic states as determined by physiological properties of PSII. Other populations with the same degree of energy transfer but with different photosynthetic states exhibited little enhancement in electron flow and, in some cases, a reduction. This nonanalytical approach provides a simple means to quantify theoretically how nonlinear dynamics in photosynthesis arise at the photosystem level and how these dynamics may act to enhance or constrain photosynthetic rates and yields. Such simulations can provide quantitative insight into different physiological bases of nonlinear light‐harvesting dynamics and identify those that would have the strongest theoretical influence and thus warrant closer experimental examination.     

Fluorescence clamp: A direct measure of fluxes into and out of the antenna pool of Photosystem II

Fluorescence clamp (FC) is a method of directly measuring the fluxes out of Photosystem II antenna. This is achieved by a feed-back loop which controls the light intensity of light emitting diodes in order to keep the amplitude of modulated chlorophyll fluorescence constant, and by taking the intensity or the current fed into the light emitting diodes as a measure of the fluxes. Saturating flashes serve to distinguish between fluxes into thermal deactivation and into the photosynthetic electron transfer chain (ETC). As FC is only active in the light period of the measuring light, the background signal (induced by actinic light) is compensated by a second feed-back loop in the dark period of the measuring light. Equations are provided for the interpretation of the FC signals. This includes the quenching parameters of chlorophyll fluorescence, the flux into the electron transfer chain and the redox state of Q_A. Experiments are presented which show that traditional fluorescence (LC) and FC measurements yield the same results. However, the FC method provides a better presentation of fluxes as the scaling factor (flux/signal) is constant for all states of Photosystem II. This leads to a simpler analysis of quenching mechanisms. Examples are given which show that the co-existing quenching mechanisms with different effects on photochemical and non-photochemical fluxes can be better identified by FC rather than by LC. This revised version was published online in June 2006 with corrections to the Cover Date.     

Photoadaptation in the Green Alga Spongiochloris Sp. A Three-Fluorometer Study

The dark-adapted cells of the green alga Spongiochloris sp. were exposed to "white light" of 1000 μmol(photon) m⁻² s⁻¹ for 2 h and then dark adapted for 1.5 h. Changes of photochemical activities during photoadaptation were followed by measurement of chlorophyll (Chl) fluorescence kinetics, 77 K emission spectra, photosynthetic oxygen evolution, and pigment composition. We observed a build-up of slowly-relaxing non-photochemical quenching which led to a decrease of the F_v/F_m parameter and the connectivity. In contrast to the depression of F_v/F_m (35 %) and the rise of non-photochemical quenching (∼ 1.6), we observed an increase in effective absorption cross-section (20 %), Hill reaction (30 %), photosynthetic oxygen evolution (80 %), and electron transport rate estimated from the Chl fluorescence analysis (80 %). We showed an inconsistency in the presently used interpretation schemes, and ascribe the discrepancy between the increase of effective absorption cross-section and the photosynthetic activities on one side and the effective non-photochemical quenching on the other side to the build-up of a quenching mechanism which dissipates energy in closed reaction centres. Such a type of quenching changes the ratio between thermal dissipation and fluorescence without any effect on photochemical yield. In this case the F_v/F_m ratio cannot be used as a measure of the maximum photochemical yield of PS2. This revised version was published online in August 2006 with corrections to the Cover Date.     

Photoacoustic measurements of the energy-conversion efficiency of photosynthesis in thalli of the green alga Bryopsis maxima

The efficiency of photosynthetic energy conversion in thalli of the green alga Bryopsis maxima was studied with the photoacoustic technique. Photosynthetic O₂ evolution did not interfere with the photoacoustic measurements in this material, most probably owing to its coenocytic cellular organization. The energy yield (defined as the fraction of absorbed photon energy that is stored in photosynthetic products or intermediates relative to the total absorbed photon energy) was estimated from the photoacoustic signals by applying the background-illumination method to obtain a reference without the photochemical capacity (Lasser-Ross, N., Malkin, S. and Cahen, D. (1980) Biochim. Biophys. Acta 593, 330–341). With the monitoring light modulated at 60 Hz, photon energy is mainly stored by redox changes in electron-transport chains because the energy yield was strongly reduced by 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea and heat treatment of the thalli, whereas KCN, an inhibitor of CO₂ reduction, had no effect, and because a significant lowering of the energy yield occurred in the presence of methyl viologen but the effect of the Photosystem I acceptor was largely reversed on the addition of an uncoupler, methylamine. The maximum energy yield of 0.4 that was obtained with a saturating background light and with a sufficiently weak monitoring light modulated at 100 Hz is explained in terms of electron transfer from electron-donor pools to acceptor pools of the two photosystems with the quantum yield close to unity. A lowering of the modulation frequency decreased the energy yield, indicating that less energy is stored in more stable intermediates.     

Chlorophyll fluorescence characteristics associated with hydration level in pea cotyledons

In order to study the effects of desiccation on a photosynthetic system, light harvesting and light-induced electron transport processes were examined in pea cotyledons at various moisture levels, using in vivo fluorescence excitation spectra and fluorescence induction kinetics. Water sorption isotherms yielded thermodynamic data that suggested very strong water binding between 4 to 11% water, intermediate sorption between water contents of 13 to 22%, and very weak binding at moisture contents between 24 to 32%. The fluorescence properties of the tissue changed with the moisture contents, and these changes correlated generally with the three regions of water binding. Peak fluorescence and fluorescence yield remained at low levels when water content was limited to the tightly bound regions, below 12%. Several new peaks appeared in the chlorophyll a excitation spectrum and both peak fluorescence and fluorescence yield increased at intermediate water-binding levels (12-22%). At moisture contents where water is weakly bound (>24%), peak fluorescence and fluorescence yield were maximum and the fluorescence excitation spectrum was unchanging with further increases in water content.

The state of water is an important component in the energy transfer and electron transport system. At hydration levels where water is most tightly bound, energy transfer from pigments is limited and electron transport is blocked. At intermediate water binding levels, energy transfer and electron transport increase and, in the region of weak water binding, energy transfer and electron transport are maximized.

     

超级高产小麦旗叶荧光动力学研究

探明超级小麦品种的旗叶光合作用与荧光动力学特性,为超级小麦品种选育利用提供理论依据。以超级小麦临麦4号为试验材料,应用CI-301PS型便携式光合作用测定系统和FMS-2便携式荧光测定仪(英国Hansatech公司)在田间试验中测定旗叶光合作用与荧光动力学参数。结果表明,与普通高产对照品种皖麦52和烟农19相比,超级小麦临麦4号的光合作用参数光合速率、光饱和点和CO2饱和点、羧化效率高,光补偿点和CO2补偿点低;光合机构系统工作参数PSII实际的光化学效率(ΦPSII)、光化学猝灭系数(qP)、PSII反应中心的激发能捕获效率(Fv/Fm)、PSⅡ潜在活性Fv/Fo和电子传递速率(ETR)值高,非光化学猝灭系数(NPQ)值低。这表明超级小麦临麦4号的光合机构系统工作能力强和工作效率高,保证旗叶光合作用的高效运行,为子粒灌浆提供充足的能量和碳水化合物。     

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