The long-wavelength limit of plant photosynthesis

It is a common knowledge that the photosynthesis efficiency drops rapidly under the long-wavelength light excitation above 680 nm. We discovered that in sunflower leaves attached to the plant the initial fall is replaced by an unexpected increase at much longer wavelengths, so that a detectable O(2) evolution is remained till 780 nm. The quantum yield of O(2) evolution at the local maximum at 745 nm reaches almost 20% of the yield at 650 nm. We conclude that extreme long-wavelength chlorophylls may be present in the intact photosystem II antenna system, similarly to photosystem I.     

Bicarbonate effects in leaf discs from spinach

In this paper, we show the unique role of bicarbonate ion in stimulating the electron transfer of photosystem II (PS II) in formate-treated leaf discs from spinach. This is referred to as the bicarbonate effect and is independent of the role of CO₂ in CO₂ fixation. It is shown to have two sites of action:

Analysis of oxygen evolution during photosynthetic induction and in multiple-turnover flashes in sunflower leaves

Exchange of CO₂ and O₂ and chlorophyll fluorescence were measured in the presence of 360 1 · 1^–1 CO₂ in nitrogen in Helianthus annuss L. leaves which had been preconditioned in the dark or at a photon flux density (PFD) of 24 mol · m^–2 · s^–1 either in 21 or 0% O₂. An initial light-dependent O₂ outburst of 6 mol · m^–2 was measured after aerobic dark incubation. It was attributed to the reduction of electron carriers, predominantly plastoquinone. The maximum initial rate of O₂ evolution at PFD 8000 mol · m^–2 · s^–1 was 170 mol · m^–2 · s^–2 or about four times the steady CO₂-and light-saturated rate of photosynthesis. Fluorescence measurements showed that the rate was still acceptor-limited. Fast O₂ evolution ceased after electron carriers were reduced in the dark-adapted leaf, but continued for a short time at the lower rate of 62 mol · m^–2 · s^–1 in the light-adapted leaf. The data are interpreted to show that enzymes involved in 3-phosphoglycerate reduction are dark-inhibited, but were fully active in low light. In a dark-adapted leaf, respiratory CO₂ evolution continued under nitrogen; it was partially inhibited by illumination. Prolonged exposure of a leaf to anaerobic conditions caused reducing equivalents to accumulate. This was shown by a slowly increasing chlorophyll fluorescence yield which indicated the reduction of the PSII acceptor Q_A in the dark. When the leaf was illuminated, no O₂ evolution was detected from short light pulses, although transient O₂ production was appreciable during longer light pulses. This indicates that an electron donor (pool size about 2–3 e/PSII reaction center) became reduced in the dark and the first photons were used to oxidise this donor instead of water.Abbreviations Chl chlorophyll - CRC carbon reduction cycle - GAPDH NADP-glyceraldehyde-phosphate dehydrogenase - PFD photon flux density - PGA 3-phosphoglycerate - RuBP ribulose bisphosphate - TCA tricarboxylic acid cycle To whom correspondence should be addressedThis work received support by the Estonian Academy of Sciences, the Gottfried-Wilhelm-Leibniz Program of the Deutsche For-schungsgemeinschaft and the Sonderforschungsbereich 251 of the University of Würzburg.     

乙草胺对葡萄叶片光合和叶绿素荧光特性及叶绿体结构的影响

以沙培1年生巨峰葡萄为材料,研究土施乙草胺对葡萄叶片光合、叶绿素荧光特性和叶绿体结构的影响.结果表明：喷施初期(处理后第13天),上部叶片净光合速率和气孔导度显著下降,PSII最大光化学效率和实际光化学效率显著低于对照,快速叶绿素荧光诱导动力学曲线中J点和K点荧光显著上升,性能指数PI_ABS显著下降,其PSII反应中心和放氧复合体受损伤程度显著高于中部叶片,但随着处理时间的延长,受损伤的程度减轻.在喷施后期(处理后第60天),上部叶片与中部叶片各指标之间的差距变小;下部叶片对除草剂的响应滞后,PSII反应中心和放氧复合体受到较大损伤,J点和K点荧光上升及PI_ABS下降的幅度高于中、上部叶片.乙草胺处理后第60天,葡萄叶片可溶性糖和淀粉含量增加,中、上部叶片色素含量显著下降,叶绿体膜受损,叶绿体变小,片层结构模糊或间隙增大.表明土施乙草胺可传导至葡萄地上部,导致叶片光合机构损伤、PSII活性下降和光合速率降低.     

Oxygen-evolving system and secondary quinonic acceptors are highly reduced in dark adapted Euglena cells: A thermoluminescence study

Characteristics of thermoluminescence glow curves were compared in three types of Euglena cells: (i) strictly autotrophic, Cramer and Myers cells; (ii) photoheterotrophic cells sampled from an exponentially growing culture containing lactate as substrate repressing the photosynthetic activity; (iii) semiautotrophic cells, sampled when the lactate being totally exhausted, the photosynthesis was enhanced.In autotrophic and semiautotrophic cells, composite curves were observed after series of two or more actinic flashes fired at –10°C, which can be deconvoluted into a large band peaking in the range 12–22°C and a smaller one near 40°C, This second band presents the characteristics of a typical B band (due to S_2/3Q_B ^- recombination), whereas the first one resembled the band, shifted by -15–20°C, which is observed in herbicide resistant plants. The amplitude of this major band, which was in all cases very low after one flash, exhibited oscillations of period four but rapidly damping, with maxima after two and six flashes. In contrast, photoheterotrophic Euglena displayed single, non-oscillating curves with maxima in the range 5–10°C.In autotrophic and semiautotrophic cells, oxidizing pretreatments by either a preillumination with one or more (up to twenty-five) flashes, or a far-red preillumination in the presence of methylviologen, followed by a short dark period, induced thermoluminescence bands almost single and shifted by +3–5°C, or +12°C, respectively. In autotrophic cells, far-red light plus methyl viologen treatment induced a band peaking at 31°C, as in isolated thylakoids from Euglena or higher plants, while it had barely any effect in photoheterotrophic cells.Due to metabolic activities in dark-adapted cells, a reduction of redox groups at the donor and acceptor sides of PS II dark-adapted cells is supposed to occur. Two different explanations can be proposed to explain such a shift in the position of the main band in dark-adapted autotrophic control. The first explanation would be that in these reducing conditions a decreasing value of the equilibrium constant for the reaction: S_nQ_A ^-Q_BS_nQ_AQ_B ^-, would determine the shift of the main TL band towards low temperatures, as observed in herbicide resistant material. The second explanation would be that the main band would correspond to peak III already observed in vivo and assigned to S_2/3Q_B ^2- recombinations.Abbreviations CM Cramer and Myers - D₁ a 32 kDa protein component of the PS II reaction center, psbA.gene product - D₂ a 34 kDa protein component of the PS II reaction center, psbD gene product - FR lar-red illumination - Lexpo and Lstat cells from lactate culture samples at exponential and stationary phase of growth - MV methylviologen - pBQ parabenzoquinone - PQ plastoquinone - PS II photosystem II - Q_A primary quinone electron acceptor - Q_B secondary quinone electron acceptor - TL thermoluminescence     

The influence of metal cations and pH on the heat sensitivity of photosynthetic oxygen evolution and chlorophyll fluorescence in spinach chloroplasts

The heat-sensitivity of photosynthetic oxygen evolution of thylakoids isolated from spinach increases by increasing the pH above neutral value. The temperature for inactivation (transition temperature) is lowered from about 45° C (pH 6.0–7.4) to 33°C (pH 8.5). Similar results are obtained with intact chloroplasts. At pH 7.0 the transition temperature of washed thylakoids decreases by lowering the salt concentration below 20 mM with monovalent cations (Li⁺, Na⁺, K⁺) and below 3–4 mM with divalent cations (Mg²⁺, Ca²⁺, Sr²⁺). Illumination decreases the heat-sensitivity of oxygen evolution in intact chloroplasts, but even increases the heat-sensitivity in uncoupled chloroplasts. In intact chloroplasts the transition temperature of the heat-induced rise in chlorophyll fluorescence yield (F_o; see Schreiber and Armond 1978) decreases from 44° C to 38° C when the pH of the suspending medium is increased from 6.5 to 8.5. At 20° C, F_o is almost insensitive to pH (6.0–8.5). At 40° C, however, F_o is constant between 6.0 and 7.0, but strongly increases by increasing the pH above neutral value. The results are discussed in terms of a close relation between electrostatic forces at the thylakoid membrane and thermal sensitivity of photosynthetic apparatus. It is suggested that the heat-sensitivity of the photosystem II complex partially depends on the ionization state of fixed groups having alkaline pK. The packed volume of thylakoids suspended in a low salt medium increases when the temperature is increased above 30° C (pH 7.0) and above 20° C (pH 8.0), respectively. This result suggests a heat-induced increase in surface charge density of the thylakoid membrane.Abbreviations HEPES N-2-hydroxyethylpiperazine-N-2-ethane sulfonic acid - MES morpholinoethane sulfonic acid - MOPS 2-N-morpholinopropane sulfonic acid - TRICIN N-[tris(hydroxymethyl)-methyl] glycine     

Changes in Photosystem II fluorescence in Chlamydomonas reinhardtii exposed to increasing levels of irradiance in relationship to the photosynthetic response to light

The effects of a 60 min exposure to photosynthetic photon flux densities ranging from 300 to 2200 mol m^–2s^–1 on the photosynthetic light response curve and on PS II heterogeneity as reflected in chlorophyll a fluorescence were investigated using the unicellular green alga Chlamydomonas reinhardtii. It was established that exposure to high light acts at three different regulatory or inhibitory levels; 1) regulation occurs from 300 to 780 mol m^–2s^–1 where total amount of PS II centers and the shape of the light response curve is not significantly changed, 2) a first photoinhibitory range above 780 up to 1600 mol m^–2s^–1 where a progressive inhibition of the quantum yield and the rate of bending (convexity) of the light response curve can be related to the loss of Q_B-reducing centers and 3) a second photoinhibitory range above 1600 mol m^–2s^–1 where the rate of light saturated photosynthesis also decreases and convexity reaches zero. This was related to a particularly large decrease in PS II centers and a large increase in spill-over in energy to PS I.Abbreviations Chl chlorophyll - DCMU 3,(3,4-dichlorophenyl)-1,1-dimethylurea - F_M maximal fluorescence yield - F_pl intermediate fluorescence yield plateau level - F₀ non-variable fluorescence yield - F_v total variable fluorescence yield (F_M-F₀) - initial slope to the light response curve, used as an estimate of initial quantum yield - convexity (rate of bending) of the light response curve of photosynthesis - LHC light-harvesting complex - P_max maximum rate of photosynthesis - PQ plastoquinone - Q photosynthetically active photon flux density (400–700 nm, mol m^–2s^–1) - PS photosystem - Q_A and Q_B primary and secondary quinone electron acceptor of PS II     

On the relationship between the quantum yield of Photosystem II electron transport,as determined by chlorophyll fluorescence and the quantum yield of CO2-dependent O2 evolution

We tested the two empirical models of the relationship between chlorophyll fluorescence and photosynthesis, previously published by Weis E and Berry JA 1987 (Biochim Biophys Acta 894: 198–208) and Genty B et al. 1989 (Biochim Biophys Acta 990: 87–92). These were applied to data from different species representing different states of light acclimation, to species with C₃ or C₄ photosynthesis, and to wild-type and a chlorophyll b-less chlorina mutant of barley. Photosynthesis measured as CO₂-saturated O₂ evolution and modulated fluorescence were simultaneously monitored over a range of photon flux densities. The quantum yields of O₂ evolution (ØO₂) were based on absorbed photons, and the fluorescence parameters for photochemical (q_p) and non-photochemical (q_N) quenching, as well as the ratio of variable fluorescence to maximum fluorescence during steady-state illumination (F'_v/F'_m), were determined. In accordance with the Weis and Berry model, most plants studied exhibited an approximately linear relationship between ØO₂/q_p (i.e., the yield of O₂ evolution by open Photosystem II reaction centres) and q_N, except for wild-type barley that showed a non-linear relationship. In contrast to the linear relationship reported by Genty et al. for q_p×F'_v/F'_m (i.e., the quantum yield of Photosystem II electron transport) and ØCO₂, we found a non-linear relationship between q_p×F'_v/F'_m and ØO₂ for all plants, except for the chlorina mutant of barley, which showed a largely linear relationship. The curvilinearity of wild-type barley deviated somewhat from that of other species tested. The non-linear part of the relationship was confined to low, limiting photon flux densities, whereas at higher light levels the relationship was linear. Photoinhibition did not change the overall shape of the relationship between q_p×F'_v/F'_m and ØO₂ except that the maximum values of the quantum yields of Photosystem II electron transport and photosynthetic O₂ evolution decreased in proportion to the degree of photoinhibition. This implies that the quantum yield of Photosystem II electron transport under high light conditions may be similar for photoinhibited and non-inhibited plants. Based on our experimental results and theoretical analyses of photochemical and non-photochemical fluoresce quenching processes, we conclude that both models, although not universal for all plants, provide useful means for the prediction of photosynthesis from fluorescence parameters. However, we also discuss that conditions which alter one or more of the rate constants that determine the various fluorescence parameters, as well as differential light penetration in assays for oxygen evolution and fluorescence emission, may have direct effect on the relationships of the two models.Abbreviations F₀ and F'₀ fluorescence when all Photosystem II reaction centres are open in dark- and light-acclimated leaves, respectively - F_m and F'_m fluorescence when all Photosystem II reaction centres are closed in dark and light, respectively - F_v variable fluorescence equal to F_m-F₀ - F_s steady state level of fluorescence in light - F'_v and F'_m variable (F'_m-F'₀) and maximum fluorescence under steady state light conditions - HEPES N-2-hydroxyethylpiperazine-N-2-ethane-sulphonic acid - Q_A the primary, stabile quinone acceptor of Photosystem II - q_N non-photochemical quenching of fluorescence - q_p photochemical quenching of fluorescence - ØO₂ quantum yield of CO₂-saturated O₂ evolution based on absorbed photons     

The quenching characteristics of potassium iridic chloride and their meaning for the origin of chlorophyll fluorescence components

To understand the origins of the different lifetime components of photosystem 2 (PS2) chlorophyll (Chl) fluorescence we have studied their susceptibility to potassium iridic chloride (K₂IrCl₆) which has been shown to bleach antenna pigments of photosynthetic bacteria (Loach et al. 1963). The addition of K₂IrCl₆ to PS2 particles gives rise to a preferential quenching of the variable Chl fluorescence (F_v). At concentrations lower than 20 M, this is brought about mainly by a decrease in the yield, but not in the lifetime, of the slowest component when all the PS2 reaction centres are closed (F_M). The yield of the middle and fast decays are not significantly altered. This type of quenching is not seen with DNB. The iridate-induced quenching of the initial fluorescence level (F₀) is due to a proportional decrease in the yield and lifetime of the three components and correlates with the observed modification in the relative quantum yield of oxygen evolution. In this concentration range a bleaching of Chl a is seen. At higher iridate levels, greater than 20 M, a proportional decrease in the lifetimes and yields of the three kinetic components is seen at F_M. These changes are associated with a carotenoid bleaching. In isolated light harvesting Chl a/b complexes of PS2 (LHC2), iridate addition converts a 4 ns decay into a 200 ps emission and both types of bleaching are observed. By also measuring the rate of PS2 trap closure versus iridate concentration, we have discussed the results in terms of excitation energy transfer.Abbreviations DNB m-dinitrobenzene - F_M maximum Chl fluorescence - F₀ initial fluorescence - F_v variable fluorescence - I pheophytin a primary electron acceptor of PS2 - P₆₈₀ chlorophyll a of photochemical centre - PS2 photosystem 2 - Q_A primary stable electron acceptor of PS2 - Chl chlorophyll - LHC2 light harvesting Chl a/b complex of PS2 - MES 2(N-morpholino) ethanesulfonic acid - DCMU 3-(3-4-dichlorophenyl) 1-1 dimethylurea - PPBQ phenyl-p-benzo-quinone - BBY PS2-enriched membranes prepared as in Berthold et al. (1981) - Q₄₀₀ PS2 electron acceptor with a midpoint potential of 400 mV     

Reduction of QA in the dark: Another cause of fluorescence Fo increases by high temperatures in higher plants

Increases in the chlorophyll fluorescence F_o (dark level fluorescence) during heat treatments were studied in various higher plants. Besides the dissociation of light-harvesting chlorophyll a/b protein complexes from the reaction center complex of PS II and inactivation of PS II, dark reduction of Q_A via plastoquinone (PQ) seemed to be related to the F_o increase at high temperatures. In potato leaves or green tobacco cultured cells, a part of the F_o increase was quenched by light, reflecting light-induced oxidation of Q_A ^- which had been reduced in the dark at high temperatures. Appearance of the F_o increase due to Q_A reduction depended on the plant species, and the mechanisms for this are proposed. The reductants seemed to be already present and formed by very brief illumination of the leaves at high temperatures. A ndhB-less mutant of tobacco showed that complex I type NAD(P)H dehydrogenase is not involved in the heat-induced reduction of Q_A. Quite strong inhibition of the Q_A reduction by diphenyleneiodonium suggests that a flavoenzyme is one of the electron mediator to PQ from the reductant in the stroma. Reversibility of the heat-induced Q_A reduction suggests that an enzyme(s) involved is activated at high temperatures and mostly returns to an inactive form at room temperature (25 °C).This revised version was published online in October 2005 with corrections to the Cover Date.     

Secondary fluorescence kinetics of spinach leaves in relation to the onset of photosynthetic carbon assimilation

When spinach leaves are re-illuminated, after dark periods of 90 s or less, an initial fluorescence peak is observed which rapidly gives way to a much lower terminal value. After 2 min or more in the dark, however, there is a secondary rise, at about 50–70 s, which then gives way, more slowly, to approximately the same low terminal value as before. The secondary rise is eliminated or disguised by feeding D,L-glyceraldehyde (a specific inhibitor of photosynthetic carbon assimilation) and by manose, 2-deoxyglucose and glucosamine, all of which are believed to sequester cytoplasmic orthophosphate. This secondary rise in fluorescence is discussed in relation to photosynthetic induction and the manner in which these compounds may modulate fluorescence by their effect on the availability of orthophosphate and their consequent impact on the adenylate status of the stroma.Abbreviations DCMU 3(3,4-dichlorophenyl)-1,1-dimethylurea - CCCP carbonylcyanidchlorophenylhydrazon     

Observation and characterisation of a transient in the yield of chlorophyll fluorescence in intact spinach chloroplasts

A transient in chlorophyll fluorescence, which is associated with a transient in 9-aminoacridine fluorescence and a perturbation in the rate of oxygen evolution, has been observed in intact spinach chloroplasts. The results indicate that changes in the redox state of Q are, at least partially, responsible for the transient in chlorophyll fluorescence. The size of the transient is highly dependent upon the concentration of inorganic phosphate and upon the pH of the medium. The properties of the transient are consistent with the suggestion that it reflects changes in the levels of stromal intermediates during induction.Abbreviations BES NN-Bis(2-hydroxyethyl)2-aminoethanesulphonic acid dihydroxyacetone-P(DHAP): dihydroxyacetone phosphate glycerate-3-P (PGA): glycerate-3-phosphate - HEPES N-2-Hydroxyethylpiperazine-N-2-ethanesulphonic acid - MES 2-(N-Morpholino)ethanesulphonic acid - Pi inorganic phosphate - qE quenching of chlorophyll fluorescence by the energisation of the thylakoid membrane - qQ quenching of chlorophyll fluorescence by oxidised Q, the electron acceptor of photosystem 2 - ribose-5-P (R5P) ribose-5-phosphate - Rbu-5-P ribulose-5-phosphate     

Effects of dehydration on the electron transport of Chlorella. An in vivo fluorescence study

Chlorella was used to study the effects of dehydration on photosynthetic activities. The use of unicellular green algae assured that the extent of dehydration was uniform throughout the whole cell population during the course of desiccation. Changes in the activities of the cells were monitored by measurements of fluorescence induction kinetics. It was found that inhibition of most of the photosynthetic activities started at a similar level of cellular water content. They included CO₂ fixation, photochemical activity of Photosystem II and electron transport through Photosystem I. The blockage of electron flow through Photosystem I was complete and the whole transition occurred within a relative short time of dehydration. On the other hand, the suppression of Photosystem II activity was incomplete and the transition took a longer time of dehydration. Upon rehydration, the inhibition of Photosystem II activity was fully reversible when samples were in the middle of the transition, but was not thereafter. The electron transport through Photosystem I was also reversible during the transition, but was only partially afterward.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl urea - F_m maximum fluorescence yield - F₀ non-variable fluorescence level emitted when all PS II centers are open - F_v variable part of fluorescence - PS photosystem - Q_A primary quinone acceptor of Photosystem II     

Regulation of photosynthetic electron-transport in Phaseolus vulgaris L., as determined by room-temperature chlorophyll a fluorescence

The regulation of photosystem II (PSII) by light-, CO₂-, and O₂-dependent changes in the capacity for carbon metabolism was studied. Estimates of the rate of electron transport through PSII were made from gas-exchange data and from measurements of chlorophyll fluorescence. At subsaturating photon-flux density (PFD), the rate of electron transport was independent of O₂ and CO₂. Feedback on electron transport was observed under two conditions. At saturating PFD and low partial pressure of CO₂, p(CO₂), the rate of electron transport increased with p(CO₂). However, at high p(CO₂), switching from normal to low p(O₂) did not affect the net rate of photosynthetic CO₂ assimilation but the rate of electron-transport decreased by an amount related to the change in the rate of photorespiration. We interpret these effects as 1) regulation of ribulose-1,5-bisphosphatecarboxylase (RuBPCase, EC 4.1.1.39) activity to match the rate of electron transport at limiting PFD, 2) regulation of electron-transport rate to match the rate of RuBPCase at low p(CO₂), and 3) regulation of the electron-transport rate to match the capacity for starch and sucrose synthesis at high p(CO₂) and PFD. These studies provide evidence that PSII is regulated so that the capacity for electron transport is matched to the capacity for other processes required by photosynthesis, such as ribulose-bisphosphate carboxylation and starch and sucrose synthesis. We show that at least two mechanisms contribute to the regulation of PSII activity and that the relative engagement of these mechanisms varies with time following a step change in the capacity for ribulose-bisphosphate carboxylation and starch and sucrose synthesis. Finally, we take advantage of the relatively slow activation of deactivated RuBPCase in vivo to show that the activation level of this enzyme can limit the rate of electron transport as evidenced by increased feedback on PSII following a step change in p(CO₂). As RuBPCase as activated, the feedback on PSII declined.Abbreviations and symbols J_C electron-transport rate calculated from CO₂-assimilation measurements - J_F electron-transport rate calculated from fluorescence parameters - PFD photon-flux density - q_E energy-dependent quenching - PSII photosystem II - q_Q Q-dependent quenching - QY quantum yield - RuBPCase ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) C.I.W. publication No. 1015     

Computer-controlled pulse modulation system for analysis of photoacoustic signals in the time domain

A newly developed photoacoustic system for measurement of photosynthetic reactions in intact leaves is described. The system is based on pulsed light-emitting diodes, the pulse program and pulse response analysis being computer controlled. Separation of various components in the overall photoacoustic signal is achieved by curve fitting analysis of the responses following individual measuring light pulses in the millisecond time domain. This procedure is in distinction to the conventionally used analysis in the frequency domain, with the advantage that various signal components are obtained by on-line deconvolution, yielding simultaneous recordings of photothermal (complement of energy storage) and photobaric (evolution and uptake) signals. The basic components of the new system are described by block diagrams and the principal steps for deconvolution of the overall photoacoustic response are outlined. An example of application with simultaneous recording of chlorophyll fluorescence is given. It is apparent that the photobaric uptake component represents a significant part of the overall signal, particularly during induction of photosynthesis after dark-adaptation. This component probably contains not only O₂-uptake but uptake of CO₂ as well.Abbreviations PA photoacoustic - LED light-emitting-diode - RAM random access memory     

Effects of drought on photosynthetic characteristics of flag leaves of a newly-developed super-high-yield rice hybrid

We investigated responses of chloroplasts from flag leaves of a newly-developed super-high-yield rice (Oryza sativa L.) hybrid LiangYouPeiJiu (LYPJ) to water stress (withholding irrigation) during the grain-filling period. In the early stage of water stress (0–6 d) only the activity of Hill reaction was inhibited, whereas activities of photophosphorylation and Ca²⁺-ATPase, and ATP content were increased and peaked in the day 6 of withholding irrigation. In the late stage of water stress (6-12 d), the activities of photosynthetic O₂ evolution, Hill reaction, photophosphorylation, and Ca²⁺- ATPase, and ATP content were significantly reduced. The membrane lipid content was sharply decreased, especially of sulfoquinovosyl-diacylglycerol (SQDG) and phosphatidylglycerol (PG). The changes in the ultrastructure of chloroplasts included mainly a decrease in number of grana and increase in number of osmiophilic granules.This revised version was published online in March 2005 with corrections to the page numbers.     

Increases in the fluorescence Fo level and reversible inhibition of Photosystem II reaction center by high-temperature treatments in higher plants

Effects of high temperatures on the fluorescence F_m (maximum fluorescence) and F_o (dark level fluorescence) levels were studied and compared with those of the photochemical reactions of PS II. These comparisons were performed during and after the high temperature treatments. The following results were obtained; (1) increases in the F_o level at high temperatures were partly reversible, (2) the F_m level in the presence of dithionite in spinach chloroplasts decreased at high temperatures and also showed a partial reversibility, (3) photoreductions of pheophytin a and Q_a were reversibly inhibited at high temperatures parallel to the decrease in the difference between the F_m and F_o levels, and (4) the decrease in the fluorescence F_m level seemed to be related to denaturation of chlorophyll-proteins. All the data suggested that, as well as the separation of light-harvesting chlorophyll a/t b protein complexes of PS II from the PS II core complexes, partly reversible inactivation of the PS II reaction center at high temperatures is the cause of the increase in the F_o level.     

Mechanism of nano-anatase TiO2 on promoting photosynthetic carbon reaction of spinach

Having a photocatalyzed characteristic, our previous research had proved that nano-anatase TiO₂ is closely related to the photosynthesis of spinach. It could not only improve the light absorbance and the transformation from light energy to electron energy and to active chemical energy but also promote carbon dioxide (CO₂) assimilation of spinach. However, the mechanism of carbon reaction promoted by nano-anatase TiO₂ remains largely unclear. By electrophoresis and Western blot methods, the results of the experiments proved that Rubisco from the nano-anatase TiO₂-treated spinach during the extraction procedure of Rubisco was found to consist of Rubisco and a heavier molecular-mass protein (about 1200 kDa) comprising both Rubisco and Rubisco activase. The Rubisco carboxylase activity was 2.67 times that of Rubisco from the control and it could hydrolyze ATP in the same manner as Rubisco activase. The total sulfhydryl groups and available sulfhydryl groups of the Rubisco were 32-SH and 21-SH per mole of enzyme more than those of the Rubisco purified from the control, respectively. The circular dichroism spectra showed that the secondary structure of Rubisco from the nano-anatase TiO₂-treated spinach was very different from Rubisco of the control. It suggested that the mechanism of nano-anatase TiO₂ activating Rubisco of spinach was that the complex of Rubsico and Rubisco activase was induced in spinach, which promoted Rubsico carboxylation and increased the rate of photosynthetic carbon reaction.     

Drought-induced changes in chlorophyll fluorescence,photosynthetic pigments,and thylakoid membrane proteins of Vigna radiata

The effects of drought on chlorophyll fluorescence characteristics of PSII, photosynthetic pigments, thylakoid membrane protein (D1), and proline content in different varieties of mung bean plants were studied. Drought stress inhibits PSII activity and induces alterations in D1 protein. We observed a greater decline in the effective quantum yield of PSII, electron transport rate, and saturating photosynthetically active photon flux density (PPFD_sat) under drought stress in var. Anand than var. K-851 and var. RMG 268. This may possibly be due to either downregulation of photosynthesis or photoinhibition process. Withholding irrigation resulted in gradual diminution in total Chl content at Day 4 of stress. HPLC analysis revealed that the quantity of β-carotene in stressed plants was always higher reaching maxima at Day 4. Photoinactivation of PSII in var. Anand includes the loss of the D1 protein, probably from greater photosynthetic damage caused by drought stress than the other two varieties.     

Effect of nano-TiO2 on strength of naturally aged seeds and growth of spinach

The effects of nano-TiO₂ (rutile) and non-nano-TiO₂ on the germination and growth of naturally aged spinach seeds were studied by measuring the germination rate and the germination and vigor indexes of aged spinach seeds. An increase of these factors was observed at 0.25–4‰ nano-TiO₂ treatment. During the growth stage, the plant dry weight was increased, as was the chlorophyll formation, the ribulosebisphosphate carboxylase/oxygenase activity, and the photosynthetic rate. The best results were found at 2.5‰ nano-TiO₂. The effects of non-nano-TiO₂ are not significant. It is shown that the physiological effects are related to the nanometer-size particles, but the mechanism by which nano-TiO₂ improves the growth of spinach seeds still needs further study.