Unit Chlorophyll Research Articles

Effects of Simulated Acid Rain on Pinus densiflora: Inhibition of Net Photosynthesis by the Pheophytization of Chlorophyll

This research addressed the effects of simulated acid rain on the chlorophyll contents of leaves of Pinus densiflora, on the net photosynthetic rate on a unit chlorophyll basis, and on chlorophyll degradation. When NO3- concentrations in rain water increased, the chlorophylla+b contents of the leaves increased in parallel, even though the pH of the simulated rain decreased. The nitrogen content of the rain was sufficient to stimulate the production of chlorophyll and was able to override any degradative effect expected due to the H+ input from acid rain. The absorption spectrum of chlorophyll extracted from the leaves exposed to acid rain showed peaks at 665, 606 and 535 nm, characteristic of pheophytin a, indicating degradation of chlorophyll to pheophytin. Net photosynthetic rate on the basis of chlorophyll content was reduced by acid rain, even though the total chlorophyll content of the leaves was increased. The acid rain does, therefore, simultaneously increase chlorophyll content and reduce the efficiency of the use of chlorophyll in photosynthesis. This reduced efficiency may be linked to the increase in the rate of degradation of chlorophyll to pheophytin.

Effects of manganese toxicity on leaf CO2 assimilation of contrasting common bean genotypes

Parameters related to leaf photosynthesis were evaluated in three genotypes of common bean (Phaseolus vulgaris L.) with contrasting tolerance to Mn toxicity. Two short‐term studies in solution culture were used to assess the effect of excess Mn on CO2 assimilation in mature and immature leaves. Mn toxicity decreased total chlorophyll content only in immature leaves, with a consequent reduction of leaf CO2 assimilation. Mature leaves that showed brown speckles characteristic of Mn toxicity, did not suffer any detriment in their capacity to assimilate CO2, at least in a 4‐day experiment. Stomatal conductance and transpiration were not affected by the presence of high levels of Mn in leaf tissue. Lower stomatal conductance and transpiration rates were observed only in leaves with advanced chlorosis. Differences among genotypes were detected as increased chlorosis in the more sensitive genotype ZPV‐292, followed by A‐283 and less chlorosis in the tolerant genotype CALIMA. Since CO2 assimilation expressed per unit of chlorophyll was not different between high‐Mn plants and control plants, we conclude that the negative effect of Mn toxicity on CO2 assimilation can be explained by a reduction in leaf chlorophyll content.

Differences in Flag Leaf Photosynthesis And Respiration In Bread Wheat

Photosynthetic oxygen evolution in light and oxygen consumption in dark were studied in two parental bread wheat cultivars and five selections initially made for stomatal frequency. Discs cut out from flag leaf blades of field grown plants were assayed in a sensitive oxygen measuring system over a period starting from flag leaf emergence. Chlorophyll content, flag leaf blade area, stomatal frequency, mesophyll cell size and lobe number were estimated. Photosynthetic rate per unit leaf area, respiration rate per unit chlorophyll, leaf area and stomatal frequency differences were significant. Correlations between different parameters were drawn. Results indicate that selections made on the basis of anatomical or morphological characters may have different photosynthetic characteristics useful in crop improvement.

PROTEINS, PHOTOSYNTHETIC PIGMENTS AND POLAR LIPIDS CHANGES OF THYLAKOID BIOMEMBRANE FROM PRUNUS AFFECTED BY IRON CHLOROSIS

ISHS III International Symposium on Mineral Nutrition of Deciduous Fruit Trees PROTEINS, PHOTOSYNTHETIC PIGMENTS AND POLAR LIPIDS CHANGES OF THYLAKOID BIOMEMBRANE FROM PRUNUS AFFECTED BY IRON CHLOROSIS

Photosynthetic carbon acquisition in the lichen photobionts Coccomyxa and Trebouxia (Chlorophyta)

Processes involved in photosynthetic CO2 acquisition were characterised for the isolated lichen photobiont Trebouxia erici (Chlorophyta, Trebouxiophyceae) and compared with Coccomyxa (Chlorophyta), a lichen photobiont without a photosynthetic CO2‐concentrating mechanism. Comparisons of ultrastructure and immuno‐gold labelling of ribulose‐1,5‐bisphosphate carboxylase‐oxygenase (Rubisco; EC 4.1.1.39) showed that the chloroplast was larger in T. erici and that the majority of Rubisco was located in its centrally located pyrenoid. Coccomyxa had no pyrenoid and Rubisco was evenly distributed in its chloroplast. Both species preferred CO2 rather than HCO3− as an external substrate for photosynthesis, but T. erici was able to use CO2 concentrations below 10–12 μM more efficiently than Coccomyxa. In T. erici, the lipid‐insoluble carbonic anhydrase (CA; EC 4.2.1.1) inhibitor acetazolamide (AZA) inhibited photosynthesis at CO2 concentrations below 1 μM, while the lipid‐soluble CA inhibitor ethoxyzolamide (EZA) inhibited CO2‐dependent O2 evolution over the whole CO2 range. EZA inhibited photosynthesis also in Coccomyxa, but to a much lesser extent below 10–12 μM CO2. The internal CA activity of Trebouxia, per unit chlorophyll (Chl), was ca 10% of that of Coccomyxa. Internal CA activity was also detected in homogenates from T. erici and two Trebouxia‐lichens (Lasallia hispanica and Cladina rangiferina). In all three, the predominating CA had α‐type characteristics and was significantly inhibited by low concentrations of AZA, having an I50 below 10–20 nM. In Coccomyxa a β‐type CA predominates, which is much less sensitive to AZA. Thus, the two photobionts differed in three major characteristics with respect to CO2 acquisition, the subcellular location of Rubisco, the relative requirement of CA and the biochemical characteristics of their predominating internal CA. These differences may be linked to the ability of Trebouxia to accumulate dissolved inorganic carbon internally, enhancing their CO2 use efficiency at and below air‐equilibrium concentrations (10–12 μM CO2) in comparison with Coccomyxa.

Analysis of diel variability in chlorophyll fluorescence

ABSTRACT Examples of the die1 variability in chlorophyll and beam attenuation are presented using data from moored in situ fluorometers and transmissometers, and from profiles of the beam attenuation coefficient and fluorescence. The data are discussed in terms of the three primary processes thought to influence the die1 variability in chlorophyll: (1) fluorescence yield per unit chlorophyll a, (2) chlorophyll a changes per cell (or, carbon), and (3) changes in phytoplankton carbon (growth). A simple, provisional, model is presented which incorporates these three biological processes, under the assumption that the corresponding die1 variability in particle attenuation represents the change in phytoplankton carbon. The analysis produces qualitative agreement with the in situ data. The model suggests that while under high-light conditions fluorescence declines, chlorophyll can still increase during the day. Under low-light conditions, the die1 variability of chlorophyll and fluorescence are in phase. The model is limited by (1) physiological understanding of the processes involved in producing fluorescence and chlorophyll changes over die1 time scales; (2) biomass-independent variations in particle attenuation; and (3) not including, for the present, physical forcing.

Seasonal changes in factors controlling phytoplankton growth in Beatrix Bay, New Zealand

Phytoplankton biomass and growth rates were measured in Beatrix Bay, Marlborough Sounds, during 1994–95. In spring and summer, nitrate and chlorophyll a concentrations in the mixed layer were low (typically < 1 mg N m3 and < 1 to 3 mg Chi. a m−3, respectively), and phytoplankton growth rates were moderate (average 0.3 d−1). Growth rates increased several‐fold in response to the experimental addition of inorganic N (but not to inorganic phosphorus). Higher nitrate concentrations were found below the pycnocline. During autumn and winter, nitrate concentrations in the mixed layer were high (up to 60 mg m3), but phytoplankton growth rates were lower than in summer (average 0.2 d−1), and did not respond to added nutrients. Chlorophyll a concentrations, however, were highest during the winter (typically 3–6 mg m−3). Even so, low light levels meant integral photosynthesis per unit chlorophyll a was low in winter. Phytoplankton growth appears to be controlled by the availability of nitrogen in summer and by light in winter.

The primary productivity of benthic and planktonic algae in a prairie wetland under controlled water-level regimes

There have been few measurements of primary productivity by benthic (periphytic) and planktonic algae in prairie wetlands so their quantitative importance relative to other primary producers is largely unknown. We measured the daily productivity (inorganic carbon assimilation per m2 of wetland area) of phytoplankton, epipelon, epiphyton, and metaphyton in ten wetland cells in Delta Marsh, Manitoba over a five-year period. Water levels in the cells were manipulated so that some cells had normal water levels for the wetland, while water depths increased 30 cm or 60 cm in other treatments. With increasing water depth, phytoplankton productivity increased while that of epipelon, epiphyton, and metaphyton decreased. Metaphyton was the largest contributor to total algal productivity (70%), followed by epiphyton (23%), phytoplankton (6%), and epipelon (1%). Phytoplankton had the highest photosynthetic efficiency (C assimilated per unit chlorophyll), despite being a minor contributor to total productivity. Variations in P-I parameters (α, β, Ik, and Pmax) were considerable, possibly due to temporal and spatial fluctuation in the abiotic environment. Algal productivity was comparable to that of submersed and emergent macrophytes, suggesting that algae are probably important resources in supporting food webs in prairie wetlands.

Foliar CO2photoassimilation and chloroplast linear electron transport rates in nitrogen-sufficient and nitrogen-limited soybean plants

Leaflets of soybean plants which are moderately inorganic nitrogen (N)-limited exhibit either no difference in the rate of net photosynthesis or as much as a 15–23% lower net photosynthesis rate per unit area than leaflets of N-sufficient plants [Robinson JM (1996) Photosynth Res 50: 133–148; Robinson JM (1997a) Int J Plant Sci 158: 32–43]. However, mature leaflets of N-limited soybean plants have a higher CO2photoassimilation rate per unit chlorophyll than leaflets of N-sufficient soybean plants at both moderate light intensity (≈500 µmol m-2s-1) and saturating light intensity (≈1200 µmol m-2s-1) [Robinson JM (1996) Photosynth Res 50: 133–148]. This study was undertaken to determine whether chloroplast thylakoids isolated from the leaflets of nitrogen-limited soybean plants displayed similar or higher linear electron transport rates (H2O → ferredoxin → NADP) per unit chlorophyll than thylakoids isolated from leaflets of N-sufficient plants. Chlorophyll concentration in reaction mixtures containing chloroplast thylakoids prepared from leaflets of N-limited plants was manipulated so that it was similar to the chlorophyll concentration in reaction mixtures of thylakoids prepared from leaflets of N-sufficient plants. Measurements of ferredoxin dependent, NADP dependent, O2photo-evolution in thylakoid isolates were carried out in saturating light (≈1500 µmol m-2s-1) and with \(NH_4^ + \)(an uncoupler) in the chloroplast reaction mixtures. Chloroplast thylakoids isolated from N-limited soybean plant leaflets routinely had a 1.5 to 1.7 times higher rate of uncoupled, whole chain electron transport per unit chlorophyll in saturating light than did chloroplast thylakoids isolated from leaflets of N-sufficient plants. The results suggest that the photosystems and photosynthetic electron transport chain components are more active per unit Chl in leaflet chloroplast thylakoids of N-limited soybean plants than in thylakoids of N-sufficient plants.

Physiological Aspects Related to Tolerance of Spring Wheat Cultivars to Septoria tritici Blotch

ABSTRACT The susceptible wheat cultivar Miriam exhibited tolerance under severe infection of Septoria tritici blotch (STB). Nethouse and greenhouse trials confirmed former field results in which losses in grain weight of 'Miriam' wheat due to STB infection were significantly lower than those of the susceptible cultivar Barkai, under equivalent severity and the same disease progress curve. Several physiological mechanisms that may explain this tolerance of 'Miriam' wheat were studied. A comparison between protected and infected plants proved that carbohydrate reserves in the culms and other vegetative plant parts did not account for the lower losses in grain weight of 'Miriam'. Each tiller was shown to be independent in its supply of carbohydrates to its grains, and no import from secondary tillers was observed. Differences in the ratio between grain weight and vegetative biomass could not explain the sustained grain filling of infected plants of 'Miriam'. The daily balance of CO(2) exchange of the ears was negative, since carbon fixation by the spike in the light was more than counterbalanced by night time spike respiration. Radioisotope studies revealed that mature, infected 'Miriam' plants maintained as large a percentage of the carbohydrates fixed at the vegetative stage and early grain filling as healthy plants. On the other hand, under the same conditions, infected 'Barkai' plants lost a larger fraction of these carbohydrates. The rate of carbon fixation per unit of chlorophyll and per residual green leaf area of infected 'Miriam' was higher than in healthy plants. It is proposed that this enhancement of photosynthesis in residual green tissue of infected plants of the tolerant cultivar Miriam compensates for the loss of photosynthesizing tissue due to STB.

Growth and photosynthesis of the 'brown tide' microalga Aureococcus anophagefferens in subsaturating constant and fluctuating irradiance

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 153:67-75 (1997) - doi:10.3354/meps153067 Growth and photosynthesis of the 'brown tide' microalga Aureococcus anophagefferens in subsaturating constant and fluctuating irradiance Milligan AJ, Cosper EM During the summer of 1985, a small (2 to 3 µm) chrysophyte, Aureococcus anophagefferens, bloomed and dominated the phytoplankton assemblage, and recurred each summer for the following 12 yr in bays of Long Island, New York, USA. Macronutrients remained at high levels throughout the years but Secchi disc depths were as low as 30 cm in some areas, corresponding to a 1% light depth of about 80 cm. In batch culture, A. anophagefferens maintained high growth rates at low irradiances. The initial slope of the growth versus irradiance curve, alphag, was 0.021 ± 0.003 divisions d-1 (µmol quanta m-2 s-1)-1 which is similar to that for bloom formers such as Skeletonema costatum. In continuous culture, constant and fluctuating irradiance regimes resulted in equivalent steady state growth rates at 0.46 divisions d-1. Short-term carbon fixation per unit chlorophyll a was about twice as high in the fluctuating irradiance regime compared with constant irradiance. Differences between short-term carbon fixation and cell growth could not be accounted for by dark fixation or exudation and are assumed to be due to photorespiration. The photosynthesis versus irradiance parameter, Pmax (light saturated photosynthetic rate), was similar in both irradiance regimes; however, alpha p (initial slope) was consistently higher in the fluctuating regime, indicating that photosynthetic efficiency is higher in fluctuating irradiance. Aureococcus anophagefferens · Photoacclimation · Brown tide · Fluctuating irradiance Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 153. Publication date: July 10, 1997 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1997 Inter-Research.

Comparative effects of benthic microalgae and phytoplankton on dissolved silica fluxes

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 13:275-284 (1997) - doi:10.3354/ame013275 Comparative effects of benthic microalgae and phytoplankton on dissolved silica fluxes Sigmon DE, Cahoon LB Microalgal effects on dissolved silica fluxes in an estuarine sandflat were investigated over an annual cycle. Benthic microalgal and phytoplankton chlorophyll a were measured at 17 times in Masonboro Sound, NC, USA, between June 1994 and May 1995. Using light and dark benthic chambers, dissolved silica (DSi) concentrations were measured in situ and used to calculate flux rates. Mean annual biomass was 64.6 and 10.0 mg chl a m-2 for benthic microalgae and phytoplankton, respectively. Benthic microalgae and phytoplankton (both containing diatoms), and cultured benthic diatoms were used to calculate Si:chl a weight ratios. Ratios of Si:chl a showed that benthic microalgae contain more silica per unit chlorophyll than phytoplankton, with mean Si:chl a ratios of 14.3, 10.9, and 2.8:1, for benthic microalgae, cultured benthic diatoms, and phytoplankton, respectively. DSi flux rates were always higher in the dark, while some negative flux rates (into the sediment) were seen in the light. DSi flux rates averaged 81.0 μmol m-2 h-1 in the dark and 37.0 μmol m-2 h-1 in the light. Lab and field experiments showed that benthic microalgae can take up water column DSi, while typical phytoplankton concentrations exert little control of water column DSi. Temperature was found to be the major regulator of DSi fluxes, with flux rates decreasing with decreasing temperature. Benthic microalgae were significant regulators of light chamber DSi flux at temperatures <20°C. Due to benthic microalgal uptake and low water column DSi concentrations observed under lower temperatures, nutrient limitation experiments were performed to determine if phytoplankton were silicate-limited. Of the 3 major nutrients, nitrogen, phosphorus, and silicon, only silicon was limiting for phytoplankton growth in every experiment. Benthic microalgae are important regulators of DSi flux, and can regulate water column DSi concentrations for at least half of the year, possibly limiting phytoplankton growth and species composition in shallow, coastal environments. Diatoms · Benthic microalgae · Phytoplankton · Dissolved silica · Sediments · Flux rates Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 13, No. 3. Publication date: September 11, 1997 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 1997 Inter-Research.

Remote sensing of sea surface Sun-induced chlorophyll fluorescence: consequences of natural variations in the optical characteristics of phytoplankton and the quantum yield of chlorophyll a fluorescence

Abstract The rate of Sun-induced chlorophyll a fluorescence (SICF) observed at sea surface is determined by chlorophyll a concentration, incident irradiance, and optical and fluorescence properties of the phytoplanktonic population. In this study, the impact of natural variations in the two latter on the use of remotely sensed SICF to determine ocean surface chlorophyll a concentration, is assessed using a simple parameterization of phytoplankton optical properties and a model describing the variations in the quantum yield of chlorophyll a fluorescence as a function of environmental factors, such as excess irradiance and nutrient limitations. It is shown that (1) variations in the optical properties of phytoplankton are the main cause of non-linearity in the relationship between SICF and chlorophyll a concentration, (2) the extent of spatial variations in the rate of fluorescence per unit chlorophyll a concentration and irradiance, at the level of a typical sensor scene, prevents the use of a linear relationship between SICF and chlorophyll a concentration even at local scales and (3) the optical properties of the ocean surface layer play an important role in modifying the SICF signal. Nevertheless, the parameterizations presented in this paper may represent a reasonably sound approach for a meaningful use of SICF in view of detecting the chlorophyll a concentration within the upper layer of the ocean. Interestingly, it is also shown that the detection threshold of SICF could be significantly lower than the one currently expected.

Oceanic primary production: 1. Adaptation of a spectral light‐photosynthesis model in view of application to satellite chlorophyll observations

A global equation, designed to estimate the column‐integrated oceanic primary production realized by a given phytoplankton biomass under various environmental conditions, is used to develop a practical method to assess the primary production (P) from the chlorophyll concentration as provided by satellite imagery. This basic equation combines three terms, namely the photosynthetically available radiation impinging at the sea surface, PAR(0+), the column‐integrated chlorophyll content, &lt;Chl&gt;tot, and the cross section for photosynthesis per unit of chlorophyll, Ψ*. Global monitoring of incident irradiance and near‐surface algal biomass is now achievable from space, and thus the next step toward a monitoring of oceanic primary production would be to dispose in parallel of a “climatological field” of the Ψ* quantity. Actually, Ψ* depends on the two other terms of the equation (PAR(0+) and &lt;Chl&gt;tot,), and, in addition, on temperature (also detectable from satellite). Therefore such a “climatological field” is variable and complex and it can be conveniently replaced by lookup tables allowing easy interpolation. The entries are date, latitude, cloudiness, temperature, and remotely sensed chlorophyll concentration. This upper layer concentration is extended downward owing to previous results of a statistical analysis of the chlorophyll vertical distribution; accordingly, two parallel tables, corresponding to well‐mixed or stratified upper layers with uniform or non uniform chlorophyll vertical profiles, respectively, are constructed. These tables are produced by systematically using a previously published spectral light‐photosynthesis model. For such extensive computations, the model necessarily relies on, and is operated with, a standard set of ecological and physiological parameters. Therefore sensitivity analyses have been carried out in view of assessing the impact on Ψ*, and on the resulting production of deviations in these parameters or parameterizations, vis‐a‐vis the standard values or formulations which were adopted when building the tables. The effects of the biomass vertical structure, of possible light and temperature adaptation, and of the presence of degraded pigments are among the sensitivity studies which have been performed. The method as proposed can accomodate any improvement and complexity in parameterization to the extent that additional computation time is faced only when generating the lookup tables, not when using them in conjunction with satellite data.

Decoupling of calcification and photosynthesis in the coccolithophore Emiliania huxleyi under steady-state light-limited growth

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 142:87-97 (1996) - doi:10.3354/meps142087 Decoupling of calcification and photosynthesis in the coccolithophore Emiliania huxleyi under steady-state light-limited growth Balch WM, Fritz J, Fernandez E Carbon fixation of Emiliania huxleyi was studied in light-limited, steady-state, continuous cultures. Six growth rates were examined ranging from 0.24 to 1.0 d-1 although the lowest may have been carbon-limited. Cell-specific and chlorophyll-specific rates of photosynthesis and calcification increased as a function of growth rate. The ratio of calcification to photosynthesis (C/P) increased from about 0.2 to 0.7 as the growth rate increased (from 0.24 to 0.75 d-1), then the C/P ratio decreased slightly as the growth rate approached washout at 1 d-1. Extrapolation of the regression data at low growth rates suggested that there should be zero calcification at a growth rate of about 0.1 d-1. Cells were also given a 30 s acidification/neutralization treatment to dissolve their coccoliths, and then carbon fixation was measured. Photosynthesis and calcification at all growth rates increased by about 0.1 pg C cell-1 h-1 following this treatment. Carbon fixation per unit chlorophyll a was predicted by multiplying the total carbon:chlorophyll a ratio by the respective culture dilution rate. These predictions were almost identical to 14C measurements of carbon fixation per unit chlorophyll. Nevertheless, if only total carbon incorporation data are available for a coccolithophore population, accurate predictions of just photosynthesis or calcification will require the function (presented in this paper) which relates the C/P ratio to growth rate. This function suggests a decoupling of photosynthesis from calcification as growth becomes progressively more light-limited. Coccolithophores · Calcification · Photosynthesis · Coccoliths · Growth rate Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 142. Publication date: October 24, 1996 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1996 Inter-Research.

Photosynthetic and bacterial production gradients in a larval fish nursery:the St. Lawrence River transition zone

Phytoplankton biomass, primary production and bacterial abundance and production were measured across the freshwater-saltwater transition zone of the St. Lawrence River [Canada) during the seasonal period of maximum concentrations of fish larvae and macrozooplankton. The estuarine front was characterized by steep gradients in b~olog~cal s well as physical properties. Maximum turbidity and high phytoplankton biomass occurred in the well-mixed, low salinity (0.2 to 4 psu) region immediately upstream of the salt wedge. Peak zooplankton and ichthyoplankton biomass occurred within and slightly downstream of this frontal region The hypothesis that lower food chain processes were controlled exclusively by allochthonous carbon and bacterial heterotrophy was not supported. Photosynthetic rates per unit chlorophyll a [chl a) remained high across the freshwater-saltwater transition and the low light penetration was offset by a shallow mean depth of mixing. Bacterial concentrations and activity remained relatively constant across the transition, while chl a declined sharply downstream of the front, consistent with grazing losses. Photosynthesis contributed 34 to 66% of the total production (bacteria + phytoplankton). Freshwater phytoplankton advected from upstream contributed another 20 to 30%. These first-order estimates underscore the combined importance of photosynthesis plus bacterial processes within the downstream food web of large river ecosystems.

Photosynthesis by developing embryos of oilseed rape (Brassica napusL.)

The aim of this study was to assess the photosynthetic potential of developing seeds of oilseed rape (Brassica napus L.) and to compare photosynthetic properties of embryo plastids with those of leaf chloroplasts from the same species. Measurements of CO 2 -dependent O 2 evolution show that developing seeds of B. napus are photosynthetically active in vitro. Essentially, all of the photosynthetic activity of the developing seed is accounted for by the embryo. The rate of photosynthesis by developing embryos increased until the onset of desiccation, after which it declined, so that by maturity embryos were no longer photosynthetically active. Photosynthetic activity was positively correlated with chlorophyll content throughout development. Comparison of the photosynthetic characteristics of leaf and embryo chloroplasts revealed that rates of uncoupled electron transport were 2.5-fold greater in those from the embryo. Light-saturated rates of CO 2 -dependent O 2 evolution, per unit chlorophyll, and CO 2 saturation points were similar for chloroplasts from both tissues. However, light-saturation points and chlorophyll a/b ratios were lower for embryo than for leaf choroplasts. Embryos and embryo chloroplasts also contained considerably less ribulose 1,5-bis-phosphate carboxylase/oxygenase protein per unit total protein, than leaves. Although excised embryos were capable of high rates of CO 2 -dependent O 2 evolution (90-100 μmol mg -1 chlorophyll h -1 ) under a saturating photosynthetic photon flux density (PPFD), low transmittance of light through the silique wall (30%), together with the high PPFD required to achieve light compensation points in developing seeds (500 μmol m -2 s -1 ), suggests that photosynthesis in vivo is unlikely to make a net contribution to carbon economy under normal environmental conditions.

The effect of light intensities on the transcript level of proteins involved in photosynthesis in mustard plants

Summary The influence of light quantity on the steady-state levels of plastid encoded transcripts was examined during the development of primary leaves from mustard ( Sinapis alba ) plants. RbcL mRNA, petA mRNA, and psbA mRNA, which encode the large subunit (LSU) of Rubisco, Cyt f of the Cyt b6/f complex, and D1 protein of PS II were investigated in leaves grown under high-light (HL) or low-light (LL) conditions. Additionally, the nuclear encoded 25 S rRNA was quantified. As a proportion of total RNA, the levels of 25 S rRNA, rbcL mRNA, petA mRNA, and psbA mRNA did not differ substantially in the HL versus LL plants. During leaf ontogenesis, though, the proportion of psbA mRNA in total RNA increased drastically, in contrast to the other transcripts. The results calculated per unit fresh weight, dry weight, leaf area, and chlorophyll closely resembled each other and gave an idea of the absolute prevalence of the transcripts in the cell. The light regime had a remarkable influence on the absolute levels of 25 S rRNA, rbcL mRNA, and petA mRNA: Paralleling total RNA content, the amounts of the three transcripts per unit fresh weight at the beginning of leaf development were, in HL plants, more than twice the amounts found in LL plants. Furthermore, during leaf ontogenesis the decrease in total RNA, 25 S rRNA, rbcL mRNA, and petA mRNA was more pronounced in HL than in LL, while the psbA mRNA content per unit fresh weight was only slightly higher in HL than in LL leaves and did not decline significantly during HL and LL leaf ontogenesis. Comparison of the absolute levels of rbcL mRNA, petA mRNA, and psbA mRNA with the amounts of the corresponding proteins revealed that the prevalence of the transcripts contributes principally to the differences in the protein content in HL and LL leaves.

Source-sink relationships in wheat leaves infected with powdery mildew. II. Changes in the regulation of the Calvin cycle

Alterations in the regulation of the Calvin cycle were studied in wheat leaves infected with powdery mildew (Erysiphe graminis f. sp. tritici ) during disease development. During infection the triazole fungicide hexaconazole (trade name ANVIL, Zeneca Agrochemicals) was applied to leaves to determine if removal of the fungal sink reversed any pathogen-induced alterations in host physiology. In mildewed and fungicide treated leaves the rate of photosynthesis per unit leaf area declined but was largely unchanged per unit chlorophyll. In addition, the proportion of excitation energy used photochemically (qP) decreased, the proportion dissipated non-photochemically (qN) increased and a loss of chlorophyll was observed in these leaves. By five days after inoculation triose-phosphates and fructose-1,6-bisphosphate had significantly increased while 3-phosphoglyceric acid, ribulose-1,5-bisphosphate and the maximum extractable activity of ribulose-1,5-bisphosphate carboxylase/oxygenase, NADP-glyceraldehyde-3-phosphate dehydrogenase and the stromal fructose-1,6-bisphosphatase had decreased in mildewed leaves. In fungicide treated leaves the amount of specific metabolites and the activity of enzymes was generally intermediate between those in a mildewed, and a healthy leaf. These data indicate a down-regulation of the Calvin cycle as a result of a decreased amount of Calvin cycle enzymes. It is suggested this caused an inhibition of the photosynthetic electron transfer chain and ultimately, the rate of photosynthesis in both mildewed and fungicide treated leaves.

Specific phytoplankton biomasses and their relation to primary production in the tropical North Atlantic

Phytoplankton pigment concentrations and primary production rates were measured in the North Tropical Atlantic Ocean (20°N, 31°W) in September–October 1991 and in May–June 1992 to provide new insights into the phytoplankton biomass and dynamics of oligotrophic environments. The overall biomass standing stocks were remarkably constant during both periods (around 23 mg chlorophyll a m −2), despite marked differences in the water column stratification. The structure of the autotrophic community was also stable: prochlorophytes, cyanobacteria and flagellates were the dominant autotrophic groups and contributed to 36, 30 and 34% of the chlorophyll a biomass in May–June and 43, 30 and 27% in September–October. The vertical distribution of these taxa was also stable with cyanobacteria dominating at the surface (100−10%) of surface irradiance), prochlorophytes at intermediate depths (10−0.1% of surface irradiance) and flagellates below the euphotic zone (0.1−0.01% of surface irradiance). Despite this qualitative and quantitative stability of the phytoplankton biomass, primary production rates were significantly higher ( p < 0.05) in May–June (352 ± 68 mg C m −2d −1) than in September–October (267 ± 53 mg C m −2d −1). The cross-section for photosynthesis per unit chlorophyll a was constant during both periods (0.063 m 2 g Chl a −1) suggesting that differences in production rates were mainly governed by variations in irradiance. The photic zone accounted for more than 80% of the integrated production, but less than 50% of the chlorophyll a biomass. Analysis of the photoadaptation characteristics of the dominant populations suggests that cyanobacteria and prochlorophyte distributions are mainly regulated by light, whereas flagellate distribution is mainly linked to nutrient availability. The respective distributions of fucoxanthin, 19′-hexanoyloxyfucoxanthin and 19′-butanoyloxyfucoxanthin suggest that, in such oligotrophic environments, a particular group of 19′-butanoyloxyfucoxanthin-containing flagellates, living close to the nitracline, is responsible for the new production associated with the regular diffusion of nitrate, but that diatoms, generally present at background levels, can be responsible for spatio-temporal events of new production.