Effects of photoinhibition on whole-plant carbon gain assessed with a photosynthesis model

A canopy photosynthesis model was modified to assess the effect of photoinhibition on whole‐plant carbon gain. Photoinhibitory changes in maximum quantum yield of photosystem II (F_v/F_m) could be explained solely from a parameter (Lflux) calculated from the light micro‐environment of the leaves. This relationship between F_v/F_m and the intercepted cumulative light dose, integrated and equally weighted over several hours was incorporated into the model. The effect of photoinhibition on net photosynthesis was described through relationships between photoinhibition and the shaping parameters of the photosynthetic light‐response curve (quantum use efficiency, convexity, and maximum capacity). This new aspect of the model was then validated by comparing measured field data (diurnal courses of F_v/F_m) with simulation results. Sensitivity analyses revealed that the extent of photoinhibitory reduction of whole‐plant photosynthesis was strongly dependent on the structural parameters (LAI and leaf angle). Simulations for a Mediterranean evergreen oak, Quercus coccifera, under climatic conditions which cause mild photoinhibition revealed a daily loss of 7·5–8·5% of potential carbon gain in the upper sunlit canopy layers, a 3% loss in the bottom canopy, and an overall loss of 6·1%. Thus, this canopy photoinhibition model (CANO‐PI) allows the quantitative evaluation of photoinhibition effects on primary production.     

A mechanistic model of algal photoinhibition induced by photodamage to photosystem-II

Photoinhibition is a central problem for the understanding of plasticity in photosynthesis vs. irradiance response. It effectively reduces the photosynthetic rate. In this contribution, we present a mechanistic model of algal photoinhibition induced by photodamage to photosystem-II. Photosystem-IIs (PSIIs) are assumed to exist in three states: open, closed and inhibited. Photosynthesis is closely associated with the transitions between the three states. The present model is defined by four parameters: effective cross section of PSII, number of PSIIs, turnover time of electron transfer chains and the ratio of rate constant of damage to that of repair of D1 proteins in PSIIs. It gives a photosynthetic response curve of phytoplankton to irradiance (PI-curve). Without photoinhibition, the PI-curve is in hyperbola with the first three parameters. The PI-curve with photoinhibition can be simplified to the same form as the hyperbola by replacing either the number of PSIIs with the number of functional PSIIs or the turnover time of electron transfer chains with the average turnover time.     

Electrochromism: a useful probe to study algal photosynthesis

In photosynthesis, electron transfer along the photosynthetic chain results in a vectorial transfer of protons from the stroma to the lumenal space of the thylakoids. This promotes the generation of an electrochemical proton gradient (Δμ _H ⁺ ), which comprises a gradient of electric potential (ΔΨ) and of proton concentration (ΔpH). The Δμ _H ⁺ has a central role in the photosynthetic process, providing the energy source for ATP synthesis. It is also involved in many regulatory mechanisms. The ΔpH modulates the rate of electron transfer and triggers deexcitation of excess energy within the light harvesting complexes. The ΔΨ is required for metabolite and protein transport across the membranes. Its presence also induces a shift in the absorption spectra of some photosynthetic pigments, resulting in the so-called ElectroChromic Shift (ECS). In this review, we discuss the characteristic features of the ECS, and illustrate possible applications for the study of photosynthetic processes in vivo.     

Elements of a dynamic systems model of canopy photosynthesis

Improving photosynthesis throughout the full canopy rather than photosynthesis of only the top leaves of the canopy is central to improving crop yields. Many canopy photosynthesis models have been developed from physiological and ecological perspectives, however most do not consider heterogeneities of microclimatic factors inside a canopy, canopy dynamics and associated energetics, or competition among different plants, and most models lack a direct linkage to molecular processes. Here we described the rationale, elements, and approaches necessary to build a dynamic systems model of canopy photosynthesis. A systems model should integrate metabolic processes including photosynthesis, respiration, nitrogen metabolism, resource re-mobilization and photosynthate partitioning with canopy level light, CO(2), water vapor distributions and heat exchange processes. In so doing a systems-based canopy photosynthesis model will enable studies of molecular ecology and dramatically improve our insight into engineering crops for improved canopy photosynthetic CO(2) uptake, resource use efficiencies and yields.     

Effect of long-term photoinhibition on growth and photosynthesis of cold-hardened spring and winter wheat

The effect of repeated exposure to high light (1200 mol · m^–2 · s^–1 photosynthetic photon flux density, PPFD) at 5° C was examined in attached leaves of cold-grown spring (cv. Katepwa) and winter (cv. Kharkov) wheat (Triticum aestivum L.) over an eight-week period. Under these conditions, Kharkov winter wheat exhibited a daily reduction of 24% in F_V/F_M (the ratio of variable to maximal fluorescence in the dark-adapted state), in contrast to 41% for cold-grown Katepwa spring wheat. Both cultivars were able to recover from this daily suppression of F_V/F_M such that the leaves exhibited an average morning F_V/F_M of 0.651 ± 0.004. Fluorescence measurements made under steady-state conditions as a function of irradiance from 60 to 2000 mol · m^–2 · s^–1 indicated that the yield of photosystem II (PSII) electron transport under light-saturating conditions was the same for photoinhibited and control cold-grown plants, regardless of cultivar. Repeated daily exposure to high light at low temperature did not increase resistance to short-term photoinhibition, although zeaxanthin levels increased by three- to fourfold. In addition, both cultivars increased the rate of dry-matter accumulation, relative to control plants maintained at 5° C and 250 mol · m^–2 · s^–1 PPFD (10% and 28% for Katepwa and Kharkov, respectively), despite exhibiting suppressed f_v/f_m and reduced photon yields for O₂ evolution following daily high-light treatments. Thus, although photosynthetic efficiency is suppressed by a longterm, photoinhibitory treatment, light-saturated rates of photosynthesis are sufficiently high during the high-light treatment to offset any reduction in photochemical efficiency of PSII. We suggest that in these cold-tolerant plants, photoinhibition of PSII may represent a longterm, stable, down-regulation of photochemistry to match the overall photosynthetic demand for ATP and reducing equivalents.Abbreviations and Symbols Chl chlorophyll - HL high light - PPFD photosynthetic photon flux density - F_O minimum fluorescence in the dark-adapted state - F_M maximum fluorescence in the dark-adapted state - F_V maximum variable fluorescence in the dark-adapted state (F_M-F_O) - F_V/F_V photosynthetic efficiency of the dark-adapted state - f_V/f_M photosynthetic efficiency of the light-adapted steady state - q_P photochemical quenching parameter - q_N non-photochemical quenching parameter - _e yield of electron transport and equals q_P · f_V/f_M - 1-q_O F_O quenching parameter - _app apparent photon yield. The assistance of Amy So is gratefully acknowledged. This research was supported by a Natural Sciences and Engineering Research Council of Canada (NSERCC) Operating Grant to N.P.A.H. G.Ö. was supported by an NSERCC International Exchange Award and the Swedish Natural Sciences Research Council.     

Improving photosynthesis for algal biofuels: toward a green revolution

Biofuels derived from marine algae are a potential source of sustainable energy that can contribute to future global demands. The realisation of this potential will require manipulation of the fundamental biology of algal physiology to increase the efficiency with which solar energy is ultimately converted into usable biomass. This 'photosynthetic solar energy conversion efficiency' sets an upper limit on the potential of algal-derived biofuels. In this review, we outline photosynthetic molecular targets that could be manipulated to increase the efficiency and yield of algal biofuel production. We also highlight modern 'omic' and high-throughput technologies that might enable identification, selection and improvement of algal cell lines on timescales relevant for achieving significant contributions to future energy solutions.     

Influence of light and temperature on photoinhibition of photosynthesis inSpirulina platensis

Photoinhibition of photosynthesis and its recovery in the cyanobacteriumSpirulina platensis was studied to find how photosynthetic rates were influenced by light and temperature. By exposing cell samples from a turbidostat culture to combinations of light and temperature, a connection between light, temperature and photoinhibition was found. The experiments showed that a 10 degree increase from 20 °C to 30 °C considerably reduced the photoinhibition. At 25 °C a photon flux density of 1720 µmol m^–2 s^–1 reduced the photosynthetic rate by 50 % in 1 h, but a similarly high photon flux density had nearly no negative effect at 35 °C. Reactivation in low light from 50% photoinhibition was fast and complete in 60 min at 30 °C, while at 20 °C only about 1/6 of the full capacity was regained in the same time. Addition of the protein synthesis inhibitor streptomycin to cultures undergoing photoinhibition and regeneration indicated the presence also in this organism of a repair mechanism based on protein synthesis.Author for correspondence     

The influence of vertical mixing on the photoinhibition of variable chlorophyll a fluorescence and its inclusion in a model of phytoplankton photosynthesis

The maximum effective quantum yield of photosystem II was estimatedfrom measurements of variable chlorophyll a fluorescence [(F'_m- F'_o)/F'_m = F'_v/F'_m] in samples of phytoplankton collectedfrom various depths in Chaffey Reservoir, Australia. Duringstratified conditions, F'_v/F'_m showed depth-dependent decreasesas irradiance increased during the morning, and increases asirradiancereduced in the afternoon. Wind-induced mixing disrupted thediel pattern, but even under well-mixed conditions a verticalgradient in F'_v/F'_m remained. Differences in F'_v/F'_m valuesbetween samples incubated at fixed depths and unconstrainedlake samples enabled identification of the phytoplankton mixingdepth. Recovery of F'_v/F'_m was modelled as a function of timeand the degree of F'_v/F'_m inhibition, while damage was considereda function of photon dose. A combined, numerical model was fittedto diel sequences of F'_v/F'_m to estimate rate constants fordamage and repair. Recovery rate constants ranged from r = 0.7x 10^-4 to 9.1 x 10^-4 s^-1, while damage rate constants rangedfrom k = 0.03 to 0.22 m² mol photon^-1. A fluorescence-basedmodel of photosynthesis was used to investigate the effectsof wind speed, euphotic depth and mixed layer depth on photoinhibition.At different mixing rates, depth-integrated photosynthesis wasenhanced by up to 16% under the conditions tested, while increasesof 9% occurred between phytoplankton with different measureddamage and repair characteristics.     

Cold acclimation and photoinhibition of photosynthesis in Scots pine

Cold acclimation of Scots pine did not affect the susceptibility of photosynthesis to photoinhibition. Cold acclimation did however cause a suppression of the rate of CO₂ uptake, and at given light and temperature conditions a larger fraction of the photosystem II reaction centres were closed in cold-acclimated than in nonacclimated pine. Therefore, when assayed at the level of photosystem II reaction centres, i.e. in relation to the degree of photosystem closure, cold acclimation caused a significant increase in resistance to photoinhibition; at given levels of photosystem II closure the resistance to photoinhibition was higher after cold acclimation. This was particularly evident in measurements at 20° C. The amounts and activities of the majority of analyzed active oxygen scavengers were higher after cold acclimation. We suggest that this increase in protective enzymes and compounds, particularly Superoxide dismutase, ascorbate peroxidase, glutathione reductase and ascorbate of the chloroplasts, enables Scots pine to avoid excessive photoinhibition of photosynthesis despite partial suppression of photosynthesis upon cold acclimation. An increased capacity for light-induced de-epoxidation of violaxanthin to zeaxanthin upon cold acclimation may also be of significance.Abbreviations APX ascorbate peroxidase - DHA dehydroascorbate - DHAR dehydroascorbate reductase - Fm maximal fluorescence when all reaction centres are closed - Fv/Fm maximum photochemical yield of PSII - GR glutathione reductase - GSH reduced glutathione - Je rate of photosynthetic electron transport - MDAR monodehydroascorbate reductase - q_N nonphotochemical quenching of fluorescence - q_P photochemical quenching of fluorescence - SOD superoxide dismutase This work was supported by the Swedish Natural Science Research Council and the National Natural Science Foundation of China.     

温州蜜柑叶片光合作用光抑制的保护机理

晴天条件下,使用便携式调制荧光仪和分光光度计观察了温州蜜柑叶片光合作用光抑制发生过程中几个主要荧光参数（初始荧光F0、最大荧光Fm、PSⅡ的光化学效率Fv/Fm、非光化学猝灭qN及其快相qNf和慢相qNs）、电子传递速率（ETR）和玉米黄素相对含量的日变化,结果表明,随着光强的增强,ETR、qN及其qNr与qNs以及玉米黄素相对含量升高,Fv/Fm、Fm和F0下降。用DTT处理后,qNs较对照明显下降,F0较对照明显上升,可以认为,柑橘在光合作用日变化中存在依赖于叶黄素循环和类囊体膜质子梯度两种非辐射能量耗散方式,而且它们在防御光破坏方面起着重要作用。     

Effect of low water potential on photosynthesis in intact lichens and their liberated algal components

Earlier experiments (T.D. Brock 1975, Planta124, 13–23) addressed the question whether the fungus of the lichen thallus might enable the algal component to function when moisture stress is such that the algal component would be unable to function under free-living conditions. It was concluded that the liberated phycobiont in ground lichen thalli could not photosynthesize at water potentials as low as those at which the same alga could when it was present within the thallus. However, our experience with lichen photosynthesis has not substantiated this finding. Using instrumentation developed since the mid-1970's to measure photosynthesis and control humidity, we repeated Brock's experiments. When applying “matric” water stress (equilibrium with air of constant relative humidity) we were unable to confirm the earlier results for three lichen species including one of the species,Letharia vulpina, had also been used by Brock. We found no difference between the effects of low water potential on intact lichens and their liberated algal components (ground thallus material and isolated algae) and no indication that the fungal component of the lichen symbiosis protects the phycobiont from the adverse effects of desiccation once equilibrium conditions are reached. The photosynthetic apparatus of the phycobiont alone proved to be highly adapted to water stress as it possesses not only the capability of functioning under extremely low degrees of hydration but also of becoming reactivated solely by water vapor uptake.     

Photoinhibition of photosynthesis in intact kiwifruit (Actinidia deliciosa) leaves: Effect of light during growth on photoinhibition and recovery

Photoinhibition of photosynthesis was induced in intact kiwifruit (Actinidia deliciosa (A. Chev.) C. F. Liang et A. R. Ferguson) leaves grown at two photon flux densities (PFDs) of 700 and 1300 mol·m^-2·s^-1 in a controlled environment, by exposing the leaves to PFD between 1000 and 2000 mol·m^-2·s^-1 at temperatures between 10 and 25°C; recovery from photoinhibition was followed at the same range of temperatures and at a PFD between 0 and 500 mol·m^-2·s^-1. In either case the time-courses of photoinhibition and recovery were followed by measuring chlorophyll fluorescence at 692 nm and 77K and by measuring the photon yield of photosynthetic O₂ evolution. The initial rate of photoinhibition was lower in the high-light-grown plants but the long-term extent of photoinhibition was not different from that in low-light-grown plants. The rate constants for recovery after photoinhibition for the plants grown at 700 and 1300 mol·m^-2·s^-1 or for those grown in shade were similar, indicating that differences between sun and shade leaves in their susceptibility to photoinhibition could not be accounted for by differences in capacity for recovery during photoinhibition. Recovery following photoinhibition was increasingly suppressed by an increasing PFD above 20 mol·m^-2·s^-1, indicating that recovery in photoinhibitory conditions would, in any case, be very slow. Differences in photosynthetic capacity and in the capacity for dissipation of non-radiative energy seemed more likely to contribute to differences in susceptibility to photoinhibition between sun and shade leaves of kiwifruit.Abbreviations and symbols F _o , F _m , F _v instantaneous, maximum, variable fluorescence - F _v /F _m fluorescence ratio - F _i =F _v at t=0 - F F _v at t= - K _D rate constant for photochemistry - k(F _p ) first-order rate constant for photoinhibition - k(F _r ) first-order rate constant for recovery - PFD photon flux density - PSII photosystem II - _i photon yield of O₂ evolution (incident light)     

Polymixis and algal production in a tropical lake: latitudinal effects on the seasonality of photosynthesis

SUMMARY. 1. Phytoplankton production was measured during 1981–82 in Bahia de Puno. a large polymictic system that is part of LakeTiticaca in the tropical Andes (Peru-Bolivia), Photosynthesis followed aweak seasonal pattern through each 12-month cycle but the largestvariations occurrud between years. 2. The large temporal variations in productivity appeared to becontrolled by biological events rather than by the direct effeets ofseasonal or aseasonal physical processes. Major shifts in production overthe 2-year period were attributable to the proportional abundance andactivity of N₂-fixing blue-green algae. Production was uncorrelated withthe seasonal radiation cycle. 3. In vivo fluorescence and photosynthetic assays revealed largechanges in phytoplankton community structure over time and depth.The shifts in vertical structure were associated with changes in transparency and water column stabilily.Physiological as well as floristiceffects dictated the variations in fluorescence per unit chlorophyll a. 4. An analysis of twenty-seven primary productivity data sets fromdifferent latitudes shtiwed that Bahia dc Puno resembled other tropicallakes in its relatively weak coupling between algal photosynthesis andseasonal variations in light. Productivity in the temperate zone correlated with the annual cycle of incoming radiation while the tropicalproduction data did not. For many of the tropical lakes, but fewtemperate lakes, the between-month variance in primary production wassignificantly higher than the between-month variance for incomingradiation. There was a significant positive relationship between maximum production variance explained hy light (maximum r² for each dataset) and latitude. 5. As in many tropical lakes. Bahia de Puno experiences moderatelylarge variations in algal photosynthesis.The muted seasonal cycle of light explains only a small portion of the total variation In productivity, and other factors such as weuther-related events are disproportionately important.     

Effect of vertical cycling on photosynthesis during the annual algal succession in the northern Baltic

During the phytoplankton succession in 1984 and 1985, the effect of fluctuating light on algal photosynthesis (incorporation of ¹⁴C, acidified water sample) was studied in the northern Baltic. Bottles were mounted on moving racks that mimicked vertical transport caused by Langmuir circulations in the trophogenic layer. Assuming that the photoinhibition observed near the surface in fixed-depth incubations (from 1 to 8% of integral photosynthesis) was avoided in cycled samples, vertical cycling conducted around noon resulted in on average 10% lower photosynthesis than fixed-depth incubations (n = 17). This difference lies within the 5% confidence limits of the measurement, and hence it was concluded that the lack of short-term fluctuations in light associated with the vertical circulation of natural phytoplankton communities does not seriously bias conventional in situ ¹⁴CO₂ fixation measurements performed at fixed depths in the study area.     

A mechanistic model of photoinhibition

A mechanistic model was developed, to simulate the main facets of photoinhibition in phytoplankton. Photoinhibition is modelled as a time dependent decrease in the initial slope of a photosynthesis versus irradiance curve, related to D1 (photosystem II reaction centre protein) damage and non-photochemical quenching. The photoinhibition model was incorporated into an existing ammonium-nitrate nutrition interaction model capable of simulating photoacclimation and aspects of nitrogen uptake and utilization. Hence the current model can simulate the effects of irradiance on photosynthesis from sub-saturating to inhibitory photon flux densities, during growth on different nitrogen sources and under nutrient stress. Model output conforms well to experimental data, allowing the extent of photoinhibition to be predicted under a range of nutrient and light regimes. The ability of the model to recreate the afternoon depression of photosynthesis and the enhancement of photosynthesis during fluctuating light suggests that these two processes are related to photoinhibition. The model may be used to predict changes in biomass and/or carbon fixation under a wide range of oceanographic situations, and it may also help to explain the progression to dominance of certain algal species, and bloom formation under defined irradiance and nutrient conditions.     

The phenomenon of photoinhibition of photosynthesis and its importance in reforestation

Photoinhibition, defined as the inhibition of photosynthesis caused by excessive radiance, affects field production to a great extent. This phenomenon is particularly relevant in reforestation practices, when one deals with forests of rapid growth such asEucalyptus. The imposition of additional stress factors during exposure to high radiance increases the potential for photoinhibitory effects, so the inhibition of photosynthesis indicates that the plant is submitted to stressful conditions. Photoinhibition can be reversible, playing a protective role for the photosynthetic systems, but it can also reflect damage that has already occurred in the photosynthetic apparatus, being irreversible in this case. In this review, we present the physiological and molecular mechanisms of photoinhibition and discuss the interaction between light and other stress factors and its effects on plants destined for reforestation. In addition, the present work analyzes some of the features and strategies that help plants avoid or restrict the occurrence of photoinhibition. For instance, pigments and enzymes which naturally occur in plants can prevent photoinhibition, while preadaptation to nonideal conditions can enhance tolerance to a certain stress factor. Most of these morphological, metabolic, and biochemical mechanisms of defense are related to the dissipation of excessive energy such as heat. Understanding these mechanisms can help improve cultivation procedures, avoid the plants’ death, and increase productivity in the field.     

Effect of cold hardening on sensitivity of winter and spring wheat leaves to short-term photoinhibition and recovery of photosynthesis

Photoinhibition of photosynthesis and its recovery were studied in wheat (Triticum aestivum L.) leaves grown at nonhardening (20°C) and cold-hardening (5°C) temperatures. Cold-hardened wheat leaves were less susceptible to photoinhibition at 5°C than nonhardened leaves, and the winter cultivars, Kharkov and Monopol, were less susceptible than the spring cultivar, Glenlea. The presence of chloramphenicol, a chloroplastic protein synthesis inhibitor, increased the susceptibility to photoinhibition, but cold-hardened leaves still remained less susceptible to photoinhibition than nonhardened leaves. Recovery at 50 μmol m⁻² s⁻¹ photosynthetic photon flux density and 20°C was at least biphasic, with a fast and a slow phase in all cultivars. Cold-hardened leaves recovered maximum fluorescence and maximum variable fluorescence in the dark-adapted state during the fast phase at a rate of 42% h⁻¹ compared with 22% h⁻¹ for nonhardened leaves. The slow phase occurred at similar rates (2% h⁻¹) in cold-hardened and nonhardened leaves. Full recovery required up to 30 h. Fast-recovery phase was not reduced by either lowering the recovery temperature to 5°C or by the presence of chloramphenicol. Slow-recovery phase was inhibited by both treatments. Hence, the fast phase of recovery does not require de novo chloroplast protein synthesis. In addition, only approximately 60% of the photochemical efficiency lost through photoinhibition at 5°C was associated with lost [¹⁴C]atrazine binding and, hence, with damage to the secondary quinone electron acceptor for photosystem II-binding site. We conclude that the decrease in susceptibility to photoinhibition exhibited following cold hardening of winter and spring cultivars is not due to an increased capacity for repair of photoinhibitory damage at 5°C but reflects intrinsic properties of the cold-hardened photosynthetic apparatus. A model to account for the fast component of recovery is discussed.     

Fluorescence quenching during photosynthesis and photoinhibition of Ulva rotundata blid.

The relationships between photoinhibition and photoprotection in high and low-light-grown Ulva were examined by a combination of chlorophyll-fluorescence-monitoring techniques. Tissues were exposed to a computer-controlled sequence of 5-min exposures to red light, followed by 5-min darkness, with stepwise increases in photon flux. Coefficients of chlorophyll fluorescence quenching (1?q_P and NPQ) were calculated following a saturating pulse of white light near the end of each 5-min light treatment. Dark-adapted chlorophyll fluorescence parameters (F₀ and F_V/F_M) were calculated from a saturating pulse at the end of each 5-min dark period. Low-light-grown Ulva showed consistently higher 1?q_P, i.e. higher reduction status of Q (high primary acceptor of photosystem II), and lower capacity for nonphotochemical quenching (NPQ) at saturating light than did high-light-grown plants. Consequently, low-light plants rapidly displayed photoinhibitory damage (increased F₀) at light saturation in seawater. Removal of dissolved inorganic carbon from seawater also led to photoinhibitory damage of high-light-grown Ulva at light saturation, and addition of saturating amounts of dissolved inorganic carbon protected low-light-grown plants against photoinhibitory damage. A large part of NPQ was abolished by treatment with 3 mM dithiothreitol and the processes so inhibited were evidently photoprotective, because dithiothreitol treatment accelerated photoinhibitory damage in both low- and high-light-grown Ulva. The extent of photoinhibitory damage in Ulva was exacerbated by treatment with chloramphenicol (1 mM) without much effect on chlorophyll-quenching parameters, evidently because this inhibitor of chloroplast protein synthesis reduced the rate of repair processes.