Action spectrum for a low intensity, rapid photoinhibition of mechanically stimulable bioluminescence in the marine dinoflagellates Gonyaulax catenella, G. acatenella, and G. tamarensis

The mechanically stimulable bioluminescence of members of the Gonyaulax catenella group can be maximally photoinhibited by exposure to as few as 10¹³ quanta/cm², a factor 10⁴ times smaller than that required for comparable photoinhibition in Gonyaulax polyedra and all other photosynthetic bioluminescent dinoflagellates investigated. Following an irradiation pulse there is an initial time lag of one minute, followed by a rapid decrease in mechanical stimulability to approximately 1% of the dark unirradiated control with a firstorder rate constant as high as 0.01 sec^?1. Action spectra for all three species imply a pigment with a single absorption band having a maximum at 562 nm and a half band width of 105 nm within the spectral range 325 nm to 775 nm. Photoinhibition appears to decrease either the sensitivity of the shear receptor mechanism or the efficiency of signal transmission in the dinoflagellates, since chemically stimulable bioluminescence is unaffected by these exposures.     

Photoadaptation, photoinhibition and productivity in the blue-green alga, Spirulina platensis grown outdoors

Two Spirulina platensis strains, SP-G and SP-RB, resistant and sensitive to photoinhibition of photosynthesis, respectively, were grown outdoors in dense cultures and under different photon fluxes provided by shading. Cultures of both strains grown under full sunlight were more resistant to photoinhibition than those grown under nets with 15–50% decreases in the incident photon flux. Cultures grown outdoors were more resistant to photoinhibition than the laboratory ones. At noon, the photosynthetic activity, as expressed by O₂ evolution, was higher for cultures grown under 50% shade, as compared with unshaded cultures. Productivity of the shaded cultures, in terms of biomass produced per day, was always higher when the cultures were protected from photoinhibition.     

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.     

Irreversible photoinhibition of photosystem II is caused by exposure of Synechocystis cells to strong light for a prolonged period

Irreversible photoinhibition of photosystem II (PSII) occurred when Synechocystis sp. PCC 6803 cells were exposed to very strong light for a prolonged period. When wild-type cells were illuminated at 20 degrees C for 2 h with light at an intensity of 2,500 micromol photons m(-2) s(-1), the oxygen-evolving activity of PSII was almost entirely and irreversibly lost, whereas the photochemical reaction center in PSII was inactivated only reversibly. The extent of irreversible photoinhibition was enhanced at lower temperatures and by the genetically engineered rigidification of membrane lipids. Western and Northern blotting demonstrated that, after cells had undergone irreversible photoinhibition, the precursor to D1 protein in PSII was synthesized but not processed properly. These observations may suggest that exposure of Synechocystis cells to strong light results in the irreversible photoinhibition of the oxygen-evolving activity of PSII via impairment of the processing of pre-D1 and that this effect of strong light is enhanced by the rigidification of membrane lipids.     

Mechanistic differences in photoinhibition of sun and shade plants

Leaf discs of the shade plant Tradescantia albiflora Kunth grown at 50 μmol · m^?2 · s^?1, and the facultative sun/shade plant Pisum sativum L. grown at 50 or 300 μmol · m^?2, s^?1, were photoinhibited for 4 h in 1700 μmol photons m^?2 · s^?1 at 22° C. The effects of photoinhibition on the following parameters were studied: i) photosystem II (PSII) function; ii) amount of D1 protein in the PSII reaction centre; iii) dependence of photoinhibition and its recovery on chloroplast-encoded protein synthesis; and, iv) the sensitivity of photosynthesis to photoinhibition in the presence or absence of the carotenoid zeaxanthin. We show that: i) despite different sensitivities to photoinhibition, photoinhibition in all three plants occurred at the reaction centre of PSII; ii) there was no correlation between the extent of photoinhibition and the degradation of the D1 protein; iii) the susceptibility to photoinhibition by blockage of chloroplas-tencoded protein synthesis was much less in shade plants than in plants acclimated to higher light; and iv) inhibition of zeaxanthin formation increased the sensitivity to photoinhibition in pea, but not in the shade plant Tradescantia. We suggest that there are mechanistic differences in photoinhibition of sun and shade plants. In sun plants, an active repair cycle of PSII replaces photoinhibited reaction centres with photochemically active ones, thereby conferring partial protection against photoinhibition. However, in shade plants, this repair cycle is less important for protection against photoinhibition; instead, photoinhibited PSII reaction centres may confer, as they accumulate, increased protection of the remaining connected, functional PSII centres by controlled, nonphotochemical dissipation of excess excitation energy.     

Photosynthesis,photoinhibition and low temperature acclimation in cold tolerant plants

Cold acclimation requires adjustment to a combination of light and low temperature, conditions which are potentially photoinhibitory. The photosynthetic response of plants to low temperature is dependent upon time of exposure and the developmental history of the leaves. Exposure of fully expanded leaves of winter cereals to short-term, low temperature shiftsinhibits whereas low temperature growthstimulates electron transport capacity and carbon assimilation. However, the photosynthetic response to low temperature is clearly species and cultivar dependent. Winter annuals and algae which actively grow and develop at low temperature and moderate irradiance acquire a resistance to irradiance 5- to 6-fold higher than their growth irradiance. Resistance to short-term photoinhibition (hours) in winter cereals is a reflection of the increased capacity to keep Q_A oxidized under high light conditions and low temperature. This is due to an increased capacity for photosynthesis. These characteristics reflect photosynthetic acclimation to low growth temperature and can be used to predict the freezing tolerance of cereals. It is proposed that the enhanced photosynthetic capacity reflects an increased flux of fixed carbon through to sucrose in source tissue as a consequence of the combined effects of increased storage of carbohydrate as fructans in the vacuole of leaf mesophyll cells and an enhanced export to the crown due to its increased sink activity. Long-term exposure (months) of cereals to low temperature photoinhibition indicates that this reduction of photochemical efficiency of PS II represents a stable, long-term down regulation of PS II to match the energy requirements for CO₂ fixation. Thus, photoinhibition in vivo should be viewed as the capacity of plants to adjust photosynthetically to the prevailing environmental conditions rather than a process which necessarily results in damage or injury to plants. Not all cold tolerant, herbaceous annuals use the same mechanism to acquire resistance to photoinhibition. In contrast to annuals and algae, overwintering evergreens become dormant during the cold hardening period and generally remain susceptible to photoinhibition. It is concluded that the photosynthetic response to low temperatures and susceptibility to photoinhibition are consequences of the overwintering strategy of the plant species.     

Pigments, photosynthesis and photoinhibition in two amphibious plants: consequences of varying carbon availability

In the present study, we investigated the effects of CO(2) availability on photosynthesis, photoinhibition and pigmentation in two species of amphibious plants, Lobelia cardinalis and Nesaea crassicaulis. The plants were grown emergent under atmospheric conditions and submerged under low and high CO(2) availability. Compared with Lobelia, Nesaea had thin leaves and few stomata in all CO(2) treatments. While Lobelia expressed no variation in anthocyanin content among treatments, Nesaea produced high concentrations of anthocyanin when submerged. Lobelia photosynthesis increased in response to increasing CO(2) availability, and photoinhibition was negatively related to xanthophyll content. By contrast, Nesaea photosynthesis was highest under submerged conditions, and there was no relationship between photoinhibition and the xanthophyll content. We conclude that the response of Lobelia to varying CO(2) availability is similar to that of terrestrial plants and that this species relies on the xanthophyll cycle for nonphotochemical quenching (NPQ) and protection against photoinhibition. By contrast, the thin leaves, few stomata and low levels of chlorophylls and accessory pigments in Nesaea, relative to Lobelia, suggest adaptation to a submerged habitat. While Nesaea does not seem to rely on the xanthophyll cycle or other xanthophylls for NPQ, some role of anthocyanins in the protection against photoinhibition cannot be ruled out, owing to its effect as a sunscreen and as an efficient quencher of free radicals.     

Irreversible photoinhibition of photosystem II is caused by exposure of Synechocystis cells to strong light for a prolonged period

Irreversible photoinhibition of photosystem II (PSII) occurred when Synechocystis sp. PCC 6803 cells were exposed to very strong light for a prolonged period. When wild-type cells were illuminated at 20 °C for 2 h with light at an intensity of 2,500 μmol photons m⁻² s⁻¹, the oxygen-evolving activity of PSII was almost entirely and irreversibly lost, whereas the photochemical reaction center in PSII was inactivated only reversibly. The extent of irreversible photoinhibition was enhanced at lower temperatures and by the genetically engineered rigidification of membrane lipids. Western and Northern blotting demonstrated that, after cells had undergone irreversible photoinhibition, the precursor to D1 protein in PSII was synthesized but not processed properly. These observations may suggest that exposure of Synechocystis cells to strong light results in the irreversible photoinhibition of the oxygen-evolving activity of PSII via impairment of the processing of pre-D1 and that this effect of strong light is enhanced by the rigidification of membrane lipids.     

Relationships between phosphatidylglycerol molecular species of thylakoid membrane lipids and sensitivities to chilling-induced photoinhibition in rice

In an attempt to explore the relationships between phosphatidylglycerol （PG） molecular species of thylakoid membrane lipids and sensitivities to chilling-induced photoinhibition, PG molecular species, D1 protein, electron transport activities of thylakoid membrane and the potential quantum yield （FvlFm） in rice treated under middle and low photon flux density （PFD） at 11℃ were analyzed by high performance liquid chromatography, enzyme hydrolysis, gas phase chromatography （GC） and so on. Results showed that the major molecular species of PGs in rice thylakoid membrane were 18：3/16：0, 18：3/16：1（3t）, 18：2/16：0, 18：2/16：1（3t）, 18：1/16：0, 18：1/16：1（3t）, 16：0/16：0, 16：0/16：1（3t）. There were large differences in the contents of unsaturated PG molecular species such as 18：1-3/16：0-16：1（3t） and saturated PG molecular species like 16：0/16：0-16：1（3t） among japonica cv 9516 0-9516）, japonica-indica hybrid F1 j-9516/i-SY63 （ji-95SY） and indica cv Shanyou 63 （i-SY63）. J-9516 containing higher contents of unsaturated PG molecular species was manifest in stable D1 protein contents under chill and tolerant to chill-induced photoinhibition. In contrast to j-9516, i-SY63 with lower contents of unsaturated PG molecular species, exhibited unstable D1 protein contents under chill and was sensitive to chill-induced photoinhibition, ji-95SY containing middle contents of unsaturated PG molecular species between those of j-9516 and i-SY63, exhibited mid extent of sensitivity to chill-induced photoinhibition. The losses in D1 protein also account for the inhibition in electron transport activity of thylakoid membrane and the observed decline in FvlFm. The PG molecular species that is efficient in raising chilling-resistant capacity were those containing unsaturated fatty acids, namely, unsaturated PG molecular species. These results implied that the substrate selectivity of the glycerol-3-phosphate acyltransferase in chloroplasts towards 16：0 or 18：1 displayed greatly the difference between japonica and indica rice. Itwas possible to enhance the capacity of resistance to chilling-induced photoinhibition by improving or modifying the GPAT gene.     

Mathematical modelling of the light response curve of photoinhibition of Photosystem II

The light response curves of the acceptor and donor side mechanisms of photoinhibition of Photosystem II were calculated, using Arabidopsis as a model organism. Acceptor-side photoinhibition was modelled as double reduction of Q_A, noting that non-photochemical quenching has the same effect on the quantum yield of Q_A double reduction in closed PSII centres as it has on the quantum yield of electron transport in open centres. The light response curve of acceptor-side photoinhibition in Arabidopsis shows very low efficiency under low intensity light and a relatively constant quantum yield above light saturation of photosynthesis. To calculate the light response curve of donor-side photoinhibition, we built a model describing the concentration of the oxidized primary donor P₆₈₀⁺ during steady-state photosynthesis. The model is based on literature values of rate constants of electron transfer reactions of PSII, and it was fitted with fluorescence parameters measured in the steady state. The modelling analysis showed that the quantum yield of donor-side photoinhibition peaks under moderate light. The deviation of the acceptor and donor side mechanisms from the direct proportionality between photoinhibition and photon flux density suggests that these mechanisms cannot solely account for photoinhibition in vivo, but contribution of a reaction whose quantum yield is independent of light intensity is needed. Furthermore, a simple kinetic calculation suggests that the acceptor-side mechanism may not explain singlet oxygen production by photoinhibited leaves. The theoretical framework described here can be used to estimate the yields of different photoinhibition mechanisms under different wavelengths or light intensities.     

Influence of photoinhibition on electron transport and photophosphorylation of isolated chloroplasts

The effects of a photoinhibition treatment (PIT) on electron transport and photophosphorylation reactions were measured in chloroplasts isolated from triazine-resistant and susceptible Chenopodium album plants grown under high and low irradiance. Electron transport dependent on photosystem I (PSI) alone was much less affected by PIT than that dependent on both photosystem II (PSII) and PSI. There was a smaller difference in susceptibility to PIT between the photophosphorylation activitity dependent on PSI alone and that dependent on both PSII and PSI. Because in all cases photophosphorylation activity decreased faster upon PIT than the rate of electron transport, we conclude that photoinhibition causes a gradual uncoupling of electron transport with phosphorylation. Since the extent of the light-induced proton gradient across the thylakoid membrane decreased upon PIT, it is suggested that photoinhibiton causes a proton leakiness of the membrane. We have found no significant differences to PIT of the various reactions measured in chloroplasts isolated from triazine-resistant and susceptible plants. We have also not observed any significant differences to PIT of the photophosphorylation reactions in chloroplasts of plants grown under low irradiance, compared with those grown under high irradiance. However, the electron transport reactions in chloroplasts from plants grown under low irradiance appeared to be somewhat less sensitive to PIT than those grown under high irradiance.     

Evidence for the role of the oxygen-evolving manganese complex in photoinhibition of Photosystem II

Photoinhibition of PSII occurs at the same quantum efficiency from very low to very high light, which raises a question about how important is the rate of photosynthetic electron transfer in photoinhibition. We modulated electron transfer rate and light intensity independently of each other in lincomycin-treated pea leaves and in isolated thylakoids, in order to elucidate the specific effects of light and PSII electron transport on photoinhibition. Major changes in the rate of electron transport caused only small changes in the rate of photoinhibition, suggesting the existence of a significant photoinhibitory pathway that contains an electron-transfer-independent phase. We compared the action spectrum of photoinhibition with absorption spectra of PSII components that could function as photoreceptors of the electron-transfer-independent phase of photoinhibition and found that the absorption spectra of Mn(III) and Mn(IV) compounds resemble the action spectrum of photoinhibition, showing a steep decrease from UV-C to blue light and a low visible-light tail. Our results show that the release of a Mn ion to the thylakoid lumen is the earliest detectable step of both UV- and visible-light-induced photoinhibition. After Mn release from the oxygen-evolving complex, oxidative damage to the PSII reaction center occurs because the Mn-depleted oxygen-evolving complex cannot reduce P680+ normally.     

Small light-harvesting antenna does not protect from photoinhibition

High-light-induced decrease in photosystem II (PSII) electron transfer activity was studied in high- and low-light-grown pumpkin (Cucurbita pepo L.) plants in vivo and in vitro. The PSII light-harvesting antenna of the low-light leaves was estimated to be twice as big as that of the high-light leaves. The low-light leaves were more susceptible to photoinhibition in vivo. However, thylakoids isolated from these two plant materials were equally sensitive to photoinhibition when illuminated in the absence of external electron acceptors. Only the intensity of the photoinhibitory light and the chlorophyll concentration of the sample, not the size of the light-harvesting antenna, determined the rate of PSII photoinhibition in vitro. Because excitation of the reaction center and not only the antenna chlorophylls is a prerequisite for photoinhibition of PSII activity, independence of photoinhibition on antenna size provides support for the hypothesis (Schatz EH, Brock H, Holzwarth AR [1988] Biophys J 54: 397-405) that the excitations of the antenna chlorophylls are in equilibrium with the excitations of the reaction centers. Better tolerance of the high-light leaves in vivo was due to a more active repair process and more powerful protective mechanisms, including photosynthesis. Apparently, some protective mechanism of the high-light-grown plants is at least partially active at low temperature. The protective mechanisms do not appear to function in vitro.     

Rapid turnover of a component required for photosynthesis explains temperature dependence and kinetics of photoinhibition in a cyanobacterium,Synechococcus 6301

Illumination of a liquid culture of Synechococcus 6301 at high photon flux density (PFD) elicits a time-dependent first-order exponential decline in relative quantum yield of photosynthetic O₂ evolution to some steady-state value. Full photosynthetic activity is restored, also as a time-dependent first-order process, when the photoinhibited culture is transferred to lower PFD. Temperature and irradiation dependence of photoinhibition were measured under conditions which precluded simultaneous recovery from photoinhibition. Also the temperature and irradiation dependence of recovery from photoinhibition were determined under conditions which precluded simultaneous photoinhibition. Kinetics of photoinhibition were sensitive to PFD but relatively independent of temperature. Kinetics of recovery saturated at low PFD but were very temperature dependent at all PFDs. A general equation can be written to predict the change in photosynthetic activity versus time when a cell culture is placed at photoinhibitory PFD, assuming that first-order exponential photoinhibition and first-order exponential recovery from photoinhibition occur simultaneously. The equation can be made specific if the values of the kinetic constant for photoinhibition and for recovery from photoinhibition are known for the particular environmental conditions to which the cells are exposed. These values can be obtained by independently measuring the kinetics of photoinhibition without simultaneous recovery and the kinetics of recovery without simultaneous photoinhibition. The curve of photosynthetic activity versus time for cells placed at high PFD, which is predicted by this equation, precisely fits the experimentally determined kinetics of photoinhibition. This correlation remains valid over a wide range of temperatures and PFDs. Identical results were obtained with the marine cyanobacterium Synechococcus 7002. We conclude that the extent of net photoinhibition over a broad range of conditions represents a sum of individual rates of simultaneous photoinhibition and recovery from photoinhibition. The results support previous proposals that a protein required for photosystem II activity becomes functionally depleted during photoinhibition because protein synthesis or assembly into the membranes cannot keep up with the rate of its inactivation at excessively high PFDs. We also conclude that photoinhibition and light-dependent chilling sensitivity are manifestations of the same phenomenon.Abbreviations CAP chloramphenicol - Chl chlorophyll - PFD photon flux density - PSII photosystem II The authors thank Rockey Butler and Donna Scott for performing many of the preliminary experiments which led to this research. This work was supported by R.A. Welch and University Research Institute Grants to J.J.B.     

Excess copper predisposes photosystem II to photoinhibition in vivo by outcompeting iron and causing decrease in leaf chlorophyll

Photoinhibition of photosystem II was studied in vivo with bean (Phaseolus vulgaris) plants grown in the presence of 0.3 (control), 4, or 15 microM Cu(2+). Although photoinhibition, measured in the presence of lincomycin to block concurrent recovery, is faster in leaves of Cu(2+)-treated plants than in control leaves, thylakoids isolated from Cu-treated plants did not show high sensitivity to photoinhibition. Direct effects of excess Cu(2+) on chloroplast metabolism are actually unlikely, because the Cu concentration of chloroplasts of Cu-treated plants was lower than that of their leaves. Excess Cu in the growth medium did not cause severe oxidative stress, collapse of antioxidative defenses, or loss of photoprotection. Thus, these hypothetical effects can be eliminated as causes for Cu-enhanced photoinhibition in intact leaves. However, Cu treatment lowered the leaf chlorophyll (Chl) concentration and reduced the thylakoid membrane network. The loss of Chl and sensitivity to photoinhibition could be overcome by adding excess Fe together with excess Cu to the growth medium. The addition of Fe lowered the Cu(2+) concentration of the leaves, suggesting that Cu outcompetes Fe in Fe uptake. We suggest that the reduction of leaf Chl concentration, caused by the Cu-induced iron deficiency, causes the high photosensitivity of photosystem II in Cu(2+)-treated plants. A causal relationship between the susceptibility to photoinhibition and the leaf optical density was established in several plant species. Plant species adapted to high-light habitats apparently benefit from thick leaves because the rate of photoinhibition is directly proportional to light intensity, but photosynthesis becomes saturated by moderate light.     

Turnover of thylakoid photosystem II proteins during photoinhibition of Chlamydomonas reinhardtii

The turnover of photosystem-II proteins during photoinhibition was analyzed in the green alga Chlamydomonas reinhardtii. Changes in the amount of photosystem II core complex polypeptides D1, D2, 44 kDa and 51 kDa, the antennae-CP-29 and light-harvesting-complex-II polypeptides and the water-oxidizing complex polypeptides of 30 kDa, 23 kDa and 16 kDa were monitored by a variety of techniques. Only the D1 and D2 polypeptides were found to turnover during photoinhibition when cells were exposed to ten fold photosynthesis-saturating light (2500 W/m2 for 90 min) at 25 degrees C. While 80% of photosystem-II activity was lost, a reduction of only 20% was observed in the total amount of D1 and D2 proteins. However, inhibition of chloroplast translation by chloramphenicol during photoinhibition resulted in the loss of about 60% of the D1 and 40% of the D2 proteins, as demonstrated by Western blotting and dot blotting of isolated thylakoids, quantitative analysis of immunogold-labeled whole-cell thin sections, and chase of radioactively prelabelled proteins during photoinhibition. We propose that the light-dependent turnover of the D1 protein is a protective mechanism against photoinhibition as far as the removal and replacement of D1 is compatible with the photoinactivation incurred by photosystem II. At light intensities at which the rate of D1 removal becomes limiting, loss of photosystem-II activity exceeds the turnover of D1 and the stability of the D2 protein is impaired as well.     

Midday photoinhibition of two newly developed super-rice hybrids

Super-rice hybrids are two-line hybrid rice cultivars with 15 to 20 % higher yields than the raditional three-line hybrid rice cultivars. Response of photosynthetic functions to midday photoinhibition was compared between seedlings of the traditional hybrid rice (Oryza sativa L.) Shanyou63 and two super-rice hybrids, Hua-an3 and Liangyoupeijiu. Under strong midday sunlight, in comparison with Shanyou63, the two super-rice hybrids were less photoinhibited, as indicated by the lower loss of the net photosynthetic rate (P _N), the quantum yield of photosystem 2 (Φ_PS2), and the maximum and effective quantum yield of PS2 photochemistry (F_v/F_m and F_v′/F_m′). They also had a much higher transpiration rate. Hence the super-rice hybrids could protect themselves against midday photoinhibition at the cost of water. The photoprotective de-epoxidized xanthophyll cycle components, antheraxanthin (A) and zeaxanthin (Z), were accumulated more in Hua-an3 and Liangyoupeijiu than in Shanyou63, but the size of xanthophyll cycle pool of the seedlings was not affected by midday photoinhibition. Compared to Shanyou63, the super-rice hybrids were better photoprotected under natural high irradiance stress and the accumulation of Z and A, not the size of the xanthophyll pool protected the rice hybrids against photoinhibition.     

Development of light-shielding hydrogel for nitrifying bacteria to prevent photoinhibition under strong light irradiation

Microalgae-nitrifying bacteria consortia have gained attention because photooxygenation of algae can supply oxygen to bacteria which eliminates the need for costly mechanical aeration. However, nitrifying bacteria are known to suffer from photoinhibition. In this study, we developed “Light-shielding hydrogel”, in which bacteria were immobilized in hydrogel and light-shielding particles (carbon black) were incorporated, and evaluated its effectiveness to mitigate photoinhibition for bacteria under strong light irradiation. For comparison, “Hydrogel”, in which bacteria were immobilized in hydrogel without carbon black, and “Dispersion” which was simply suspended bacteria were prepared. At 1600 μmol photons m⁻² s^–1, the nitrification performance markedly decreased to 15.1 and 48.0% compared to the dark condition in the Dispersion and the Hydrogel, respectively. Meanwhile, it was successfully maintained for the Light-shielding hydrogel. Our results showed that the effectiveness of light-shielding hydrogel to mitigate photoinhibition on nitrifying bacteria even under strong light irradiation.     

ABA对水稻幼苗叶片光抑制的光保护作用

经ABA处理的水稻幼苗叶片和对照相比 ,PSII光化学效率 (Fv/Fm)和非光化学猝灭系数 (qN)显著受抑制。经高光处理 1h后 ,ABA处理的水稻幼苗叶片光抑制程度比对照小 ,这暗示ABA对高光光抑制具有一定的光保护作用 ,且间接表明ABA提高水稻幼苗抗光抑制的能力与叶黄素循环密切相关。     

Intra-leaf gradients of photoinhibition induced by different color lights: implications for the dual mechanisms of photoinhibition and for the application of conventional chlorophyll fluorometers

? We studied how different color lights cause gradients of photoinhibition within a leaf, to attempt to resolve the controversy of whether photon absorption by chlorophyll or by manganese (Mn) is the primary cause of photoinhibition, as suggested by the excess-energy hypothesis or the two-step hypothesis, respectively. ? Lincomycin-treated leaf discs were photoinhibited by white, blue, green or red light. Combining a microfiber fluorometer, a fiber-thinning technique and a micro-manipulator enabled us to measure the chlorophyll fluorescence signals within a leaf. Photoinhibition gradients were also compared with results from various conventional fluorometers to estimate their depth of signal detection. ? The severity of photoinhibition was in the descending order of blue, red and green light near the adaxial surface, and in the descending order of blue, green and red light in the deeper tissue, which correlated with the chlorophyll and the Mn absorption spectrums, respectively. These results cannot be explained by either hypothesis alone. ? These data strongly suggest that both the excess-energy and the two-step mechanisms occur in photoinhibition, and fluorometers with red or blue measuring light give overestimated or underestimated F(v)/F(m) values of photoinhibited leaves compared with the whole tissue average, respectively; that is, they measured deeper or shallower leaf tissue, respectively.