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

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

Salt stress and fluctuating light have separate effects on photosynthetic acclimation,but interactively affect biomass

In nature, soil salinity and fluctuating light (FL) often occur concomitantly. However, it is unknown whether salt stress interacts with FL on leaf photosynthesis, architecture, biochemistry, pigmentation, mineral concentrations, as well as whole-plant biomass. To elucidate this, tomato (Solanum lycopersicum) seedlings were grown under constant light (C, 200 μmol m⁻² s⁻¹) or FL (5–650 μmol m⁻² s⁻¹), in combination with no (0 mM NaCl) or moderate (80 mM NaCl) salinity, for 14 days, at identical photoperiods and daily light integrals. FL and salt stress had separate effects on leaf anatomy, biochemistry and photosynthetic capacity: FL reduced leaf thickness as well as nitrogen, chlorophyll and carotenoid contents per unit leaf area, but rarely affected steady-state and dynamic photosynthetic properties along with abundance of key proteins in the electron transport chain. Salt stress, meanwhile, mainly disorganized chloroplast grana stacking, reduced stomatal density, size and aperture as well as photosynthetic capacity. Plant biomass was affected interactively by light regime and salt stress: FL reduced biomass in salt stressed plants by 17%, but it did not affect biomass of non-stressed plants. Our results stress the importance of considering FL when inferring effects of salt-stress on photosynthesis and productivity under fluctuating light intensities.     

Adjustment of photosynthetic activity to drought and fluctuating light in wheat

Drought is a major cause of losses in crop yield. Under field conditions, plants exposed to drought are usually also experiencing rapid changes in light intensity. Accordingly, plants need to acclimate to both, drought and light stress. Two crucial mechanisms in plant acclimation to changes in light conditions comprise thylakoid protein phosphorylation and dissipation of light energy as heat by non-photochemical quenching (NPQ). Here, we analyzed the acclimation efficacy of two different wheat varieties, by applying fluctuating light for analysis of plants, which had been subjected to a slowly developing drought stress as it usually occurs in the field. This novel approach allowed us to distinguish four drought phases, which are critical for grain yield, and to discover acclimatory responses which are independent of photodamage. In short-term, under fluctuating light, the slowdown of NPQ relaxation adjusts the photosynthetic activity to the reduced metabolic capacity. In long-term, the photosynthetic machinery acquires a drought-specific configuration by changing the PSII-LHCII phosphorylation pattern together with protein stoichiometry. Therefore, the fine-tuning of NPQ relaxation and PSII-LHCII phosphorylation pattern represent promising traits for future crop breeding strategies.     

Balancing the energy flow from captured light to biomass under fluctuating light conditions

The balance of energy flow from light absorption into biomass was investigated under simulated natural light conditions in the diatom Phaeodactylum tricornutum and the green alga Chlorella vulgaris. The energy balance was quantified by comparative analysis of carbon accumulation in the new biomass with photosynthetic electron transport rates per absorbed quantum, measured both by fluorescence quenching and oxygen production. The difference between fluorescence- and oxygen-based electron flow is defined as 'alternative electron cycling'. The photosynthetic efficiency of biomass production was found to be identical for both algae under nonfluctuating light conditions. In a fluctuating light regime, a much higher conversion efficiency of photosynthetic energy into biomass was observed in the diatom compared with the green alga. The data clearly show that the diatom utilizes a different strategy in the dissipation of excessively absorbed energy compared with the green alga. Consequently, in a fluctuating light climate, the differences between green algae and diatoms in the efficiency of biomass production per photon absorbed are caused by the different amount of alternative electron cycling.     

水淹导致皇冠草光合机构发生变化并加剧其出水后光抑制

通过气体交换和叶绿素荧光等方法研究了水淹及胁迫解除后皇冠草不同功能叶的光合特性及光抑制的变化.结果表明:与对照相比,气生叶(全淹组淹水前形成的功能叶)在水淹条件下叶片大小和气孔没有明显变化,但沉水叶(全淹组淹水后新生的功能叶)的叶面积增加,气孔变小,上表皮气孔密度增加.水淹导致气生叶碳同化能力、光化学效率和叶绿素含量下降.沉水叶在发育过程中碳同化能力、光化学效率和叶绿素逐渐升高.气生叶和沉水叶出水后其活体叶片在强光下的相对含水量急剧下降,发生明显的光抑制;而弱光下无明显光抑制发生.出水后离体叶片强光照射下6h后两种功能叶均发生严重光抑制,且弱光下不能恢复.因此,可以认为淹水条件下,沉水叶上表皮气孔密度的增加使其蒸腾速率提高;沉水叶较强的碳同化能力和增加的叶面积是确保其植株水下生存的重要因素;强光使气生叶和沉水叶出水后均发生严重光抑制,导度和蒸腾速率提高导致的叶片失水则加剧了这一过程,两者共同作用导致自然条件下两种功能叶的出水死亡.     

Forest biomass energy: assessing atmospheric carbon impacts by discounting future carbon flows

Although forest biomass energy was long assumed to be carbon neutral, many studies show delays between forest biomass carbon emissions and sequestration, with biomass carbon causing climate change damage in the interim. While some models suggest that these primary biomass carbon effects may be mitigated by induced market effects, for example, from landowner decisions to increase afforestation due to higher biomass prices, the delayed carbon sequestration of biomass energy systems still creates considerable scientific debate (i.e., how to assess effects) and policy debate (i.e., how to act given these effects). Forests can be carbon sinks, but their carbon absorption capacity is finite. Filling the sink with fossil fuel carbon thus has a cost, and conversely, harvesting a forest for biomass energy – which depletes the carbon sink – creates potential benefits from carbon sequestration. These values of forest carbon sinks have not generally been considered. Using data from the 2010 Manomet Center for Conservation Sciences ‘Biomass sustainability and carbon policy study’ and a model of forest biomass carbon system dynamics, we investigate how discounting future carbon flows affects the comparison of biomass energy to fossil fuels in Massachusetts, USA. Drawing from established financial valuation metrics, we calculate internal rates of return (IRR) as explicit estimates of the temporal values of forest biomass carbon emissions. Comparing these IRR to typical private discount rates, we find forest biomass energy to be preferred to fossil fuel energy in some applications. We discuss possible rationales for zero and near‐zero social discount rates with respect to carbon emissions, showing that social discount rates depend in part on expectations about how climate change affects future economic growth. With near‐zero discount rates, forest biomass energy is preferred to fossil fuels in all applications studied. Higher IRR biomass energy uses (e.g., thermal applications) are preferred to lower IRR uses (e.g., electricity generation without heat recovery).     

闽楠幼树光合特性及生物量分配对光环境的响应

设置100%光环境(L100)、40%光环境(L40)、8%光环境(L8)3种光照梯度,分析万木林闽楠(Phoebe bournei)幼树的光合特性及生物量分配特征对不同光环境的响应。结果表明:(1)闽楠幼树在不同光环境下的最大净光合速率(Pnmax),表观量子效率(AQY),光饱和点(LSP),光补偿点(LCP),暗呼吸速率(Rd)均有显著差异(P<0.05),且随着光照强度的降低,Pnmax,LSP,LCP,Rd随之降低,而AQY却呈现升高的趋势。(2)总生物量、茎生物量、叶生物量、根生物量均表现为L100最大,根冠比、叶生物量比、茎生物量比及根生物量比在3种光环境下无显著差异。(3)闽楠幼树通过改变光合特性,生物量积累来适应光环境的变化,其中光照强度的降低限制了生物量的积累,但并未显著改变生物量地上地下分配比例。闽楠幼树在3种光环境下生物量分配比例并无显著的改变,生物量分配可塑性极低可能是闽楠零散分布的一个重要非人为干扰因素。(4)闽楠幼树所采取的生存策略以地上部分生长为主,光照强度降低时则采取保守策略进行缓慢的资源获取和消耗,全光照条件下采取快速的资源获取和消耗策略。未来造林时可以将闽楠与毛竹(Phyllostachys pubescens)混交来减少郁闭度,促进闽楠幼树的生长。     

Minimal requirements for in vitro reconstitution of the structural subunit of light-harvesting complexes of photosynthetic bacteria

Unlike the and polypeptides of the core light-harvesting complex (LH1) of Rhodobacter (Rb.) sphaeroides, the and polypeptides of the peripheral light-harvesting complex (LH2) of this organism will not form a subunit complex by in vitro reconstitution with bacteriochlorophyll. Guided by prior experiments with the LH1 polypeptides of Rb. sphaeroides and Rhodospirillum rubrum, which defined a set of interactions required to stabilize the subunit complex, a series of mutations to the Rb. sphaeroides LH2 polypeptide was prepared and studied to determine the minimal changes necessary to enable it to form a subunit-type complex. Three mutants were prepared: Arg at position –10 was changed to Asn (numbering is from the conserved His residue which is known to be coordinated to bacteriochlorophyll); Arg at position –10 and Thr at position +7 were changed to Asn and Arg, respectively; and Arg at position –10 was changed to Trp and the C-terminus from +4 to +10 was replaced with the amino acids found at the corresponding positions in the LH1 polypeptide of Rb. sphaeroides. Only this last multiple mutant polypeptide formed subunit-type complexes in vitro. Thus, the importance of the C-terminal region, which encompasses conserved residues at positions +4, +6 and +7, is confirmed. Two mutants of the LH1 polypeptide of Rb. sphaeroides were also constructed to further evaluate the interactions stabilizing the subunit complex and those necessary for oligomerization of subunits to form LH1 complexes. In one of these mutants, Trp at position –10 was changed to Arg, as found in LH2 at this position, and in the other His at position –18 was changed to Val. The results from these mutants allow us to conclude that the residue at the –10 position is unimportant in subunit formation or oligomerization, while the strictly conserved His at –18 is not required for subunit formation but is very important in oligomerization of subunits to form LH1.     

Photosynthesis research in the People's Republic of China1

The first research paper on photosynthesis in China was published by T.T. Li² in 1929. Two photosynthesis laboratories were established in Shanghai and Beijing in the 1950s and the 1960s, respectively. A photophosphorylation `intermediate' was discovered after the energy conversion process was separated into light and dark phases in the 1960s. Since the 1980s, research has accelerated at several different levels through efforts of a large number of scientists in China. This revised version was published online in August 2006 with corrections to the Cover Date.     

Detection of photosynthetic energy storage in a photosystem I reaction center preparation by photoacoustic spectroscopy

Thermal emission and photochemical energy storage were examined in photosystem I reaction center/core antenna complexes (about 40 Chl a/P700) using photoacoustic spectroscopy. Satisfactory signals could only be obtained from samples bound to hydroxyapatite and all samples had a low signal-to-noise ratio compared to either PS I or PS II in thylakoid membranes. The energy storage signal was saturated at low intensity (half saturation at 1.5 W m^-2) and predicted a photochemical quantum yield of >90%. Exogenous donors and acceptors had no effect on the signal amplitudes indicating that energy storage is the result of charge separation between endogenous components. Fe(CN)₆ ^-3 oxidation of P700 and dithionite-induced reduction of acceptors F_A-F_B inhibited energy storage. These data are compatible with the hypothesis that energy storage in PS I arises from charge separation between P700 and Fe-S centers F_A-F_B that is stable on the time scale of the photoacoustic modulation. High intensity background light (160 W m^-2) caused an irreversible loss of energy storage and correlated with a decrease in oxidizable P700; both are probably the result of high light-induced photoinhibition. By analogy to the low fluorescence yield of PS I, the low signal-to-noise ratio in these preparations is attributed to the short lifetime of Chl singlet excited states in PS I-40 and its indirect effect on the yield of thermal emission.Abbreviations FFT fast Föurier transform - HA hydroxyapatite - I₅₀ half saturation intensity for energy storage - PA photoacoustic - PS photosystem - PS I-40 photosystem I reaction center/core antenna complex containing about 40 Chl a/P700 - 201-1 photoacoustic energy storage signal - S/N signal-to-noise     

Utility-scale solar impacts to volant wildlife

To reduce carbon emissions from fossil fuel combustion, United States government agencies, including those in California, initiated aggressive programs to hasten development of utility-scale solar energy. Much of California's early development of solar energy occurred in deserts and annual grasslands, much of it on public land. Measurement of solar energy's impacts to wildlife has been limited to mortality caused by features of solar facilities, and has yet to include impacts from habitat loss and energy transmission. To estimate species-specific bird and bat fatality rates and statewide mortality, I reviewed reports of fatality monitoring from 1982 to 2018 at 14 projects, which varied in duration, level of sampling, search interval, search method, and carcass detection trials. Because most monitors performed carcass detection trials using species of birds whose members were larger than birds and bats found as fatalities, I bridged the monitors' onsite trial results to offsite trial results based on the same methods but which also measured detection probabilities across the full range of body sizes of species represented by fatalities. This bridge preserved the project site's effects on detection probabilities while more fully adjusting for the effects of body size. My fatality estimates consistently exceeded those reported. Projected to California's installed capacity of 1,948.8 MW of solar thermal and 12,220 MW of photovoltaic (PV) panels in 2020 (14,168.8 MW total), reported estimates would support an annual statewide fatality estimate of 37,546 birds and 207 bats, whereas I estimated fatalities of 267,732 birds and 11,418 bats. Fatalities/MW/year averaged 11.61 birds and 0.06 bats at PV projects and 64.61 birds and 5.49 bats at solar thermal projects. Fatalities/km/year averaged 113.16 birds and zero bats at generation tie-ins, and 14.44 birds and 2.56 bats along perimeter fences. Bird fatality rates averaged 3 times higher at PV projects searched by foot rather than car. They were usually biased low by insufficient monitoring duration and by the 22% of fatalities that monitors could not identify to species. I estimated that construction grading for solar projects removed habitat that otherwise would have supported nearly 300,000 birds/year. I recommend that utility-scale solar energy development be slowed to improve project decision-making, impacts assessment, fatality monitoring, mitigation efficacy, and oversight.     

Evaluation of maize productivity considering solar energy use limitation by environmental factors

Evaluation of maize productivity under different climatic conditions was made by determination of the amount of the incident solar radiation energy in the PAR range, which can be potentially used by plants for photosynthesis. An irradiance, which can be stored in primary photosynthesis, designated as photosynthetic energy, W _ph, was estimated taking into account the action spectra of photosynthesis. Limitation of the W _ph usage, owing to unfavorable environmental factors was considered. Quantitative evaluation of limitations by two such factors, air temperature and soil water potential, was made by means of the coefficients F(i), which were defined as the ratio between the photosynthetic rate at a given value of a particular environmental factor and that at the optimal value for this factor. The coefficients F(i), were determined from the dependencies of the photosynthesis rate on air temperature and soil water potential as obtained in chamber and field experiments. In general terms, the fraction of W _ph, which can be utilized under a given climatic condition, was named bioclimatic potential, W _pc. In our model, the effect of monodominancy, when strong action of one factor suppresses the influence of any other factor, was considered. In this case, the bioclimatic potential, designated W′_pc, was calculated by multiplying W _ph times the coefficient F, for the factor which was most limiting during the period of measurement. There was close correlation between values of bioclimatic potential for the period of vegetation, W′_pc,v, and total dry matter. W′_pc,v use efficiency in the maize crop was also evaluated for five variants of mineral nutrition. This revised version was published online in June 2006 with corrections to the Cover Date.     

A highly efficient heptamethine cyanine antenna for photosynthetic Reaction Center: From chemical design to ultrafast energy transfer investigation of the hybrid system

The photosynthetic Reaction Center (RC) from the purple bacterium Rhodobacter sphaeroides has unique photoconversion capabilities, that can be exploited in assembly biohybrid devices for applications in solar energy conversion. Extending the absorption cross section of isolated RC through covalent functionalization with ad-hoc synthesized artificial antennas is a successful strategy to outperform the efficiency of the pristine photoenzyme under visible light excitation. Here we report a new heptamethine cyanine antenna that, upon covalent binding to RC, forms a biohybrid (hCyN7-RC) which, under white light excitation, has doubled photoconversion efficiency versus the bare photoenzyme. The artificial antenna hCyN7 successfully meets appropriate optical properties, i.e. peak position of absorption and emission maximum in the visible and NIR region respectively, large Stokes shift, and high fluorescence quantum yield, required for improving the efficiency of the biohybrid in the production of the charge-separated state in the RC. The kinetics of energy transfer and charge separation of hCyN7-RC studied via ultrafast visible and IR spectroscopies are here presented. The antenna transfers energy to RC chromophores within <10?ps and the rate of Q_A reduction is doubled compared to the native RC. These experiments further demonstrate hCyN7-RC, besides being an extremely efficient white light photoconverter, fully retains the charge separation mechanism and integrity of the native RC photoenzyme, thus allowing to envisage its suitability as biohybrid material in bioinspired systems for solar energy conversion.     

Culture on sugar medium enhances photosynthetic capacity and high light resistance of plantlets grown in vitro

The significance of photosynthetic photon flux (PPF) and sugar feeding for the production of plants in vitro is only poorly understood. Nicotiana tabacum L. plantlets were grown photoautotrophically and photomixotrophically (3% sucrose) at two different PPFs (60 µmol m⁻² s⁻¹ and 200 µmol m⁻² s⁻¹) to investigate the effect of these culture parameters on photosynthetic performance and growth. Photomixotrophically‐grown plantlets showed an increase in carbohydrate content, mainly in glucose and fructose. Plant growth, dry matter accumulation and total leaf area were higher under photomixotrophic than photoautotrophic conditions. Not only biomass formation but also photosynthesis was positively affected by exogenous sucrose; the chlorophyll (Chl) content and the light‐saturated rate of photosynthetic oxygen evolution were higher in photomixotrophic plantlets. Photoinhibition occurred in plantlets that were grown photoautotrophically at the higher PPF. It became apparent as a loss in Chl content and photochemical efficiency. Photoinhibited plantlets showed a decrease in the D2/LHCII and CP47/LHCII ratios, suggesting a preferential loss of proteins from the photosystem II (PSII) core. The increased content of xanthophyll cycle pigments in photoinhibited plantlets indicated that also protective mechanisms were activated. Photomixotrophic growth of the plantlets prevented the occurrence of photoinhibitory symptoms. Therefore, we conclude that culture on sugar medium increases not only the photosynthetic potential but also the high light resistance of plantlets grown in vitro.     

Annual patterns of phytoplankton density and primary production in a large, shallow lake: the central role of light

1. We studied the seasonal dynamics of suspended particulate matter in a turbid, large shallow lake during an annual period (2005–06). We relate the patterns of seston concentration (total suspended solids), phytoplankton biomass and water transparency to the seasonal pattern of incident solar radiation (I₀). We also report the seasonal trends of phytoplankton primary production (PP) and photosynthesis photoinhibition due to photosynthetically active radiation (PAR) and ultraviolet radiation (UVR) (I_β and UV₅₀). 2. We first collected empirical evidence that indicated the conditions of light limitation persisted during the study period. We found that the depth‐averaged irradiance estimated for the time of the day of maximum irradiance (I_mean–noon) was always lower than the measured onset of light saturation of photosynthesis (I_k). 3. We then contrasted the observations with theoretical expectations based on a light limitation scenario. The observed temporal patterns of seston concentration, both on a volume and area basis, were significantly explained by I₀ (R² = 0.39 and R² = 0.37 respectively). The vertical diffuse attenuation coefficient (kd_PAR) (R² = 0.55) and the depth‐averaged irradiance (I_mean) (R² = 0.66), significantly increased with the I₀; while the irradiance reaching the lake bottom (I_out) significantly decreased with the incident irradiance (R² = 0.49). However, phytoplankton biovolume maxima were not coincident with the time of the year of maximum irradiance. 4. A significant positive relationship was observed between PP estimated on an area basis and I₀ (R² = 0.51, P < 0.001). In addition, the parameters describing the photosynthetic responses to high irradiances displayed marked seasonal trends. The photosynthesis photoinhibition due to PAR as well as to UV were significantly related to incident solar radiation (PAR: R² = 0.73; UV: R² = 0.74). These results suggest adaptation of the phytoplankton community in response to changes in incident solar radiation.     

太阳辐射减弱对冬小麦旗叶光合速率的影响

以冬小麦扬麦13号为供试材料,设计了15%(T₁₅)、20%(T₂₀)、40%(T₄₀)、60%(T₆₀)和100%（CK）自然光5个处理,在大田条件下研究了模拟太阳辐射减弱对
冬小麦旗叶光合速率日变化及其影响因素的影响.结果表明：太阳辐射减弱显著提高了冬小麦叶绿素和叶黄素含量,降低了光合速率（P_n）.不同辐射减弱条件下冬小麦P_n日变化差异较大,日最高值表现为CK＞60%自然光＞40%自然光＞20%自然光＞15%自然光,其中CK呈双峰曲线变化,有明显的“午休”现象,其他各处理均呈单峰型曲线变化,“午休”现象不明显,但峰值出现时间滞后.相关分析表明,太阳辐射减弱是影响P_n日变化的主导因子,但其他因子也显著影响P_n.与CK相比,60%和40%自然光处理中光合有效辐射（PAR）、叶温（T_l）、气孔导度（G_s）和蒸腾速率（T_r）与P_n均呈显著正相关,表明上述因子对P_n有正效应;冬小麦叶片胞间二氧化碳浓度（C_i）和气孔导度限制值（L_s）在60%和40%自然光处理中与P_n呈显著负相关,但在20%和15%自然光处理中与P_n呈显著正相关,说明太阳辐射强度高于40%自然光时C_i和L_s对P_n有负效应,太阳辐射强度低于40%自然光时则为正效应.     

试析光合作用的研究动向

光合作用被称为＂地球上最重要的化学反应＂和＂生命界最重大的顶极创造之一＂,在生物演化、生物圈形成和运转及人类诞生与经济和社会的可持续发展等过程中都处于非常关键的地位。从最近召开的国际和国内光合作用会议来看,当前进行的研究呈现出领域越来越宽广、层次越来越深入、技术越来越先进的特点;研究重点集中在探讨光合作用反应机理、结构与功能,揭示光合机构组装、运转与调节机制及光合作用与人类可持续发展3个方面。     

Biotechnological hydrogen production: research for efficient light energy conversion

The study of biological hydrogen production by using photosynthetic bacteria and cyanobacteria is described based on a national R&D project in Japan. We describe here the subjects examined in the research for photosynthetic bacteria: analysis of the relationship between the penetration of light to photobioreactor and hydrogen production, genetic engineering of photosynthetic bacteria to control the pigment content for making the light penetration easy. Examples of bench-scale reactors are shown. Genetic manipulation of hydrogenase to enhance the protein expression is also studied.     

The effects of elevated light on Photosystem II function: A thermoluminescence study

We have used the technique of thermoluminescence (TL) to investigate high-light-induced chlorophyll fluorescence quenching phenomena in barley leaves, and have shown it to be a powerful tool in such investigations. TL measurements were taken from wild-type and chlorina f2 barley leaves which had been dark-adapted or exposed to 20 min illumination of varying irradiance or given varying periods of recovery following strong irradiance. We have found strong evidence that there is a sustained trans-thylakoid pH in leaves following illumination, and that this pH gives rise to quenching of chlorophyll fluorescence which has previously been identified as a slowly-relaxing component of antenna-related protective energy dissipation; we have identified a state of the PS II reaction centre resulting from high light treatments which is apparently able to perform normal charge separation and electron transport but which is non-photochemically quenched, in that the application of a light pulse of high irradiance cannot cause the formation of a high fluorescent state; and we have provided evidence that a transient state of the PS II reaction centre is formed during recovery from such high light treatments, in which electron transport from Q_Ato Q_Bis apparently impaired.