Photosynthetic Light Reactions Research Articles

Inhibition of respiration and nitrate assimilation enhances photohydrogen evolution under low oxygen concentrations in Synechocystis sp. PCC 6803

In cyanobacterial membranes photosynthetic light reaction and respiration are intertwined. It was shown that the single hydrogenase of Synechocystis sp. PCC 6803 is connected to the light reaction. We conducted measurements of hydrogenase activity, fermentative hydrogen evolution and photohydrogen production of deletion mutants of respiratory electron transport complexes. All single, double and triple mutants of the three terminal respiratory oxidases and the ndhB-mutant without a functional complex I were studied. After activating the hydrogenase by applying anaerobic conditions in the dark hydrogen production was measured at the onset of light. Under these conditions respiratory capacity and amount of photohydrogen produced were found to be inversely correlated. Especially the absence of the quinol oxidase induced an increased hydrogenase activity and an increased production of hydrogen in the light compared to wild type cells. Our results support that the hydrogenase as well as the quinol oxidase function as electron valves under low oxygen concentrations. When the activities of photosystem II and I (PSII and PSI) are not in equilibrium or in case that the light reaction is working at a higher pace than the dark reaction, the hydrogenase is necessary to prevent an acceptor side limitation of PSI, and the quinol oxidase to prevent an overreduction of the plastoquinone pool (acceptor side of PSII). Besides oxygen, nitrate assimilation was found to be an important electron sink. Inhibition of nitrate reductase resulted in an increased fermentative hydrogen production as well as higher amounts of photohydrogen.

Light acclimation in rose (Rosa hybrida cv. Grand Gala) leaves after pruning: Effects on chlorophyll a fluorescence, nitrate reductase, ammonium and carbohydrates

Abstract Changes in light intensity have a particular effect on the photosynthetic apparatus. Most of the studies on light acclimation in higher plants have focused on the effects of fixed light intensity. Few works deal on sudden changes in light intensity due to pruning for long periods of time. Pruning the bent shoots in roses (cv. Grand Gala) can modify the light interception in internal leaves and change their photosynthetic reactions. Before pruning, internal leaves were acclimated to low light intensity and after pruning, internal and external leaves received the same light intensity. The aim of this work was to find out how the photosynthetic light reaction, measured by chlorophyll (Chl) a fluorescence in internal and external leaves of arched shoot could be modulated by light changes. Other parameters described as light dependent were also measured: nitrate reductase (NR) activity, NH 4 + , sucrose and starch levels. Internal and external leaves in rose have shown a high plasticity, rapid and dynamical acclimation, in response to changes in incident sunlight produced by pruning, that can be explained by Chl a fluorescence parameters. The modified NR activity, NH 4 + , sucrose and starch levels were difficult to associated with light intensity changes, and their modulation could be the response to long-time light acclimation.

Effects of water deficit on photosystem II photochemistry and photoprotection during acclimation of lyreleaf sage ( Salvia lyrata L.) plants to high light

Acclimation of photosynthetic light reactions to high light requires adjustments in photosystem II (PSII) photochemistry and may be affected by environmental stresses, such as water deficit. In this study, we examined the effects of this stress on PSII photochemistry and photoprotection, with an emphasis on the role of carotenoids and tocopherols, during acclimation of lyreleaf sage ( Salvia lyrata L.) plants to high light. Violaxanthin was rapidly converted to zeaxanthin under high light, the de-epoxidation state of the xanthophyll cycle reaching maximum levels of 0.97 after 10 days of high light exposure. Under a higher photoprotective demand caused by water deficit, plants showed significant decreases in β-carotene and enhanced oxidation of α-tocopherol to α-tocopherol quinone, which was followed by decreases in the F v/ F m ratio. The levels of β-carotene decreased more in water-stressed than irrigated plants during acclimation to high light, being particularly degraded (up to 73%) after 14 days of water deficit. Tocopherol levels increased significantly during acclimation to high light, particularly under water deficit, which caused 6.6- and 10-fold increases in α-tocopherol and α-tocopherol quinone, respectively. We conclude that when xanthophyll cycle-dependent excess energy dissipation could not afford further protection during high light acclimation and the photoprotective demand increased in lyreleaf sage plants by water deficit, enhanced oxidation of α-tocopherol and β-carotene occurred. As stress persisted, enhanced formation of reactive oxygen species might ultimately damage the PSII, as indicated by the reductions in the F v/ F m ratio.

Biomass and photosynthetic productivity of water hyacinth (Eichhornia crassipes) as affected by nutrient supply and mirid (Eccritotarus catarinensis) biocontrol

The biological control of water hyacinth is affected by water nitrogen and phosphorus content and this was investigated experimentally at five levels of nutrient supply by measuring plant photosynthetic and growth responses, and mirid reproduction and herbivory of nutrient treated plants. Low nitrogen (2–0.2 mg L −1 ) and phosphorus (0.2–0.01 mg L −1 ) supply decreased hyacinth photosynthesis, growth and biomass accumulation relative to plants supplied 200 mg L −1 N and 20 mg L −1 P. This effect depended more on nitrogen supply than phosphorus supply. Chlorophyll fluorescence showed that the photosynthetic light reactions of low nutrient plants were affected and leaves had decreased chlorophyll content, density of functional photosystems II and dissipated a greater proportion of absorbed energy as heat. Gas exchange parameters showed reduced carboxylation efficiency, rates of RuBP regeneration and light saturated photosynthetic rates, but not quantum yields. Effects on photosynthesis translated into lower plant dry biomass. Mirid herbivory exacerbated the effects of low nutrients noted for chlorophyll fluorescence, gas exchange parameters and biomass accumulation, however, these effects were not always significant and there was no obvious correlation between the level of nutrients supplied and the effect of mirid herbivory. Low nutrient supply did, however, affect mirid performance reducing the number of adult insects, nymphs and herbivory intensity suggesting that in the long-term mirid populations would be significantly affected by water nutrient status.

Nuclear Photosynthetic Gene Expression Is Synergistically Modulated by Rates of Protein Synthesis in Chloroplasts and Mitochondria

Arabidopsis thaliana mutants prors1-1 and -2 were identified on the basis of a decrease in effective photosystem II quantum yield. Mutations were localized to the 5'-untranslated region of the nuclear gene PROLYL-tRNA SYNTHETASE1 (PRORS1), which acts in both plastids and mitochondria. In prors1-1 and -2, PRORS1 expression is reduced, along with protein synthesis in both organelles. PRORS1 null alleles (prors1-3 and -4) result in embryo sac and embryo development arrest. In mutants with the leaky prors1-1 and -2 alleles, transcription of nuclear genes for proteins involved in photosynthetic light reactions is downregulated, whereas genes for other chloroplast proteins are upregulated. Downregulation of nuclear photosynthetic genes is not associated with a marked increase in the level of reactive oxygen species in leaves and persists in the dark, suggesting that the transcriptional response is light and photooxidative stress independent. The mrpl11 and prpl11 mutants are impaired in the mitochondrial and plastid ribosomal L11 proteins, respectively. The prpl11 mrpl11 double mutant, but neither of the single mutants, resulted in strong downregulation of nuclear photosynthetic genes, like that seen in leaky mutants for PRORS1, implying that, when organellar translation is perturbed, signals derived from both types of organelles cooperate in the regulation of nuclear photosynthetic gene expression.

Functional flexibility and acclimation of the thylakoid membrane

Light is an elusive substrate for the function of photosynthetic light reactions of photosynthesis in the thylakoid membrane. Therefore structural and functional dynamics, which occur in the timescale from seconds to several days, are required both at low and high light conditions. The best characterized short-time regulation mechanism at low light is a rapid state transition, resulting in higher absorption cross section of PSI at the expense of PSII. If the low light conditions continue, activation of the lhcb-genes and synthesis of the light-harvesting proteins will occur to optimize the functions of PSII and PSI. At high light, the transition to state 2 is completely inhibited, but the feedback de-excitation of absorbed energy as heat, known as the energy-dependent quenching (q(E)), is rapidly set up. It requires, at least, the DeltapH-dependent activation of violaxanthin de-epoxidase and involvement of the PsbS protein. Another crucial mechanism for protection against the high light stress is the PSII repair cycle. Furthermore, the water-water cycle, cyclic electron transfer around PSI and chlororespiration are important means induced under high irradiation, functioning mainly to avoid an excess production of reactive oxygen species.

The mechanism of photosynthetic water splitting

Oxygenic photosynthesis, which provides the biosphere with most of its chemical energy, uses water as its source of electrons. Water is photochemically oxidized by the protein complex photosystem II (PSII), which is found, along with other proteins of the photosynthetic light reactions, in the thylakoid membranes of cyanobacteria and of green plant chloroplasts. Water splitting is catalyzed by the oxygen-evolving complex (OEC) of PSII, producing dioxygen gas, protons and electrons. O(2) is released into the atmosphere, sustaining all aerobic life on earth; product protons are released into the thylakoid lumen, augmenting a proton concentration gradient across the membrane; and photo-energized electrons pass to the rest of the electron-transfer pathway. The OEC contains four manganese ions, one calcium ion and (almost certainly) a chloride ion, but its precise structure and catalytic mechanism remain unclear. In this paper, we develop a chemically complete structure of the OEC and its environment by using molecular mechanics calculations to extend and slightly adjust the recently-obtained X-ray crystallographic model with reference to this structure and to some important recent experimental results.

Effect of lanthanide chloride on photosynthesis and dry matter accumulation in tobacco seedlings.

To investigate the physiological effects of rare earth ions, we have studied the effect of LaCl3 on the photosynthetic light reactions in tobacco (Nicotiana tobacum). When treated with 5-20 mg/L LaCl3 in Hoagland solution by water culture, the dry matter accumulation of different parts in tobacco, the content of chlorophyll increased gradually, but decreased when the concentration of LaCl3 was > or = 50 mg/L. The optimum concentration for growth appeared to be about 20 mg/L of LaCl3 in nutrient solution. La3+ promoted the activities of the Hill reaction, Mg2(+)-ATPase, and stimulated the rate of photophosphorylation in chloroplast at low concentrations, but inhibited them at high concentrations. It is concluded that La3+ stimulated the growth of tobacco seedlings and accelerated the photosynthetic light reactions at suitable concentration in vivo.

Light-dependent herbicides: an overview

Activities of a surprisingly large number of herbicide families are directly or indirectly influenced by light. Herbicide mechanisms that are dependent on light or are enhanced by light fall into several different categories. These herbicides inhibit electron flow in photosystem II in the photosynthetic light reaction (e.g., triazines, phenyl ureas, and uracils), capture electrons in photosystem I in the photosynthetic light reaction (e.g., bipyridiniums such as paraquat), inhibit glutamine synthetase in the nitrogen assimilation pathway (e.g., glufosinate), inhibit protoporphyrinogen oxidase during chlorophyll biosynthesis (e.g., diphenyl ethers [DPEs] and N-phenyl heterocycles), directly inhibit carotenoid biosynthesis by inhibiting one of the desaturase enzymes (e.g., norflurazon and fluridone), or indirectly inhibit carotenoid biosynthesis by inhibiting a quinone cofactor involved in the desaturase reaction (e.g., triketones and isoxazoles). Surprisingly, in all instances the involvement of light in herbicide action is not a direct interaction of light and herbicide at the primary site of action. In addition, the events that cause tissue damage (necrosis) are not associated with the primary target and are always due to membrane damage caused by lipid peroxidation of polyunsaturated fatty acids.

Interactions of chronic exposure to elevated CO2 and O3 levels in the photosynthetic light and dark reactions of European beech (Fagus sylvatica)

Young trees of European beech (Fagus sylvatica) acclimated for one growing season to ambient (c. 367 μl l−1) or elevated CO2 levels (c. 660 μl l−1) were exposed during the subsequent year to combinations of the same CO2 regimes and ambient or twice‐ambient ozone (O3) levels (generated from the database of a rural site). By the end of June, before the development of macroscopic leaf injury, the raised O3 levels had not affected the light and dark reactions of photosynthesis. However, acclimation to elevated CO2 had resulted in lowered chlorophyll and nitrogen concentrations, whereas photosynthetic performance, examined over a wide range of parameters from light and dark reactions, remained unchanged or showed only slight reductions (e.g. apparent electron transport rate, ETR; apparent quantum yield of CO2 gas exchange, ΦCO2; apparent carboxylation efficiency, CE; and photosynthetic capacity at light and CO2 saturation, PC). In August, after the appearance of leaf necroses, plants grown under ambient CO2 and twice‐ambient O3 conditions declined in both the photosynthetic light reactions (optimum electron quantum yield, Fv/Fm, non‐photochemical energy quenching, NPQ, reduction state of QA, apparent electron quantum yield, ΦPSII, maximum electron transport rates) and the dark reactions as reflected by CE, ΦCO2, as well as the maximum CO2 uptake rate (i.e. PC). CE, ΦCO2 and PC were reduced by c. 75, 40 and 75%, respectively, relative to plants exposed to ambient CO2 and O3 levels. By contrast, plants exposed to twice‐ambient O3 and elevated CO2 levels maintained a photosynthetic performance similar to individuals grown either under ambient CO2 and ambient O3, or elevated CO2 and ambient O3 conditions. The long‐term exposure to elevated CO2 therefore tended to counteract adverse chronic effects of enhanced O3 levels on photosynthesis. Possible reasons for this compensatory effect in F. sylvatica are discussed.

A nuclear-encoded protein of prokaryotic origin is essential for the stability of photosystem II in Arabidopsis thaliana.

To understand the regulatory mechanisms underlying the biogenesis of photosystem II (PSII) we have characterized the nuclear mutant hcf136 of Arabidopsis thaliana and isolated the affected gene. The mutant is devoid of any photosystem II activity, and none of the nuclear- and plastome-encoded subunits of this photosystem accumulate to significant levels. Protein labelling studies in the presence of cycloheximide showed that the plastome-encoded PSII subunits are synthesized but are not stable. The HCF136 gene was isolated by virtue of its T-DNA tag, and its identity was confirmed by complementation of homozygous hcf136 seedlings. Immunoblot analysis of fractionated chloroplasts showed that the HCF136 protein is a lumenal protein, found only in stromal thylakoid lamellae. The HCF136 protein is produced already in dark-grown seedlings and its levels do not increase dramatically during light-induced greening. This accumulation profile confirms the mutational data by showing that the HCF136 protein must be present when PSII complexes are made. HCF136 homologues are found in the cyanobacterium Synechocystis species PCC6803 (slr2034) and the cyanelle genome of Cyanophora paradoxa (ORF333), but are lacking in the plastomes of chlorophytes and metaphytes as well as from those of rhodo- and chromophytes. We conclude that HCF136 encodes a stability and/or assembly factor of PSII which dates back to the cyanobacterial-like endosymbiont that led to the plastids of the present photosynthetic eukaryotes.

Impact of temperature acclimation on photosynthesis in the toxic red-tide dinoflagellate Alexandriumfundyense (Ca28)

This study assessed the impact of temperature on the photosynthetic activity in the dino- flagellate Alexandrium fundyense (Ca28) for cultures grown at 75 umol photons rrr2 s~' over a range of temperatures. Increasing light intensity under static temperatures caused a S-fold decrease in the maximum quantum yield for photosystem II (PSII) (FJFm). Carbon fixation rates mirrored high-light depressions in FJFm. Cells in the presence of streptomycin showed an 83% recovery in FJFm; therefore, only a minor proportion of the decline in FJFm was attributable to PSII damage by bright light. For cells transferred to higher temperatures, FJFm was less sensitive to high light, decreasing only 20-40% compared to the 80-90% decrease observed for cells incubated at their ambient growth temperature. For cells shifted to higher temperatures, the rapid recovery phase of FJFm was not present; therefore, cells did not initiate downregulation of PSII. Higher capacity to maintain electron transport, as indicated by the quantum yields, was confirmed by enhanced carbon fixation. Shifts to lower temperatures significantly increased PSII sensitivity to high light. Overall, these relationships reflect the synergy between photosynthetic light and dark reactions which are differentially impacted by changes in temperature.

Light Duration Alters [14C] Photosynthate Partitioning into Sorbitol in Apple

Sorbitol synthesis in apple leaves requires NADPH, a product of photosynthetic light reactions. Light duration directly affects NADPH production, thus, variation in light duration may influence sorbitol synthesis and alter the ratios of sorbitol to other carbohydrates. Our experiments were designed to determine whether light duration altered photosynthate partitioning among carbohydrates and whether longer light durations favored sorbitol synthesis. One-year-old `Gala' apple plants that experienced a 1-, 4-, 7-, 10-, or 14-hour photoperiod for 7 days were exposed to 14CO2 for 15 minutes. Individual carbohydrate concentrations and the activity of newly-fixed [14C] carbohydrates were analyzed during different intervals for 24 hours after labeling. Sorbitol and fructose concentrations increased significantly as light duration increased from 1 to 10 hours, then remained stable at a light duration of 14 hours, whereas starch concentration increased linearly over increasing light durations. Sucrose and glucose concentrations were not significantly altered by light duration. The sorbitol: sucrose ratios increased while sorbitol: starch ratios decreased as light duration increased from 1 to 14 hours. Newly-fixed [14C] carbohydrates varied diurnally. Similar changes during the different light duration periods were found for individual [14C] carbohydrates and unlabeled carbohydrate concentrations. Our results suggest that longer light durations favored sorbitol synthesis over sucrose, and NADPH concentration may become a rate-limiting factor for sorbitol synthesis during short light durations.

Effects of elevated CO2 concentration on leaf characteristics and photosynthetic capacity of beech (Fagus sylvatica) during the growing season

Two-year-old beech (Fagus sylvatica L.) saplings were planted directly in the ground at high density (100 per m(2)), in an experimental design that realistically mimicked field conditions, and grown for two years in air containing CO(2) at either ambient or an elevated (ambient + 350 ppm) concentration. Plant dry mass and leaf area were increased by a two-year exposure to elevated CO(2). The saplings produced physiologically distinct types of sun leaves associated with the first and second growth flushes. Leaves of the second flush had a higher leaf mass per unit area and less chlorophyll per unit area, per unit dry mass and per unit nitrogen than leaves of the first flush. Chlorophyll content expressed per unit nitrogen decreased over time in plants grown in elevated CO(2), which suggests that, in elevated CO(2), less nitrogen was invested in machinery of the photosynthetic light reactions. In early summer, the photosynthetic capacity measured at saturating irradiance and CO(2) was slightly but not significantly higher in saplings grown in elevated CO(2) than in saplings grown in ambient CO(2). However, a decrease in photosynthetic capacity was observed after July in leaves of saplings grown in CO(2)-enriched air. The results demonstrate that photosynthetic acclimation to elevated CO(2) can occur in field-grown saplings in late summer, at the time of growth cessation.

Stomatal and nonstomatal limitations of photosynthesis in relation to the drought and shade tolerance of tree species in open and understory environments

Light saturated photosynthesis (A) in field saplings of shade tolerant, intermediate, and intolerant tree species was analyzed for stomatal and nonstomatal limitations to test differences between species and sun and shade phenotypes during drought. Throughout the study, photosynthesis was highest and mesophyll limitations of A (Lm) lowest in the intolerant species in both open and understory habitats. The shade tolerant species exhibited the only drought-related decreased A and increased Lm in the open, and the greatest drought-related decreased A and increased Lm in the understory. Few species exhibited significant habitat or drought-related differences in stomatal conductance to CO2 (gc), but even slight decreases in gc during drought were associated with large increases in stomatal limitations to A (Lg). Combined changes in Lm and Lg resulted in increased relative stomatal limitation to A (l g) in several species during drought. Nevertheless, the overall lack of stomatal closure allowed for nonstomatal limitations to play a major role in reduced A during drought. Higher leaf N was associated with shallower slope of the l g versus gc relationship, an indication of greater A capacity. Photosynthetic capacity tended to be greater in the intolerant species than the tolerant species, and it tended to decrease during drought primarily in the shade tolerant species in the understory. Findings in the literature suggest that carbon reduction reactions may be more susceptible to drought than photosynthetic light reactions. If so, reduced carbon reduction capacity of shade tolerant species or shade phenotypes may predispose them to drought conditions, which suggests a mechanism behind the well-recognized tradeoff between drought tolerance and shade tolerance of temperate tree species.

The Nostoc‐Gunnera symbiosis: carbon fixation and translocation

The in vitro specific activity of ribulose‐1,5‐bisphosphate carboxylase (Rubisco; EC 4. 1. 1. 39) and the dark and light in vivo CO2 fixation activities were determined in the cyanobiont of Gunnera. Compared to the free‐living isolate Nostoc PCC 9231, the in vitro Rubisco activity was high, while the in vivo CO2 fixation was very low. Light did not significantly influence CO2 fixation if the cyanobiont was left in the sliced Gunnera tissues, while a small light stimulation was found for CO2 fixation of the freshly‐isolated cyanobiont. The adjacent non‐infected Gunnera tissue showed a very low CO2 fixation. A rapid translocation of fixed 14CO2 from leaves towards apical parts of the plant was apparent, in particular to the symbiotic tissue. The 14C label appeared mainly in soluble form in this tissue and was rapidly catabolised as shown by 14C chase experiments. Also, short‐term experiments revealed that maximum 14C accumulation occurred in the symbiotic tissue showing the highest rates of nitrogen fixation (Söderbäck et al. 1990), about 10–15 mm from the plant apex. The data were taken to indicate that there is a modification in the photosynthetic light reaction of the cyanobiont and that the cyanobiont lives heterotrophically in the dark on photo‐synthate rapidly delivered from nearby leaves of the host plant.

Acclimation of Photosynthetic Light Reactions during Induction of Inorganic Carbon Accumulation in the Green Alga Chlamydomonas reinhardtii

Cells of the unicellular green algae Chlamydomonas reinhardtii were grown in high dissolved inorganic carbon (DIC) concentrations (supplied with 50 milliliters per liter CO(2)[g]) and transferred to low DIC concentrations (supplied with </= 100 microliters per liter CO(2)[g]). Immediately after transfer from high to low DIC the emission of photosystem II related chlorophyll a fluorescence was substantially quenched. It is hypothesized that the suddenly induced inorganic carbon limitation of photosynthesis resulted in a phosphorylation of LHCII, leading to the subsequent state 1 to state 2 transition. After 2 hours of low-DIC acclimation, 77 K fluorescence measurements revealed an increase in the fluorescence emitted from photosystem I, due to direct excitation, suggesting a change in photosystem II/photosystem I stoichiometry or an increased light harvesting capacity of photosystem I. After 5 to 6 hours of acclimation a considerable increase in spillover from photosystem II to photosystem I was observed. These adjustments of the photosynthetic light reactions reached steady-state after about 12 hours of low DIC treatment. The quencher of fluorescence could be removed by 5 minutes of dark treatment followed by 5 minutes of weak light treatment, of any of four different light qualities. It is hypothesized that this restoration of fluorescence was due to a state 2 to state 1 transition in low-DIC acclimated cells. A decreased ratio of violaxanthin to zeaxanthin was also observed in 12 hour low DIC treated cells, compared with high DIC grown cells. This ratio was not coupled to the level of fluorescence quenching. The role of different processes during the induction of a DIC accumulating mechanism is discussed.

Photosynthese und Wasserhaushalt der immergrünen mediterranen Hartlaubptlanze Arbutus unedo L. im Jahresverlauf am Freilandstandort in Portugal: III. Einzelfaktorenanalyse zur Licht-, Temperatur- und CO2-Abhängigkeit der Nettophotosynthese

Summary Steady state field measurements of the photosynthetic light-, temperature- and humidity-response of intact Arbutus unedo leaves were carried out parallel to the 1982/83 diurnal course measurements of CO 2 and H 2 O-gas exchange described in parts I and II of the present series. The following physiologically meaningful parameters were calculated from these single factor response curves, and subsequently related to the annual course of microclimate conditions of the growing site and plant water status: – Maximum light utilization efficiency α (initial slope of the net photosynthesis (NP) response curve to incident photosynthetically active radiation (PAR), measured under saturating CO 2 partial pressure): the annual course of this parameter (which is well known from the literature to be a general stress indicator and also an indicator for the activity of the photosynthetic light reaction) showed two marked phases of depression which occurred (i) during the low temperature period in the winter an (ii) during summer drought. The annual maximum was reached in newly flushed leaves in the spring. – Maximum photosynthetic capacity P ML (net photosynthesis rate at saturating PAR and under saturating CO 2 partial pressure, which frequently has been interpreted as a measure of the RuBP-regeneration capacity of the C alvin cycle): High values were exhibited in the late autumn, the P ML -rates then declined during winter time, with the lowest winter values occuring at the end of February, when the lowest temperatures of the year were recorded (see part I of the present series). As in the case of α there was another decline during the period of summer drought, when at all temperatures the P ML -values were found to be markedly lower than in February. The temperature optimum of P ML remained at approx. 30°C during most of the entire year. During the dry period, however, the temperature optimum was shifted to approx. 25°C. This effect was not observed in watered plants growing at the same site. Throughout the year, P ML was further found to be negatively and non-linearily correlated with the water status of the leaves (expressed as predawn water potential of comparable leaves) taken from the same tree outside of the gas exchange cuvette. – Carboxylation efficiency CE (initial slope of the photosynthetic CO 2 -response plotted vs. leaf internal CO 2 -partial pressure, Pi) which has frequently been interpreted as a measure for the activity of the CO 2 -binding enzyme RuBP-carboxylase-oxygenase: CE showed similar annual changes to those of P ml , which was also reflected in a significant linear correlation of the parameters throughout the seasons. Throughout the entire year, the leaves were always operating on the linear part of the photosynthetic CO 2 -response curve and CE was the major parameter determining the net CO 2 -uptake rate under normal ambient conditions.

Light-Induced Polar pH Changes in Leaves of Elodea canadensis

Leaves of the submerged aquatic Elodea canadensis Michx. exhibit a light induced polar pH reaction. In this study, the effects of light intensity and dissolved inorganic carbon concentration on this polar reaction were examined. At a light intensity of 100 watts per square meter the leaf showed a polar pH response when the dissolved inorganic carbon concentration was less than about 1 millimolar. The polar reaction was suppressed at a higher dissolved inorganic carbon concentration. This suppression was not due to the buffering capacity of bicarbonate. Because another weak acid, acetate, did not inhibit the polarity, but even had a small stimulatory effect, the effect of bicarbonate is also not due to acidification of the cytoplasm. The suppression of the polar reaction by CO(2)/HCO(3) (-) was relieved when the light intensity was increased. Apparently there is competition for product(s) of the photosynthetic light reactions between processes generating the polar reaction and the carbon fixation reactions. The possibility that the redox state of the cell regulates the generation of the polar reaction is discussed.

Flexibility and Rigidity of Proteins and Protein–Pigment Complexes

AbstractProteins may be rigid or flexible to various degrees as required for optimal function. Flexibility of large parts of a protein, which rearrange or move, is particularly interesting and will be discussed in this article. We differentiate between several categories, although the boundaries between them are diffuse: flexibility of peptide segments, order–disorder transitions of spatially contiguous regions, and domain motions. The domains may be flexibly linked to allow rather unrestricted motions or the motions may be constrained to certain modes. The various categories of large‐scale flexibility will be illustrated with the following examples: (1) Small protein proteinase inhibitors are rather rigid molecules which provide binding surfaces complementary to their cognate proteases but show also limited segmental flexibility and adaptation. (2) Large plasma proteinase inhibitors exhibit large conformational changes after interaction with proteases probably for regulatory purposes. (3) Pancreatic serine proteases employ a disorder–order transition of their activation domain as a means to regulate enzymic activity. (4) Immunoglobulins show rather unrestricted and also hinged domain motions in different parts of the molecule probably to allow binding to antigens in different arrangements. (5) Citrate synthase adopts open and closed forms by a hinged domain motion to bind substrates and release products and to perform the catalytic condensation reaction, respectively. (6) Riboflavin synthase, a bifunctional multienzyme complex, catalyzes two consecutive reactions by means of two subunits, α and β. The β‐subunits form a shell, in which the α‐subunits are enclosed. Diffusional motion of the catalytic intermediates is therefore restricted. In addition, rearrangement of the N‐terminal segment occurs during the assembly of the β‐subunit. In contrast, rigidity is dominant in the structures of the light‐harvesting complexes and the photosynthetic reaction centers involved in photosynthetic light reactions. These are large protein–pigment complexes in which the proteins serve as matrices to hold the pigments in the appropriate conformation and relative arrangement. Since motion would contribute to deactivation of the photoexcited states of the pigments and diminish the efficiency of light‐energy and electron transfer, the functional role of rigidity is easy to rationalize for these proteins.