Saturating Light Intensity Research Articles

Carbon to chlorophyll-a ratio in modeling long-term eutrophication phenomena

Carbon to chlorophyll-a ratio (CCHL) is formulated based on the assumption that adaptive changes in carbon to chlorophyll occur so as to maximize the specific growth rate for ambient conditions, including solar radiation and water temperature. With the dynamic CCHL, an unsteady two-layered, two-dimensional eutrophication numerical model for density stratified coastal waters has been developed. Saturated light intensity (IS) is determined as weighted average of the light intensity for previous three days to incorporate light acclimation by phytoplankton. The bottom water anoxic condition during summer in Tolo Harbour, Hong Kong is successfully reproduced by the present method. Otherwise, the simulation with a constant CCHL gave a wrong result.

Physiological characteristics of tobacco cultivars with contrasting sensitivity to ozone

Two cultivars of tobacco with contrasting sensitivity to ozone have similar photosynthesis at low light intensity, but at high light, photosynthesis of the sensitive cultivar Bel-W3 was lower than that of the resistant cultivar Havana. In contrast, the electron transport rate at photosynthesis saturated light intensity was higher in Bel-W3 than in Havana. This excess may require efficient mechanisms to dissipate energy and oxygen radicals. We found that the non-radiative dissipation was lower in Bel-W3 than in Havana and that the conversion between violaxanthin and anther-axanthin + zeaxanthin was not complete in the ozone sensitive cultivar. Violaxanthin may have been partly converted in ABA. The high content of ABA may have caused stomatal closure and explain why photosynthesis at high light was reduced in Bel-W3 in comparison to Havana. β-Carotene content was also lower in Bel-W3 than in Havana. A low β-carotene content coupled with an inefficient use of the xanthophyll mechanism may contribute to determine the ozone sensitivity of Bel-W3 leaves

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.

The H +/ATP ratio of the ATP synthase from the cyanobacterium Synechococcus 6716 varies with growth temperature and light intensity

The proton translocation stoichiometry (H +/ATP ratio) and other bioenergetic features were investigated in membrane vesicles from the moderately thermophile Synechococcus 6716 grown at 38°C and 50°C with saturating light intensity, and at 38°C with limiting light intensity. At 50°C growth is slower but proceeds to a higher cell density than at 38°C. Increasing the growth temperature from 38°C to 50°C resulted in an altered membrane fatty acid composition, with increased length and saturation of the acyl chains. At 38°C and lower light intensity chain length was somewhat decreased and saturation increased to a small extent. Membrane vesicles from cells grown at 50°C performed cyclic photophosphorylation at lower light intensities and lower threshold Δ μ ̄ H + than vesicles from cells grown at 38°C. The 50°C vesicles also displayed a diminished light-induced proton uptake, but ATP synthesis activity and the attained Δ G p remained constant. Moreover, ATP synthesis became more resistant to uncoupling. From acid–base transition induced ATP synthesis experiments the H +/ATP ratios were determined to be 3.9, 3.1 and 3.3 for membrane vesicles from cells grown at 50°C, 38°C and light-limited 38°C, respectively. In vesicles from cells grown at 50°C, ATP hydrolysis is inhibited by a lower valinomycin-induced K +-diffusion potential than in vesicles from cells grown at 38°C. A molecular mechanism to explain changes in H +/ATP as well as the physiological implications are discussed.

Photosynthetic Gas Exchange and Discrimination against 13CO2 and C18O16O in Tobacco Plants Modified by an Antisense Construct to Have Low Chloroplastic Carbonic Anhydrase.

The physiological role of chloroplastic carbonic anhydrase (CA) was examined by antisense suppression of chloroplastic CA (on average 8% of wild type) in Nicotiana tabacum. Photosynthetic gas-exchange characteristics of low-CA and wild-type plants were measured concurrently with short-term, on-line stable isotope discrimination at varying vapor pressure deficit (VPD) and light intensity. Low-CA and wild-type plants were indistinguishable in the responses of assimilation, transpiration, stomatal conductance, and intercellular CO2 concentration to changing VPD or light intensity. At saturating light intensity, low-CA plants had lower discrimination against 13CO2 than wild-type plants by 1.2 to 1.8[per mille (thousand) sign]. Consequently, tissue of the low-CA plants was higher in 13C than the control plants. It was calculated that low-CA plants had chloroplast CO2 concentrations 13 to 22 [mu]mol mol-1 lower than wild-type plants. Discrimination against C18O16O in low-CA plants was 20% of that of the wild type, confirming a role of chloroplastic CA in the mechanism of discrimination against C18O16O ([delta]C18O16O). As VPD increased, stomatal closure caused a reduction in chloroplastic C02 concentration, and since VPD and chloroplastic CO2 concentration act in opposing directions on [delta]C18O16O, no effect of VPD was seen on [delta]C18O16O.

Inorganic carbon uptake for photosynthesis by the symbiotic coral/dinoflagellate association I. Photosynthetic performances of symbionts and dependence on sea water bicarbonate

The aim of this and the accompanying paper is to investigate the mechanisms of dissolved inorganic carbon (DIC) uptake by the scleractinian coral Galaxea fascicularis, and its delivery to the endosymbiotic photosynthetic dinoflagellates (zooxanthellae). For this purpose, a comparison was made between the photosynthetic performance of zooxanthellae in intact symbiosis within microcolonies of Galaxea fascicularis, freshly isolated zooxanthellae (FIZ) and cultured zooxanthellae (CZ) under different conditions. Discrimination between CO 2 or HCO 3 − uptake was achieved by several means including changes in DIC concentration, pH variations, pharmacology or modifications of ion concentration in seawater. In this paper, the photosynthesis/irradiance curves of G. fascicularis microcolonies, FIZ and CZ are presented. It is shown that zooxanthellae inside their host have lower photosynthetic performance than isolated zooxanthellae. Light saturation ( I k ) occurred at higher irradiance in the intact association than in isolated symbionts. Light utilization efficiency (α) was minimum in the intact association and increased in FIZ and CZ. G. fascicularis microcolonies, FIZ and CZ were tested for their ability to utilize HCO 3 − as a source of DIC for photosynthesis. Two main approaches were used, the first consisting of changing the bicarbonate concentration by adding HCC 3 − to bicarbonate-free artificial seawater at constant pH, and the second of modifying the pH of the seawater in a closed or open system. At saturating light intensity, the DIC concentration saturating for photosynthesis is no more than that of normal sea water. At pH 8.2, a half-maximal rate of photosynthetic O 2 evolution is achieved at 408, 71 and 178 μM HCO 3 − for coral, FIZ and CZ respectively. The photosynthetic O 2 production with constant inorganic carbon but varying pH reached an optimum at pH 8 to 9 suggesting that HCO 3 − is the main species taken up initially. FIZ and CZ possess the ability to utilize both CO 2 and HCO 3 − as substrates for transport. The rate of non-enzymatic dehydration of HCO 3 − exceeds the rate of photosynthesis in coral and FIZ, but not in CZ. The results presented in this paper suggest that G. fascicularis microcolonies are able to take up bicarbonate to supply symbiont photosynthesis, although zooxanthellae in hospite seems DIC-limited. FIZ seem to absorb CO 2 and HCO 3 − indiscriminately while CZ use HCO 3 −.

Growth and Photosynthetic Responses of Wheat Plants Grown in Space

Growth and photosynthesis of wheat (Triticum aestivum L. cv Super Dwarf) plants grown onboard the space shuttle Discovery for 10 d were examined. Compared to ground control plants, the shoot fresh weight of space-grown seedlings decreased by 25%. Postflight measurements of the O2 evolution/photosynthetic photon flux density response curves of leaf samples revealed that the CO2-saturated photosynthetic rate at saturating light intensities in space-grown plants declined 25% relative to the rate in ground control plants. The relative quantum yield of CO2-saturated photosynthetic O2 evolution measured at limiting light intensities was not significantly affected. In space-grown plants, the light compensation point of the leaves increased by 33%, which likely was due to an increase (27%) in leaf dark-respiration rates. Related experiments with thylakoids isolated from space-grown plants showed that the light-saturated photosynthetic electron transport rate from H2O through photosystems II and I was reduced by 28%. These results demonstrate that photosynthetic functions are affected by the microgravity environment.

Changes in the expression of photosynthetic genes precede loss of photosynthetic activities and chlorophyll when glucose is supplied to mature spinach leaves

To study the molecular mechanism of ‘sink’ regulation on photosynthesis, thylakoid protein synthesis and content of photosynthetic proteins were compared with changes in photosynthetic activities and in leaf chlorophyll (chl) content in detached leaves kept on a diluted nutrient solution in the presence or absence of 50 mM glucose for 4 days at 60 μmol m −2 s −1. During the experiment, leaf chl content decreased in glucose-supplied leaves about 50%, in detached control leaves about 20%. The degradation of the photosynthetic apparatus was not connected with any substantial changes in photosynthetic parameters, when determined by pulse-modulated chl fluorescence at growth light conditions. However, at saturating light intensities of 1000 μmol m −2 s −1, leaf oxygen production was inhibited about 43% in leaves supplied with glucose, leading to a substantial increase in membrane energetization, as indicated by the increase of the non-photochemical quench, q n, and a down-regulation of photosystem (PS) II activity in the light, as determined by ΔF F′ m (yield of variable fluorescence under steady state conditions/fluorescence yield when all reaction centres are closed (after a saturating flash) under steady-state conditions). No substantial changes were observed in the detached control leaves. The reduction of leaf oxygen production under external glucose supply was probably caused by a 60% loss of ribulose-1,5-bisphosphat-carboxylase, as revealed by Western blotting. In the detached control leaves, the content of Rubisco stayed stable. The content of D1 protein of the PS II reaction centre was reduced by 43% in the glucose-supplied leaves, when related to chl. However, a substantial loss of D1 protein of 30% was observed in the detached control leaves. As in other systems, the specific loss of D1 protein initiated the degradation of PS II and via the breakdown of light harvesting complex (LHC) II the loss of leaf chl. By measuring thylakoid protein synthesis, it could be shown that the changes in D1 protein content were caused by an inhibition of its synthesis, together with an increase in its degradation. In the glucose-supplied leaves, the expression of the 25 kDa polypeptide of the LHC II was also inhibited.

ATP synthesis by the F0F1 ATP synthase from thermophilic Bacillus PS3 reconstituted into liposomes with bacteriorhodopsin. 2. Relationships between proton motive force and ATP synthesis.

The correlation between the rate of ATP synthesis and light-induced proton flux was investigated in proteoliposomes reconstituted with bacteriorhodopsin and ATP synthase from thermophilic Bacillus PS3. By variation of the actinic light intensity it was found that ATP synthase activity depended in a sigmoidal manner on the amplitude of the transmembrane light-induced pH gradient. Maximal rates of ATP synthesis (up to to 200 nmol ATP x min(-1) x mg protein (-1) were obtained at saturating light intensities under a steady-state pH gradient of about pH 1.25. It was demonstrated that this was the maximal deltapH attainable at 40 degrees C in reconstituted proteoliposomes, due to the feedback inhibition of bacteriorhodopsin by the proton gradient it generates. In the absence of valinomycin, a small but significant transmembrane electrical potential could develop at 40 degrees C, contributing to an increase in the rate of ATP synthesis. The H+/ATP stoichiometry was measured at the static-head (equilibrium) conditions from the ratio of the phosphate potential to the size of the light-induced pH gradient and a value of about four was obtained under the maximal electrochemical proton gradient. Increasing the amount of bacteriorhodopsin in the proteoliposomes at a constant F0F1 concentration led to a large increase in the rate of ATP synthesis whereas the magnitude of delta pH remained the same or, at very high bacteriorhodopsin levels, decreased. Consequently the H+/ATP stoichiometry was found to increase significantly with increasing bacteriorhodopsin content. Reconstitutions with mixtures of native and impaired bacteriorhodopsin (Asp96-->Asn mutated bacteriorhodopsin) further demonstrated that this increase in the coupling efficiency could not be related to protein-protein interactions but rather to bacteriorhodopsin donating H+ to the ATP synthase. Increasing the amount of negatively charged phospholipids in the proteoliposomes also increased the coupling efficiency between bacteriorhodopsin and ATP synthase at a constant transmembrane pH gradient. Similar results were obtained with chloroplast ATP synthase. Furthermore, ATP synthase activities induced by delta pH/delta psi transitions were independent of bacteriorhodopsin or anionic lipid levels. These observations were interpreted as indicating that, in bacteriorhodopsin/ATP synthase, proteoliposomes, a localized pathway for coupling light-driven H+ transport by bacteriorhodopsin to ATP synthesis by F0F1 might exist under specific experimental conditions.

CO2 fixation and photoevolution of H2 and O2 in a mutant of Chlamydomonas lacking photosystem I

ALTHOUGH mutant B4 of Chlamydomonas reinhardtii lacks photo-system I (PS I), it is capable of photoautotrophic assimilation of atmospheric carbon dioxide and sustained simultaneous photo-evolution of molecular oxygen and hydrogen. Here we report that at saturating light intensities, carbon dioxide reduction is stable under anaerobiosis but unstable in air. At lower light intensities, carbon dioxide reduction is stable in both atmospheres. The data indicate that PS I is not necessary for autotrophic photosynthesis. One interpretation of these results is that oxygenic photosynthesis developed as a single-light-reaction process, presumably from a bacterium with a phaeophv tin-quinone reaction centre, but became unstable as oxygen in the Eartha¤™s atmosphere accumulated. PS I was the second light reaction, added to confer stability in oxygen-containing atmospheres. Viewed from this perspective, the well-known Z scheme of modern photosynthesis is seen as a specialized adaptation for performing low-potential reductive photochemistry in oxygen-containing atmospheres, but is not an irreducible necessity for satisfying the thermodynamic and mechanistic requirements of carbon dioxide photoreduction using water as the source of reductant.

Photooxidative damage to the cyanobacterium Spirulina platensis mediated by singlet oxygen

Experiments on the specific growth rate, bleaching of pigments, O2 evolution, lipid peroxidation, and loss of sulfhydryl (-SH) content in response to the varying light intensities (2–28 W/m2) suggested that photodamage to the Spirulina cells was maximum at or beyond the photosynthesis saturating light intensity (12 W/m2). However, photobleaching of the chlorophyll a was relatively higher than β carotenoid. The results on the N,N-dimethyl-p-nitrosoaniline (RNO) bleaching in the presence of oxygen radical quenchers exhibited maximum effect of sodium azide and indicated about the generation of singlet oxygen. The chlorophyll a-sensitized production of singlet oxygen by a type II reaction cannot be ruled out because of maximum oxidative damage to the cells at or beyond the photosynthesis saturating light intensity, i.e., 12 W/m2, when the availability of triplet chlorophyll is maximum.

The Role of Cytochrome c-550 as Studied through Reverse Genetics and Mutant Characterization in Synechocystis sp. PCC 6803

The gene coding for cytochrome c-550 in Synechocystis sp. PCC 6803 was cloned based on the N-terminal sequence of the mature polypeptide. Using the most probable translation start codon, the gene is expected to code for 160 amino acid residues. This includes a cleavable N-terminal leader sequence of 25 residues. This leader sequence has an Arg-Asn-Arg sequence immediately before the cleavage site; this is characteristic for transit peptides in prokaryotes. Comparison of this sequence with the leader sequence of the photosystem II-associated extrinsic 33-kDa protein from the same cyanobacterium showed an identity of 13 out of 25 residues. These results suggest that after synthesis of the apoprotein, cytochrome c-550 is transported into the thylakoid lumen. Using the cloned gene, insertion and deletion mutants of Synechocystis sp. PCC 6803 were constructed. In the absence of cytochrome c-550, both mutants were capable of photoautotrophic growth but at a significantly reduced rate. Atrazine bindng and Western blot analysis showed that these mutants on a per-chlorophyll basis contained 53-67% of the amount of photosystem II as compared with wild type. The photosystem II-specific oxygen-evolving activity at saturating light intensity was reduced to about 40% of that in the wild type strain. Taken together, these results indicate that the cytochrome c-550 is transported into the thylakoid lumen and contributes to optimal functional stability of photosystem II in cyanobacteria. This supports our biochemical evidence that cytochrome c-550 is associated with the lumenal side of photosystem II as one of the extrinsic proteins enhancing oxygen evolution (Shen, J.-R., Ikeuchi, M., and Inoue, Y. (1992) FEBS Lett. 301, 145-149; Shen, J.-R., and Inoue, Y. (1993) Biochemistry 32, 1825-1832). Based on these results, the gene for cytochrome c-550 was named psbV. The possible evolutionary relationship among extrinsic proteins of the photosystem II donor side is discussed.

Some difficulties in modelling chlorophyll a evolution in a high rate algal pond ecosystem

The High Rate Algal Pond (HRAP) is an efficient treatment for controlling wastewater pollution by reducing the organic matter and the inorganic nutrient content. Deterministic modelling of temporal evolution of algae could provide a rational basis for pond management. An experimental HRAP was set up in Mèze (France) and sampled weekly over 24 months. Models simulating the evolution of chlorophyll a concentration and nutrients (N and P) were constructed using Stella II software. The seasonal pattern of chlorophyll a concentrations results from the annual cycle of solar irradiance and temperature, whereas shorter trends (1 to 4 weeks) are dependent on the evolution of zooplankton groups. The first difficulty is to determine the functional relationships of the phytoplankton and zooplankton groups. In the model the evolution of the phytoplankton taxa is considered to be dependent on (i) inherent parameters of phytoplankton taxa (mortality rate, growth rate, saturating light intensity, etc.) and on (ii) parameters of zooplankton taxa (filtration rate, size selectivity, etc.). To take all these taxa as state variables, and all the associated parameters, into account is impossible: to solve such a problem, we forced the evolution of the biomass of the phytoplankton and zooplankton taxa. This approach improves the agreement between the simulated and observed chlorophyll a concentration values. The second difficulty concerns the determinism of appearance of the phytoplankton and zooplankton taxa used in the model: up to now we are only able to force these appearances. Thus, even in quite simple ecosystems, using deterministic modelling as a predictive tool requires a full understanding of the exact biological succession and interaction processes.

Deep‐water aquatic plant communities in an oligotrophic lake: physiological responses to variable light

SUMMARY1. Oligotrophic Lake Waikaremoana, New Zealand, is used for hydroelectric power generation and the lake levels are manipulated within an operating range of 3 m. There was concern that rapidly changing water levels adversely affected the littoral zone by decreasing light availability in two ways: local turbidity caused by shoreline erosion at low water levels; and decreased light penetration to the deep littoral zone caused by high water levels in summer.2. The littoral zone was dominated by native aquatic plants with vascular species to 6 m and a characean meadow below this to 16 m. The biomass and heights of the communities in the depth zone 0–6 m were reduced at a site exposed to wave action relative to those at a sheltered site. However, the community structure below 6 m was similar at exposed and sheltered sites. The lower boundary of the littoral zone was sharply delimited at 16 m and this bottom boundary remained constant throughout the year despite large seasonal changes in solar radiation and the 3 m variation in lake level.3. There was evidence that the deep‐water community consisting of Chara corallina had adapted physiologically to low‐light conditions. Net light saturated photosynthesis (CO2 exchange) per unit chlorophyll a (Chl a) was reduced to 1.7 μg C (μg Chl a)−1 h−1 at the lower boundary, half of that recorded at 5 m. The concentration of Chi a per gram of biomass (dry weight), was considerably greater at the lower boundary than higher in the profile [c. 7 mg Chl a (g dry wt)−1 at 16 m vs. 4 mg Chl a (g dry wt)−1 at 5 m]. Chl b also increased with depth and there was no change in the ratio of Chl a and Chl b with increasing depth. The saturation light intensity (Ik) of the community at the lower boundary was only 78 μmol photons m−2 s−1. Photosynthetic parameters (Ik and α) as well as the Chl a content remained relatively constant throughout the seasonal and short‐term changes in radiation.4. The photosynthetic characteristics of the littoral community were therefore not greatly affected by the lake level change caused by the present hydroelectric operations. However, the sharpness of the lower boundary and its extreme shade characteristics imply that the deep‐water community would be sensitive to any further changes in underwater light availability.

Effect of cell movement by random mixing between the surface and bottom of photobioreactors on algal productivity

Effect of algae movement, as a result of random mixing, between the surface and bottom zones of shallow, moderately deep and deep photobioreactors (incident light intensities per unit volume were 8125, 4062 and 2031 μmol·m −3·s −1, respectively) on the reactor productivity was investigated. The results showed that at low cell concentrations, movement of cells between the surface and bottom zones of shallow and moderately deep reactors had no significant effect on Chlorella pyrenoidosa C-212 growth and productivity. However, as the cell concentration in the reactors increased, cell movement between the two zones resulted in increased productivity of the shallow reactor but decreased productivity of the moderately deep reactor. On the other hand, in the deep reactor, random movement of cells between the two zones resulted in decreased Chlorella growth rate regardless of the cell concentration. This may be attributed to the fact that at high cell concentration or in a deep reactor, if the cells move between the surface and bottom of the reactor, they spend too long a time in the dark part of the reactor where there is no cell growth, and endogenous respiration as well as cell death may lead to a decrease in cell concentration. When Spirulina platensis M-135 cells were cultivated in the deep reactor, even at high cell concentration, movement of cells between the surface and bottom zones of the reactor led to an increase in the reactor productivity. The reasons for the difference in the results obtained with these two strains of algae could be attributed to the difference in their light requirements since it was found that the saturation light intensity and specific decrease in cell concentration when incubated in the dark were lower for Spirulina than for Chlorella cells.

Constraints to dry matter production in fibre hemp (Cannabis sativa L.)

Fibre hemp (Cannabis sativa L.) may be an alternative to wood as a raw material for pulp and paper production. Research was done to assess the potential productivity of fibre hemp and to identify constraints to that productivity. Growth analyses were done on hemp crops in three consecutive years, using several cultivars and seeding rates. In 1987 the crops suffered severely from fungal diseases ; stem dry matter yields were 9.4 t ha−1. In 1988 and 1989 fungicides were applied and stem dry matter yields were respectively 11.9 t ha−1 and 13.6 t ha−1. The number of living plants m−2 ranged from 86 to 823 at emergence, depending on treatment, and from 38 to 102 at final harvest. Increased seeding rates led to earlier canopy closure and higher initial biomass production, but more plants died during the growing season and the stem yield at final harvest was not affected by seeding rate. Average radiation-use efficiency (RUE; above-ground accumulated dry matter divided by intercepted photosynthetically active radiation) for the entire growing season under favourable growing conditions was 1.9 g MJ−1, which is low compared to other C3 crops. Leaf photosynthesis rates at saturating light intensities, however, were high: 30 kg CO2ha−1 h−1. The light extinction coefficient of hemp canopies was high (0.96), and may have reduced canopy photosynthesis rate and RUE. The high lignin content of the stems, and the high fat and protein content in the seed, also reduced RUE. Furthermore, dry matter invested in the tap root and in shed dead leaves was ignored. The RUE was also underestimated because an appreciable proportion of the biomass of plants that died during the growing season could not be collected. More research is needed to optimize cultural practices in fibre hemp.

Mutation of chlorophyll ligands in the chlorophyll-binding CP47 protein as studied in a Synechocystis sp. PCC 6803 photosystem I-less background.

Site-directed mutations have been introduced to replace conserved histidine residues in the chlorophyll-binding protein CP47 of photosystem II (PS II) in a PS I-less/apcE-background strain of the cyanobacterium Synechocystis sp. PCC 6803. In thylakoids isolated from such a system, the degree of loss of the 695-nm fluorescence emission maximum at 77 K compared to that at 685 nm generally was consistent with the decrease in oxygen evolution rates measured at saturating light intensity. Taking into account that in the absence of CP47 and PS I some chlorophyll remains detectable in cells, the relative 695-nm fluorescence emission and the rate of oxygen evolution also correlate with the relative amount of chlorophyll per cell and with the number of PS II reaction centers on a chlorophyll basis. Interestingly, the 77 K fluorescence excitation spectra monitoring 695-nm emission of thylakoids from the CP47 His-to-Tyr mutants in a photosystem I-less/apcE-background showed increases in the 413- and 531-nm absorption regions, compared to spectra of thylakoids from the background strain. These wavelengths coincide with absorption maxima of pheophytin. No increase in the 531-nm excitation band was observed in thylakoids from mutants lacking PS II or with a His-to-Asn mutation. These results are interpreted to indicate that replacement of conserved histidine residues by tyrosine in CP47 leads to the loss of Mg2+ from chlorophyll, resulting in the formation of pheophytin, or to the binding of pheophytin (rather than chlorophyll) at a particular pigment-binding site of CP47 during biogenesis and assembly of the protein. It was observed that the light-harvesting efficiency of CP47 His mutants was lower judging from the light intensity dependence of electron transport and analysis of fluorescence decay kinetics. This suggests that the presence of pheophytin in the antenna decreases antenna efficiency.

Time-course of photoadaptation in the symbiotic sea anemone Anemonia viridis

The time-course of photoadaptation in the symbiotic sea anemone Anemonia viridis (Forskal) was determined on freshly collected anemones which were maintained under low light (LL: 10 μE m-2s-1) or high light (HL: 300 μE m-2s-1) over a period of 35 d. Photosynthesis versus irradiance curves (P vs I) normalised to anemone dry weight were determined for anemones at the start of the experiment and for both groups at 7 d intervals. Photosynthetic parameters derived from the P vs I curves, P max gross (the maximum photosynthetic capacity of the zooxanthellae), α (an estimate of photosynthetic efficiency) and I k(the light-saturating irradiance), and the respiration rate, showed that anemones were high-light adapted before the experiment began. Thereafter, any observed changes occurred only in anemones adapting to the low-light regime. P max gross remained relatively constant over 35 d and there was no significant difference between LL and HL groups. In the LL group α increased and both I kand the respiration rate decreased significantly. These photoadaptive responses were most rapid in the first 7 d, after which change was gradual until about Day 28, when photoadaptation appeared to be complete. In order to predict the physiological mechanisms of photoadaptation to low light, P gross vs I curves normalised to zooxanthellae biomass were constructed for LL and HL anemones at 35 d. Zooxanthellae density and photosynthetic pigment content were also analysed. For LL-adapted anemones, α and chlorophyll a were higher, there were no differences in algal density and chlorophyll c 2 and I kwas lower. It is proposed that the size of the peridinin-chlorophyll a-protein complex (PCP) of the light-harvesting complex was increased to facilitate photoadaptation. Plotting P net vs I normalised to anemone dry weight on the 35 d-photoadapted anemones gave a measure of changes in the excess photosynthetically fixed carbon that was potentially available to the host. Net photosynthesis of the LL group was higher at subsaturating light intensities as a result of the increase in α, and at saturating light intensities P max net was higher as a result of a decrease in the rate of respiration of the whole symbiotic association.

Synechocystis sp PCC 6803 strains lacking photosystem I and phycobilisome function.

To design an in vivo system allowing detailed analysis of photosystem II (PSII) complexes without significant interference from other pigment complexes, part of the psaAB operon coding for the core proteins of photosystem I (PSI) and part of the apcE gene coding for the anchor protein linking the phycobilisome to the thylakoid membrane were deleted from the genome of the cyanobacterium Synechocystis sp strain PCC 6803. Upon transformation and segregation at low light intensity (5 microE m-2 sec-1), a PSI deletion strain was obtained that is light tolerant and grows reasonably well under photoheterotrophic conditions at 5 microE m-2 sec-1 (doubling time approximately 28 hr). Subsequent inactivation of apcE by an erythromycin resistance marker led to reduction of the phycobilin-to-chlorophyll ratio and to a further decrease in light sensitivity. The resulting PSI-less/apcE- strain grew photoheterotrophically at normal light intensity (50 microE m-2 sec-1) with a doubling time of 18 hr. Deletion of apcE in the wild type resulted in slow photoautotrophic growth. The remaining phycobilins in apcE- strains were inactive in transferring light energy to PSII. Cells of both the PSI-less and PSI-less/apcE- strains had an approximately sixfold enrichment of PSII on a chlorophyll basis and were as active in oxygen evolution (on a per PSII basis) as the wild type at saturating light intensity. Both PSI-less strains described here are highly appropriate both for detailed PSII studies and as background strains to analyze site- and region-directed PSII mutants in vivo.

Photosynthetic characteristics of seagrasses (Cymodocea serrulata, Thalassia hemprichii and Zostera capricornia) in a low-light environment, with a comparison of leaf-marking and lacunal-gas measurements of productivity

We describe the photosynthetic characteristics of three seagrasses and the relationship between their production and natural light intensities (photosynthesis-irradiance response, i.e. PI curves). Seagrass production (gross production minus shoot respiration) was measured in the field by the rate of gas release from the lacuna1 space of whole seagrass shoots and compared with net leaf production. Field work was carried out on the seagrasses Cymodocea serrulata (R. Br.) Aschers, and Magnus, Thalassia hemprichii (Ehrenb.) Aschers., and Zostera capricornia Aschers. in the turbid, warm waters of Cairns Harbour, Queensland, Australia. The photosynthetic efficiencies (the initial slope of the PI curves) of all of the seagrass species were 10 times greater than any previously measured for the same species in higher-light environments. The high compensating light intensities (80-92 �E m-2 s-1) showed that the plants have high respiration rates that were probably due to the high water temperatures (29-33�C) of the harbour. The seagrasses responded to small increases of light at low light intensities by rapidly reaching saturating light intensities, and the maximum rates of production were between 0.4 and 0.6 mg C h-1 shoot-1. The average period of exposure to saturating light intensity was 2 h day-1. One-quarter of the gross production was lost to plant respiration. The net productivity and respiration of all three seagrasses was calculated from this photoperiod. Net leaf production in situ compared well with the seagrass production estimates that were measured with the lacunal-gas technique. Most of the production appeared to be allocated to the above-ground tissue, a feature consistent with seagrasses growing in low-light and terrigenous sediments.