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Irradiance
Kinds of Irradiance Terms modified by Irradiance Selected AbstractsXanthophyll Cycle and Inactivation of Photosystem II Reaction Centers Alleviating Reducing Pressure to Photosystem I in Morning Glory Leaves under Short-term High IrradianceJOURNAL OF INTEGRATIVE PLANT BIOLOGY, Issue 7 2007Xin-Guo Li Abstract Under 30-min high irradiance (1500 ,mol m,2 s,1), the roles of the xanthophyll cycle and D1 protein turnover were investigated through chlorophyll fluorescence parameters in morning glory (Ipomoea setosa) leaves, which were dipped into water, dithiothreitol (DTT) and lincomycin (LM), respectively. During the stress, both the xanthophyll cycle and D1 protein turnover could protect PSI from photoinhibition. In DTT leaves, non-photochemical quenching (NPQ) was inhibited greatly and the oxidation level of P700 (P700+) was the lowest one. However, the maximal photochemical efficiency of PSII (Fv/Fm) in DTT leaves was higher than that of LM leaves and was lower than that of control leaves. These results suggested that PSI was more sensitive to the loss of the xanthophyll cycle than PSII under high irradiance. In LM leaves, NPQ was partly inhibited, Fv/Fm was the lowest one among three treatments under high irradiance and P700+ was at a similar level as that of control leaves. These results implied that inactivation of PSII reaction centers could protect PSI from further photoinhibition. Additionally, the lowest of the number of active reaction centers to one inactive reaction center for a PSII cross-section (RC/CSo), maximal trapping rate in a PSII cross-section (TRo/CSo), electron transport in a PSII cross-section (ETo/CSo) and the highest of 1-qP in LM leaves further indicated that severe photoinhibition of PSII in LM leaves was mainly induced by inactivation of PSII reaction centers, which limited electrons transporting to PSI. However, relative to the LM leaves the higher level of RC/CSo, TRo/CSo, Fv/Fm and the lower level of 1-qP in DTT leaves indicated that PSI photoinhibition was mainly induced by the electron accumulation at the PSI acceptor side, which induced the decrease of P700+ under high irradiance. [source] Physiological Responses of Acropora cervicornis to Increased Solar Irradiance,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 4 2007Juan L. Torres ABSTRACT The effects of increased UV radiation (UV-B [280,320 nm] + UV-A [320,400 nm]; hereafter UVR) on the growth, production of photosynthetic pigments and photoprotective mycosporine-like amino acids (MAAs) were studied in the threatened Caribbean coral Acropora cervicornis transplanted from 20 to 1 m depth in La Parguera, Puerto Rico. The UVR exposure by the transplanted colonies was significantly higher than that at 20 m, while photosynthetically active radiation (PAR) only increased by 9%. Photosynthetic pigments, quantified with HPLC, as well as linear extension rates and skeletal densities, were significantly reduced 1 month after transplantation to 1 m depth, while MAAs increased significantly despite immediate paling experienced by transplanted colonies. While these colonies showed a significant reduction in photosynthetic pigments, there were no significant reductions in zooxanthellae densities suggesting photoacclimation of the coral's symbionts to the new radiation conditions. The results suggest that while corals might be able to survive sudden increases in UVR and PAR, their skeletal structure can be greatly debilitated due to a reduction in the photosynthetic capacity of their symbionts and a possible relocation of resources. [source] The Effect of Irradiance on Carboxylating/Decarboxylating Enzymes and Fumarase Activities in Mesembryanthemum crystallinum L. Exposed to Salinity StressPLANT BIOLOGY, Issue 1 2001Z. Miszalski Abstract: In Mesembryanthemum crystallinum plants, treated for 9 days with 0.4 M NaCl at low light intensities (80 - 90 or 95 - 100 ,E m -2 s -1; , = 400 - 700 nm), no day/night malate level differences (,malate) were detected. At high light (385 - 400 ,E m -2 s -1) strong stimulation of PEPC activity, accompanied by a ,malate of 11.3 mM, demonstrated the presence of CAM metabolism. This indicates that, to evolve day/night differences in malate concentration, high light is required. Salt treatment at low light induces and increases the activity of NAD- and NADP-malic enzymes by as much as 3.7- and 3.9-fold, while at high light these values reach 6.4- and 17.7-fold, respectively. The induction of activity of both malic enzymes and PEPC (phospoenolpyruvate carboxylase) take place before ,malate is detectable. An increase in SOD (superoxide dismutase) was observed in plants cultivated at high light in both control and salt-treated plants. However, in salt-treated plants this effect was more pronounced. Carboxylating and decarboxylating enzymes seem to be induced by a combination of different signals, i.e., salt and light intensity. Plants performing CAM, after the decrease of activity of both the decarboxylating enzymes at the beginning of the light period, showed an increase in these enzymes in darkness when the malate pool reaches higher levels. In CAM plants the activity of fumarase (Krebs cycle) is much lower than that in C3 plants. The role of mitochondria in CAM plants is discussed. [source] Acclimation of photosynthetic capacity to irradiance in tree canopies in relation to leaf nitrogen concentration and leaf mass per unit areaPLANT CELL & ENVIRONMENT, Issue 3 2002P. Meir Abstract The observation of acclimation in leaf photosynthetic capacity to differences in growth irradiance has been widely used as support for a hypothesis that enables a simplification of some soil-vegetation-atmosphere transfer (SVAT) photosynthesis models. The acclimation hypothesis requires that relative leaf nitrogen concentration declines with relative irradiance from the top of a canopy to the bottom, in 1 : 1 proportion. In combination with a light transmission model it enables a simple estimate of the vertical profile in leaf nitrogen concentration (which is assumed to determine maximum carboxylation capacity), and in combination with estimates of the fraction of absorbed radiation it also leads to simple ,big-leaf' analytical solutions for canopy photosynthesis. We tested how forests deviate from this condition in five tree canopies, including four broadleaf stands, and one needle-leaf stand: a mixed-species tropical rain forest, oak (Quercus petraea (Matt.) Liebl), birch (Betula pendula Roth), beech (Fagus sylvatica L.) and Sitka spruce (Picea sitchensis (Bong.) Carr). Each canopy was studied when fully developed (mid-to-late summer for temperate stands). Irradiance (Q, µmol m,2 s,1) was measured for 20 d using quantum sensors placed throughout the vertical canopy profile. Measurements were made to obtain parameters from leaves adjacent to the radiation sensors: maximum carboxylation and electron transfer capacity (Va, Ja, µmol m,2 s,1), day respiration (Rda, µmol m,2 s,1), leaf nitrogen concentration (Nm, mg g,1) and leaf mass per unit area (La, g m,2). Relative to upper-canopy values, Va declined linearly in 1 : 1 proportion with Na. Relative Va also declined linearly with relative Q, but with a significant intercept at zero irradiance (P < 0·01). This intercept was strongly related to La of the lowest leaves in each canopy (P < 0·01, r2 = 0·98, n= 5). For each canopy, daily lnQ was also linearly related with lnVa(P < 0·05), and the intercept was correlated with the value for photosynthetic capacity per unit nitrogen (PUN: Va/Na, µmol g,1 s,1) of the lowest leaves in each canopy (P < 0·05). Va was linearly related with La and Na(P < 0·01), but the slope of the Va : Na relationship varied widely among sites. Hence, whilst there was a unique Va : Na ratio in each stand, acclimation in Va to Q varied predictably with La of the lowest leaves in each canopy. The specific leaf area, Lm(cm2 g,1), of the canopy-bottom foliage was also found to predict carboxylation capacity (expressed on a mass basis; Vm, µmol g,1 s,1) at all sites (P < 0·01). These results invalidate the hypothesis of full acclimation to irradiance, but suggest that La and Lm of the most light-limited leaves in a canopy are widely applicable indicators of the distribution of photosynthetic capacity with height in forests. [source] On the observation of traveling acoustic waves in the solar atmosphere using a magneto-optical filterASTRONOMISCHE NACHRICHTEN, Issue 3-4 2007M. Haberreiter Abstract In contrast to low-frequency waves that are trapped in the cavity of the Sun, high-frequency waves can travel freely in the solar atmosphere. By modelling the observed intensity signal in the red and blue wings of K I 7699 Å and Na I 5890 Å, we aim to better understand the measurements carried out with the Magneto-Optical Filter at Two Heights (MOTH) experiment. We model the observed intensity signal with radiative transfer calculations carried out with the COde for Solar Irradiance (COSI). Furthermore, we derive the formation height of the lines in order to analyze to what extent the contribution functions are modulated by the acoustic waves. We find a phase lag between the red and blue filter for acoustic waves with a frequency above ,7 mHz and conclude that a frequency dependent data analysis is required for higher frequencies. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Energy input and zooplankton species richnessECOGRAPHY, Issue 6 2007Dag O. Hessen What are the relative contribution of temperature and solar irradiance as types of energy deliveries for species richness at the ecosystem level? In order to reveal this question in lake ecosystems, we assessed zooplankton species richness in 1891 Norwegian lakes covering a wide range in latitude, altitude, and lake area. Geographical variables could largely be replaced by temperature-related variables, e.g. annual monthly maximum temperature or growth season. Multivariate analysis (PCA) revealed that not only maximum monthly temperature, but also energy input in terms of solar radiation were closely associated with species richness. This was confirmed by stepwise, linear regression analysis in which lake area was also found to be significant. We tested the predictive power of the "metabolic scaling laws" for species richness by regressing Ln of species richness over the inverse of the air temperature (in Kelvin), corrected for the activation energy (eV) as predicted by the Boltzmann constant. A significant, negative slope of 0.78 for ln richness over temperature, given as 1/kT, was found, thus slightly higher than the range of slopes predicted from the scaling law (0.60,0.70). Temperature basically constrained the upper bound of species number, but it was only a modest predictor of actual richness. Both PCA-analysis and linear regression models left a large unexplained variance probably due to lake-specific properties such as catchment influence, lake productivity, food-web structure, immigration constraints or more stochastic effects. [source] Large-scale distribution and activity patterns of an extremely low-light-adapted population of green sulfur bacteria in the Black SeaENVIRONMENTAL MICROBIOLOGY, Issue 5 2010Evelyn Marschall Summary The Black Sea chemocline represents the largest extant habitat of anoxygenic phototrophic bacteria and harbours a monospecific population of Chlorobium phylotype BS-1. High-sensitivity measurements of underwater irradiance and sulfide revealed that the optical properties of the overlying water column were similar across the Black Sea basin, whereas the vertical profiles of sulfide varied strongly between sampling sites and caused a dome-shaped three-dimensional distribution of the green sulfur bacteria. In the centres of the western and eastern basins the population of BS-1 reached upward to depths of 80 and 95 m, respectively, but were detected only at 145 m depth close to the shelf. Using highly concentrated chemocline samples from the centres of the western and eastern basins, the cells were found to be capable of anoxygenic photosynthesis under in situ light conditions and exhibited a photosynthesis,irradiance curve similar to low-light-adapted laboratory cultures of Chlorobium BS-1. Application of a highly specific RT-qPCR method which targets the internal transcribed spacer (ITS) region of the rrn operon of BS-1 demonstrated that only cells at the central station are physiologically active in contrast to those at the Black Sea periphery. Based on the detection of ITS-DNA sequences in the flocculent surface layer of deep-sea sediments across the Black Sea, the population of BS-1 has occupied the major part of the basin for the last decade. The continued presence of intact but non-growing BS-1 cells at the periphery of the Black Sea indicates that the cells can survive long-distant transport and exhibit unusually low maintenance energy requirements. According to laboratory measurements, Chlorobium BS-1 has a maintenance energy requirement of ,1.6,4.9·10,15 kJ cell,1 day,1 which is the lowest value determined for any bacterial culture so far. Chlorobium BS-1 thus is particularly well adapted to survival under the extreme low-light conditions of the Black Sea, and can be used as a laboratory model to elucidate general cellular mechanisms of long-term starvation survival. Because of its adaptation to extreme low-light marine environments, Chlorobium BS-1 also represents a suitable indicator for palaeoceanography studies of deep photic zone anoxia in ancient oceans. [source] Differential expression of antenna and core genes in Prochlorococcus PCC 9511 (Oxyphotobacteria) grown under a modulated light,dark cycleENVIRONMENTAL MICROBIOLOGY, Issue 3 2001Laurence Garczarek The continuous changes in incident solar light occurring during the day oblige oxyphototrophs, such as the marine prokaryote Prochlorococcus, to modulate the synthesis and degradation rates of their photosynthetic components finely. How this natural phenomenon influences the diel expression of photosynthetic genes has never been studied in this ecologically important oxyphotobacterium. Here, the high light-adapted strain Prochlorococcus sp. PCC 9511 was grown in large-volume continuous culture under a modulated 12 h,12 h light,dark cycle mimicking the conditions found in the upper layer of equatorial oceans. The pcbA gene encoding the major light-harvesting complex showed strong diel variations in transcript levels with two maxima, one before the onset of illumination and the other near the end of the photoperiod. In contrast, the mRNA level of psbA (encoding the reaction centre II subunit D1), the monocistronic transcript of psbD (encoding D2) and the dicistronic transcript of psbDC were all tightly correlated with light irradiance, with a minimum at night and a maximum at noon. The occurrence of a second peak during the dark period for the monocistronic transcript of psbC (encoding one of the PS II core Chl a antenna proteins) suggested the involvement of post-transcriptional regulation. Differential expression of the external antenna and core genes may constitute a mechanism of regulation of the antenna size to cope with the excess photon fluxes that Prochlorococcus cells experience in the upper layer of oceans around midday. The 5, ends of all transcripts were mapped, and a conserved motif, 5,-TTGATGA-3,, was identified within the putative psbA and pcbA promoters. [source] The responses of photosynthesis and oxygen consumption to short-term changes in temperature and irradiance in a cyanobacterial mat (Ebro Delta, Spain)ENVIRONMENTAL MICROBIOLOGY, Issue 4 2000Eric Epping We have evaluated the effects of short-term changes in incident irradiance and temperature on oxygenic photosynthesis and oxygen consumption in a hypersaline cyanobacterial mat from the Ebro Delta, Spain, in which Microcoleus chthonoplastes was the dominant phototrophic organism. The mat was incubated in the laboratory at 15, 20, 25 and 30°C at incident irradiances ranging from 0 to 1000 µmol photons m,2 s,1. Oxygen microsensors were used to measure steady-state oxygen profiles and the rates of gross photosynthesis, which allowed the calculation of areal gross photosynthesis, areal net oxygen production, and oxygen consumption in the aphotic layer of the mat. The lowest surface irradiance that resulted in detectable rates of gross photosynthesis increased with increasing temperature from 50 µmol photons m,2 s,1 at 15°C to 500 µmol photons m,2 s,1 at 30°C. These threshold irradiances were also apparent from the areal rates of net oxygen production and point to the shift of M. chthonoplastes from anoxygenic to oxygenic photosynthesis and stimulation of sulphide production and oxidation rates at elevated temperatures. The rate of net oxygen production per unit area of mat at maximum irradiance, J0, did not change with temperature, whereas, JZphot, the flux of oxygen across the lower boundary of the euphotic zone increased linearly with temperature. The rate of oxygen consumption per volume of aphotic mat increased with temperature. This increase occurred in darkness, but was strongly enhanced at high irradiances, probably as a consequence of increased rates of photosynthate exudation, stimulating respiratory processes in the mat. The compensation irradiance (Ec) marking the change of the mat from a heterotrophic to an autotrophic community, increased exponentially in this range of temperatures. [source] Tolerance of Antarctic cyanobacterial mats to enhanced UV radiationFEMS MICROBIOLOGY ECOLOGY, Issue 1 2001Alison L. George Abstract To assess the biological implications of ozone depletion over the Antarctic Peninsula, the ultraviolet (UV) regime of two Antarctic cyanobacterial communities (composed of Leptolyngbya sp. and Phormidium sp.) was manipulated using screens that cut out UV radiation and a lamp which enhanced the dose of UV-B radiation (280,315 nm). The biological response of the cyanobacterial mats was monitored by measurement of chlorophyll fluorescence and pigment concentrations. The Leptolyngbya mat showed significant photochemical inhibition due to increased UV-B relative to photosynthetically active radiation (400,700 nm). The effect of UV on the Phormidium mat was less pronounced and dependent on the method of analysis: significantly lower photochemical yields were observed in UV-enhanced Phormidium mats compared to UV-excluded treatment, but if the yield data relative to the time zero control were considered then no effect of UV enhancement was observed. The Phormidium mat contained over 25 times the absolute concentration of UV-protecting mycosporine-like amino acid (MAA) and double the concentration of carotenoids compared to the Leptolyngbya mat, but the latter contained a higher ratio of carotenoids+MAAs to chlorophyll. There were no significant treatment-related changes in the concentrations of MAA, carotenoids and chlorophyll a in the Phormidium mat. The Leptolyngbya mat showed significantly lower chlorophyll a concentrations under UV enhancement, which could account for the lower photochemical yield in this sample. Our results show that different cyanobacterial species have differing photochemical sensitivity to UV-B radiation, which may confer a subtle advantage to the UV-B tolerant species over the less tolerant type during a period of high UV-B irradiance. [source] BDMC interlaboratory cone calorimeter test programmeFIRE AND MATERIALS, Issue 1 2002Joe Urbas In the spring of 1997, seven companies and industry associations from the USA and Canada decided to sponsor the cone calorimeter interlaboratory test programme. Reproducibility and repeatability were determined for the scalar variables measured in the cone calorimeter (ASTM E1354) according to the protocol developed by the Board for the Coordination of the Model Codes. The main requirement of the protocol was that the sample irradiance should be 75 kW/m2. The purpose of the project was to assist the model building code organizations, NFPA and various other groups in the development of a system to determine degrees of combustibility of building materials. Three US and one Canadian laboratory agreed to conduct tests on 16 materials. The results of this round robin show that the cone calorimeter, following the Board for the Coordination of the Model Codes protocol, can provide precision similar to that cited in the current cone calorimeter standards. It is recommended that further improvements of the standards are pursued and provisions are made to improve the quality of operation of the cone calorimeter in commercial laboratories to maintain and possibly improve its repeatability and reproducibility. Copyright © 2002 John Wiley & Sons, Ltd. [source] UV-B radiation constrains the photosynthesis of Quercus robur through impacts on the abundance of Microsphaera alphitoidesFOREST PATHOLOGY, Issue 5 2000By K. K. Newsham Summary Quercus robur saplings were exposed at an outdoor facility in the UK to supplemental levels of UV-B radiation (280,315 nm) under arrays of cellulose diacetate-filtered fluorescent lamps which also produced UV-A radiation (315,400 nm). Saplings were also exposed to supplemental UV-A radiation under arrays of polyester-filtered lamps and to ambient levels of solar radiation under arrays of unenergized lamps. The UV-B treatment was modulated to maintain a 30% elevation above the ambient level of erythemally weighted UV-B radiation. Naturally occurring infections by oak powdery mildew (Microsphaera alphitoides) were more abundant, and developed more rapidly, on lammas leaves of saplings which were exposed to treatment levels of UV-B radiation than on leaves of saplings exposed to supplemental UV-A or to ambient levels of solar radiation over 12 weeks in summer and autumn 1996. An analysis of leaf photosynthetic capacities revealed that M. alphitoides infection reduced the quantum efficiency of photosystem (PS) II by 14% at moderate irradiance. Although there was no direct effect of UV-B radiation on PSII photochemistry, exposure of saplings to supplemental UV-A radiation under polyester-filtered lamps resulted in a 17.5% decrease in PSII quantum efficiency, compared with saplings exposed to ambient solar radiation. The results from our study suggest that photosynthesis of Q. robur may be constrained by exposure to UV-B radiation in the natural environment through impacts on the abundance of M. alphitoides. [source] Effects of sediment resuspension on phytoplankton production: teasing apart the influences of light, nutrients and algal entrainmentFRESHWATER BIOLOGY, Issue 2 2004Marc Schallenberg Summary 1. Wind-induced sediment resuspension can affect planktonic primary productivity by influencing light penetration and nutrient availability, and by contributing meroplankton (algae resuspended from the lake bed) to the water column. We established relationships between sediment resuspension, light and nutrient availability to phytoplankton in a shallow lake on four occasions. 2. The effects of additions of surficial sediments and nutrients on the productivity of phytoplankton communities were measured in 300 mL gas-tight bottles attached to rotating plankton wheels and exposed to a light gradient, in 24 h incubations at in situ temperatures. 3. While sediment resuspension always increased primary productivity, resuspension released phytoplankton from nutrient limitation in only two of the four experiments because the amount of available nitrogen and phosphorus entrained from the sediments was small compared with typical baseline levels in the water column. In contrast, chlorophyll a entrainment was substantial compared with baseline water column concentrations and the contribution of meroplankton to primary production was important at times, especially when seasonal irradiance in the lake was high. 4. Comparison of the in situ light climate with the threshold of light-limitation of the phytoplankton indicated that phytoplankton in the lake were only likely to be light-limited at times of extreme turbidity (e.g. >200 nephelometric turbidity units), particularly when these occur in winter. Therefore, resuspension influenced phytoplankton production mainly via effects on available nutrients and by entraining algae. The importance of each of these varied in time. 5. The partitioning of primary productivity between the water column and sediments in shallow lakes greatly influences the outcome of resuspension events for water column primary productivity. [source] Increase in photosynthetic efficiency as a strategy of planktonic organisms exploiting deep lake layersFRESHWATER BIOLOGY, Issue 2 2004Beatriz Modenutti Summary 1. The photosynthetic efficiencies of the mixotrophic ciliate Ophrydium naumanni and the autotrophic dinoflagellate Gymnodinium paradoxum were investigated using laboratory and field experiments in Lake Moreno Oeste (41°5,S and 71°33,W, 758 m a.s.l.), in the Nahuel Huapi System (North Patagonia, Argentina). 2. The effect of different underwater light intensities on net primary production (NPP) was assessed during one summer. Additionally, laboratory experiments were carried out to obtain photosynthesis-irradiance response curves for each species. 3. Ophrydium naumanni and G. paradoxum dominated the metalimnetic (30 m depth) deep chlorophyll maximum (DCM) in the lake. 4. Despite these deep higher abundances, the cell-specific production of both species was higher at 10 m than at 30 m (DCM) depth. In addition, at 5 m depth, NPP was reduced by PAR + UV-A radiation. 5. Both species exhibited a positive NPP at very low irradiance but the mixotrophic ciliate was more efficient in exploiting the DCM irradiance level both in situ and at comparable light intensities in laboratory experiments. Light acclimatised O. naumanni showed a higher NPP at lower irradiances and photoinhibition at medium and high irradiances. 6. Under the strong wind-driven turbulence commonly found in Patagonian lakes, organisms cannot select their position in the epilimnetic water column and will be dragged to potentially harmful UV radiation levels. Thus, metalimnetic DCM colonisation by these two species represents a tradeoff between higher survival and lower cell-specific NPP. [source] Photoheterotrophy and light-dependent uptake of organic and organic nitrogenous compounds by Planktothrix rubescens under low irradianceFRESHWATER BIOLOGY, Issue 10 2003Tatiana Zotina Summary 1. Planktothrix rubescens is the dominant photoautotrophic organism in Lake Zürich, a prealpine, deep, mesotrophic freshwater lake with an oxic hypolimnion. Over long periods of the year, P. rubescens accumulates at the metalimnion and growth occurs in situ at irradiance near the photosynthesis compensation point. Experiments were conducted to evaluate the contribution of photoheterotrophy, heterotrophy and light-dependent uptake of nitrogenous organic compounds to the carbon and nitrogen budget of this cyanobacterium under conditions of restricted availability of light quanta. 2. We used both purified natural populations of P. rubescens from the depth of 9 m and an axenic culture grown under low irradiance at 11 ,mol m,2 s,1 on a light : dark cycle (10 : 14 h) to determine the uptake rates of various amino acids, urea, glucose, fructose, acetate and inorganic carbon. The components were added to artificial lake water in low amounts that simulated the naturally occurring potential concentrations. 3. The uptake rates of acetate and amino acids (glycine, serine, glutamate and aspartate) were strongly enhanced at low irradiance as compared with the dark. However, no difference was observed in the uptake of arginine, which was taken up at high rates under both treatments. The uptake rates of glucose, fructose and urea were very low under all conditions. Similar results were obtained for both axenic P. rubescens and for purified natural populations of P. rubescens that were separated from bacterioplankton and other phytoplankton. 4. Metalimnetic P. rubescens that was stratified at low irradiance for weeks exhibited much higher uptake rates than filaments that were entrained in the deepening surface mixed layer and experienced higher irradiance. The added organic compounds contributed up to 62% to the total carbon uptake of metalimnetic P. rubescens. On the basis of a molar C : N ratio of 4.9, the nitrogen uptake as organic compounds satisfied up to 84% of the nitrogen demand. 5. The experiments indicate that photoheterotrophy and light-dependent uptake of nitrogenous organic compounds may contribute significantly to the carbon and nitrogen budget of filaments at low irradiance typical for growth of P. rubescens in the metalimnion and at the bottom of the surface mixed layer. [source] Growth rates of phytoplankton under fluctuating lightFRESHWATER BIOLOGY, Issue 2 2000Elena Litchman Summary 1The effect of light fluctuations on the growth rates of four species of freshwater phytoplankton was investigated. Experimental light regimes included constant irradiance and fluctuations of a step function form, with equal proportion of high (maximum of 240 µmol photons m -2 s -1) and low light (minimum of 5 µmol photons m -2 s -1) (or dark) in a period. Fluctuations of 1, 8 and 24-h periods were imposed over several average irradiances (25, 50, 100 and 120 µmol photons m -2 s -1). 2Growth rate responses to fluctuations were species-specific and depended on both the average irradiance and the period of fluctuations. Fluctuations at low average irradiances slightly increased growth rate of the diatom Nitzschia sp. and depressed growth of the cyanobacterium Phormidium luridum and the green alga Sphaerocystis schroeteri compared to a constant irradiance. 3Fluctuations at higher average irradiance did not have a significant effect on the growth rates of Nitzschia sp. and Sphaerocystis schroeteri (fluctuations around saturating irradiances) and slightly increased the growth rates of the cyanobacteria Anabaena flos-aquae and Phormidium luridum (when irradiance fluctuated between limiting and inhibiting levels). 4In general, the effect of fluctuations tended to be greater when irradiance fluctuated between limiting and saturating or inhibiting levels of a species growth-irradiance curve compared to fluctuations within a single region of the curve. 5The growth rates of species under fluctuating light could not always be predicted from their growth-irradiance curves obtained under constant irradiance. When fluctuations occur between limiting and saturating or inhibiting irradiances for the alga and when the period of fluctuations is long (greater than 8 h), steady-state growth-irradiance curves may be insufficient to predict growth rates adequately. Consequently, additional data on physiological acclimation, such as changes in photosynthetic parameters, may be required for predictions under non-constant light supply in comparison to constant conditions. [source] Light gains and physiological capacity of understorey woody plants during phenological avoidance of canopy shadeFUNCTIONAL ECOLOGY, Issue 4 2005C. K. AUGSPURGER Summary 1Carbon gain during phenological avoidance of canopy shade by an understorey plant depends on the extent of avoidance, the leaf stage during avoidance, and whether young and old leaves can exploit greater light availability in spring and autumn. 2For Asimina triloba (L.) Dunal., Aesculus glabra Willd., Acer saccharum Marsh., Lindera benzoin (L.) Blume and Carpinus caroliniana Walt. in a deciduous forest in Illinois, USA, spring avoidance with leaves at full size ranged from 0 days for Asimina to 24 days for Aesculus, and brought 36,98% of estimated total annual irradiance. Autumn avoidance was non-existent to minimal in all species. 3Total chlorophyll reached maximum concentrations at the middle of leaf life span, and declined well before senescence. Leaf nitrogen concentrations and net photosynthetic capacity both peaked in youngest leaves during spring avoidance, and were low in old leaves during autumn avoidance. 4Aesculus had especially high photosynthetic capacity during precanopy closure, while Asimina had relatively low capacity in its later developing leaves. 5Young leaves of species with phenological avoidance can enhance C gain, while old leaves in autumn do not. Thus phenological avoidance in spring may enhance the persistence of understorey woody individuals of some species. [source] Allocation of above-ground growth is related to light in temperate deciduous saplingsFUNCTIONAL ECOLOGY, Issue 4 2003D. A. King Summary 1Allocational shifts in response to light may be an important factor in allowing plants to survive in shade, while increasing their extension rates and competitive ability in sun. To investigate this response, the allocation of above-ground growth between leaves, branches and stems was studied in saplings of Acer pensylvanicum L. and Castenea dentata (Marsh.) Borkh. in the Appalachian mountains of western Virginia, USA. Measurements of current leaf biomass, current and past year leaf numbers and the growth ring widths of branches and stem were used to estimate biomass partitioning for saplings growing in locations ranging from forest understorey to large openings. 2Both species showed higher leaf area per unit leaf biomass (SLA) and higher allocation of above-ground growth to leaves in shade than in sun. 3There were no differences between species in the slopes of the relationships of allocation and SLA vs estimated irradiance, but SLA was significantly greater in A. pensylvanicum than in C. dentata at a given light level. Hence, somewhat lower production per unit leaf area is required to maintain the canopy in A. pensylvanicum, consistent with foresters' ratings of greater shade tolerance for this species. 4Greater foliar allocation in shade than sun has also been observed in broad-leaved evergreen saplings, but generally not in seedlings. This difference is probably related to differences in size and age between seedlings and saplings. Young seedlings typically show exponential growth with no immediate foliar losses, while shaded saplings lie closer to the steady state where new leaves replace old ones with little additional stem growth. 5Thus trees shift their allocation patterns in an acclimatory fashion, depending on their size and light environment, with the costs of replacing senesced leaves becoming of consequence as juveniles age. [source] Crossovers in seedling relative growth rates between low and high irradiance: analyses and ecological potentialFUNCTIONAL ECOLOGY, Issue 2 2003L. Sack First page of article [source] Why do species of woody seedlings change rank in relative growth rate between low and high irradiance?FUNCTIONAL ECOLOGY, Issue 2 2001L. Sack First page of article [source] Gender, light and water effects in carbon isotope discrimination, and growth rates in the dioecious tree Ilex aquifoliumFUNCTIONAL ECOLOGY, Issue 5 2000R. Retuerto Abstract 1.,Detailed understanding of the specific physiology of sexes in dioecious species is required to explain patterns in gender dimorphism. Under controlled-environment conditions we tested the hypothesis that sexes of the dioecious tree holly Ilex aquifolium L. (Aquifoliaceae) differed in growth and long-term potential water-use efficiency, as measured by carbon isotope discrimination (,13C), and that these differences were dependent on the environmental context. 2.,Patterns of response in ,13C to the various combinations of light and water were gender-specific. Under more xeric conditions, females maintained significantly higher ,13C than males. 3.,Female plants exhibited significantly greater relative diameter growth rates than male plants. 4.,As expected, ,13C significantly increased with decreasing irradiance, and decreased with increasing limitation in water supply. Light and water effects were not independent, with a more pronounced drought effect in decreasing leaf ,13C under unshaded than under shaded conditions. 5.,Our results suggest that between-sex differences in physiology are context-dependent. Future studies attempting to assess gender dimorphism should take more account of gender-specific interactions with the environment. Gender-specific efficiency in water use could play a decisive role in explaining gender differences in growth and ecological interactions. [source] A geochronological approach to understanding the role of solar activity on Holocene glacier length variability in the Swiss AlpsGEOGRAFISKA ANNALER SERIES A: PHYSICAL GEOGRAPHY, Issue 4 2006Anne Hormes ABSTRACT. We present a radiocarbon data set of 71 samples of wood and peat material that melted out or sheared out from underneath eight presentday mid-latitude glaciers in the Central Swiss Alps. Results indicated that in the past several glaciers have been repeatedly less extensive than they were in the 1990s. The periods when glaciers had a smaller volume and shorter length persisted between 320 and 2500 years. This data set provides greater insight into glacier variability than previously possible, especially for the early and middle Holocene. The radiocarbon-dated periods defined with less extensive glaciers coincide with periods of reduced radio-production, pointing to a connection between solar activity and glacier melting processes. Measured long-term series of glacier length variations show significant correlation with the total solar irradiance. Incoming solar irradiance and changing albedo can account for a direct forcing of the glacier mass balances. Long-term investigations of atmospheric processes that are in interaction with changing solar activity are needed in order to understand the feedback mechanisms with glacier mass balances. [source] Photodegradation leads to increased carbon dioxide losses from terrestrial organic matterGLOBAL CHANGE BIOLOGY, Issue 11 2010SUSANNA RUTLEDGE Abstract CO2 production in terrestrial ecosystems is generally assumed to be solely biologically driven while the role of abiotic processes has been largely overlooked. In addition to microbial decomposition, photodegradation , the direct breakdown of organic matter (OM) by solar irradiance , has been found to contribute to litter mass loss in dry ecosystems. Previous small-scale studies have shown that litter degradation by irradiance is accompanied by emissions of CO2. However, the contribution of photodegradation to total CO2 losses at ecosystems scales is unknown. This study determined the proportion of the total CO2 losses caused by photodegradation in two ecosystems: a bare peatland in New Zealand and a seasonally dry grassland in California. The direct effect of solar irradiance on CO2 production was examined by comparing daytime CO2 fluxes measured using eddy covariance (EC) systems with simultaneous measurements made using an opaque chamber and the soil CO2 gradient technique, and with night-time EC measurements under the same soil temperature and moisture conditions. In addition, a transparent chamber was used to directly measure CO2 fluxes from OM caused by solar irradiance. Photodegradation contributed 19% of the annual CO2 flux from the peatland and almost 60% of the dry season CO2 flux from the grassland, and up to 62% and 92% of the summer mid-day CO2 fluxes, respectively. Our results suggest that photodegradation may be important in a wide range of ecosystems with exposed OM. Furthermore, the practice of partitioning daytime ecosystem CO2 exchange into its gross components by assuming that total daytime CO2 losses can be approximated using estimates of biological respiration alone may be in error. To obtain robust estimates of global ecosystem,atmosphere carbon transfers, the contribution of photodegradation to OM decomposition must be quantified for other ecosystems and the results incorporated into coupled carbon,climate models. [source] Decadal change in wetland,woodland boundaries during the late 20th century reflects climatic trendsGLOBAL CHANGE BIOLOGY, Issue 8 2010DAVID A. KEITH Abstract Wetlands are important and restricted habitats for dependent biota and play vital roles in landscape function, hydrology and carbon sequestration. They are also likely to be one of the most sensitive components of the terrestrial biosphere to global climate change. An understanding of relationships between wetland persistence and climate is imperative for predicting, mitigating and adapting to the impacts of future climate change on wetland extent and function. We investigated whether mire wetlands had contracted, expanded or remained stable during 1960,2000. We chose a study area encompassing a regional climatic gradient in southeastern Australia, specifically to avoid confounding effects of water extraction on wetland hydrology and extent. We first characterized trends in climate by examining data from local weather stations, which showed a slight increase in precipitation and marked decline in pan evaporation over the relevant period. Remote sensing of vegetation boundaries showed a marked lateral expansion of mires during 1961,1998, and a corresponding contraction of woodland. The spatial patterns in vegetation change were consistent with the regional climatic gradient and showed a weaker co-relationship to fire history. Resource exploitation, wildland fires and autogenic mire development failed to explain the observed expansion of mire vegetation in the absence of climate change. We therefore conclude that the extent of mire wetlands is likely to be sensitive to variation in climatic moisture over decadal time scales. Late 20th-century trends in climatic moisture may be related primarily to reduced irradiance and/or reduced wind speeds. In the 21st century, however, net climatic moisture in this region is projected to decline. As mires are apparently sensitive to hydrological change, we anticipate lateral contraction of mire boundaries in coming decades as projected climatic drying eventuates. This raises concerns about the future hydrological functions, carbon storage capacity and unique biodiversity of these important ecosystems. [source] Responses of plants in polar regions to UVB exposure: a meta-analysisGLOBAL CHANGE BIOLOGY, Issue 11 2009KEVIN K. NEWSHAM Abstract We report a meta-analysis of data from 34 field studies into the effects of ultraviolet B (UVB) radiation on Arctic and Antarctic bryophytes and angiosperms. The studies measured plant responses to decreases in UVB radiation under screens, natural fluctuations in UVB irradiance or increases in UVB radiation applied from fluorescent UV lamps. Exposure to UVB radiation was found to increase the concentrations of UVB absorbing compounds in leaves or thalli by 7% and 25% (expressed on a mass or area basis, respectively). UVB exposure also reduced aboveground biomass and plant height by 15% and 10%, respectively, and increased DNA damage by 90%. No effects of UVB exposure were found on carotenoid or chlorophyll concentrations, net photosynthesis, Fv/Fm or ,PSII, belowground or total biomass, leaf mass, leaf area or specific leaf area (SLA). The methodology adopted influenced the concentration of UVB absorbing compounds, with screens and natural fluctuations promoting significant changes in the concentrations of these pigments, but lamps failing to elicit a response. Greater reductions in leaf area and SLA, and greater increases in concentrations of carotenoids, were found in experiments based in Antarctica than in those in the Arctic. Bryophytes typically responded in the same way as angiosperms to UVB exposure. Regression analyses indicated that the percentage difference in UVB dose between treatment and control plots was positively associated with concentrations of UVB absorbing compounds and carotenoids, and negatively so with aboveground biomass and leaf area. We conclude that, despite being dominated by bryophytes, the vegetation of polar regions responds to UVB exposure in a similar way to higher plant-dominated vegetation at lower latitudes. In broad terms, the exposure of plants in these regions to UVB radiation elicits the synthesis of UVB absorbing compounds, reduces aboveground biomass and height, and increases DNA damage. [source] Ecophysiological controls over the net ecosystem exchange of mountain spruce stand.GLOBAL CHANGE BIOLOGY, Issue 1 2007Comparison of the response in direct vs. diffuse solar radiation Abstract Cloud cover increases the proportion of diffuse radiation reaching the Earth's surface and affects many microclimatic factors such as temperature, vapour pressure deficit and precipitation. We compared the relative efficiencies of canopy photosynthesis to diffuse and direct photosynthetic photon flux density (PPFD) for a Norway spruce forest (25-year-old, leaf area index 11 m2 m,2) during two successive 7-day periods in August. The comparison was based on the response of net ecosystem exchange (NEE) of CO2 to PPFD. NEE and stomatal conductance at the canopy level (Gcanopy) was estimated from half-hourly eddy-covariance measurements of CO2 and H2O fluxes. In addition, daily courses of CO2 assimilation rate (AN) and stomatal conductance (Gs) at shoot level were measured using a gas-exchange technique applied to branches of trees. The extent of spectral changes in incident solar radiation was assessed using a spectroradiometer. We found significantly higher NEE (up to 150%) during the cloudy periods compared with the sunny periods at corresponding PPFDs. Prevailing diffuse radiation under the cloudy days resulted in a significantly lower compensation irradiance (by ca. 50% and 70%), while apparent quantum yield was slightly higher (by ca. 7%) at canopy level and significantly higher (by ca. 530%) in sun-acclimated shoots. The main reasons for these differences appear to be (1) more favourable microclimatic conditions during cloudy periods, (2) stimulation of photochemical reactions and stomatal opening via an increase of blue/red light ratio, and (3) increased penetration of light into the canopy and thus a more equitable distribution of light between leaves. Our analyses identified the most important reason of enhanced NEE under cloudy sky conditions to be the effective penetration of diffuse radiation to lower depths of the canopy. This subsequently led to the significantly higher solar equivalent leaf area compared with the direct radiation. Most of the leaves in such dense canopy are in deep shade, with marginal or negative carbon balances during sunny days. These findings show that the energy of diffuse, compared with direct, solar radiation is used more efficiently in assimilation processes at both leaf and canopy levels. [source] High rates of net ecosystem carbon assimilation by Brachiara pasture in the Brazilian CerradoGLOBAL CHANGE BIOLOGY, Issue 5 2004Alexandre J.B. Santos Abstract To investigate the consequences of land use on carbon and energy exchanges between the ecosystem and atmosphere, we measured CO2 and water vapour fluxes over an introduced Brachiara brizantha pasture located in the Cerrado region of Central Brazil. Measurements using eddy covariance technique were carried out in field campaigns during the wet and dry seasons. Midday CO2 net ecosystem exchange rates during the wet season were ,40 ,mol m,2 s,1, which is more than twice the rate found in the dry season (,15 ,mol m,2 s,1). This was observed despite similar magnitudes of irradiance, air and soil temperatures. During the wet season, inferred rates of canopy photosynthesis did not show any tendency to saturate at high solar radiation levels, with rates of around 50 ,mol m,2 s,1 being observed at the maximum incoming photon flux densities of 2200 ,mol m,2 s,1. This contrasted strongly to the dry period when light saturation occurred with 1500 ,mol m,2 s,1 and with maximum canopy photosynthetic rates of only 20 ,mol m,2 s,1. Both canopy photosynthetic rates and night-time ecosystem CO2 efflux rates were much greater than has been observed for cerrado native vegetation in both the wet and dry seasons. Indeed, observed CO2 exchange rates were also much greater than has previously been reported for C4 pastures in the tropics. The high rates in the wet season may have been attributable, at least in part, to the pasture not being grazed. Higher than expected net rates of carbon acquisition during the dry season may also have been attributable to some early rain events. Nevertheless, the present study demonstrates that well-managed, productive tropical pastures can attain ecosystem gas exchange rates equivalent to fertilized C4 crops growing in the temperate zone. [source] The contribution of bryophytes to the carbon exchange for a temperate rainforestGLOBAL CHANGE BIOLOGY, Issue 8 2003Evan H. DeLucia Abstract Bryophytes blanket the floor of temperate rainforests in New Zealand and may influence a number of important ecosystem processes, including carbon cycling. Their contribution to forest floor carbon exchange was determined in a mature, undisturbed podocarp-broadleaved forest in New Zealand, dominated by 100,400-year-old rimu (Dacrydium cupressimum) trees. Eight species of mosses and 13 species of liverworts contributed to the 62% cover of the diverse forest floor community. The bryophyte community developed a relatively thin (depth <30 mm), but dense, canopy that experienced elevated CO2 partial pressures (median 46.6 Pa immediately below the bryophyte canopy) relative to the surrounding air (median 37.6 Pa at 100 mm above the canopy). Light-saturated rates of net CO2 exchange from 14 microcosms collected from the forest floor were highly variable; the maximum rate of net uptake (bryophyte photosynthesis , whole-plant respiration) per unit ground area at saturating irradiance was 1.9 ,mol m,2 s,1 and in one microcosm, the net rate of CO2 exchange was negative (respiration). CO2 exchange for all microcosms was strongly dependent on water content. The average water content in the microcosms ranged from 1375% when fully saturated to 250% when air-dried. Reduction in water content across this range resulted in an average decrease of 85% in net CO2 uptake per unit ground area. The results from the microcosms were used in a model to estimate annual carbon exchange for the forest floor. This model incorporated hourly variability in average irradiance reaching the forest floor, water content of the bryophyte layer, and air and soil temperature. The annual net carbon uptake by forest floor bryophytes was 103 g m,2, compared to annual carbon efflux from the forest floor (bryophyte and soil respiration) of ,1010 g m,2. To put this in perspective of the magnitude of the components of CO2 exchange for the forest floor, the bryophyte layer reclaimed an amount of CO2 equivalent to only about 10% of forest floor respiration (bryophyte plus soil) or ,11% of soil respiration. The contribution of forest floor bryophytes to productivity in this temperate rainforest was much smaller than in boreal forests, possibly because of differences in species composition and environmental limitations to photosynthesis. Because of their close dependence on water table depth, the contribution of the bryophyte community to ecosystem CO2 exchange may be highly responsive to rapid changes in climate. [source] Alumina Optical Surface Heat Shield for Use in Near-Solar EnvironmentINTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 3 2009Don E. King Experiments indicate that coating a heat shield with alumina can significantly reduce spacecraft temperature during operation near the sun. A thin alumina (Al2O3) coating applied to carbon,carbon (C,C) reflects the majority of the visible solar irradiance while reemitting absorbed energy in the infrared. Testing on Al2O3 -coated C,C coupons using visible and NIR lasers (from ambient to 1773 K) show that the solar-absorptance-to-IR emittance ratio (,S/,IR) of the Al2O3 -coated heat shield was 0.6 or less. Compared with an uncoated carbon,carbon heat shield, the coated version is at least 12% cooler, enabling thermal insulation mass reductions, improved scientific measurements, and the use of less exotic thermal protection materials. [source] An investigation of solar erythemal ultraviolet radiation in the tropics: a case study at four stations in ThailandINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 12 2010Serm Janjai Abstract Our study examines a 5-year data set of erythemal ultraviolet radiation (EUV) collected at four locations in Thailand: Chiang Mai (18.78°N, 98.98°E), Ubon Ratchathani (15.25°N, 104.87°E), Nakhon Pathom (13.82°N, 100.04°E) and Songkhla (7.20°N, 100.60°E). Seasonal changes are strongly influenced by the wet and dry season in this tropical environment, with maximum daily and noontime irradiances being reached in April or May, prior to the onset of the wet season. Transmission by aerosols, estimated by comparison of cloudless measured irradiance with a cloudless sky model, ranges from 51% at Chiang Mai to 83% at Songkhla during the dry season. By contrast, higher transmissions characterise the wet season when values around 90% are reached. Cloud cover further depletes the EUV irradiance and wet season transmissions range from 68% at Chiang Mai to 79% at Songkhla. Three of the four stations record increases in EUV irradiance over the study period. Copyright © 2009 Royal Meteorological Society [source] | ||||||||||||||||||||||||||||||||||||||||||||||