Phytoplankton as a factor in the decline of the submersed macrophyte Myriophyllum spicatum L. in Lake Wingra,Wisconsin, U.S.A.

A dramatic decline in biomass and areal coverage of the submersed macrophyte Myriophyllum spicatum in Lake Wingra, Wisconsin, USA during the mid-1970's is documented using aerial photography, vegetation surveys, and quadrat biomass sampling. Over the same period, light penetration as measured by Secchi disc transparency and extinction coefficient decreased substantially. During this period, extinction coefficient was closely correlated with chlorophyll a levels implicating phytoplankton as the major source of decreased light penetration. A growth model for M. spicatum predicts a substantial decline in macrophyte biomass when extinction coefficient is increased to the levels reported since 1977. Available data do not show whether the phytoplankton increase preceeded or followed the macrophyte decline. Nonetheless it is clear that phytoplankton growth can account for a substantial portion of the decline and that macrophyte recovery will be difficult given the phytoplankton-induced decrease in water clarity.     

Pigments and photosynthesis of understory grasses: Light irradiance and soil moisture effects

Phleum alpinum L. and Poa pratensis L. are major forage species that often grow in various environments in Tierra del Fuego, Argentina. We performed a greenhouse experiment to investigate how these species acclimate to the different irradiance of the microenvironments where they grow. Both grass species were exposed to three levels of incident irradiance (I4: 4%; I26: 26%, or I64: 64% of ambient sunlight) and two levels of soil moisture content (M30: 30–50% or M60: 60–80% of field capacity) under greenhouse conditions. As irradiance levels increased, the contents of chlorophyll per unit surface area and fresh weight basis increased, and the chlorophyll a/b and carotenoids/chlorophyll ratio also increased. Maximum photosynthetic rate and the light compensation point increased with increasing light availability. Values for these variables varied with time. However, the relationship of these values was not modified in P. alpinum between the irradiance treatments. Contrarily, temporal changes of those variables showed that the maximum photosynthetic rate was similar to that in March in all treatments in P. pratensis. Results indicated that P. alpinum and P. pratensis were able to acclimate to the various experimental environments.     

An LED-based fluorometer for chlorophyll quantification in the laboratory and in the field

The chlorophyll content is an important experimental parameter in agronomy and plant biology research. In this report, we explore the feasibility of determining total concentration of extracts containing chlorophyll a and chlorophyll b by chlorophyll fluorescence. We found that an excitation at 457?nm results in the same integrated fluorescence emission for a molecule of chlorophyll a and a molecule of chlorophyll b. The fluorescence yield induced by 457?nm is therefore proportional to total molar chlorophyll concentration. Based on this observation, we designed an instrument to determine total chlorophyll concentrations. A single light emitting diode (LED) is used to excite chlorophyll extracts. After passing through a long-pass filter, the fluorescence emission is assessed by a photodiode. We demonstrate that this instrument facilitates the determination of total chlorophyll concentrations. We further extended the functionality of the instrument by including LEDs emitting at 435 and 470?nm wavelengths, thereby preferentially exciting chlorophyll a and chlorophyll b. This instrument can be used to determine chlorophyll a and chlorophyll b concentrations in a variety of organisms containing different ratios of chlorophylls. Monte-Carlo simulations are in agreement with experimental data such that a precise determination of chlorophyll concentrations in carotenoid-containing biological samples containing a concentration of less than 5?nmol/mL total chlorophyll can be achieved.     

Use of a SPAD meter to estimate chlorophyll content in Eugenia uniflora L. leaves as affected by contrasting light environments and soil flooding

In three separate experiments, the effectiveness of a SPAD-502 portable chlorophyll (Chl) meter was evaluated for estimating Chl content in leaves of Eugenia uniflora seedlings in different light environments and subjected to soil flooding. In the first experiment, plants were grown in partial or full sunlight. In the second experiment plants were grown in full sunlight for six months and then transferred to partial sunlight or kept in full sunlight. In the third experiment plants were grown in a shade house (40% of full sunlight) for six months and then transferred to partial shade (25–30% of full sunlight) or full sunlight. In each experiment, plants in each light environment were either flooded or not flooded. Non-linear regression models were used to relate SPAD values to leaf Chl content using a combination of the data obtained from all three experiments. There were no significant effects of flooding treatments or interactions between light and flooding treatments on any variable analyzed. Light environment significantly affected SPAD values, chlorophyll a (Chl a), chlorophyll b (Chl b), and total chlorophyll [Chl (a+b)] contents in Experiment I (p≤0.01) and Experiment III (p≤0.05). The relationships between SPAD values and Chl contents were very similar among the three experiments and did not appear to be influenced by light or flooding treatments. There were high positive exponential relationships between SPAD values and Chl (a+b), Chl a, and Chl b contents.     

Response of the Photosynthetic Apparatus in Dunaliella salina (Green Algae) to Irradiance Stress

The response of the photosynthetic apparatus in the green alga Dunaliella salina, to irradiance stress was investigated. Cells were grown under physiological conditions at 500 millimoles per square meter per second (control) and under irradiance-stress conditions at 1700 millimoles per square meter per second incident intensity (high light, HL). In control cells, the light-harvesting antenna of photosystem I (PSI) contained 210 chlorophyll a/b molecules. It was reduced to 105 chlorophyll a/b in HL-grown cells. In control cells, the dominant form of photosystem II (PSII) was PSII_α(about 63% of the total PSII) containing >250 chlorophyll a/b molecules. The smaller antenna size PSII_β centers (about 37% of PSII) contained 135 ± 10 chlorophyll a/b molecules. In sharp contrast, the dominant form of PSII in HL-grown cells accounted for about 95% of all PSII centers and had an antenna size of only about 60 chlorophyll a molecules. This newly identified PSII unit is termed PSII_γ. The HL-grown cells showed a substantially elevated PSII/PSI stoichiometry ratio in their thylakoid membranes (PSII/PSI = 3.0/1.0) compared to that of control cells (PSII/PSI = 1.4/1.0). The steady state irradiance stress created a chronic photoinhibition condition in which D. salina thylakoids accumulate an excess of photochemically inactive PSII units. These PSII units contain both the reaction center proteins and the core chlorophyll-protein antenna complex but cannot perform a photochemical charge separation. The results are discussed in terms of regulatory mechanism(s) in the plant cell whose function is to alleviate the adverse effect of irradiance stress.     

Spectral Irradiance and Distribution of Pigments in a Highly Layered Marine Microbial Mat

The spectral irradiance from 400 to 1,100 nm was measured with depth in the intertidal sand mats at Great Sippewissett Salt Marsh, Mass. These mats contained at least four distinct layers, composed of cyanobacteria, purple sulfur bacteria containing bacteriochlorophyll a (Bchl a), purple sulfur bacteria containing Bchl b, and green sulfur bacteria. Spectral irradiance was measured directly by layering sections of mat on a cosine receptor. Irradiance was also approximated by using a calibrated fiber-optic tip. With the tip, irradiance measurements could be obtained at depth intervals less than 250 μm. The irradiance spectra were correlated qualitatively and quantitatively with the distribution of the diverse chlorophyll pigments in this mat and were compared with spectra recorded in plain sand lacking pigmented phototrophs. We found that the shorter wavelengths (400 to 550 nm) were strongly attenuated in the top 2 mm of the mat. The longer wavelengths (red and near infrared) penetrated to much greater depths, where they were attenuated by Bchl a, b, and c-containing anoxygenic phototrophic bacteria. The specific attenuation bands in the irradiance spectra correlated with the specific in vivo absorption bands of the Bchl-protein complexes in the bacteria. We concluded that the pigments in the phototrophs had a profound affect on the light environment within the mat. It seems likely that the diverse Bchl-protein complexes found in the anoxygenic phototrophs evolved in dense mat environments as a result of competition for light.     

Structure of the Red Fluorescence Band in Chloroplasts

Using Weber's method of "matrix analysis" for the estimation of the number of fluorescent species contributing to the emission of a sample, it is shown that the fluorescence¹ band in spinach chloroplast fragments at room temperature originates in two species of chlorophyll a. Emission spectra obtained upon excitation with different wavelengths of light (preferentially absorbed in chlorophyll a or b) are presented. Upon cooling to - 196°C, the fluorescence efficiency increases about twentyfold. Two additional bands, that now appear at 696 and 735 mµ, suggest the participation of four molecular species. Emission spectra observed at different concentrations of chloroplast fragments with excitation in chlorophyll a and b and excitation spectra for different concentrations of chloroplast fragments and measurements at 685 and 760 mµ are presented. Two of the four emission bands may belong to pigment system I and two to system II. The 685, 696, and 738 mµ bands respond differently to temperature changes. In the -196°C to -150°C range, the intensity of the 685 mµ band remains constant, and that of the 696 mµ band decreases twice as fast as that of the 738 mµ band.     

Chlorophyll-protein complexes from higher plants: A procedure for improved stability and fractionation

An improved procedure for the electrophoretic fractionation of higher plant chlorophyllprotein complexes is described. Compared with currently used systems, it greatly reduces the amount of chlorophyll that is found unassociated with protein after electrophoresis and resolves four chlorophyll-protein complexes. The slowest migrating band has a red adsorption maximum at 674 nm or greater, contains chlorophyll a but not chlorophyll b, and has a molecular weight equivalency of 110,000. These properties are similar to the previously described CPI or P700-chlorophyll a-protein complex. The amount of the total chlorophyll in this material is increased by two to three fold over that present in the equivalent complex fractionated by previous procedures. The other three chlorophyll-protein complexes contain both chlorophylls a and b, and have molecular weight equivalencies of 80,000, 60,000, and 46,000. None of these complexes seems to correspond directly to the previously characterized light-harvesting chlorophyll $\text{a}\text{b}$-protein complex.     

Spectral characterization of five chlorophyll-protein complexes

Sodium dodecyl sulfate-solubilized chloroplast internal membranes of higher plants (cowpea [Vigna unguiculata L. Walp], chinese cabbage [Brassica chinensi L.], and tobacco [Nicotiana tabacum L.]) are resolved by polyacrylamide gel electrophoresis into two chlorophyll a- and three chlorophyll a,b-proteins. A small portion (about 15%) of the membrane chlorophyll migrates as a component of high electrophoretic mobility and presumably consists of detergent-complexed, protein-free pigment.

One of the chlorophyll a-proteins is qualitatively similar to the P₇₀₀ chlorophyll a-protein but contains a much larger proportion of total chlorophyll (about 30%) than previously reported. The second chlorophyll a-protein is a recently discovered component of the membrane and accounts for about 7% of the total chlorophyll. The absorption and fluorescence emission spectra of these two chlorophyll a-proteins differ.

The three chlorophyll a,b-proteins are components of the chloroplast membrane chlorophyll a,b-light-harvesting complex which was previously resolved as a single chlorophyll-protein band. The two additional chlorophyll a,b-proteins observed in our work probably represent larger aggregates contained within that membrane complex which are preserved under the solubilization and electrophoretic conditions used here.

     

Photosynthetic performance of the aquatic macrophyte <Emphasis Type="Italic">Althenia orientalis</Emphasis> to solar radiation along its vertical stems

We have studied the plasticity of the photosynthetic apparatus in the endangered aquatic macrophyte Althenia orientalis to the gradient of light availability within its meadow canopy. We determined diurnal change in situ irradiance, light quality, in vivo chlorophyll a fluorescence, ex situ oxygen evolution rates, respiration rate and pigment concentration. The levels of photosynthetic photon flux density (PFD) and ultraviolet radiation (UVR) and the red/far-red ratio decreased with depth within the canopies of A. orientalis. Apical leaves had a greater decrease of the maximal quantum yield (F _v/F _m) in the morning and a faster recovery rate in the afternoon than those in the basal ones. The relative electron transport rate (ETRr) was not saturated at any time of the day, even in the apical leaves that received the highest light. The maximum light-saturated rate of gross photosynthesis (GP_max) took place in apical leaves around noon. The chlorophyll a/b ratio values were higher, and the chlorophyll/carotenoid ratio values lower, in apical leaves than basal ones. The highest concentrations in total carotenoids were reached in the apical leaves around noon. A. orientalis has a high capacity to acclimatize to the changes in the light environment, both in quality and quantity, presenting sun and shade leaves in the same stem through the vertical gradient in the canopy.     

Dynamics of Photosystem II and Its Light Harvesting System in Response to Light Changes in the Halotolerant Alga Dunaliella salina

A photosystem two (PSII) core complex consisting of five major polypeptides (47, 40, 32, 30, and 10 kilodaltons) and a light harvesting chlorophyll a/b complex (LHC-2) have been isolated from the halotolerant alga Dunaliella salina. The chlorophyll and polypeptide composition of both complexes were compared in illuminated and dark-adapted cultures. Dark adaptation is accompanied by a decrease in the chlorophyll a to chlorophyll b (Chl a/Chl b) ratio of intact thylakoids without any change in total chlorophyll. These changes occur with a half-time of 3 hours and are reversed upon reillumination. Analyses of PSII enriched membrane fragments suggest that the decrease in the Chl a/Chl b is due partly to an increase in the Chl b content of LHC-2 and partly to changes in the relative levels of the two complexes. Apparently during dark adaptation there is: (a) a net synthesis of chlorophyll b, (b) removal of PSII core complexes resulting in a 2-fold drop in the PSII cores to LHC-2 chlorophyll ratio. These changes should dramatically increase the light harvesting capacity of the remaining PSII reaction centers. Presumably this adjustment of antenna size and composition is a physiological mechanism necessary for responding to shade conditions. Also detected, using ³²P, are light-induced phosphorylation of the LHC-2 (consistent with the ability to undergo State transitions) and of the 40 and 30 kilodalton subunits of the PSII core complex. These observations indicate that additional mechanisms may also exist to help optimize the interception of quanta during rapid changes in illumination conditions.     

Seasonal reversibility of acclimation to irradiance in leaves of common box (Buxus sempervirens L.) in a deciduous forest

Understorey shade plants are seasonally exposed to dramatic changes in light conditions in deciduous forests related with the dynamics of the overstorey leaf phenology. These transitions are commonly followed by changes in herb plant communities, but shade-tolerant evergreen species must be able to adapt to changing light conditions. In this work we checked the photoprotective responses of evergreen species to acclimate to the shady summer environment and reversibly de-acclimate to a more illuminated environment after leaf fall on deciduous overstoreys. For that purpose we have followed the process of light acclimation in leaves of common box (Buxus sempervirens) during the winter to spring transition, which decrease irradiance in the understorey, and conversely during the transition from summer to autumn. Four parameters indicative of the structure and degree of acclimation of the photosynthetic apparatus have been studied: chlorophyll a/b ratio which is supposed to be inversely proportional to the antenna size, α/β-carotene which increases in shade acclimated leaves and the pools of α-tocopherol and xanthophyll cycle pigments (VAZ) which are two of the main photoprotection mechanisms in plants. Among these parameters, chlorophyll a/b ratio and VAZ pool responded finely to changes in irradiance indicating that modifications in the light harvesting size and photoprotective capacity contribute to the continuous acclimation and de-acclimation of long-lived evergreen leaves.     

Differential responses of antioxidant enzymes in pioneer and late-successional tropical tree species grown under sun and shade conditions

To investigate the ability of pioneer and late-successional species to adapt to a strong light environment in a reforestation area, we examined the activities of antioxidant enzymes in relation to photosystem II, chlorophyll a fluorescence and photosynthetic pigment concentration for eight tropical tree species grown under 100% (sun) and 10% (shade) sunlight irradiation. The pioneer (early-succession) species (PS) were Cecropia pachystachya, Croton urucurana, Croton floribundus and Schinus terebinthifolius. The non-pioneer (late succession) species (LS) were Hymenaea courbaril L. var. stilbocarpa, Esenbeckia leiocarpa, Cariniana legalis and Tabebuia roseo-alba. We observed a greater decline in the ratio of variable to maximum chlorophyll a fluorescence (F_v/F_m) under full sunlight irradiation in the late-successional species than in the pioneer species. The LS species most sensitive to high irradiance were C. legalis and H. courbaril. In LS species, chlorophyll a, chlorophyll b and total chlorophyll concentrations were higher in the shade-grown plants than in plants that developed under full sunlight, but in the PS species C. floribundus and C. pachystachya, we did not observe significant changes in chlorophyll content when grown in the two contrasting environments. The carotenoids/total chlorophyll ratio increased significantly when plants developed under high-sunlight irradiation, but this response was not observed in the PS species S.terebinthifolius and C. pachystachya. The improved performance of the pioneer species in high sunlight was accompanied by an increase in superoxide dismutase (SOD, EC 1.15.1.1) activity, though no light-dependent increase in the activity of ascorbate peroxidase (APX, EC 1.11.1.11) was observed. The activity of catalase (CAT, EC 1.11.1.6) was reduced by high irradiation in both pioneer and late-successional species. Our results show that pioneer species perform better under high-sunlight irradiation than late-successional species, as indicated by increased SOD activity and a higher F_v/F_m ratio. C. legalis was the LS species most susceptible to photoinhibition under full sunlight conditions. These results suggest that pioneer plants have more potential tolerance to photo-oxidative damage than late-successional species associated with the higher SOD activity found in pioneer species. Reduced photoinhibition in pioneer species probably results from their higher photosynthetic capacities, as has been observed in a previous survey carried out by our group.     

Environmental relevance of heavy metal-substituted chlorophylls using the example of water plants

Following experiments which studied the substitution of thecentral ion of isolated chlorophylls by heavy metal ions invitro, in vivo experiments with submersed water plants werecarried out. It was discovered that the substitution of thecentral atom of chlorophyll, magnesium, by heavy metals (mercury,copper, cadmium, nickel, zinc, lead) in vivo is an importantdamage mechanism in stressed plants. This substitution preventsphotosynthetic light-harvesting in the affected chlorophyllmolecules, resulting in a breakdown of photosynthesis. The reactionvaries with light intensity. In low light irradiance all thecentral atoms of the chlorophylls are accessible to heavy metals,with heavy metal chlorophylls being formed, some of which aremuch more stable towards irradiance than Mg-chlorophyll. Consequently,plants remain green even when they are dead. In high light,however, almost all chlorophyll decays, showing that under suchconditions most of the chlorophylls are inaccessible to heavymetal ions. Key words: Heavy metal chlorophylls, submersed water plants, antenna pigments, copper, zinc     

Microscale Vertical Profiles of N2 Fixation, Photosynthesis, O2, Chlorophyll a, and Light in a Cyanobacterial Assemblage

Profiles of ¹⁵N₂ fixation, O₂ production (gross photosynthesis), O₂ concentration, chlorophyll a concentration, and photon fluence rates were measured with 50-μm resolution in colonies of the heterocyst-forming cyanobacterium Nostoc parmelioides. Microelectrode measurements were made after 20 h of incubation under ¹⁵N₂ gas. Colonies were frozen, and 50-μm sections were prepared by using a freezing microtome and analyzed for ¹⁵N enrichment and chlorophyll a concentration. Colonies exhibited steep spatial gradients in rates of gross photosynthesis, O₂ concentration, and irradiance, with the highest values generally occurring at the surface. O₂ concentration, photosynthesis, and irradiance all showed positive correlations, but chlorophyll a concentrations varied independently of photosynthesis and irradiance. Forty-four percent of the variation in ¹⁵N incorporation was explained by gross photosynthesis (a positive correlation) when incorporation of ¹⁵N was expressed per unit of biomass (chlorophyll a).     

荆条叶性状对野外不同光环境的表型可塑性

光照是影响植物生长和分布的重要环境因子。对生长在野外5种不同光环境下(林外、阔叶林林缘、阔叶林林下、针叶林林窗和针叶林林下)的荆条的叶片进行取样研究,通过对光合作用光响应曲线、叶绿素荧光、叶绿素含量、叶片氮磷含量以及叶片形态的测量,来反映荆条对不同光环境的表型可塑性。研究结果表明,荆条叶片对于野外不同的光环境具有很好的适应机制,叶片功能性状受到结构性状的调节。低光下通过高的比叶面积(SLA)、单位质量叶绿素含量、光系统II最大量子产量,低的暗呼吸速率、光饱和点、光补偿点、叶绿素a,b的比值来提高对光能的利用效率,维持生长;高光下则通过与SLA有关的叶片结构的变化对光合作用进行调节。大多数的叶性状只受到日光照总量的影响,SLA的大小与日最高光强有关,可以对不同日变化模式的光照做出迅速的响应,是适应不同光照的敏感指标。尽管光照是不同光环境下影响荆条叶性状的主要环境因子,土壤养分含量同样会对叶性状产生影响,高土壤养分下的高叶长与叶柄长的比值体现了植物对资源获取和支撑结构之间分配的权衡。     

Intramembranous particles and chlorophyll complexes in chloroplasts

The size and population density of large and small particles from freeze-fractured chloroplasts of three wild-type algae and of normal spinach were determined.Computer analyses of low-temperature absorption spectra of chloroplast preparations from these species were performed, and a possible correlation between the occurrence of seven chlorophyll complexes and the aforementioned properties of the intramembranous particles was studied.It was found that only single-sized particles occur in a species containing neither chlorophyll b nor chlorophyll a-685 complexes. The three remaining species carry particles of two sizes, termed large and small particles. However, from quantitative considerations it is concluded that the chlorophyll content of none of the various pigment complexes is related to the size and the population density of the studied particles. If such a relationship exists, it seems likely to be due to the carrier moiety of the chlorophyll b · chlorophyll a-685 complex.     

Antenna ring around trimeric Photosystem I in chlorophyll b containing cyanobacterium Prochlorothrix hollandica

Prochlorothrix hollandica is one of the three known species of an unusual clade of cyanobacteria (formerly called “prochlorophytes”) that contain chlorophyll a and b molecules bound to intrinsic light-harvesting antenna proteins. Here, we report the structural characterization of supramolecular complex consisting of Photosystem I (PSI) associated with the chlorophyll a/b-binding Pcb proteins. Electron microscopy and single particle image analysis of negatively stained preparations revealed that the Pcb-PSI supercomplex consists of a central trimeric PSI surrounded by a ring of 18 Pcb subunits. We conclude that the formation of the Pcb ring around trimeric PSI represents a mechanism for increasing the light-harvesting efficiency in chlorophyll b-containing cyanobacteria.     

Conversion of chlorophyll b to chlorophyll a precedes magnesium dechelation for protection against necrosis in Arabidopsis

Chlorophyll is a deleterious molecule that generates reactive oxygen species and must be converted to non‐toxic molecules during plant senescence. The degradation pathway of chlorophyll a has been determined; however, that of chlorophyll b is poorly understood, and multiple pathways of chlorophyll b degradation have been proposed. In this study, we found that chlorophyll b is degraded by a single pathway, and elucidated the importance of this pathway in avoiding cell death. In order to determine the chlorophyll degradation pathway, we first examined the substrate specificity of 7‐hydroxymethyl chlorophyll a reductase. 7‐hydroxymethyl chlorophyll a reductase reduces 7‐hydroxymethyl chlorophyll a but not 7‐hydroxymethyl pheophytin a or 7‐hydroxymethyl pheophorbide a. These results indicate that the first step of chlorophyll b degradation is its conversion to 7‐hydroxymethyl chlorophyll a by chlorophyll b reductase, although chlorophyll b reductase has broad substrate specificity. In vitro experiments showed that chlorophyll b reductase converted all of the chlorophyll b in the light‐harvesting chlorophyll a/b protein complex to 7‐hydroxymethyl chlorophyll a, but did not completely convert chlorophyll b in the core antenna complexes. When plants whose core antennae contained chlorophyll b were incubated in the dark, chlorophyll b was not properly degraded, and the accumulation of 7‐hydroxymethyl pheophorbide a and pheophorbide b resulted in cell death. This result indicates that chlorophyll b is not properly degraded when it exists in core antenna complexes. Based on these results, we discuss the importance of the proper degradation of chlorophyll b.     

Adaptation of epiphytic bryophytes in the understorey attributing to the correlations and trade-offs between functional traits

This study explores adaptive strategies of epiphytic bryophytes in the understorey by investigating the photosynthetic characteristics, pigment concentrations and nutrient stoichiometry, as well as other functional traits of three trunk-dwelling bryophytes in a subtropical montane cloud forest in SW China. The results showed that their light-saturated net photosynthetic rate (A_nmax?L), light saturation point (I_sat), light compensation point (I_c) and dark respiration rate (R_d) were ca 0.55, 106.72, 4.17 and 0.25?μmol?m^?2?s^?1, respectively. Furthermore, the samples demonstrated photosynthetic down-regulation under high irradiance. These photosynthetic characteristics can be explained by higher total chlorophyll concentrations, specific leaf area, chlorophyll per unit leaf N (Chl/N), lower ratio of chlorophyll a to chlorophyll b (Chl a/b) and photosynthetic nitrogen-use efficiency. We suggest that the bryophytes adapted to the shaded understorey microhabitats through a series of correlations and trade-offs between functional traits.