Evaluation of the role of damage to photosystem II in the inhibition of CO2 assimilation in pea leaves on exposure to UV-B radiation

Mature pea (Pisum sativum L., cv. Meteor) leaves were exposed to two levels of UV-B radiation, with and without supplementary UV-C radiation, during 15 h photoperiods. Simultaneous measurements of CO₂ assimilation and modulated chlorophyll fluorescence parameters demonstrated that irradiation with UV-B resulted in decreases in CO₂ assimilation that are not accompanied by decreases in the maximum quantum efficiency of photosystem II (PSII) primary photochemistry. Increased exposure to UV-B resulted in a further loss of CO₂ assimilation and decreases in the maximum quantum efficiency of PSII primary photochemistry, which were accompanied by a loss of the capacity of thylakoids isolated from the leaves to bind atrazine, thus demonstrating that photodamage to PSII reaction centres had occurred. Addition of UV-C to the UV-B treatments increased markedly the rate of inhibition of photosynthesis, but the relationships between CO₂ assimilation and PSII characteristics remained the same, indicating that UV-B and UV-C inhibit leaf photosynthesis by a similar mechanism. It is concluded that PSII is not the primary target site involved in the onset of the inhibition of photosynthesis in pea leaves induced by irradiation with UV-B.     

植物光合作用模型参数的温度依存性研究进展

综述了植物光合作用与温度响应模型研究的进展,围绕光合作用生化模型的4个主要参数：胞间CO₂浓度、RuBP最大碳同化速率(V_{c max})的活化能、RuBP最大再生速率(J_max)的活化能和J_max/V_{c max},讨论了影响光合作用 温度响应曲线的内在机理.随着生长温度的升高,所有物种的V_{c max}活化能均呈增加趋势,而其他参数的变化因物种不同而存在明显差异,说明V_{c max}的活化能可能是决定光合作用温度依存性的首要参数.最后分析了研究中存在的问题并提出研究展望,认为应整合叶片与群落水平的光合作用模型,从叶面积、太阳辐射、冠层结构、冠层小气候和光合能力等方面研究植物群落对全球变化的响应机理.这对于人们理解和准确估算植物生长、群落碳收支和生态系统初级生产力具有重要意义.     

High nitrogen inhibits photosynthetic performance in a shade-tolerant and N-sensitive species Panax notoginseng

Photosynthesis Research - Nitrogen (N) is a primary factor limiting leaf photosynthesis. However, the mechanism of high-N-driven inhibition on photosynthetic efficiency and photoprotection is still...     

Biochemical and molecular basis for impairment of photosynthetic potential

Ozone induces reductions in net photosynthesis in a large number of plant species. A primary mechanism by which photosynthesis is reduced is through impact on carbon dioxide fixation. Ozone induces loss in Rubisco activity associated with loss in concentration of the protein. Evidence is presented that ozone may induce oxidative modification of Rubisco leading to subsequent proteolysis. In addition, plants exposed to ozone sustain reduction in rbcS, the mRNA for the small subunit of Rubisco. This loss in rbcS mRNA may lead to a reduced potential for synthesis of the protein. The regulation of O₃-induced loss of Rubisco, and implications of the decline in this protein in relation to accelerated senescence are discussed.     

Mechanism of Senescence Induction by Silicone Oil

The physiological basis for senescence symptoms induced by spraying attached primary leaves of bean plant (Phaseolus vulgaris L. cv. Brittlewax) with silicone oil emulsions was investigated. It was found that chlorophyll levels declined significantly between 24 and 48 h after spraying, whereas neither protein levels nor respiration rates were affected over a 7-day period. Rates of photosynthesis and reducing sugar levels in silicone-sprayed leaves were however significantly reduced after 1 and 24 h respectively. These responses were obtained using plants grown under low stress controlled environment conditions. By contrast, leaves on greenhouse plants did not senesce in response to silicone sprays. A possible mechanism of silicone oil action based on selective leaf penetration and inhibition of photosynthesis is postulated.     

固氮蓝细菌束毛藻生物固氮策略研究进展

固氮蓝细菌束毛藻(Tricodesmium)是海洋中丰度最高的固氮微生物,贡献了约42%的海洋生物固氮,为海洋生态系统提供了新的氮源,驱动海洋初级生产力和食物网,在海洋生物地球化学循环中发挥重要作用。作为海洋中“新氮”主要贡献者,束毛藻是一种不产生异形胞的丝状固氮蓝细菌。因为生物固氮的关键酶固氮酶对氧气十分敏感,一般固氮蓝细菌通常产生异形胞或采用夜间固氮的方式进行生物固氮,避免氧气对固氮酶的抑制作用。近年来研究发现,束毛藻具有一套独特的生物固氮体系,能够使同一藻丝在白天同时完成光合作用和生物固氮,并具有复杂的调控机制。本文综述了近年来束毛藻生物固氮策略的最新研究进展,介绍了其生物固氮和光合作用之间的精密调控机制,对拓展固氮微生物尤其是海洋蓝细菌固氮机制的认识具有借鉴意义。     

Carbon Dioxide Exchange in an Incompatible Barley/Powdery Mildew Combination

CO₂ exchanges in intact barley leaves were studied by infra red gas analysis after inoculation with an incompatible race of powdery mildew. Net photosynthesis was inhibited, and dark and photorespiration were stimulated, by inoculation. The increase in respiratory processes was approximately equal to the change in net photosynthesis indicating that CO₂ fixation, “gross photosynthesis”, was unaffected by inoculation. It is concluded that changes in net photosynthesis may be the primary cause of the reduced quantity and quality of grain in incompatible barley/mildew combinations.     

Energy upconversion theory of the primary photochemical reaction in plant photosynthesis

In this paper we suggest a basic mechanism for the utilization of light quanta in photosynthesis. Through interactions between the lowest lying triplet state of the reaction-center chlorophylls and the first excited singlet state of the antenna chlorophylls, absorbed light quanta are upconverted to a higher-lying charge transfer state of the reaction-center Chl molecules. It is shown that the efficiency of the upconversion process is maximized by the parallel configuration of the two Chl porphyrin rings in the reaction-center water adduct proposed by the writer. Steady-state solutions are obtained, and the theoretical results are shown to account for a variety of crucial experimental observations including (1) the doubling (in whole cells) of in vivo fluorescence quantum yield of system II in strong light, (2) the observation by Dutton et al. of the light-induced triplet-state reaction-center bacteriochlorophyll when the primary electron acceptor is reduced and (3) despite the apparent involvement of two excitations in the energy upconversion process, only one quantum is needed for the transfer of one electron in the primary photo-chemical reaction, satisfying the eight-quanta requirement for the evolution of one O₂ molecule in photosynthesis.     

Photosynthesis, leaf resistances, and ribulose-1,5-bisphosphate carboxylase degradation in senescing barley leaves

The relationship between loss of ribulose-1,5-bisphosphate carboxylase (RuBPCase) and the decline in photosynthesis during the senescence of barley primary leaves was assessed. Loss of RuBPCase accounted for about 85% of the decrease in soluble protein. RuBPCase was highly correlated with in vitro RuBPCase activity (r = 0.95) and gross photosynthesis (r = 0.96). However, the rate of photosynthesis per milligram RuBPCase increased during the early stages of leaf senescence. The concentration of nonreducing sugars was negatively correlated (1% level) with photosynthesis. Free α-amino N, in contrast to nonreducing sugars, declined markedly during senescence. A decrease in chlorophyll and an increase in in vitro protease activity was observed, but these changes did not appear to be closely related to the decline in photosynthesis and RuBPCase. Mesophyll resistance increased at the same rate that photosynthesis and RuBPCase declined. Stomatal resistance increased more rapidly than mesophyll resistance and accounted for about 24% of the total increase in resistance to CO₂ diffusion. The concentration of CO₂ in the intercellular air spaces decreased during the last stage of senescence. Although loss of RuBPCase probably is the primary event responsible for the decline in photosynthesis during leaf senescence, other factors such as in vivo regulation and stomatal aperture must also be considered.     

Operation of dual mechanisms that both lead to photoinactivation of Photosystem II in leaves by visible light

Photosystem II (PS II) is photoinactivated during photosynthesis, requiring repair to maintain full function during the day. What is the mechanism(s) of the initial events that lead to photoinactivation of PS II? Two hypotheses have been put forward. The 'excess-energy hypothesis' states that excess energy absorbed by chlorophyll (Chl), neither utilized in photosynthesis nor dissipated harmlessly in non-photochemical quenching, leads to PS II photoinactivation; the 'Mn hypothesis' (also termed the two-step hypothesis) states that light absorption by the Mn cluster in PS II is the primary effect that leads to dissociation of Mn, followed by damage to the reaction centre by light absorption by Chl. Observations from various studies support one or the other hypothesis, but each hypothesis alone cannot explain all the observations. We propose that both mechanisms operate in the leaf, with the relative contribution from each mechanism depending on growth conditions or plant species. Indeed, in a single system, namely, the interior of a leaf, we could observe one or the other mechanism at work, depending on the location within the tissue. There is no reason to expect the two mechanisms to be mutually exclusive.     

非充分灌溉及其生理基础

介绍了非充分灌溉的概念及内涵,主要阐述了在非充分灌溉条件下,作物体内产生的适应性生理反应,经非充分灌溉及轻度干旱处理,作物气孔阻力增加,蒸腾失水减少,作物水分散失对气孔开度的依赖性大于光合对其的依赖性。可通过气孔调节作物光合与水分的关系,最终提高作物的水分利用效率;有限度的水分亏缺,有利于同化物向籽粒调运,利用^14CO2标记研究表明,生长后期水分亏缺下,小麦体内存在对花前营养器官“临时库”同化物的再动员和对产量的补偿机制;适度水分亏缺促进了小麦等作物初生根的生长发育,增加深层土壤中的根系与根系活性,防止后期根系早衰。总之,在非充分灌溉条件下,作物能够在营养生长,物质运输和产量形成等方面产生有效的补偿机制,可作为非充分灌溉的重要理论基础。     

Exploring the Conformational Behavior of Rigid Porphyrin-Quinone Systems by High-Temperature MD Simulations and Temperature-Dependent 1H-NMR Experiments

Photoinduced electron transfer reactions play an important role in the primary step of the biological photosynthesis process. In an attempt to understand better the mechanism of the charge separation organic donor-acceptor molecules containing porphyrins and quinones were designed as photosynthesis models. In order to study the structure dependence of the photoinduced electron transfer twofold and fourfold bridged porphyrin-quinone systems with increasing donor-acceptor distance were synthesized (Figure 1) [1, 2, 3]. It was assumed that in these molecules the porphyrin and quinone should be linked in a rigid and well-defined orientation. To verify this assumption the conformational behavior of these systems was studied by high-temperature MD simulations in combination with conformational analysis of selected minimized structures [4, 5].     

Photosynthetic Production of a Brackish Water Community of Chara tomentosa L. and its Dependence on Environmental Conditions

Net photosynthesis and dark respiration of a natural Chara tomentosa community were continuously recorded. There is a close correlation between net rate of photosynthesis and variations in the PhAR flux under water. The net photosynthesis fluence Px correlates with the PhAR fluence Ix, but the quotient Px/Ix varies seasonally, apparently as a consequence of variations in chlorophyll content following long-range changes in radiation fluence. No influence of temperature, salinity or pH on net photosynthesis was detected, but water temperature stimulates dark respiration and consequently diminishes net primary production.     

Clonal differences in the response of dark and light reactions of photosynthesis to elevated atmospheric CO2 in poplar

Two hybrid poplar (Populus) clones (i.e., fast growing clone Beauprè and slow growing clone Robusta) were grown for two years from cuttings at close spacings in open top chambers (OTCs) under ambient (AC) and elevated [EC = AC + 350 μmol(CO2) mol-1] CO2 treatments. For clone Beauprè no down-regulation of photosynthesis was observed. Two years of growing under EC resulted in an increase in quantum yield of photosystem 2 (PS2), steady state irradiance saturated rate of net photosynthesis (P _Nmax), chlorophyll (Chl) content, and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPC) activity for this clone. We suppose that under non-limiting conditions of nitrogen and phosphorus content the response to EC was by building up light-harvesting complexes of PS2 and increasing photochemical efficiency of PS2. Due to a high rate of the primary reactions of photosynthesis and a high RuBPCO activity the end product of the response to EC was an increase in PNmax and a larger saccharides content. The Robusta clone showed a depression in the primary reactions of photosynthesis under EC. We found a decrease in quantum yield of PS2, Chl and phosphorus contents, and in RuBPCO activity. However, an increase in PNmax, saccharides content and Chl a/b ratio was observed. We speculate (1) that the phosphorus deficiency in combination with an increase in CO2 concentrations may lead to a potential damage of the assimilation apparatus of the primary reactions of photosynthesis and to a decrease in photochemical efficiency of PS2; (2) that the primary target of "down-regulation" takes place at PS2 for irradiances above 150 μmol m-2 s-1. This revised version was published online in September 2006 with corrections to the Cover Date.     

叶绿素荧光主动与被动联合观测应用前景

叶绿素荧光是研究植物光合生理机制、量化植被光合作用时空格局以及准确理解气候变化背景下陆地生态系统生产力的关键。然而, 目前对于叶绿素荧光主动与被动联合观测的研究还较少。该文对比了叶绿素荧光主动观测与被动观测的优缺点, 展示了叶片尺度和冠层尺度主动与被动联合观测的仪器设备组成, 探讨了主动与被动联合观测在探索叶绿体尺度-叶片尺度-冠层尺度能量在光合、荧光以及热耗散中的分配, 阐明叶绿素荧光与总初级生产力的关联机制, 验证星基日光诱导叶绿素荧光, 解译叶绿素荧光光谱形状4个方面的应用前景。综上, 叶绿素荧光的主动与被动联合观测对于揭示各尺度上荧光与光合作用之间的关联机制, 改善全球尺度植被生产力模型至关重要。     

On Photosynthesis and Organic Carbon, Carbon Dioxide, and Oxygen Flows in the Ocean

The modern concept of photosynthesis as a mechanism for utilizing the energy of solar radiation is used as the basis for assessing the scale of photosynthetic production of initial organic matter in the ocean (primary biological production), its destruction, the carbon and carbon dioxide cycles (flows) involved in this process, and the size of oil- and gas-bearing hydrocarbonaceous formations originating in sedimentary deposits.     

Photocontrol of sorghum leaf phosphoenolpyruvate carboxylase : characterization of messenger RNA and of photoreceptor

The mechanism underlying the light effect on phosphoenolpyruvate carboxylase (PEPC) from the C₄ plant sorghum (Sorghum vulgare Pers., var Tamaran) leaves was investigated. Following exposure to light a new isozyme of PEPC, specific for the green leaf and responsible for primary CO₂ fixation in photosynthesis, was established. Northern blot experiments revealed the presence of PEPC mRNA showing a molecular weight of 3.4 kilobases. During the greening process, concomitant to enzyme activity, PEPC protein and PEPC messenger RNA amounts increased considerably. This photoresponse was shown to be under phytochrome control.     

Proteomics analysis reveals multiple regulatory mechanisms in response to selenium in rice

Selenium (Se) shows both beneficial and toxic effects on plant growth. Rice (Oryza sativa L.) seedlings cultivated under lower concentrations of sodium selenite showed enhanced growth, whereas higher concentrations of sodium selenite repressed seedling growth. To acquire detailed regulatory mechanisms underlying these effects, a comparative proteomics study using 2-dimensional gel electrophoresis and MALDI-TOF/TOF MS was performed. By comparison of gel images between Se treatments and control, 66 and 97 differentially expressed proteins were identified in shoot and root, respectively under at least one of the Se treatment concentrations. Gene Ontology and Clustering analysis reveal primary metabolism, photosynthesis and redox homeostasis are the most highly affected biological processes by Se treatments. Lower Se treatments (2 and 6 mg/L sodium selenite) activated antioxidative system, enhanced photosynthesis and primary metabolism. However, higher Se treatment (10 mg/L sodium selenite) damaged photosynthesis apparatus, inhibited photosynthesis and primary metabolism. Protein ubiquitination and phosphorylation may also play important roles in Se response in rice. In conclusion, our study provided novel insights into Se response in rice at the proteome level, which are expected to be highly useful for dissecting the Se response pathways in higher plants and for producing Se enriched rice cultivars in the future.     

Inhibition of photosynthesis by heavy metals

Inhibition of photosynthesis by heavy metals is well documented. In this review the results are compared between in vitro experiments on isolated systems (chloroplasts, enzymes ­.), experiments on excised leaves and intact plants and algae in vivo. In vitro experiments suggest potential sites of heavy metal interaction with photosynthesis at several levels of organisation, which are not necessarily confirmed in vivo. Analytical data on subcellular heavy metal level are generally missing to discuss their mechanism of action in the intact organism. In the field factors such as soil characteristics and air pollution have to be taken into account for assessing the mechanism of action of heavy metals on photosynthesis in plants, growing in a polluted erea.paper presented at the FESPP meeting in Strasbourg (1984)