Effects Of Photoinhibition Research Articles

Outdoor microalgae cultivation in airlift photobioreactor at high irradiance and temperature conditions: effect of batch and fed-batch strategies, photoinhibition, and temperature stress.

The microalgae Scenedesmus abundans cultivated in five identical airlift photobioreactors (PBRs) in batch and fed-batch modes at the outdoor tropical condition. The microalgae strain S. abundans was found to tolerate high temperature (35-45°C) and high light intensity (770-1690µmolm- 2s- 1). The highest biomass productivities were 152.5-162.5mgL- 1day- 1 for fed-batch strategy. The biomass productivity was drastically reduced due to photoinhibition effect at a culture temperature of > 45°C. The lipid compositions showed fatty acids mainly in the form of saturated and monounsaturated fatty acids (> 80%) in all PBRs with Cetane number more than 51. The fed-batch strategies efficiently produced higher biomass and lipid productivities at harsh outdoor conditions. Furthermore, the microalgae also accumulated omega-3 fatty acid (C18:3) up to 14% (w/w) of total fatty acid at given outdoor condition.

Biophysical modeling of microalgal cultivation in open ponds

Microalgal biomass is currently recognized as a promising sustainable source for biofuel production and carbon dioxide (CO2) sequestration. Utilization of biophysical models are emerging to access the real-time feasibility of microalgal technology. In this present work, a comprehensive mathematical model based on the site-specific meteorological variables is formulated using MATLAB ODE 45 s solver to estimate the microalgal productivity. The predictive model framework utilized material balance equations with basic laws of physics, known constants and conservative assumptions to evaluate the water temperature that influences the microalgal viability. The dynamic behaviour of algal ponds considering the operating variables like light intensity (including the effects of photoinhibition), water temperature, and design criteria like pond depth, microalgal concentration, was used to estimate the performance of T. pseudonana in open ponds. Maximum growth was projected in September accounting to the biomass and lipid productivity of 170.28 kg (dry mass) ha−1 d−1 and 39.42 l ha−1 d−1 respectively with a CO2 capture potential of 224.77 kg (CO2) ha−1 d−1 based on the influence of water temperature. Optimal pond depth and operational conditions to achieve the desired productivity for the specific site were estimated. The maximum annual areal productivity dropped by 19% from 62.18 tons (dry mass) ha−1 yr−1 due to photoinhibition. The simulated biophysical model as a tool could be used to evaluate the biokinetic processes affecting the algal pond performance for further facilitation of effective decision making on scaling up of microalgae cultivation.

Tidal modulation of seabed light and its implications for benthic algae

AbstractThe temporal behavior of seabed light in a shallow, tidal sea is set largely by the interaction of the solar elevation cycle with tidal cycles in water depth and temporal variability in water clarity. The effect of tidal modulation on seabed light often does not simply average out, producing instead a net effect (either an amplification or a reduction of seabed light, integrated over time) compared to a tideless, but otherwise equivalent, scenario. Observations of this phenomenon from the Bay of Brest (France) show reasonable agreement with predictions based on an earlier theoretical framework, confirming that the key physics has been understood and that the important parameters are tidal amplitude, timing of low waters, diffuse attenuation coefficient, and daylength. Implications for benthic macroalgae living in the bay's shallow subtidal zone are investigated using a simple numerical model. The effects of the tide on time‐integrated seabed light and, in turn, time‐integrated macroalgal community photosynthesis in the Bay of Brest correspond closely at three timescales: annual, springs‐neaps (i.e., approximately fortnightly), and daily. Tidal amplification of both parameters occurs over the year, during winter months generally, and at spring tides during winter specifically (slight reduction occurs at neaps during winter). For an individual, isolated thallus, the relationship between tidal modulation of seabed light and photosynthesis is complicated by more pronounced light‐saturation and photoinhibition effects. Demonstrated here for the first time, neglecting tidal effects on seabed light is likely to result in erroneous estimates (and, for many sites, underestimation) of subtidal benthic productivity.

Quantifying phytoplankton productivity and photoinhibition in the Ross Sea Polynya with large eddy simulation of Langmuir circulation

AbstractSouthern Ocean phytoplankton assemblages acclimated to low‐light environments that result from deep mixing are often sensitive to ultraviolet and high photosynthetically available radiation. In such assemblages, exposures to inhibitory irradiance near the surface result in loss of photosynthetic capacity that is not rapidly recovered and can depress photosynthesis after transport below depths penetrated by inhibitory irradiance. We used a coupled biophysical modeling approach to quantify the reduction in primary productivity due to photoinhibition based upon experiments and observations made during the spring bloom in Ross Sea Polynya (RSP). Large eddy simulation (LES) was used to generate depth trajectories representative of observed Langmuir circulation that were passed through an underwater light field to yield time series of spectral irradiance representative of what phytoplankton would have experienced in situ. These were used to drive an assemblage‐specific photosynthesis‐irradiance model with inhibition determined from a biological weighting function and repair rate estimated from shipboard experiments on the local assemblage. We estimate the daily depth‐integrated productivity was 230 mmol C m−2. This estimate includes a 6–7% reduction in daily depth‐integrated productivity over potential productivity (i.e., effects of photoinhibition excluded). When trajectory depths were fixed (no vertical transport), the reduction in productivity was nearly double. Relative to LES estimates, there was slightly less depth‐integrated photoinhibition with random walk trajectories and nearly twice as much with circular rotations. This suggests it is important to account for turbulence when simulating the effects of vertical mixing on photoinhibition due to the kinetics of photodamage and repair.

DAYTIME VARIATIONS OF CHLOROPHYLL A FLUORESCENCE IN PAU D'ALHO SEEDLINGS

ABSTRACT Analysis of transient and modulated fluorescence of chlorophyll a were made at one-hour intervals during an eight-hour period starting at 07:30h aiming to study mechanisms of photoprotection against high radiation and temperature in Gallesia integrifolia plants. Seeds were germinated inside plastic pots containing soil as substrate. At 120 days after emergence, chlorophyll fluorescence measurements were performed using Handy-PEA and FMS2 fluorometers. During the course of a day, an increase and a subsequent decrease in temperature and in photosynthetic active radiation were observed until 12:30h. Changes in transient kinetic curves of chlorophyll a fluorescence were identified. This resulted in changes in JIP test parameters. An increase during the period of high radiation and temperature may be stressed in relation to variables related to dissipation flux and appearance of positive -K and -L bands. Considering the modulated fluorescence, high values of non-photochemical quenching coefficients associated with lower values of effective photochemical efficiency of the photosystem II (FV'/ FM') and current photochemical efficiency of PSII (φPSII) could be observed during early morning. This was probably the result of an inhibition of the biochemical phase of photosynthesis. It can be concluded that Gallesia integrifolia decrease its photochemical activity with the increase in the photosynthetic active radiation, demonstrating a photoinhibitory effect under high irradiance conditions, but without irreversible damages to the photosynthetic apparatus.

Photoinhibition on natural ammonia oxidizers biofilm populations and implications for nitrogen uptake in stream biofilms

AbstractWe investigated photoinhibition on natural communities of ammonia oxidizing (AO) archaea (AOA) and bacteria (AOB) embedded in complex stream biofilms, and its implications on nitrogen uptake at biofilm scale. Based on the strong photoinhibition previously exhibited by free living and cultured AOA and AOB, we expected AO activity to decrease in biofilms experimentally exposed to light, reducing the contribution of microbial nitrification to ammonium uptake. To test it, we conducted light manipulation experiments in mesocosms using biofilms naturally developed on stream cobbles sides both exposed to light (light‐side) and facing the sediment (dark‐side). We observed a strong AO photoinhibition in dark‐side biofilms, accompanied by either biofilm‐scale decreases in nitrification or increases in nitrogen uptake likely by heterotrophic activity. Conversely, in light‐side biofilms photoinhibition was not observed suggesting that photoautotrophic layers may protect AO in situ by a sunshade effect. Experimental light and dark alternation cycles stimulated AO, enhancing both nitrification and ammonium uptake. These results support photoinhibition effects on natural AO communities, especially in biofilms developed under ambient dark conditions, whereas this effect seems to be buffered in biofilms developed under light conditions. Therefore, the contribution of nitrification to ammonium uptake at biofilm scale should consider not only the physiological study of AOA and AOB, but also the environmental conditions and community structure at the habitat microscale, since they may counterbalance, among others, the damaging light effects under natural conditions.

Model-based optimization of microalgae areal productivity in flat-plate gas-lift photobioreactors

The productivity of microalgae used for the production of biofuels is primarily influenced by the availability of exploitable light. To overcome the effects of limitation and photoinhibition, an optimized radiation profile was implemented for the cultivation of Nannochloropsis salina 40.85 and Nannochloropsis gaditana 2.99 in LED-illuminated flat-plate gas-lift photobioreactors with a depth of 2cm. For this purpose the light-dependent growth kinetics were characterized for both strains with variable incident photon flux densities up to 2750μmolm−2s−1. The mean integral photon flux density I⁎⁎ was established to describe the light every single microalgal cell is exposed to on average during exponential growth. By implementing a phototrophic growth model based on the light attenuation described by Lambert and Beer, the kinetic growth parameters have been determined including the effects of photoinhibition. Based on the defined optimal range of I⁎⁎ for both Nannochloropsis strains optimum radiation profiles were developed: Up to an identified upper limit of light, the incident irradiation is increased with rising biomass concentration while the mean integral photon flux density is kept constant at the chosen optimum. Thus, the biomass areal productivity could be increased significantly compared to constant irradiation by 113% to 10.4gm−2d−1 with Nannochloropsis salina and by 107% to 14.8gm−2d−1 with Nannochloropsis gaditana, respectively. The areal lipid productivity was likewise distinctly improved by the implementation of the radiation profiles up to 11.0gm−2d−1 (+59%) with Nannochloropsis salina and up to 6.5gm−2d−1 (+83%) with Nannochloropsis gaditana. The concept of optimized radiation profiles may be easily transferred to other microalgal strains as well as other reactor types due to the system-independence of the mean integral photon flux density.

Selenium delays leaf senescence in oilseed rape plants

Effect of selenium on leaf senescence was studied in oilseed rape plants treated with 10 μM Na2SeO4 at a rosette growth stage. In addition to developmental senescence, N deficiency and leaf detachment were used for induction of senescence. Nonphotochemical quenching declined in old leaves as senescence became more advancing but rose progressively in the plants supplied by Se. The total carbohydrate and protein pools decreased with leaf age, while increased by the Se treatment. However, during senescence induced by N deficiency, Se did not change remarkably the C and N metabolism, but delayed senescence mainly through protection of plants from photoinhibitory effects. After detachment, untreated leaves became chlorotic and necrotic, while the Se-treated ones remained fairly green. Our results demonstrated that Se delayed leaf senescence by a maintaining or even improving photochemical activities. During developmental senescence, the Se effect on the extending life span of the leaves was additionally linked to the metabolic regulation of senescence.

Reduced light and moderate water deficiency sustain nitrogen assimilation and sucrose degradation at low temperature in durum wheat

The rate of carbon and nitrogen assimilation is highly sensitive to stress factors, such as low temperature and drought. Little is known about the role of light in the simultaneous effect of cold and drought. The present study thus focused on the combined effect of mild water deficiency and different light intensities during the early cold hardening in durum wheat (Triticum turgidum ssp. durum L.) cultivars with different levels of cold sensitivity.The results showed that reduced illumination decreased the undesirable effects of photoinhibition in the case of net photosynthesis and nitrate reduction, which may help to sustain these processes at low temperature. Mild water deficiency also had a slight positive effect on the effective quantum efficiency of PSII and the nitrate reductase activity in the cold. Glutamine synthesis was affected by light rather than by water deprivation during cold stress. The invertase activity increased to a greater extent by water deprivation, but an increase in illumination also had a facilitating effect on this enzyme. This suggests that both moderate water deficiency and light have an influence on nitrogen metabolism and sucrose degradation during cold hardening. A possible rise in the soluble sugar content caused by the invertase may compensate for the decline in photosynthetic carbon assimilation indicated by the decrease in net photosynthesis. The changes in the osmotic potential can be also correlated to the enhanced level of invertase activity. Both of them were regulated by light at normal water supply, but not at water deprivation in the cold. However, changes in the metabolic enzyme activities and osmotic adjustment could not be directly contributed to the different levels of cold tolerance of the cultivars in the early acclimation period.

Resveratrol Prevents High Fluence Red Light-Emitting Diode Reactive Oxygen Species-Mediated Photoinhibition of Human Skin Fibroblast Migration.

BackgroundSkin fibrosis is a significant medical problem that leads to a functional, aesthetic, and psychosocial impact on quality-of-life. Light-emitting diode-generated 633-nm red light (LED-RL) is part of the visible light spectrum that is not known to cause DNA damage and is considered a safe, non-invasive, inexpensive, and portable potential alternative to ultraviolet phototherapy that may change the treatment paradigm of fibrotic skin disease.ObjectiveThe goal of our study was to investigate the how reactive oxygen species (ROS) free radicals generated by high fluence LED-RL inhibit the migration of skin fibroblasts, the main cell type involved in skin fibrosis. Fibroblast migration speed is increased in skin fibrosis, and we studied cellular migration speed of cultured human skin fibroblasts as a surrogate measure of high fluence LED-RL effect on fibroblast function. To ascertain the inhibitory role of LED-RL generated ROS on migration speed, we hypothesized that resveratrol, a potent antioxidant, could prevent the photoinhibitory effects of high fluence LED-RL on fibroblast migration speed.MethodsHigh fluence LED-RL generated ROS were measured by flow cytometry analysis using dihydrorhodamine (DHR). For purposes of comparison, we assessed the effects of ROS generated by hydrogen peroxide (H2O2) on fibroblast migration speed and the ability of resveratrol, a well known antioxidant, to prevent LED-RL and H2O2 generated ROS-associated changes in fibroblast migration speed. To determine whether resveratrol could prevent the high fluence LED-RL ROS-mediated photoinhibition of human skin fibroblast migration, treated cells were incubated with resveratrol at concentrations of 0.0001% and 0.001% for 24 hours, irradiated with high fluences LED-RL of 480, 640, and 800 J/cm2.ResultsHigh fluence LED-RL increases intracellular fibroblast ROS and decreases fibroblast migration speed. LED-RL at 480, 640 and 800 J/cm2 increased ROS levels to 132.8%, 151.0%, and 158.4% relative to matched controls, respectively. These LED-RL associated increases in ROS were prevented by pretreating cells with 0.0001% or 0.001% resveratrol. Next, we quantified the effect of hydrogen peroxide (H2O2)-associated ROS on fibroblast migration speed, and found that while H2O2–associated ROS significantly decreased relative fibroblast migration speed, pretreatment with 0.0001% or 0.001% resveratrol significantly prevented the decreases in migration speed. Furthermore, we found that LED-RL at 480, 640 and 800 J/cm2 decreased fibroblast migration speed to 83.0%, 74.4%, and 68.6% relative to matched controls, respectively. We hypothesized that these decreases in fibroblast migration speed were due to associated increases in ROS generation. Pretreatment with 0.0001% and 0.001% resveratrol prevented the LED-RL associated decreases in migration speed.ConclusionHigh fluence LED-RL increases ROS and is associated with decreased fibroblast migration speed. We provide mechanistic support that the decreased migration speed associated with high fluence LED-RL is mediated by ROS, by demonstrating that resveratrol prevents high fluence LED-RL associated migration speed change. These data lend support to an increasing scientific body of evidence that high fluence LED-RL has anti-fibrotic properties. We hypothesize that our findings may result in a greater understanding of the fundamental mechanisms underlying visible light interaction with skin and we anticipate clinicians and other researchers may utilize these pathways for patient benefit.

Discerning the effects of photoinhibition and photoprotection on the rate of oxygen evolution in Arabidopsis leaves

Higher plants possess a set of interconnected processes to regulate light harvesting. Non-photochemical quenching of chlorophyll a fluorescence (NPQ) is the fastest process activated to protect the photosystem (PS) II from the absorption of excess light energy. However, damage of PSII reaction centers (RCIIs) is often inevitable, a phenomenon known as photoinhibition. Both NPQ and photoinhibition undermine PSII quantum yield (ΦPSII). Recently, we devised a fluorescence-based methodology that uses the coefficient of photochemical quenching measured in the dark following illumination (qPd) to assess the intactness of RCIIs. This procedure enables to express ΦPSII as a function (ƒ) of NPQ and qPd, ΦPSII=ƒ(NPQ,qPd), thus allowing to efficiently discern between the effects of protective NPQ and photoinhibition upon the efficiency of electron transport. In this study, we addressed the relationship between qPd and ΦPSII measured by photosynthetic oxygen evolution in intact leaves of Arabidopsis. We found a linear correlation between qPd and ΦPSII of oxygen evolution (as well as Fv/Fm). This relates to the fact that qPd reflects the onset of photoinhibition. These results further demonstrate the validity of the qPd parameter and underlying theory in quantitatively assessing PSII efficiency solely by using this effective and simple fluorescence technique.

High-Resolution Three-Dimensional Structural Data Quantify the Impact of Photoinhibition on Long-Term Carbon Gain in Wheat Canopies in the Field.

Photoinhibition reduces photosynthetic productivity; however, it is difficult to quantify accurately in complex canopies partly because of a lack of high-resolution structural data on plant canopy architecture, which determines complex fluctuations of light in space and time. Here, we evaluate the effects of photoinhibition on long-term carbon gain (over 1 d) in three different wheat (Triticum aestivum) lines, which are architecturally diverse. We use a unique method for accurate digital three-dimensional reconstruction of canopies growing in the field. The reconstruction method captures unique architectural differences between lines, such as leaf angle, curvature, and leaf density, thus providing a sensitive method of evaluating the productivity of actual canopy structures that previously were difficult or impossible to obtain. We show that complex data on light distribution can be automatically obtained without conventional manual measurements. We use a mathematical model of photosynthesis parameterized by field data consisting of chlorophyll fluorescence, light response curves of carbon dioxide assimilation, and manual confirmation of canopy architecture and light attenuation. Model simulations show that photoinhibition alone can result in substantial reduction in carbon gain, but this is highly dependent on exact canopy architecture and the diurnal dynamics of photoinhibition. The use of such highly realistic canopy reconstructions also allows us to conclude that even a moderate change in leaf angle in upper layers of the wheat canopy led to a large increase in the number of leaves in a severely light-limited state.

Repetitive light pulse-induced photoinhibition of photosystem I severely affects CO2 assimilation and photoprotection in wheat leaves.

It was previously found that photosystem I (PSI) photoinhibition represents mostly irreversible damage with a slow recovery; however, its physiological significance has not been sufficiently characterized. The aim of the study was to assess the effect of PSI photoinhibition on photosynthesis in vivo. The inactivation of PSI was done by a series of short light saturation pulses applied by fluorimeter in darkness (every 10s for 15min), which led to decrease of both PSI (~60%) and photosystem II (PSII) (~15%) photochemical activity. No PSI recovery was observed within 2days, whereas the PSII was fully recovered. Strongly limited PSI electron transport led to an imbalance between PSII and PSI photochemistry, with a high excitation pressure on PSII acceptor side and low oxidation of the PSI donor side. Low and delayed light-induced NPQ and P700(+) rise in inactivated samples indicated a decrease in formation of transthylakoid proton gradient (ΔpH), which was confirmed also by analysis of electrochromic bandshift (ECSt) records. In parallel with photochemical parameters, the CO2 assimilation was also strongly inhibited, more in low light (~70%) than in high light (~45%); the decrease was not caused by stomatal closure. PSI electron transport limited the CO2 assimilation at low to moderate light intensities, but it seems not to be directly responsible for a low CO2 assimilation at high light. In this regard, the possible effects of PSI photoinhibition on the redox signaling in chloroplast and its role in downregulation of Calvin cycle activity are discussed.

A model for autotrophic growth of Chlorella vulgaris under photolimitation and photoinhibition in cylindrical photobioreactor

A model for the autotrophic growth of Chlorella vulgaris (C. vulgaris) under photolimitation and photoinhibition in cylindrical photobioreactor was developed by taking into account the interactions between the biomass concentration and light intensity. The growth kinetic model for a batch system was developed by coupling the light distribution under photolimitation and photoinhibition effects with the biomass growth with respect to incident light intensity on the surface of the reactor. The developed model could successfully simulate the specific growth rates, light intensity distribution in the reactor, average light intensities, and biomass concentrations at a given incident light intensity with good accuracy. The prediction of the cell density primarily depended on the determination of the parameters included in the specific growth rate expression, which is a function of the light intensity distribution. Despite the limitation of the semi-empirical specific growth rate expression, the coupled model could predict the biomass growth in different reactor sizes.

A metabolomic study in oats (Avena sativa) highlights a drought tolerance mechanism based upon salicylate signalling pathways and the modulation of carbon, antioxidant and photo-oxidative metabolism.

Although a wealth of information is available on the induction of one or several drought-related responses in different species, little is known of how their timing, modulation and crucially integration influence drought tolerance. Based upon metabolomic changes in oat (Avena sativa L.), we have defined key processes involved in drought tolerance. During a time course of increasing water deficit, metabolites from leaf samples were profiled using direct infusion-electrospray mass spectroscopy (DI-ESI-MS) and high-performance liquid chromatography (HPLC) ESI-MS/MS and analysed using principal component analysis (PCA) and discriminant function analysis (DFA). The involvement of metabolite pathways was confirmed through targeted assays of key metabolites and physiological experiments. We demonstrate an early accumulation of salicylic acid (SA) influencing stomatal opening, photorespiration and antioxidant defences before any change in the relative water content. These changes are likely to maintain plant water status, with any photoinhibitory effect being counteracted by an efficient antioxidant capacity, thereby representing an integrated mechanism of drought tolerance in oats. We also discuss these changes in relation to those engaged at later points, consequence of the different water status in susceptible and resistant genotypes.

Evaluation of maintenance energy requirements in the cultivation of Scenedesmus obliquus: effect of light intensity and regime

Most relevant factors influencing microalgae growth are light availability and utilization. In view of maximizing the photosynthetic efficiency of a photobioreactor, the evaluation of maintenance energy is essential to understand the energy requirement for cell growth and it is also a key parameter of most mathematical growth models. In this work, steady-state continuous experiments were carried out in a flat-bed photobioreactor, at different light intensities and irradiation regimes, measuring growth rate, productivity, lipid content, and photosynthetic efficiency of Scenedesmus obliquus at non-limiting N and P condition. Energy balance was applied to measure the maintenance term as a function of irradiation, under both continuous and simulated seasonal irradiation at middle latitudes. The energy requirement for maintenance was found to be greater under high irradiances as a result of the photoinhibition effect. Experimental data were correlated following a substrate inhibition model. The nutrient uptakes were found to depend on growth rate and light intensity and to be affected by the maintenance requirement.

Effects of light and temperature on the photoautotrophic growth and photoinhibition of nitrogen-fixing cyanobacterium Cyanothece sp. ATCC 51142

The unicellular, nitrogen-fixing cyanobacterium Cyanothece sp. ATCC 51142 is a promising strain with a remarkable capability of producing large quantities of hydrogen, an energy carrier long being promoted as an ideal fuel. Under extreme environmental conditions, significant reduction of cellular photosynthetic capability is commonly observed in algae and cyanobacteria. Even less severe conditions can induce photo-inhibitive growth dynamics, which in turn result in a marked decrease in biomass and gas productivities. In this study a detailed analysis of the effect of two extrinsic parameters, namely light intensity and temperature, on the photoautotrophic growth of Cyanothece was performed in order to reveal critical conditions that would lead to undesired photoinhibition. A high degree of coherence between cyanobacterial growth and nutrient uptake kinetics was observed, as well as a strong dependence on the change of the two parameters. Nitrogen depletion was confirmed as a trigger, which transforms an exponential into a stationary growth phase. A non-linear relationship between the maximum specific growth rate and the irradiance up to 320μEm−2s−1 was identified and found to be dominated by light saturation rather than photoinhibition. The relationship between the specific growth rate and the temperature was found to be linear until a remarkable drop in the final biomass productivity and cyanobacterial photosynthetic capability was observed at 40°C. The cause of this is a high temperature-induced photoinhibition effect.

Photoinhibition of germination in grass seed – Implications for prairie revegetation

Germination photoinhibition is not a recognized cause of revegetation failure; yet prolonged sunlight exposure can inhibit germination of several grass species. This research addressed susceptibility to photoinhibition of selected native grass species used to restore Canadian prairies, and reclamation treatments to alter environmental conditions in order to release seeds from photoinhibition. Under laboratory conditions effects of photoinhibition were tested on the ability of seeds to germinate at low water potential and effects of daily alternating temperatures and nitrates to break photoinhibition. Whether surficial mulch can release seeds from photoinhibition was assessed in a field experiment. Germination photoinhibition was evident in Festuca hallii and Koeleria macrantha seeds even under very low irradiances. The prolonged exposure to light decreased germination rates and ability of seeds to germinate at low water potentials. Daily fluctuating temperatures released a fraction of Bromus carinatus and Elymus trachycaulus seeds from photoinhibition yet did not improve F. hallii or K. macrantha germinability. Nitrates failed to break seed photoinhibition in all species tested. In the field experiment, mulched F. hallii seeds (covered with an erosion control blanket) showed a tenfold increase in germination percentages relative to seeds exposed to direct sunlight, indicating the facilitative effects of mulching on attenuation of the light environment. We conclude that germination photoinhibition as a cause of emergence failures in land reclamation where seed is broadcast or shallow seeded should be recognized and germination photoinhibition included in the decision making process to select revegetation seeding techniques.

Increasing canopy shading reduces growth but not establishment of Elodea nuttallii and Myriophyllum spicatum in stream channels

Submersed macrophytes are often important drivers of instream structure and function, but can be problematic when overabundant. The establishment success, growth rates, and morphology of submersed macrophytes could be affected by alteration of instream light levels during riparian restoration (via removing or planting canopy-forming vegetation), potentially influencing the success of riparian restoration projects aimed at improving aquatic habitats. To examine the effects of canopy shading on two common submersed macrophytes—Elodea nuttallii (native) and Myriophyllum spicatum (non-native)—I conducted experiments in artificial stream channels in two locations in California, USA. Initial establishment of stem fragments of both species was close to 100% in all shade levels, including shade that reduced incident light by 94%. Growth rates of the two species were similar across shade levels, and lowest in the highest shade. Full light appeared to have a photoinhibitory effect on E. nuttallii at the higher elevation site. Higher shade increased the length:biomass ratio and decreased the branching of E. nuttallii. My findings suggest that altering canopy cover during riparian restoration is unlikely to affect the ability of these species to establish, but higher shade levels should slow their growth and create more favorable conditions for other instream organisms.

Donor Side-Induced Photoinhibition in Photosystem II

To investigate the photoinhibition sites in PSII at different pH (pH 5.5, 7.0 and 8.5) and at the action of heavy metals (Cd2+ and Co2+) were monitored by delayed fluorescence of chlorophyll a in the millisecond range (ms-DF). During photoinhibition of PSII membranes by excess light (4000 μmol photons m−2 s−1) the fast and steady-state phases of ms-DF induction curve were monitored. We have observed strong photoinhibition effect on phases of ms-DF at the elevated pH values. At acidic pH photoinhibition of the fast and steady-state phases were less pronounced and, in addition, protective role of an exogenous acceptor PpBQ has been observed. In view of our previous results we suggest that change in equilibrium between YZ [(P680QA−) ↔ (YZ•+)] and Mn4O5Ca-cluster [(P680QA−) ↔ (Si+1+)] taking place when pH of medium is changed from low to high values. At high pH this leads to the donor side induced photoinhibition and the destruction of the Mn4O5Ca-cluster. Under simultaneously action of Cd2+ and Co2+ ions and photoinhibition PSII become very sensitive to action of inhibitory illumination against the background of heavy metals. This inhibitory effect appears very strongly in the case of inhibition of donor side of PSII by Cd2+. The main targets of the metals action may be partners for recombination with P680+, depend on medium pH, either Yz or Mn4O5Ca-cluster on the donor site for Cd2+ and Co2+ ions and between QA and QB on the acceptor site for Co2+ ions.