Saturating Light Conditions Research Articles

Growth and Biochemical Composition of the Diatom Phaeodactylum tricornutum at Different pH and Inorganic Carbon Levels under Saturating and Subsaturating Light Regimes

The effects of pH and total dissolved inorganic carbon (DIC) availability on the growth and biochemical composition of Phaeodactylum tricornutum Bohlin acclimated to restricted and saturated light conditions have been investigated. Diluted, unbuffered and nutrient enriched batch cultures were grown at several pHs (different [Co 2 ( a q ) ] for the same DIC) and various DIC conditions (different [Co 2 ( a q ) ] for an equal pH) under saturating (150 μmol m - 2 s - 1 ) and subsaturating (30 pmol m - 2 s - 1 ) light levels. The growth rate was unaffected by pH treatments at saturating light. In contrast, under subsaturating light conditions, the growth rate was significantly reduced at pHs over 8.5 and also at low DIC levels to a similar extent. Biochemical composition revealed that the growth rate was limited by different resources. At natural DIC (DIC = 2.1 mM) and low light conditions, there was an increase of the C:N ratio under high pH (low [Co 2 ( a q ) ]) associated with a reduction of nitrogen, chlorophyll a and total protein content, resembling the biochemical composition of a diatom cultured under nitrogen limitation. On the contrary, under low light and low DIC conditions, in which both [CO 2 ( a q ) ] and [HCO 3 -] are reduced, cellular carbon content decreased for a relatively constant nitrogen content, decreasing the C:N ratio. The results are discussed in terms of competition between carbon and nitrogen metabolism for energy, and showed that the imposition of different [CO 2 ( a q ) ] by changing pH or DIC levels of seawater affected in different ways the final biochemical composition of Phaeodactylum tricornutum.

Limitations to leaf photosynthesis in field-grown grapevine under drought - metabolic and modelling approaches.

The effects of a slowly-imposed drought stress on gas-exchange, chlorophyll a fluorescence, biochemical and physiological parameters of Vitis vinifera L. leaves (cv. Aragonez, syn. Tempranillo) growing in a commercial vineyard (South Portugal) were evaluated. Relative to well-watered plants (predawn water potential, ΨPD = -0.13 ± 0.01 MPa), drought-stressed plants (ΨPD = -0.97 ± 0.01 MPa) had lower photosynthetic rates (ca 70%), stomatal conductance, and PSII activity (associated with a higher reduction of the quinone A pool and lower efficiency of PSII open centres). Stomatal limitation to photosynthesis was increased in drought-stressed plants relative to well-watered plants by ca 44%. Modelled responses of net photosynthesis to internal CO2 indicated that drought-stressed plants had significant reductions in maximum Rubisco carboxylation activity (ca 32%), ribulose-1,5-bisphosphate regeneration (ca 27%), and triose phosphate (triose-P) utilization rates (ca 37%) relative to well-watered plants. There was good agreement between the effects of drought on modelled biochemical parameters, and in vitro activities of key enzymes of carbon metabolism, namely Rubisco, glyceraldehyde-3-phosphate dehydrogenase, ribulose-5-phosphate kinase and fructose-1,6-bisphosphate phosphatase. Quantum yields measured under both ambient (35 Pa) and saturating CO2 (100 Pa) for drought-stressed plants were decreased relative to well-watered plants, as well as maximum photosynthetic rates measured at light and CO2 saturating conditions (three times ambient CO2 levels). Although stomatal closure was a strong limitation to CO2 assimilation under drought, comparable reductions in electron transport, CO2 carboxylation, and utilization of triose-P capacities were also adaptations of the photosynthetic machinery to dehydration that slowly developed under field conditions. Results presented in this study confirm that modelling photosynthetic responses based on gas-exchange data can be successfully used to predict metabolic limitations to photosynthesis.

Cold acclimation of Arabidopsis thaliana results in incomplete recovery of photosynthetic capacity, associated with an increased reduction of the chloroplast stroma.

The effects of short-term cold stress and long-term cold acclimation on the light reactions of photosynthesis were examined in vivo to assess their contributions to photosynthetic acclimation to low temperature in Arabidopsis thaliana (L.) Heynh.. All photosynthetic measurements were made at the temperature of exposure: 23 degrees C for non-acclimated plants and 5 degrees C for cold-stressed and cold-acclimated plants. Three-day cold-stress treatments at 5 degrees C inhibited light-saturated rates of CO2 assimilation and O2 evolution by approximately 75%. The 3-day exposure to 5 degrees C also increased the proportion of reduced QA by 50%, decreased the yield of PSII electron transport by 65% and decreased PSI activity by 31%. In contrast, long-term cold acclimation resulted in a strong but incomplete recovery of light-saturated photosynthesis at 5 degrees C. The rates of light-saturated CO2 and O2 gas exchange and the in vivo yield of PSII activity under light-saturating conditions were only 35-40% lower, and the relative redox state of QA only 20% lower, at 5 degrees C after cold acclimation than in controls at 23 degrees C. PSI activity showed full recovery during long-term cold acclimation. Neither short-term cold stress nor long-term cold acclimation of Arabidopsis was associated with a limitation in ATP, and both treatments resulted in an increase in the ATP/NADPH ratio. This increase in ATP/NADPH was associated with an inhibition of PSI cyclic electron transport but there was no apparent change in the Mehler reaction activity in either cold-stressed or cold-acclimated leaves. Cold acclimation also resulted in an increase in the reduction state of the stroma, as indicated by an increased total activity and activation state of NADP-dependent malate dehydrogenase, and increased light-dependent activities of the major regulatory enzymes of the oxidative pentose-phosphate pathway. We suggest that the photosynthetic capacity during cold stress as well as cold acclimation is altered by limitations at the level of consumption of reducing power in carbon metabolism.

Light responses in the green sulfur bacterium Prosthecochloris aestuarii: changes in prosthecae length, ultrastructure, and antenna pigment composition.

The morphology (mainly prosthecae length), ultrastructure, and antenna pigment composition of the green sulfur bacterium Prosthecochloris aestuarii changed when grown under different light intensities. At light intensities of 0.5 and 5 micromol quanta m(-2) s(-1), the cells had a star-like morphology. Prosthecae, the characteristic appendages of the genus Prosthecochloris, were 232 nm and 194 nm long, respectively. In contrast, when grown at 100 micromol quanta m(-2) s(-1), these appendages were shorter (98 nm) and the cells appeared more rod-shaped. Transmission electron microscopy revealed a significant decrease in the cell perimeter to area ratio and in the number of chlorosomes per linear microm of membrane as light intensity increased. In addition to these morphological and ultrastructural responses, Prosthecochloris aestuarii exhibited changes in its pigment composition as a function of light regime. Lower specific pigment content and synthesis rates were found in cultures grown at light intensities above 5 micromol quanta m(-2) s(-1). A blue shift in the bacteriochlorophyll (BChl) c Q(y) absorption maximum of up to 17.5 nm was observed under saturating light conditions (100 micromol quanta m(-2) s(-1)). This displacement was accompanied by changes in the composition of BChl c homologs and by a very low carotenoid content. The morphological, ultrastructural and functional changes exhibited by Prosthecochloris aestuarii revealed the strong light-response capacity of this bacterium to both high and low photon-flux densities.

Consequences of impaired microcystin production for light-dependent growth and pigmentation of Microcystis aeruginosa PCC 7806

Microcystis aeruginosa PCC 7806 produces potent inhibitors of eukaryotic protein phosphatases called microcystins, whose function to the organism is presently unknown. Mutants with impaired microcystin biosynthesis should provide a useful tool for investigations of microcystin function. This study has focussed on the comparison of growth and pigment content of strain PCC 7806 and its mcyB − mutant deficient in microcystin biosynthesis, under semicontinuous culture conditions. Both wild-type and mutant are characterised by a very low light demand and low-maximum specific growth rates in comparison to other Microcystis strains studied. While growth of wild-type and mutant were similar under different light conditions, the mutant cells showed significantly higher specific absorbances in the range of photosynthetic active radiance 420–700 nm, under light-limiting conditions. The mutant cells possess lower contents of chlorophyll a, β-carotene, zeaxanthin and echinenone under light limitation and of myxoxanthophyll under saturated light conditions. Though microcystins are clearly not essential for growth, the observed effects of the mutation are a first indication of their involvement in intracellular processes.

Consequences of impaired microcystin production for light-dependent growth and pigmentation of Microcystis aeruginosa PCC 7806

Microcystis aeruginosa PCC 7806 produces potent inhibitors of eukaryotic protein phosphatases called microcystins, whose function to the organism is presently unknown. Mutants with impaired microcystin biosynthesis should provide a useful tool for investigations of microcystin function. This study has focussed on the comparison of growth and pigment content of strain PCC 7806 and its mcyB− mutant deficient in microcystin biosynthesis, under semicontinuous culture conditions. Both wild-type and mutant are characterised by a very low light demand and low-maximum specific growth rates in comparison to other Microcystis strains studied. While growth of wild-type and mutant were similar under different light conditions, the mutant cells showed significantly higher specific absorbances in the range of photosynthetic active radiance 420–700 nm, under light-limiting conditions. The mutant cells possess lower contents of chlorophyll a, β-carotene, zeaxanthin and echinenone under light limitation and of myxoxanthophyll under saturated light conditions. Though microcystins are clearly not essential for growth, the observed effects of the mutation are a first indication of their involvement in intracellular processes.

Bio-optical modeling of primary production on regional scales: the Bermuda BioOptics project

Regional to global scale estimates of primary production must rely on remotely sensed quantities. Here, we characterize in situ light}primary production relationships and assess the predictive capability of several global primary production models using a 6-yr time series collected as part of the US JGOFS Bermuda Atlantic Time Series (BATS). The consistency and longevity of this data set provide an excellent opportunity to evaluate bio-optical modeling methodologies and their predictive capabilities for estimating rates of water-column-integrated primary production, PP, for use with satellite ocean-color observations. We nd that existing and regionally tuned parameterizations for vertically integrated chlorophyll content and euphotic zone depth do not explain much of the observed variability at this site. Fortunately, the use of these parameterizations for light availability and harvesting capacity has little in#uence upon modeled rates of PP. Site-specic and previously published global models of primary production both perform poorly and account for less than 40% of the variance in PP. A sensitivity analysis is performed to demonstrate the importance of light-saturated rates of primary production, PH , compared with other photophysiological parameters. This is because nearly one-half of PP occurs under light-saturated conditions. Unfortunately, we were unable to derive a simple parameterization for PH that signicantly improves prediction of PP. The failure of global PP models to encapsulate a major portion of the observed variance is due in part to the restricted range of PP observations for this site. A similar result is found comparing global chlorophyllre#ectance algorithms to the present observations. More importantly, we demonstrate that there exists a time-scale (roughly 200 d) above which the modeled distributions of PP are consistent with the observational data. By low-passltering the observed and modeled PP time series, the model's predictive skill levels

Short-term changes in free radical scavengers and chloroplast pigments in Pinus canariensis needles as affected by mild drought stress

Summary Potted Pinus canariensis seedlings were subjected to mild drought by withholding irrigation for one week. This treatment induced a reduction in maximum stomatal conductance (50 mmol m −2 s −1 ) compared to irrigated controls (130 mmol m −2 s −1 ). Needle water potentials of non-irrigated trees were maintained at control level (-0.44 MPa). Such a mild drought is a potential oxidative stressor due to the production of active oxygen species (AOS) in illuminated chloroplasts which lack CO 2 due to stomatal closure. Photoprotective pigments (e.g. the xanthophyll cycle) may avoid this situation through light energy dissipation, and antioxidants such as ascorbic acid, tocopherols, and glutathione, may detoxify AOS. Concentrations of ascorbate, glutathione, chlorophyll, and the xanthophyll cycle carotenoids were minimal in the evening (under low light) compared to light-saturated conditions. α-Carotene was highest in the evening. These short-term changes were not affected by drought. The xanthophyll cycle pool tended to be more de-epoxidized in strongly illuminated needles of non-irrigated trees at the beginning of the experiment, but this effect was transient. The glutathione pool was more oxidized in needles of non-irrigated trees (up to 20 percnt; of total vs. 10 percnt; at control) after the xanthophyll changes took place, whereas the redox state of ascorbate remained stable.

Microsensor studies of photosynthesis and respiration in larger symbiotic foraminifera. I The physico-chemical microenvironment of Marginopora vertebralis, Amphistegina lobifera and Amphisorus hemprichii

The physico-chemical microenvironment of larger benthic foraminifera was studied with microsensors for O2, CO2, pH, Ca2+ and scalar irradiance. Under saturating light conditions, the photosynthetic activity of the endosymbiotic algae increased the O2 up to 183% air saturation and a pH of up to 8.6 was measured at the foraminiferal shell surface. The photosynthetic CO2 fixation decreased the CO2 at the shell down to 4.7 μM. In the dark, the respiration of host and symbionts decreased the O2 level to 91% air saturation and the CO2 concentration reached up to 12 μM. pH was lowered relative to the ambient seawater pH of 8.2. The endosymbionts responded immediately to changing light conditions, resulting in dynamic changes of O2, CO2 and pH at the foraminiferal shell surface during experimentally imposed light–dark cycles. The dynamic concentration changes demonstrated for the first time a fast exchange of metabolic gases through the perforate, hyaline shell of Amphistegina lobifera. A diffusive boundary layer (DBL) limited the solute exchange between the foraminifera and the surrounding water. The DBL reached a thickness of 400–700 μm in stagnant water and was reduced to 100–300 μm under flow conditions. Gross photosynthesis rates were significantly higher under flow conditions (4.7 nmol O2 cm−3 s−1) than in stagnant water (1.6 nmol O2 cm −3 s−1), whereas net photosynthesis rates were unaffected by flow conditions. The Ca2+ microprofiles demonstrated a spatial variation in sites of calcium uptake over the foraminiferal shells. Ca2+ gradients at the shell surface showed total Ca2+ uptake rates of 0.6 to 4.2 nmol cm−2 h−1 in A. lobifera and 1.7 to 3.6 nmol cm−2 h−1 in Marginopora vertebralis. The scattering and reflection of the foraminiferal calcite shell increased the scalar irradiance at the surface up to 205% of the incident irradiance. Transmittance measurements across the calcite shell suggest that the symbionts are shielded from higher light levels, receiving approximately 30% of the incident light for photosynthesis.

Sulfide effects on Thalassia testudinum carbon balance and adenylate energy charge

Low iron content in tropical carbonate sediments limits the formation of iron–sulfide compounds such as pyrite. Thus, seagrasses in the tropics may be more susceptible to sulfide toxicity. Sediment sulfide levels greater than 2 mM and up to 13 mM have been hypothesized to cause widespread ‘die-back’ of the tropical seagrass Thalassia testudinum in a subtropical lagoon, Florida Bay. Hydroponic chambers were used to determine the effects of sulfide (0.0, 2.0, 4.0, 6.0 and 10.0 mM) on root and leaf adenylate ratios, energy charge (EC), leaf O 2 flux, and growth of T. testudinum under light-saturated conditions. T. testudinum did not suffer mortality under short-term (48 h) exposure to sulfide concentrations up to 10 mM, but several metabolic stress responses were observed. Root ATP and energy charge significantly declined ( P<0.05) as a function of increasing sulfide concentrations. Root EC was reduced from 0.78 in the control to 0.63–0.59 in the 2.0–6.0 mM treatments, and fell to 0.43 in the 10.0 mM treatment. Leaf elongation rates declined ( P<0.05) by an average of 43% in 2.0–6.0 mM sulfide and 67% in 10 mM sulfide. Although root EC, root ATP production and leaf elongation rates significantly declined under root sulfide exposure, sulfide concentrations of 2.0–10.0 mM failed to produce visual signs of acute sulfide toxicity, such as leaf chlorosis, leaf or root necrotic tissue development, or loss of leaf or root turgor. Photosynthesis and leaf EC remained high after sulfide treatments, suggesting a resilience of T. testudinum leaf metabolism to short-term sulfide exposure. Our data do not support the hypothesis that sulfide initiates rapid ‘die-off’ episodes of T. testudinum in Florida Bay, although this phytotoxin may play a critical role as a root carbon drain over long-term exposures.

Ontogenetic patterns of leaf CO2 exchange, morphology and chemistry in Betula pendula trees

In order to explore ontogenetic variation in leaf-level physiological traits of Betula pendula trees, we measured changes in mass- (Amass) and area-based (Aarea) net photosynthesis under light-saturated conditions, mass- (RSmass) and area-based (RSarea) leaf respiration, relative growth rate, leaf mass per area (LMA), total nonstructural carbohydrates (TNC), and macro- and micronutrient concentrations. Expanding leaves maintained high rates of Aarea, but due to high growth respiration rates, net CO2 fixation occurred only at irradiances >200 µmol photons m–2 s–1. We found that full structural leaf development is not a necessary prerequisite for maintaining positive CO2 balance in young birch leaves. Maximum rates of Aarea were realized in late June and early July, whereas the highest values of Amass occurred in May and steadily declined thereafter. The maintenance respiration rate averaged ≈8 nmol CO2 g–1 s–1, whereas growth respiration varied between 0 and 65 nmol CO2 g–1 s–1. After reaching its lowest point in mid-June, leaf respiration increased gradually until the end of the growing season. Mass and area-based dark respiration were significantly positively correlated with LMA at stages of leaf maturity, and senescence. Concentrations of P and K decreased during leaf development and stabilized or increased during maturity, and concentrations of immobile elements such as Ca, Mn and B increased throughout the growing season. Identification of interrelations between leaf development, CO2 exchange, TNC and leaf nutrients allowed us to define factors related to ontogenetic variation in leaf-level physiological traits and can be helpful in establishing periods appropriate for sampling birch leaves for diagnostic purposes such as assessment of plant and site productivity or effects of biotic or abiotic factors.

Light inhibition of bovine retinal rod guanylyl cyclase mediated by beta gamma-transducin.

Photoreceptor guanylyl cyclase (ROS-GC), converting GTP into cGMP and pyrophosphate, is a key enzyme in the regulation of the visual transduction cascade. ROS-GC requires GC-activating proteins (GCAPs) and low free [Ca] for full activity. We found that when choline or potassium were the major cations present, light caused a 70% inhibition of stimulated ROS-GC in native unstripped membranes. In the presence of sodium ions, however, no inhibition was observed. ROS-GC activity of ROS membranes, stripped of transducin and other components, was not affected by light when reconstituted with GCAP1 only. However, when stripped ROS membranes were reconstituted with both GCAP1 and either transducin (T alpha beta gamma) or the T beta gamma-subunits, the inhibition of ROS-GC by light was restored. The T alpha-subunit alone was ineffective. These results suggest that under saturating light conditions, ROS-GC may be regulated by T beta gamma and cations, providing a possible mechanism of desensitization and light adaptation.

Significance of pelagic and benthic primary production in two shallow coastal lagoons of different degrees of eutrophication in the southern Baltic Sea

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 20:273-284 (1999) - doi:10.3354/ame020273 Significance of pelagic and benthic primary production in two shallow coastal lagoons of different degrees of eutrophication in the southern Baltic Sea Jürgen Meyercordt*, Sabine Gerbersdorf, Lutz-Arend Meyer-Reil Institut für Ökologie, Ernst-Moritz-Arndt-Universität Greifswald, 18565 Kloster/Hiddensee, Germany *E-mail: meyercor@rz.uni-greifswald.de ABSTRACT: Primary production by pelagic and benthic microalgae was measured in 2 shallow coastal lagoons in the southern Baltic Sea by in situ and laboratory incubations to quantify the significance of these 2 major groups of primary producers in absolute and relative terms. Pelagic primary production rates per volume at saturating light conditions were 7 to 12 times higher at the polytrophic location Kirr-Bucht (Darss-Zingst Bodden) than at the meso-/eutrophic location Rassower Strom (Nordrügensche Bodden). Chlorophyll a specific primary production rates, however, were in the same range at both locations, except for 1 situation in spring which is explicitly discussed. The pelagic chlorophyll a specific maximum light utilization coefficient (a*) tended towards higher values in Kirr-Bucht samples, which can be interpreted as an adaptation to a lower supply of irradiance. Pelagic and benthic primary production varied seasonally, with highest rates occurring in June/July. A calculation of short-term variability due to cloud-modulated availability of irradiance revealed reductions of pelagic and benthic gross production (GP) rates on overcast days, compared to bright days, in the range of 26 to 69% and 62 to 82%, respectively. Net community production (NP) rates were more severely reduced than GP rates on overcast days. Benthic NP was generally reduced to below zero on overcast days, whereas it was mostly positive on bright days at both locations. To evaluate the significance of the different optical properties of both types of water (diffuse vertical attenuation coefficient, k = 3.3 to 3.7 and 0.5 to 0.7 m-1 at Kirr-Bucht and Rassower Strom, respectively) for primary production, the benthic and pelagic contributions at both sites were calculated regarding a water depth of 2 m, which is about the mean depth of the Bodden systems. This standardization revealed obvious differences in basic conditions for primary production which remained undetected when the comparison of the 2 sites was based on the water depths in which the in situ incubations had been carried out (0.6 and 3.4 m at Kirr-Bucht and Rassower Strom, respectively). In meso-/eutrophic Rassower Strom at a depth of 2 m, benthic GP would contribute about 30 to 45% to the total GP of microalgae, depending on weather conditions and seasons. At the same depth in polytrophic Kirr-Bucht, the microalgae on the sediment surface would account for only 0.3% or less of total GP. This demonstrates that optical properties of the water, which are closely related to the trophic state of the system, exert strong control on varying proportions of productivity, attributable to planktonic versus benthic microalgal communities. It can be concluded from the modeled data that the difference between both locations will be most prominent during periods of low supply of radiant energy, like on overcast days. KEY WORDS: Primary production · Phytoplankton · Microphytobenthos · Irradiance · Eutrophication · Estuary · Baltic Sea Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 20, No. 3. Publication date: December 30, 1999 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 1999 Inter-Research.

Sex ratios, size distributions, and sexual dimorphism in the dioecious tree Ilex aquifolium (Aquifoliaceae)

Sex ratio and sexual dimorphism in physiology and growth were studied in the dioecious tree Ilex aquifolium at two localities in northern Spain. Genet sex ratio was significantly male biased in one locality but not in the other. However, ramet and flowering ramet sex ratios were male biased at both study sites. Males had significantly thicker main trunks than females in one locality and produced more ramets in the other. Growth rate, estimated from mean width of annual rings, did not differ between localities, but males produced wider rings than females at both sites. Mean annual growth rates over the last 10, 20, and 30 yr were significantly higher for males. Measurements of chlorophyll fluorescence indicated that the efficiency of photosynthesis of leaves on nonfruiting branches of females was higher than for leaves on branches of male plants under low-light conditions, though not under saturating-light conditions. Efficiency of photosynthesis was significantly lower on fruiting branches of female plants than on nonfruiting branches. We discuss whether the observed between-sex differences are attributable to the higher cost of reproduction in females and/or to pollen competition.

Diffusional permeability of dissolved inorganic carbon through the isolated oral epithelial layers of the sea anemone, Anemonia viridis

In a recent report, Bénazet-Tambutté et al. (1996a) showed that the major source of dissolved inorganic carbon for photosynthesis of the sea anemone Anemonia viridis was external seawater and a minor source was the internal medium of the tentacles. In order to determine the mechanism of DIC transfer from the external seawater to the endodermal cells, we explored the different mechanisms potentially involved in this transfer: transepithelial passive diffusion of DIC across the oral epithelial layers, renewal of coelenteric fluid by the mouth or transepithelial active transport. In addition, we also studied the role of expansion/contraction of the tentacle in the transepithelial transport of DIC. We investigated diffusion of DIC, using isolated perfused tentacles, by measuring transepithelial H 14CO 3 flux. We found that the unidirectional transport of DIC was light-sensitive: the transepithelial flux was about three-fold lower in light than in dark conditions. The flux was insensitive to the anion transport inhibitor, DIDS, and to the carbonic anhydrase inhibitor, Diamox. It was directly proportional to the concentration of bicarbonate in the surrounding seawater. These results suggest that this flux is simply driven by diffusion via a paracellular pathway. Use of inhibitors and inside-out tentacles showed that the transepithelial flux measured in the dark was depressed in the light due to symbiont photosynthesis which diverts about 15% of the DIC flux. Consequently, the passive diffusional flux of DIC can account for less than 1/6 of the dissolved inorganic carbon used for symbiont photosynthesis under saturating light conditions. Expansion/contraction state of the tentacles did not significantly modify the permeability coefficient ( P d=2.28×10 −5 and 2.50×10 −5 cm s −1 respectively for expanded and contracted tentacles) in spite of a change of tissue thickness of a factor of about 3. This result suggests that the mesoglea could represent a significant barrier to transepithelial diffusion of molecules, due to change in its water content level upon contraction. Measurement of water intake by whole sea anemones, using 14C-Dextran, showed that the renewal of the coelenteric fluid which bathes the dinoflagellate-containing endodermal cells cannot provide DIC to symbiont photosynthesis. Therefore it is concluded that a carrier-mediated DIC transporter located on the ectodermal host cells, supplies the bulk of dissolved inorganic carbon used for symbiont photosynthesis.

Comparative growth and physiology of selected one-year-old red- and green-fruited European pear cultivars

Red-fruited pear trees are reported to be more difficult to grow, less vigorous and less productive than green-fruited pear trees. This study was undertaken to examine some hypotheses that have been proposed to explain the observed differences in red- vs. green-fruited pear trees. Several growth and gas exchange parameters plus ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity, relative total soluble protein, and total chlorophyll were measured on potted, 1-yr-old greenhouse-grown trees of the green-fruited pear cultivars, ‘Bartlett’, ‘d'Anjou’, and ‘Clapp's Favorite’, and their respective red-fruited sports, ‘Red Bartlett’, ‘Red d'Anjou’ and ‘Kalle’. Green-fruited ‘d'Anjou’ trees had double the mean total leaf area of ‘Red d'Anjou’ ( P < 0.05). Specific leaf area of ‘Clapp's Favorite’ was 77% that of ‘Kalle’. The asymptotic maximum net photosynthetic rates ( P max) for red-fruited sports were 69% of those of their green-fruited parents. Stomatal conductance to water vapor of the sports was 78% of that of the parents under light-saturated conditions. Dark respiration of green-fruited ‘d'Anjou’ leaves was 77% of that of ‘Red d'Anjou’ leaves. Intercellular CO 2 concentration was not significantly different between green-fruited parents and red-fruited sports. Although the relative total soluble leaf protein content and total chlorophyll content did not differ significantly between green-fruited parents and red-fruited sports, both characteristics tended to be higher in the parental series, which was consistent with higher net photosynthesis ( P n) in the green-fruited parents than in the red-fruited sports. The ratio of chlorophyll a to chlorophyll b in “Red Bartlett” and ⋚d d'Anjou” was 62% and 54% of that in “Bartlett” and “d'Anjou”, respectively. Rubisco activity in the red-fruited sports was 60% of that in the green-fruited parents. On the basis of these observations, we cannot reject the hypothesis that lower chlorophyll content, particularly chlorophyll a, and lower P n contribute to the reduced vigor in trees of red-fruited sports compared to their green-fruited parents.

Photoelectrochemistry of thylakoid membranes

A one-compartment three-electrode photoelectrochemical cell, first designed by Allen and Crane and recently modified by Carpentier, was used to study the chronoamperometric behavior of a suspension of thylakoids under saturating light conditions. The experimental results were modeled successfully by considering the processes occurring in the diffusion layer of the electrode. In a different experimental configuration, the thylakoid membranes were electrodeposited onto the electrode. The voltammetric and chronoamperometric curves indicate clearly the role of photosystem II as the photocatalyst of oxygen evolution at the electrode. The use of suitable mediators of electron transfer to the electrode allows evaluation of the sites of interaction of the membranes with the electrode itself.

Sink-stimulated photosynthesis, increased transpiration and increased demand-dependent stimulation of nitrate uptake: nitrogen and carbon relations in the parasitic associationCuscuta reflexa-Coleus blumei

When parasitizing Coleus blumei Benth., grown in quartz sand culture and fed with 0.2,1 or 5 mM nitrate, the biomass production of Cuscuta reflexa Roxb. was inhibited to a similar extent as that of the host supplied limiting concentrations of nitrate. In the presence of Cuscuta the growth and dry matter increase of the host plant was severely inhibited. However, dry matter production of host plus parasite was only slightly less than or at 0.2 mM nitrate almost the same as that of uninfected Coleus plants. Under all conditions of nitrate nutrition, parasitism by Cuscuta led to a substantial increase in photosynthesis in host leaves under light-saturating conditions and in transpiration . Particularly with 0.2 mM and mM NO 3~ parasitism by Cuscuta reflexa also led to an increase in the nitrogen concentration in almost all host tissues and to a substantial increase in the chlorophyll concentration of leaves. The total nitrogen content in the system, host plus parasite, was as large with 0.2 and 1 mM NO3 or only somewhat smaller with 5 mM NO3~ than that of the unparasitized host. Since the root weight of infected plants was considerably decreased, it followed that nitrate uptake by roots of parasitized plants was substantially increased compared to the uninfected Coleus. Increased photosynthesis and accelerated nitrate uptake are discussed in relation to an increased sink for assimilates and to higher demand for nitrogen in the parasitized plants and to opposing effects of Cuscuta on symbioticalty nitrogen-fed hosts.

Factors influencing the allelopathic activity of the planktonic cyanobacterium Trichormus doliolum

An axenic culture of Trichormus doliolum Bharadwaja, Komárek et Anagnostidis (Nostocales, Cyanophyta) showed growth inhibition of several axenic cyanobacteria and chlorophytes. Allelopathic activity was observed in both phosphorus-limited and -replete chemostat cultures. Phosphorus limitation led to a change in the composition of the released organic compounds and to increased allelopathic activity of the exudate. Allelopathic activity of exudates of the phosphorus-limited and -replete chemostat was confined to the same high-performance liquid chromatography fraction, indicating that the allelopathic compounds released may be similar if not identical. The exudates inhibited photosynthetic oxygen production and enhanced chlorophyll fluorescence in other cyanobacteria. Growth-inhibiting effects showed a linear dose dependency, indicating that even small concentrations of allelopathic compounds influenced the growth of other cyanobacteria and algae. A biomass of Trichormus doliolum 20 times less than that needed for growth inhibition led to an increase in chorophyll fluorescence of other algae. This suggests that, under light-limited growth conditions, target organisms are more susceptible to allelochemical compounds that interfere with the photosynthetic electron transport. Experiments conducted under light-saturated conditions are likely to underestimate the potential of allelopathic interactions.

Diurnal changes in leaf gas exchange characteristics in the uppermost canopy of a rain forest tree, Dryobalanops aromatica Gaertn. f.

Dryobalanops aromatica Gaertn. f. is a major tropical canopy species in lowland tropical rain forests in Peninsular Malaysia. Diurnal changes in net photosynthetic rate (A) and stomatal conductance to water vapor (g(s)) were measured in fully expanded young and old leaves in the uppermost canopy (35 m above ground). Maximum A was 12 and 10 micro mol m(-2) s(-1) in young and old leaves, respectively; however, because of large variation in A among leaves, mean maximum A in young and old leaves was only 6.6 and 5.5 micro mol m(-2) s(-1), respectively. Both g(s) and A declined in young leaves when T(leaf) exceeded 34 degrees C and leaf-to-air vapor pressure deficit (DeltaW) exceeded 0.025, whereas in old leaves, g(s) and A did not start to decline until T(leaf) and DeltaW exceeded 36 degrees C and 0.035, respectively. Under saturating light conditions, A was linearly related to g(s). The coefficient of variation (CV) for the difference between the CO(2) concentrations of ambient air and the leaf intercellular air space (C(a) - C(i)) was smaller than the CV for A or g(s), suggesting that maximum g(s) was mainly controlled by mesophyll assimilation (A/C(i)). Minimum C(i)/C(a) ratios were relatively high (0.72-0.73), indicating a small drought-induced stomatal limitation to A and non-conservative water use in the uppermost canopy leaves.