Protection effect of nitric oxide on photosynthesis in rice under heat stress

The effect of exogenous applied nitric oxide on photosynthesis under heat stress was investigated in rice seedlings. High temperature resulted in significant reductions of the net photosynthetic rate (P _N) due to non-stomatal components. Application of nitric oxide donors, sodium nitroprusside (SNP) or S-nitrosoglutathione (GSNO), dramatically alleviated the decrease of P _N induced by high temperature. Chlorophyll fluorescence measurement revealed that high temperature caused significant increase of the initial fluorescence (F _o) and non-photochemical quenching (NPQ) whereas remarkable decrease of the maximal fluorescence (F _m), the maximal efficiency of PSII photochemistry (F _v/F _m), the actual PSII efficiency (Φ_PSII), and photochemical quenching (q _p). In the presence of SNP or GSNO pretreatment, the increase of F _o and decrease of F _m, F _v/F _m, Φ_PSII and q _p were markedly mitigated, but NPQ was further elevated. Moreover, with SNP or GSNO pretreatment, H₂O₂ accumulation and electrolyte leakage induced by heat treatment were significantly reduced, whereas zeaxanthin content and carotenoid content relative to chlorophyll were elevated. The potassium salt of 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), a specific NO scavenger, arrested NO donors mediated effects. These results suggest that NO can effectively protect photosynthesis from damage induced by heat stress. The activation effect of NO on photosynthesis may be mediated by acting as ROS scavenging, or/and alleviating oxidative stress via maintaining higher carotenoid content relative to chlorophyll or/and enhancing thermal dissipation of excess energy through keeping higher level of zeaxanthin content under heat stress.     

Sensitivity of the relative Fpl level of chlorophyll fluorescence induction in leaves to the heat stress

The (F_pl-F_o)/F_v value of the fluorescence induction curve is shown to be a more suitable parameter to detect a wider range of heat stress damage to thylakoid membranes as compared to quantities t _1/2 (time of fluorescence rise from F_o to (F_o+F_m)/2 level) and % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0Jf9crFfpeea0xh9v8qiW7rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaacq% aHepaDaaaaaa!39D5!\[\overline \tau \] (the fluorescence induction time defined as the area above the induction curve normalized to F_v=1). A method for exact and automatic F_pl determination is presented.A break point in the quality and behaviour of the fluorescence induction curve of barley leaves incubated at 49°C was reached at the moment (about 240 s) when the transformation of PS II active (Q_B-reducing) to PS II inactive (Q_B-non-reducing) centres was completed. The meaning of the standard F_v and F_v/F_m parameter was then changed.The method of F_pl determination described here may help to increase the analytical value of the standard chlorophyll fluorometers.Abbreviations F_o initial fluorescence - F_m maximal fluorescence - F_pl fluorescence at first inflection point (plateau) - F_v variable fluorescence (F_v=F_m–F_o) - PSM plant stress meter - SD standard deviation     

Chlorophyll fluorescence and leaf chlorophyll content of bean leaves injured by spider mites (Acari: Tetranychidae)

The use of chlorophyll fluorescence as a method for detecting and monitoring plant stress arising from Tetranychus urticae (Koch) feeding injury was investigated. The effect of mite density (1–32 mites per 1.5 cm² of leaf) and the duration of the feeding period (1–5 days) on the chlorophyll fluorescence parameters of bean (Phaseolus vulgaris) leaves were examined. Changes in chlorophyll fluorescence parameters were dependent both on mite density and duration of feeding. Decreases in F _o, the initial fluorescence and F _m, the maximum fluorescence led to a decrease in the ratio of variable to maximum fluorescence, F _v/F _m. The decrease in F _v/F _m is typical of the response of many plants to a wide range of environmental stresses and indicates a reduced efficiency of photosystem II (PSII) photochemistry. T _1/2, which is proportional to the pool size of electron acceptors on the reducing side of PSII, was also reduced in response to mite-feeding injury. The leaf chlorophyll content decreased with increasing mite density and duration of feeding but did not appear to contribute to the decrease in F _v/F _m. Chlorophyll fluorescence is an effective method for detecting and monitoring stress in T. urticae-injured bean leaves.     

低温胁迫下丛枝菌根真菌对玉米光合特性的影响

利用盆栽试验,在15 ℃和5 ℃低温胁迫下研究了丛枝菌根（AM）真菌对玉米生长、叶绿素含量、叶绿素荧光和光合作用的影响.结果表明：低温胁迫抑制了AM真菌的侵染;接种AM真菌的玉米地上部和地下部干物质量、相对叶绿素含量高于不接种植株.与非菌根玉米相比,菌根玉米具有较高的最大荧光（F_m）、可变荧光（F_v）、最大光化学效率（F_v/F_m）和潜在光化学效率（F_v/F_o）及较低的初始荧光（F_o）,并且在5 ℃处理中差异显著.接种AM真菌使玉米叶片的净光合速率（P_n）和蒸腾速率（T_r）显著增强;低温胁迫下,菌根植株的气孔导度（G_s）显著高于非菌根植株;而胞间CO₂浓度（C_i）显著低于非菌根植株.表明AM真菌可通过提高叶绿素含量及改善叶片叶绿素荧光和光合作用来减轻低温胁迫对玉米植株造成的伤害,提高玉米耐受低温的能力,进而提高玉米的生物量,促进玉米生长.     

Chlorophyll fluorescence parameters and drought tolerance in a mapping population of winter bread wheat in the highlands of Iran

The usefulness of fluorescence parameters as drought tolerance selection criteria for winter bread wheat in the highlands of Iran was studied. A population of 142 recombinant inbred lines, derived from a cross between two common wheat varieties, Azar2 (winter type) and 87Zhong291 (facultative type), was used to analyze the correlation between grain yield and chlorophyll fluorescence parameters at the grain-filling stage under drought stress and supplementary irrigation conditions during 2006–2007 and 2007–2008 seasons at Maragheh experiment station of the Dryland Agricultural Research Institute (DARI) using a RCBD with three replications. The results showed significant differences among the lines in the grain yield and all fluorescence parameters under rainfed and irrigation conditions. The values of chlorophyll content, F ₀, F _m, F _v, F _v/F _m, LWP, YPEC, NPQ, and PI in the drought-tolerant genotypes were significantly higher than those in drought-sensitive genotypes under drought stress. Significant differences were observed between slope coefficients under drought, but not under supplementary irrigation conditions except NPQ (P = 5%). It was concluded that chlorophyll content, F ₀, F _m, F _v, F _v/F _m, LWP, YPEC, NPQ, and PI could be used as additional indicators in screening wheat germplasm for drought tolerance.     

Chlorophyll fluorescence as a tool for evaluation of drought stress in strawberry

The effect of water deficit on chlorophyll fluorescence, sugar content, and growth parameters of strawberry (Fragaria×ananassa Duch. cv. Elsanta) was studied. Drought stress caused significant reductions in leaf water potential, fresh and dry masses, leaf area, and leaf number. A gradual reduction of photochemical quenching (q_P) and quantum efficiency (Φ_PS2) was observed under drought stress while non-photochemical quenching (q_N) increased. Maximum efficiency of photosystem 2 (F_v/F_m) was not affected by drought stress.     

Chlorophyll fluorescence performance of sweet almond [Prunus dulcis (Miller) D. Webb] in response to salinity stress induced by NaCl

One-year old sweet almond (Prunus dulcis) seedlings were submitted to four levels of salt stress induced by NaCl, namely 0.3, 0.5, 0.7, and 1.0 S m⁻¹. Effects of salt stress on a range of chlorophyll (Chl) fluorescence parameters (Chl FPs) and Chl contents were investigated in order to establish an eco-physiological characterization of P. dulcis to salinity. Salt stress promoted an increase in F₀, F_s, and F₀/F_m and a decrease in F_m, F′_m, F_v/F_m, q_P, ΔF/F′_m, F_v/F₀, and UQF_(rel), in almost all Chl fluorescence yields (FY) and FPs due to its adverse effect on activity of photosystem 2. No significant changes were observed for quenchings q_N, NPQ, and q_N(rel). The contents of Chl a and b and their ratio were also significantly reduced at increased salt stress. In general, adverse salinity effects became significant when the electric conductivity of the nutrient solution (EC_n) exceeded 0.3 S m⁻¹. The most sensitive salt stress indicators were F_v/F₀ and Chl a content, and they are thus best used for early salt detection in P. dulcis. Monitoring of a simple Chl FY, such as F₀, also gave a good indication of induced salt stress due to the significant correlations observed between the different Chl FYs and FPs. Even essential Chl FYs, like F₀, F_m, F′_m, and F_s, and mutually independent Chl FPs, like F_v/F₀ and q_P, were strongly correlated with each other.     

两种生境条件下6种牧草叶绿素含量及荧光参数的比较

昆仑山前山牧场海拔较高, 策勒绿洲海拔相对较低, 两者生境差异较大。以昆仑山前山牧场和策勒绿洲边缘两种不同生境条件下生长的6种牧草: 冰草(Agropyron cristatum)、无芒雀麦(Bromus inermis)、矮生高羊茅(Festuca elata)、披碱草(Elymus dahuricus )、红豆草(Onobrychis pulchella)及和田大叶(Medicago sativa var. luxurians)为试验材料, 研究了不同生境条件下牧草叶片叶绿素含量及叶绿素荧光动力学参数的变化情况。结果显示: (1)在两种生境条件下, 昆仑山前山牧场生境生长的牧草叶绿素a、叶绿素b、总叶绿素的含量明显较高, 生长在策勒绿洲生境的牧草品种叶绿素a/b值较高; (2)昆仑山前山牧场生境牧草最大荧光、光系统II (PSII)最大光化学效率、PSII潜在活性和单位面积反应中心的数量的值明显高于策勒绿洲生境品种, 而初始荧光、单位反应中心吸收的光能、单位反应中心捕获的能量、单位反应中心耗散的能量、荧光诱导曲线初始斜率值则低于策勒绿洲生境品种。因此, 两种生境下环境因子发生了改变, 对牧草产生综合的胁迫作用; 策勒绿洲生境明显对牧草生长产生了抑制, 策勒绿洲生境牧草的色素含量降低以及PSII的机构遭到损坏, 导致反应中心一部分失活或裂解, 剩余有活性的反应中心的效率增加, 昆仑山生境则相对比较适宜牧草生长; 两种生境不同牧草叶绿素含量和叶绿素荧光参数的变化幅度不同。     

Lanthanum improves salt tolerance of maize seedlings

In this study, the effects of lanthanum were investigated on contents of pigments, chlorophyll (Chl) fluorescence, antioxidative enzymes, and biomass of maize seedlings under salt stress. The results showed that salt stress significantly decreased the contents of Chl and carotenoids, maximum photochemical efficiency of PSII (F_v/F_m), photochemical quenching (q_P), and quantum efficiency of PSII photochemistry (Φ_PSII), net photosynthetic rate (P_N), and biomass. Salt stress increased nonphotochemical quenching (q_N), the activities of ascorbate peroxidase, catalase, superoxide dismutase, glutathione peroxidase, and the contents of malondialdehyde and hydrogen peroxide compared with control. Pretreatment with lanthanum prior to salt stress significantly enhanced the contents of Chl and carotenoids, F_v/F_m, q_P, q_N, Φ_PSII, P_N, biomass, and activities of the above antioxidant enzymes compared with the salt-stressed plants. Pretreatment with lanthanum also significantly reduced the contents of malondialdehyde and hydrogen peroxide induced by salt stress. Our results suggested that lanthanum can improve salt tolerance of maize seedlings by enhancing the function of photosynthetic apparatus and antioxidant capacity.     

Use of simultaneous analysis of gas-exchange and chlorophyll fluorescence quenching for analysing the effects of water stress on photosynthesis in apple leaves

Summary A convenient system for the rapid simultaneous measurement of both chlorophyll fluorescence quenching using a modulated light system, and of CO₂, and water vapour exchange by leaves is described. The system was used in a study of the effects of water deficits on the photosynthesis by apple leaves (Malus x domestica Borkh.). Apple leaves were found to have low values of steady-state variable fluorescence, and the existence of significant fluorescence with open traps (F_o) quenching necessitated the measurement and use of a corrected F_o in the calculation of quenching components. Long-term water stress had a marked effect on both gas-exchange and chlorophyll fluorescence quenching. Non-photochemical quenching (qn) in particular was increased in water-stressed leaves, and it was particularly sensitive to incident radiation in such leaves. In contrast, rapid dehydration only affected gas exchange. Relaxation of qn quenching in the dark was slow, taking approximately 10 min for a 50% recovery, in well-watered and in draughted plants, and whether or not the plants had been exposed to high light.     

Chilling stress and chilling tolerance of sweet potato as sensed by chlorophyll fluorescence

We studied changes in the chlorophyll (Chl) fluorescence components in chilling-stressed sweet potato (Ipomoea batatas L. Lam) cv. Tainung 57 (TN57, chilling-tolerant) and cv. Tainung 66 (TN66, chilling-susceptible). Plants under 12-h photoperiod and 400 μmol m⁻² s⁻¹ irradiance at 24/20 °C (day/night) were treated by a 5-d chilling period at 7/7 °C. Compared to TN66, TN57 exhibited a significantly greater basic Chl fluorescence (F₀), maximum fluorescence (F_m), maximum fluorescence yield during actinic irradiation (F_m′ ), and the quantum efficiency of electron transport through photosystem 2, PS2 (Φ_PS2). Chilling stress resulted in decrease in the potential efficiency of PS2 (F_v/F_m), Φ_PS2, non-photochemical fluorescence quenching (NPQ), non-photochemical quenching (q_N), and the occurrence of chilling injury in TN66. Chilling increased the likelihood of photoinhibition, characterized by a decline in the Chl fluorescence of both cultivars, and photoinhibition during low temperature stress generally occurred more rapidly in TN66.     

Changes in Chlorophyll Fluorescence During the Course of Photoperiod and in Response to Drought in Casuarina equisetifolia Forst. and Forst.

The effects of drought and the diurnal changes in photosynthetic electron transport were studied in non-nodulated plants of Casuarina equisetifolia. The induction of fluorescence showed a slightly higher I step in water-stressed than control plants, and the time from the start of irradiation to the P step of induction was significantly shortened by drought. The quantum efficiency of photosystem 2 (PS2) in the dark-adapted state (F_v/F_m) was generally not affected by drought, whereas it decreased during the central hours of the day. The decrease in quantum yield of PS2 electron transport (₂) in water-stressed plants was associated with decreases in the photochemical efficiency of open (oxidised) PS2 centres (F_v'/F_m') and increases in non-photochemical quenching (q_N) rather than with increased closure of PS2 centres (lowered photochemical quenching, q_P). In contrast, the changes in quantum yield of electron transport during the day were related to changes in q_P rather than in F_v'/F_m'. When chlorophyll fluorescence was measured at the same irradiance during the day, a greater q_N was observed at the end of the drying cycle than after watering, and early and late in the photoperiod than in the central hours of the day. The greater q_N at the beginning and end of the day did not prevent an increase in energy not used photochemically nor dissipated non-photochemically. Drought did not affect this excess of photon energy.     

Short-term responses of photosynthetic membrane lipids and photochemical efficiency in plants of <Emphasis Type="Italic">Phaseolus vulgaris</Emphasis> and <Emphasis Type="Italic">Vigna unguiculata</Emphasis> submitted to high irradiance

Primary leaves of young plants of common bean (Phaseolus vulgaris cv. Carioca and Negro Huasteco) and cowpea (Vigna unguiculata Walp cv. Epace 10) were exposed to high irradiance (HI) of 2 000 μmol m⁻² s⁻¹ for 10, 20, and 30 min. The initial fluorescence (F₀) was nearly constant in response to HI in each genotype except for Carioca. A distinct reduction of maximum fluorescence (F_m) was clearly observed in stressed genotypes of beans after 20 min followed by a slight recovery for the longer stress times. In common bean, the maximum quantum yield (F_v/F_m) was reduced slowly from 10 to 30 min of HI. In cowpea, only a slight reduction of F_v/F_m was observed at 20 min followed by recovery to normal values at 30 min. HI resulted in changes in the photochemical (q_P) and non-photochemical (q_N) quenching in both species, but to a different extent. In cowpea plants, more efficiency in the use of the absorbed energy under photoinhibitory conditions was related to increase in q_P and decrease in q_N. In addition, lipid peroxidation changed significantly in common bean genotypes with an evident increase after 20 min of HI. Hence the photosynthetic apparatus of cowpea was more tolerant to HI than that of common bean and the integrity of cowpea cell membranes was apparently maintained under HI.     

Effect of Glomus mosseae on chlorophyll content, chlorophyll fluorescence parameters, and chloroplast ultrastructure of beach plum (Prunus maritima) under NaCl stress

The present study was undertaken to investigate the effect of Glomus mosseae on chlorophyll (Chl) content, Chl fluorescence parameters and chloroplast ultrastructure of beach plum seedlings under 2% NaCl stress. The results showed that compared to control, both Chl a and Chl b contents of NaCl + G. mosseae treatment were significantly lower during the salt stress, while Chl a/b ratio increased significantly. The increase of minimal fluorescence of darkadapted state (F₀), and the decrease of maximal fluorescence of dark-adapted state (F_m) and variable fluorescence (F_v) values were inhibited. The maximum quantum yield of PSII photochemistry (F_v/F_m), the maximum energy transformation potential of PSII photochemistry (F_v/F₀) and the effective quantum yield of PSII photochemistry (??_PSII) increased significantly, especially the latter two variables. The values of the photochemical quenching coefficient (q_P) and the nonphotochemical quenching (NPQ) were similar between G. mosseae inoculation and noninoculation. It could be concluded that G. mosseae inoculation could protect the photosystem II (PSII) of beach plum, enhance the efficiency of primary light energy conversion and improve the primitive response of photosynthesis under salinity stress. Meanwhile, G. mosseae inoculation was beneficial to maintain the integrity of thylakoid membrane and to protect the structure and function of chloroplast, which suggested that G. mosseae can alleviate the damage of NaCl stress to chloroplast.     

Development of photoautotrophy and photoinhibition of Gardenia jasminoides plantlets during micropropagation

This paper reports on the fast fluorescence responses of Gardenia jasminoides Ellis plantlets, at two successive stages (shoot multiplication and root induction) of culture in vitro. We test whether plantlets in vitro suffer photoinhibition during culture and whether the degree of photoautotrophy of these mixotrophic plantlets has any effect on the extent of photoinhibitory impairment. In this regard the effects of different sucrose levels in the medium and PPFD during growth on the development of photoautotrophy and the extent of photoinhibition were evaluated. Plantlets were grown under low, intermediate, and high (50, 100, and 300 mol m^-2 s^-1) PPFD, and at 3 different sucrose concentrations (0.5, 1.5, and 3.0%, w/v) in the medium, during shoot multiplication. During root induction the same growth conditions were assayed except for the high PPFD. The development of photoautotrophy was assessed via the difference between the stable carbon isotope composition of sucrose used as heterotrophic carbon source and that of leaflets grown in vitro. Plantlets from root induction showed more developed photoautotrophy than those from shoot multiplication. For both stages the low-sucrose medium stimulated the photoautotrophy of plantlets in vitro. In addition, intermediate PPFD induced photoautotrophy during shoot multiplication. For plantlets of both culture stages at the lowest PPFD no photoinhibition occurred irrespective of the sucrose concentration in media. However, during the shoot multiplication stage chlorophyll fluorescence measurements showed a decrease in F _v /F _m and in t _1/2 as growing PPFD increased, indicating photoinhibitory damage. The decline of F _v /F _m was caused mostly by an increase in F _o , indicating the inactivation of PSII reaction centers. However plantlets growing under low sucrose showed reduced susceptibility to photoinhibition. During root induction, only plantlets cultured with high sucrose showed a decrease in F _v /F _m as PPFD increased, although t _1/2 remained unchanged. In this case, the decline of F _v /F _m was mostly due to a decrease in F _m , which indicates increased photoprotection rather than occurrence of photodamage. Therefore, growth in low-sucrose media had a protective effect on the resistance of PSII to light stress. In addition, plantlets were more resistant to photoinhibition during root induction than during shoot multiplication. Results suggest that increased photoautotrophy of plantlets reduces susceptibility to photoinhibition during gardenia culture in vitro.Abbreviations AP apparent photosynthesis - Chl total chlorophyll content - Chl a/b chlorophyll a-to-b ratio - Chl/Car total chlorophyll-to-carotenoids ratio - ¹³C ratio of ¹³C/¹²C relative to PeeDee belemnite standard - F _m maximum chlorophyll fluorescence - F _o fluorescence emission when all reaction centres are open and the photochemical quenching is minimal - F _v variable chlorophyll fluorescence (F _m -F _o ) - F _v /F _m the ratio of variable to maximum chlorophyll fluorescence, indicator photochemical efficiency of PSII - MS medium Murashige and Skoog (1962) medium - PPFD photosynthetic photon flux density - Rd dark respiration, t _1/2 the half-time of the increase from F _o to F _m - IAA indole butyric acid     

Photoinhibition in the Antarctic moss Grimmia antarctici Card when exposed to cycles of freezing and thawing

Freezing and thawing of the endemic moss species Grimmia antarctici Card, caused photoinhibition. When snow cover was removed from moss in the field, resulting in exposure to fluctuating temperatures and light conditions, photoinhibition, measured as a reduction in the ratio of variable to maximum chlorophyll a fluorescence (F_v/F_m), was observed. The extent of photoinhibition was highly variable and appeared to be reversible during periods of warmer temperatures. A series of controlled laboratory studies found that the light conditions that prevail between freezing and thawing events influenced the recovery from photoinhibition observed during freezing and thawing, with low light conditions facilitating the greatest rates of recovery. After four cycles of freezing and thawing, recovery from photoinhibition in hydrated moss was achieved within 12 h of transfer to 5°C and 15 μmol quanta m^?2 s^?1. These results favour the hypothesis that photoinhibition observed during freezing represents a protective process involving the down-regulation of photo-system II when photosynthetic carbon assimilation is limited by low temperatures.     

Chlorophyll fluorescence as a tool for evaluation of viability in freeze-stressed grapevine buds

This study examined the utility of the ratio of variable fluorescence to maximum fluorescence (F_v/F_m) to detect freezing injury on buds of two Vitis vinifera cultivars: Pinot noir and Pinot gris. Freezing treatments on buds caused a decrease both in F_v/F_m and percentage of budburst, more severely on Pinot gris than Pinot noir, specifically at the lower temperature (−20°C). F_v/F_m ratio showed a close correlation with percentage of budburst, and a threshold of the lethal F_v/F_m was proposed as an indicator of bud mortality.     

Changes in D1-protein turnover and recovery of photosystem II activity precede accumulation of chlorophyll in plants after release from mineral stress

After seven weeks of a combined magnesium and sulphur deficiency, spinach (Spinacea oleracea L.) plants showed a substantial accumulation of inactivated photosystem II (PSII) centres as indicated by a 40% decrease of the chlorophyll (Chl) fluorescence parameter F_v/F_m (F_v being the yield of variable fluorescence and F_m the yield of maximal fluorescence when all reaction centres are closed) together with a severe loss of leaf Chl content of 75%. The responses of the photosynthetic apparatus were examined when the deficient plants were transferred back to a rich nutrient medium. During the first 24 h of the recovery phase, thylakoid protein synthesis measured as incorporation of [¹⁴C]leucine per unit of Chl increased substantially. The synthesis rate of the D1 reaction-centre polypeptide of PSII, which in the deficient plants was reduced to 50% of the non-deficient control, was stimulated eight- to ninefold. D1-protein content, which in the deficient plants was reduced to 40% of the non-deficient control, started to increase 2 d later. Thus, D1-protein degradation was also enhanced. The increased D1-protein turnover led to a rapid repair of the existing PSII centres as indicated by the rise of F_v/F_m. It was completed at day 7 of the recovery phase. At day 2 of the recovery phase, the synthesis of other thylakoid proteins such as the D2 protein, cytochrome b ₅₅₉, CP 47 and the 33-kDa polypeptide of the water-splitting system, became stimulated. This process resulted in an accumulation of new PSII centres. During the first week, formation of new PSII centres was not associated with an increase in leaf Chl content. The Chl content of the recovering leaves only started to increase when the ratio of PSII polypeptides versus LHCII (light-harvesting complex of PSII), which was substantially diminished in the deficient plants, became comparable to that of the control. The recovery process was accompanied by substantial changes in thylakoid protein phosphorylation. Their relevance to thylakoid protein turnover and stability is discussed.Abbreviations Chl chlorophyll - cyt cytochrome - F_o yield of intrinsic fluorescence when all PSII centres are open in the dark - F_m yield of maximal fluorescence when all reaction centres are closed - F_m fluorescence yield when all reaction centres are closed (after a saturating flash) under steady-state conditions - F_v yield of variable fluorescence, (difference between F_oand F_m) - F yield of variable fluorescence under steady state conditions - LHC light-harvesting complex - PQ plastoquinone - Q_A primary quinone acceptor of PSII - Q_B secondary quinone acceptor of PSII - q_P photochemical quenching - q_n non-photochemical quenching The authors like to thank Dipl. Biol. Britta Untereiser for determining the chlorophyll fluorescence quenching factors. This work was supported by grants from the Bundesminister für Forschung und Technologie, the Project Europäisches Forschungszentrum and the German Israeli Foundation in cooperation with Prof. I. Ohad, Hebrew University, Jerusalem, Israel.     

Light-induced increase in initial chlorophyll fluorescence Fo level and the reversible inactivation of PS II reaction centers in soybean leaves

With a portable PAM-2000 fluorometer it was observed that responses of initial chlorophyll fluorescence F_o level to strong light were different in various plant species examined. When the photochemical efficiency of Photosystem II, F_v/F_m, declined, F_o increased significantly in leaves of some plants such as soybean and cotton, while F_o decreased remarkably in other plants such as wheat and barley. In order to explore the mechanism of the increase in F_o in soybean leaves, the change in D1 protein amount and effects of lincomycin and far-red light on these fluorescence parameters were observed by SDS–PAGE combined with gel scanning and chlorophyll fluorescence analysis. The following results were obtained. (1) The amount of inactive PS II reaction centers increased under strong light and decreased during subsequent dark recovery [Hong and Xu (1997) Chinese Sci Bull 42(8): 684–689]. (2) No net loss of D1 protein occurred after strong light treatment. (3) Lincomycin taken up through petioles following strong light treatment had no significant effect on D1 protein level and the decay of F_o in the dark. (4) Far-red light applied after strong light treatment could largely attenuate the increase in F_o and accelerate F_o decay in the dark. Based on these results, it is deduced that the increase in F_o under strong light is mainly due to reversible inactivation of part of PS II reaction centers, rather than the net loss of D1 protein and that reversible inactivation of PS II is prevalent in some plants.     

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.