Heat shock increases thermotolerance of photosynthetic electron transport and the content of chloroplast membranes and lipids in wheat leaves

Preliminary heating of 15-16-day-old wheat (Triticum aestivum L.) plants for 3 h at 37–38°C (heat shock, HS) increased the tolerance of photosynthetic electron transport (determined as the reduction of 2,6-dichlorophenol indophenol by isolated chloroplasts) toward heating of leaves at 42–48°C in high light (100 klx). At the same time, HS did not affect the activity of the xanthophyll cycle reactions in the 30–48°C temperature range. HS exposure induced an increase in the thylakoid length, the number of grana, and the average number of thylakoids per granum. The volume of the thylakoid system increased 1.4-fold. Such indices as the total content of chlorophylls (a + b), the chlorophyll a/b ratio, as well as the contents of individual carotenoids, chloroplast membrane proteins, and the soluble leaf proteins remained unchanged. The de novo photosynthetic membrane formation was accompanied by the 1.5-fold increase in major chloroplast lipids. It was concluded that, in mature wheat chloroplasts, HS induced the formation of thylakoids characterized by a changed molecular structure and by increased lipid/protein and lipid/chlorophyll ratios.     

Effects of ozone on the photosynthetic apparatus and leaf proteins during leaf development in wheat

Leaves of Triticum aestivum cv. Avalon were grown in an atmosphere that contained 150 nmole mol^-1 ozone for 7h each day. After leaves had reached maximum size, the leaf blade was divided into three sections to provide tissue of different age, the youngest at the base of the blade and the oldest at the leaf tip. The ozone treatment was found to decrease significantly the light-saturated rate and quantum yield of CO₂ assimilation and the maximum quantum yield of photosystem II photochemistry in the oldest leaf section. No effects were found on the basal and middle sections of the leaf. These ozone-induced decreases in the photosynthetic parameters were associated with decreases in the efficiency of utilization of light for CO₂ assimilation at the photon flux density under which the leaves were grown. The depression in photosynthetic performance of tissue near the leaf tip was accompanied by large decreases in the contents of total, soluble and thylakoid proteins and chlorophyll. There was also found to be a preferential loss of ribulose-1,5-carboxylase-oxygenase. These ozone-induced changes in chlorophyll and protein contents and the photosynthetic activities of the leaf tissue were similar to changes normally associated with leaf senescence. Two-dimensional polyacrylamide gel analyses of leaf proteins demonstrated the loss of some minor, and unidentified, proteins, whilst another group of minor proteins appeared. It is concluded that daily exposure of the leaf to 150 nmol mol^-1 ozone for 7h had no effect on the development of the photosynthetic apparatus and its activities during leaf expansion, but it did promote the onset of premature senescence in fully expanded tissue that resulted in a loss of pigments, proteins and photosynthetic capacity and efficiency.     

Quantifying the dynamics of light tolerance in Arabidopsis plants during ontogenesis

The amount of light plants can tolerate during different phases of ontogenesis remains largely unknown. This was addressed here employing a novel methodology that uses the coefficient of photochemical quenching (qP) to assess the intactness of photosystem II reaction centres. Fluorescence quenching coefficients, total chlorophyll content and concentration of anthocyanins were determined weekly during the juvenile, adult, reproductive and senescent phases of plant ontogenesis. This enabled quantification of the protective effectiveness of non‐photochemical fluorescence quenching (NPQ) and determination of light tolerance. The light intensity that caused photoinhibition in 50% of leaf population increased from ～70 μmol m^?2 s^?1, for 1‐week‐old seedlings, to a maximum of 1385 μmol m^?2 s^?1 for 8‐week‐old plants. After 8 weeks, the tolerated light intensity started to gradually decline, becoming only 332 μmol m^?2 s^?1 for 13‐week‐old plants. The dependency of light tolerance on plant age was well‐related to the amplitude of protective NPQ (pNPQ) and the electron transport rates (ETRs). Light tolerance did not, however, show a similar trend to chlorophyll a/b ratios and content of anthocyanins. Our data suggest that pNPQ is crucial in defining the capability of high light tolerance by Arabidopsis plants during ontogenesis.     

The role of thylakoid lipids in the photodamage of photosynthetic activity

The effect of excess light at 10 or 30°C under aerobic or low O₂ condition on peroxidation of thylakoid lipids and primary photochemistry of photoinsynthesis was studied in wheat ( Triticum aestivum L. cv. HD 2329). Photoinhibitory damage to photosythesis was directly proportional to the peroxidation of thylakoid lipids. Photoinhibitory treatment given under low O₂ conditions resulted in significantly less peroxidation of the primary photochemistry of photosythesis measured using chlorophyll fluorescence and photosythetic electron trasport. Short term recovery of F_v/F_m ratio was fast while thylakoid lipids did not show much recovry. However, recovery (of F_v/F_m ratio and thylakoid lipids) was almost complete within 12 h after photoinhibition treatment. A possible relationship between peroxidation of thylakoid lipids and photodamage to photosynthesis is discussed.     

小麦小孢子发生过程的超微结构

采用透镜电镜技术对小麦（Ｔｒｉｔｉｃｕｍ ａｅｓｔｉｖｕｍ）小孢子发生过程进行了超微结构观察。造孢细胞时期细胞质中含有丰富的核糖体,质体和线粒体。在小孢子母细胞减数分裂过程中,有核糖体数量存在逐渐减少的现象,质体和线袜体结构在双线期／终变期和中期１简化,二分体时期,质体和线粒体形态结构基本恢复正常。结果表明,啵麦小孢子母细胞减数分裂过程中存在质体和线粒体的脱人化与再分化过程,但该过程与核糖体数量增     

Mode of high temperature injury to wheat during grain development

High temperature stress adversely affects wheat growth in many important production regions, but the mode of injury is unclear. Wheat ( Triticum aestivum L. cv. Newton) was grown under controlled conditions to determine the relative magnitude and sequences of responses of source and sink processes to high temperature stress during grain development. Regimes of 25°C day/15°C night, 30°C day/20°C night, and 35°C day/25°C night from 5 days after anthesis to maturity differentially affected source and sink processes. High temperatures accelerated the normal decline in viable leaf blade area and photosynthetic activities per unit leaf area. Electron transport, as measured by Hill reaction activity, declined earlier and faster than other photosynthetic processes at the optimum temperature of 25/15 °C and at elevated temperatures. Changes in RUBP carboxylase activities were similar in direction but smaller in magnitude than changes in photosynthesic rate. Increased protease activity during senscence was markedly accentuated by high temperature stress. Specific protease activity increased 4-fold at 25/15 °C and 28-fold at 35/25 °C from 0 to 21 days after initiation of temperature treatments. Grain-filling rate decreased from the lowest to the highest temperature, but the change was smaller than the decrease in grain-filling duration at the same temperatures. We concluded that a major effect of high temperature is acceleration of senescence, including cessation of vegetative and reproductive growth, deterioration of photosynthetic activities, and degradation of proteinaceous constituents.     

Mechanisms shaping the synergism of zeaxanthin and PsbS in photoprotective energy dissipation in the photosynthetic apparatus of plants

Safe operation of photosynthesis is vital to plants and is ensured by the activity of processes protecting chloroplasts against photo-damage. The harmless dissipation of excess excitation energy is considered to be the primary photoprotective mechanism and is most effective in the combined presence of PsbS protein and zeaxanthin, a xanthophyll accumulated in strong light as a result of the xanthophyll cycle. Here we address the problem of specific molecular mechanisms underlying the synergistic effect of zeaxanthin and PsbS. The experiments were conducted with Arabidopsis thaliana, using wild-type plants, mutants lacking PsbS (npq4), and mutants affected in the xanthophyll cycle (npq1), with the application of molecular spectroscopy and imaging techniques. The results lead to the conclusion that PsbS interferes with the formation of densely packed aggregates of thylakoid membrane proteins, thus allowing easy exchange and incorporation of xanthophyll cycle pigments into such structures. It was found that xanthophylls trapped within supramolecular structures, most likely in the interfacial protein region, determine their photophysical properties. The structures formed in the presence of violaxanthin are characterized by minimized dissipation of excitation energy. In contrast, the structures formed in the presence of zeaxanthin show enhanced excitation quenching, thus protecting the system against photo-damage.     

Short-term effects of aluminium at alkaline pH on the structure and function of the photosynthetic apparatus

A 24 h exposure of the salt-tolerant grass Thinopyrum bessarabicum (Savul. and Rayss) A. Love seedlings to 1 mM aluminium (Al) in nutrient solution at pH of 9.0 resulted in a significant reduction of the biomass. In control samples the mesophyll chloroplasts exhibited the usual lens shape with most grana arranged in straight or slightly curving lines, and only 6.5 % of the grana were out of order. In Al-treated plants the mesophyll chloroplasts displayed a slightly distorted shape and distended size with most grana arranged in bow-like lines, while in the central region of the organelle as many as 26.7 % of the grana were independent and out of order in relation to the long axis. The morphological changes in the chloroplast shape and grana arrangement were probably due to swelling and distension of the chloroplasts in consequence to the altered membrane permeability. The initial in vivo chlorophyll (Chl) fluorescence FO, as well as the intermediate FI and peak fluorescence FP were increased under the Al stress: this indicated a destruction of photosystem (PS) 2 reaction centres and increased reduction of QA. The (FI-FO)/(FP-FO) ratio exhibited a significant increase indicating higher proportion of PS2 centres unable to reduce QB. Changes in the chloroplast ultrastructure seemed to be the reason of photosynthetic electron transport inhibition. Yet all these changes in the photosynthetic performance and chloroplast ultrastructure were considered as indirect effects of Al treatment since Al concentration in the leaves was undetectable. Disturbances in the chloroplast ultrastructure could be caused by a reduced uptake and/or transport of other nutrients. This revised version was published online in June 2006 with corrections to the Cover Date.     

Modifications to the photosynthetic apparatus of higher plants in response to changes in the light environment

A brief review of the photosynthetic apparatus of higher plants is given, followed by a consideration of the modifications induced in this apparatus by changes in light intensity and light quality. Possible strategies by which plants may optimize photosynthetic activity by both long- and short-term modifications of their photosynthetic apparatus in response to changing light regimes are discussed.     

The kinetics of photoinhibition of the photosynthetic apparatus in pea chloroplasts

Abstract The kinetics of a range of chlorophyll fluorescence parameters, non-cyclic electron transport and the capacity of the thylakoids to bind Atrazine were examined during photoinhibition treatment of intact pea chloroplasts. Parameters of fluorescence induction of chloroplasts in the presence and absence of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea at 20 °C and at 77 K were determined. The contributions of photochemical and non-photochemical quenching processes to the loss of fluorescence during photoinhibitory treatment were assessed. Two distinct phases of photoinhibitory damage were observed. During the initial 5 min period of exposure to light the minimal fluorescence level (F_o) increased, whilst the maximal fluorescence level (F_P) decreased, both coupled and uncoupled non-cyclic electron transport to methyl viologen decreased and the ability to bind Atrazine to the thylakoids decreased. Fluorescence analyses demonstrated that during this period thylakoids were becoming increasingly less efficient at generating and maintaining a transmembrane proton electrochemical gradient. Photoinhibitory damage that occurred at later times between 5 and 20 min was of a very different nature. Both F_o and F_P declined, a loss of coupled and uncoupled non-cyclic electron transport was observed together with a loss of the capacity to photo-oxidize water. However, no further loss of Atrazine-binding was associated with such changes. A consistent decrease in the quantum yield of non-cyclic electron transport was also observed throughout photoinhibition treatment. The possibility of two distinct mechanisms of photoinhibitory damage to the photosynthetic apparatus is discussed.     

Electron transport and chloroplast ultrastructure of a chlorophyll deficient mutant of wheat

A non-lethal chlorophyll deficient mutation was induced by use of the chemical mutagen ethyl methanesulfonate. Chloroplasts from the control and mutant plants were found to be very similar ultrastructurally. Thylakoid membrane volume was only slightly greater in plastids from the control as compared with plastids from the mutant. The chlorophyll content of the mutant was reduced by over 60%. This decrease in chlorophyll was not accompanied by a similar decrease in electron transport. Uncoupled electron transport rate based on a unit chlorophyll basis was nearly twice as great for mutant chloroplasts as for control plastids. However, electron transport rate based on a unit membrane volume was similar in mutant and control plants. At high irradiance the relative quantum requirement of the control and mutant was similar when expressed on membrane volume.     

Excitation energy flow at 77 K in the photosynthetic apparatus of overwintering evergreens

The flow of excitation energy from the antennae to photosynthetic reaction centre complexes at 77 K was studied in leaves of two evergreen species, namely, snow gum (Eucalyptus pauciflora Sieb. ex Spreng.) and a hemiparasitic mistletoe (Amyema miquelii, Lehm. ex Miq.). The leaves that were naturally acclimated to winter conditions of freezing temperatures and high irradiance displayed the recently discovered cold‐hard‐band or CHB feature of the chlorophyll a fluorescence spectra (Gilmore & Ball, Proc. Nat. Acad. Sci. USA 97:11098–11101, 2000). A streak‐camera‐spectrograph was used and the double convolution integral method for global analysis was applied to simultaneously acquire and simulate, respectively, the time‐ and wavelength‐dependence of all major chlorophyll a components (Gilmore et al. Phil. Trans. Roy. Soc. B‐London 355:1371–1384, 2000). The CHB coincided with changed amplitudes and decreased excited state lifetimes for the main F685 nm and F695 nm emission bands from the photosystem II (PSII) core‐inner‐antenna. The CHB dissipates energy as heat separate from PSII while also reducing the PSII quantum yield by competing for both photon absorption and antenna excitation. The CHB did not correlate with changes in the decay kinetics of the PSI antenna F740 nm band. The spectral‐kinetic features of the altered energy flow were similar in the unrelated evergreen species. These results are consistent with a functional association between the CHB, PSII energy dissipation and protective storage of chlorophyll in overwintering evergreens.     

Response of the photosynthetic apparatus in maize leaves grown at low temperature on transfer to normal growth temperature

Low temperatures are known to restrict chloroplast development and prevent the attainment of photosynthetic competence in maize leaves. The responses of the photosynthetic apparatus of mature maize leaves grown at 14°C on transfer of the plants to 25°C are examined. The synthesis of thylakoid proteins increased immediately on transfer of leaves from 14 to 25°C, with a dramatic accumulation of thylakoid proteins and chlorophylls occurring after 3 d at 25°C. Thylakoid structure and organization also became similar to those observed in leaves grown at 25°C over this period. However, no comparable development of photosynthetic competence in photosystems I and II or in the rate of CO₂ assimilation was observed on transfer of leaves from 14 to 25°C. Immunocytological analyses demonstrated heterogeneity in the distribution of a range of thylakoid proteins (cy tochrome f, the α and β subunits of the coupling factor, Dl of the photosytem II reaction centre, the 33kDa protein of the extrinsic oxygen-evolving complex of photosystem II, and subunit II of photosystem I between mesophyll cells in leaves grown at 14°C, and in the responses of individual proteins to transfer of the leaves to 25°C. Such heterogeneity between mcsophyll cells would account for the inability of the leaves to develop the expected degree of photosynthetic competence on transfer to 25°C. The effects of low growth temperatures on chloroplast biogenesis are complex, as are the changes induced by the transfer ofleaves grown at low temperatures to optimal growth temperature, and both these factors may limit the canopy development and photosynthetic productivity of crops in temperate regions.     

Adaptation of the photosynthetic apparatus of Nitellopsis obtusa to changing light intensity at the molecular level: different pathways of a singlet excitation quenching

The effect of prolonged illumination (60 min) with photosynthetically active monochromatic radiation of low intensity (3 μmol m⁻² s⁻¹) and high intensity (60 μmol m⁻² s⁻¹), corresponding to the physiological conditions and light stress conditions, respectively, was studied in the algae Nitellopsis obtusa. Illumination of Nitellopsis obtusa cells with strong light was associated with activation of the xanthophyll cycle, manifested by the deepoxidation of violaxanthin and accumulation of antheraxanthin and zeaxanthin. At the same time, the efficient singlet excitation quenching in the photosynthetic apparatus was activated, as demonstrated by the decrease in the intensity of the chlorophyll a fluorescence emission by ca 50 %. The difference of the fluorescence excitation spectra recorded before and after the light treatment match the difference absorption spectrum of the xanthophyll cycle pigments. The illumination with low light intensity resulted also in the chlorophyll a fluorescence quenching but the effect was very small (less than 10 %). The fluorescence quenching is interpreted in terms of the energy transfer between the Q_y energy level of chlorophyll a and the 2¹ A_g ⁻ energy level of zeaxanthin. The singlet energy levels of carotenoids, corresponding to the green spectral region, are also taken into consideration in the interpretation of the excitation energy exchange between the carotenoids and chlorophylls. Possible molecular mechanisms involved in the activation of the strong and the weak excitation quenching, including violaxanthin isomerization, and possible physiological functions of such pathways of energy transfer are discussed.     

Effects of water deficit on photosynthetic capacity

Under drought, CO₂ assimilation rates decrease already at small leaf water deficits. At least part of the inhibition is attributed to non-stomata1 effects at the chloroplast level, with electron transport and phosphorylation being main targets of inhibition. These findings are questioned by direct measurements of photosynthetic capacity with systems that are not Limited by stomata, e.g. leaf slices in solution or leaves at ex-ternal CO₂ concentrations exceeding 5%. Here, photosynthesis was rather insensitive to dehydration down to 50–70% relative water content, and different plant species re-sponded in a very similar way. More severe dehydration affected not only pboto-synthesis, but also dark CO₂ fixation and presumably also photorespiration. Rever-sible and unspecific inhibition is thought to be mediated mainly by increased concen-trations of solutes in dehydrated cells. Inhibition of photorespiration might favour photoinhibition when long-term water stress is coupled with full sunlight. Photo-inhibition, together with general senescence phenomena might be involved in long-term effects of water stress under natural drought conditions. This offers an explanation for the conflicting results of short-term water stress experiments and studies carried out under field conditions.     

Response of photosynthetic apparatus to moderate high temperature in contrasting wheat cultivars at different oxygen concentrations

The photosynthetic responses to moderately high temperatures (38 degrees C, imposed at 21% or 2% O(2) in air and 1500 mumol m(-2) s(-1)) were compared in wheat (Triticum aestivum L.) cultivars grown in northern regions of Ukraine and expected to be relatively sensitive to high temperatures ('North' cultivars) and in cultivars grown in southern regions and expected to be relatively heat-tolerant ('South' cultivars). Heating intact leaves in 21% O(2) for 1 h decreased CO(2) assimilation by c. 63% in 'North' cultivars and only c. 32% in 'South' cultivars, with a decrease in PSII activity being observed in only one of the 'North' cultivars. Carboxylation efficiency was decreased by about 2.7-fold in 'North' cultivars with no significant effect in 'South' cultivars. The maximum rates of carboxylation by Rubisco in vivo, V(cmax), estimated from Farquhar's model, increased more than 2-fold in 'South' cultivars and remained unchanged in 'North' cultivars while the maximum rate of RuBP regeneration, J(max), decreased by 53% and 21% in 'North' and 'South' cultivars, respectively. Where the heat treatment was imposed in 2% O(2) this increased (as compared with treatment at 21% O(2)) the inhibitory effect on CO(2) assimilation in tolerant cultivars, but decreased it in sensitive ones. The results suggested that differences in tolerance of moderately high temperatures in wheat relate to the stability of the Rubisco function and to RuBP regeneration activity rather than to the effects on PSII activity or stomatal control.     

Genetic analysis of biomass and photosynthetic parameters in wheat grown in different light intensities

Growth light intensities largely determine photosynthesis, biomass, and grain yield of cereal crops. To explore the genetic basis of light responses of biomass and Researchphotosynthetic parameters in wheat(Triticum aestivum L.),a quantitative trait locus(QTL) analysis was carried out in a doubled haploid(DH) population grown in low light(LL),medium light(ML), and high light(HL), respectively. The results showed that the wheat seedlings grown in HL produced more biomass with lower total chlorophyll content(Chl), carotenoid content, and maximum photochemical efficiency of photosystem II(Fv/Fm) while the wheat seedlings grown in LL produced less biomass with higher Chl compared with those grown in ML. In total, 48 QTLs were identified to be associated with the investigated parameters in relation to growth light intensities. These QTLs were mapped to 15 chromosomes which individually explained6.3%–36.0% of the phenotypic variance, of which chromo-somes 3A, 1D, and 6B were specifically involved in LL response, 5D and 7A specifically involved in ML response,and 4B specifically involved in HL response. Several light-responsive QTLs were co-located with QTLs for photosynthetic parameters, biomass, and grain weight under various conditions which may provide new hints to uncover the genetic control of photosynthesis, biomass, and grain weight.     

Protein kinases and phosphatases involved in the acclimation of the photosynthetic apparatus to a changing light environment

Photosynthetic organisms are subjected to frequent changes in light quality and quantity and need to respond accordingly. These acclimatory processes are mediated to a large extent through thylakoid protein phosphorylation. Recently, two major thylakoid protein kinases have been identified and characterized. The Stt7/STN7 kinase is mainly involved in the phosphorylation of the LHCII antenna proteins and is required for state transitions. It is firmly associated with the cytochrome b₆f complex, and its activity is regulated by the redox state of the plastoquinone pool. The other kinase, Stl1/STN8, is responsible for the phosphorylation of the PSII core proteins. Using a reverse genetics approach, we have recently identified the chloroplast PPH1/TAP38 and PBPC protein phosphatases, which counteract the activity of STN7 and STN8 kinases, respectively. They belong to the PP2C-type phosphatase family and are conserved in land plants and algae. The picture that emerges from these studies is that of a complex regulatory network of chloroplast protein kinases and phosphatases that is involved in light acclimation, in maintenance of the plastoquinone redox poise under fluctuating light and in the adjustment to metabolic needs.     

The influence of reduced photorespiration on long-term growth and development of wheat

The long-term role of photorespiration was investigated by comparing growth, development, gas exchange characteristics and mineral nutrition of a wheat crop ( Triticum aestivum L. cv. Courtot) cultivated in a culture chamber during a life cycle, either in 4% O₂ or in normal O₂ Low O₂ pressure reduced photorespiration, but CO₂ was controlled so that net photosynthesis remained the same as in the control crop. The growth and development of the low O₂ crop was slowed down. Ear appearance was 16 days late, but the rate of tillering was the same as in the control and was maintained longer so that the final number of tillers was doubled. Pigment, ribulose bisphosphate carboxylase (EC 4.1.1.39) and soluble sugar contents were similar. The response of photosynthesis to CO₂ and O₂ was not appreciably changed by the low O₂ treatment. There was almost no seed formation, and the senescence of the leaves was delayed. It appears that in non-stress conditions most of the photorespiration can be suppressed without damage to the photosynthetic apparatus. Retardation of development and inhibition of reproduction are likely due to other effects of O₂.     

Heat-Induced Increase in the Tolerance of the Wheat Photosynthetic Apparatus to Combined Action of High Temperature and Visible Light: CO2 Fixation,Photosynthetic Pigments,and Chloroplast Ultrastructure

Heating of the leaves of 15-day-old wheat (Triticum aestivum L.) plants at 42°C in the light (370 W/m² PAR) suppressed their ability to fix CO₂ twice stronger than heating in darkness. Heat hardening (3 h at 38–39°C) improved the tolerance of photosynthesis to combined action of high light and temperature but did not affect the tolerance to photoinhibition at 30°C. Hardening did not induce changes in the levels of photosynthetic pigments and their ratios. De-epoxidation of violaxanthin turned out to be more tolerant to photoinhibition at 42°C than CO₂ fixation. Protective effect of hardening was not related to the accumulation of zeaxanthin and activation of the xanthophyll cycle. Hardening protected the most sensitive population of chloroplasts against heat-induced photodamage and simultaneously increased the number and length of thylakoids. An increase in the volume of the thylakoid system was also induced by heating at 42°C and exposure to high light at 30°C. The formation of additional thylakoids and grana of shade type was not associated with improved tolerance of photosynthesis to heat and light stresses.