Showing papers on "Photosynthesis published in 1994"

Production and action of active oxygen species in photosynthetic tissues.

Abstract: The production of active oxygen species is inevitable in chloroplasts even under favorable environmental conditions. The photoreduction of oxygen has been observed even in washed thylakoids, depleted in ferredoxin. The overall stoichiometry of the thylakoid scavenging system shows no net change of oxygen and electron. Ground-state dioxygen occurs in the triplet state, and this is the reason why the reactivity of dioxygen with the singlet states of cellular components is very low, i.e., due to the difficulty arising from the addition of an electron to the antibonding orbital that is partially occupied. The photoreduction of oxygen has been observed even in washed thylakoids, depleted in ferredoxin. Therefore, membrane-bound reductants participate in the reduction of oxygen. Hydrogen peroxide is photooxidized, producing oxygen, in a reaction catalyzed by the photosystem II-water oxidase complex under flashed light, but not under continuous light.

Effects of UV-B radiation on photosynthesis and growth of terrestrial plants.

Abstract: The photosynthetic apparatus of some plant species appears to be well-protected from direct damage from UV-B radiation. Leaf optical properties of these species apparently minimizes exposure of sensitive targets to UV-B radiation. However, damage by UV-B radiation to Photosystem II and Rubisco has also been reported. Secondary effects of this damage may include reductions in photosynthetic capacity, RuBP regeneration and quantum yield. Furthermore, UV-B radiation may decrease the penetration of PAR, reduce photosynthetic and accessory pigments, impair stomatal function and alter canopy morphology, and thus indirectly retard photosynthetic carbon assimilation. Subsequently, UV-B radiation may limit productivity in many plant species. In addition to variability in sensitivity to UV-B radiation, the effects of UV-B radiation are further confounded by other environmental factors such as CO2, temperature, light and water or nutrient availability. Therefore, we need a better understanding of the mechanisms of tolerance to UV-B radiation and of the interaction between UV-B and other environmental factors in order to adequately assess the probable consequences of a change in solar radiation.

Photoinhibition of photosynthesis

Abstract: Photoinhibition of photosynthesis is a widespread phenomenon in oxygenic photosynthetic organisms that can result in large decreases in photosynthetic productivity. In the widest context photoinhibition of photosynthesis can be defined as the light-induced decrease in CO2 assimilation, which would include the effects of photo-oxidative radical damage to many components of the photosynthetic apparatus that can occur in environmentally stressed organisms at high irradiances. However, quite frequently photoinhibition of photosynthesis is used to refer specifically to light-induced damage to the PSII reaction centre, which precedes the onset of the more severe radical-induced damage to other components of the photosynthetic apparatus. In the past decade it has become established that an essential, intrinsic feature of the photosynthetic apparatus is the light-induced decrease in the quantum efficiency of photosynthesis as irradiance increases; when leaves are exposed to increasing photon flux densities an increasing proportion of the absorbed energy is lost as heat, thereby reducing the quantum efficiency of the photosynthetic processes. Although this light-induced decrease of photosynthetic efficiency is not associated with damage to components of the photosynthetic apparatus and is, in fact, a mechanism to protect from the damaging effects of high light, it does constitute a light-dependent depression of photosynthetic potential and warrants consideration in any treatise on photoinhibition of photosynthesis. In 1956 Kok defined photoinhibition as the light-dependent reduction in photosynthetic efficiency and this still perhaps provides the most useful working definition of photoinhibition of photosynthesis for leaves and whole organisms.

Does Rubisco control the rate of photosynthesis and plant growth? An exercise in molecular ecophysiology

Abstract: Experiments are described in which tobacco (Nicotiana tabacum L.) transformed with antisense rbcS to decrease expression of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) was used to evaluate the contribution of Rubisco to the control of photosynthetic rate, and the impact of a changed rate of photosynthesis on whole plant composition, allocation and growth. (1) The concept of flux control coefficients is introduced. It is discussed how, with adequate precautions, a set of wild-type and transgenic plants with varying expression of an enzyme can be used to obtain experimental values for its flux control coefficient. (2) The flux control coefficient of Rubisco for photosynthesis depends on the short-term conditions. It increases in high light, or low CO2. (3) When plants are grown under constant irradiance, the flux control coefficient in the growth conditions is low (<0.2) at irradiances of up to 1000μmol quanta m−2 s−1. In a natural irradiance regime exceeding 1500μmol quanta m−2 s−2 over several hours the flux coefficient rose to 0.8–0.9. It is concluded that plants are able to adjust the balance between Rubisco and the remainder of the photosynthetic machinery, and thereby avoid a one-sided limitation of photosynthesis by Rubisco over a wide range of ambient growth irradiance regimes. (4) When the plants were grown on limiting inorganic nitrogen, Rubisco had a higher flux control coefficient (0.5). It is proposed that, in many growth conditions, part of the investment in Rubisco may be viewed as a nitrogen store, albeit bringing additional marginal advantages with respect to photosynthetic rate and water use efficiency. (5) A change in the rate of photosynthesis did not automatically translate into a change in growth rate. Several factors are identified which contribute to this buffering of growth against a changed photosynthetic rate. (6) There is an alteration in whole plant allocation, resulting in an increase in the leaf area ratio. The increase is mainly due to a higher leaf water content, and not to changes in shoot/root allocation. This increased investment in whole plant leaf area partly counteracts the decreased efficiency of photosynthesis at the biochemical level. (7) Plants with decreased Rubisco have a lower intrinsic water use efficiency and contain high levels of inorganic cations and anions. It is proposed that these are a consequence of the increased rate of transpiration, and that the resulting osmotic potential might be a contributory factor to the increased water content and expansion of the leaves. (8) Starch accumulation in source leaves is decreased when unit leaf photosynthesis is reduced, allowing a more efficient use of the fixed carbon. (9) Decreased availability of carbohydrates leads to a down-regulation of nitrate assimilation, acting via a decrease in nitrate reductase activity.

The Relationship Between CO2 Transfer Conductance and Leaf Anatomy in Transgenic Tobacco With a Reduced Content of Rubisco

Abstract: The CO2 transfer conductance in leaves quantifies the ease with which CO2 can diffuse from sub-stomatal cavities to sites of carboxylation within the chloroplast. The aim of this work was to test the hypothesis that the CO2 transfer conductance is proportional to the surface area of chloroplasts exposed to intercellular airspaces. We compared two genotypes, wild-type and transgenic tobacco, that had been transformed with an antisense gene directed at the mRNA of the Rubisco small subunit. Transgenic tobacco had lower rates of CO2 assimilation than wild-type but similar chlorophyll contents. Leaf anatomy was altered by growing plants in two different environments: a high daily irradiance in a growth cabinet (12 h photoperiod of 1 mmol quanta m-2 s-1) and a sunlit glasshouse. The growth cabinet gave at least twice the daily irradiance compared to the glasshouse. The CO2 transfer conductance was calculated from combined measurements of gas exchange and carbon isotope discrimination measured in 2% oxygen. Following gas exchange measurement, leaves were sampled for biochemical and anatomical measure- ment. In transgenic tobacco plants, Rubisco content was 35% of that found in the wild-type tobacco, the CO2 assimilation rate was 50% of the wild-type rate and the chlorophyll content was unaltered. While leaf mass per unit leaf area of transgenic tobacco was 82% of that of the wild-type, differences in leaf thickness and surface area of mesophyll cells exposed to intercellular airspace per unit leaf area (Smes) were small (92 and 87% of wild-type, respectively). Leaves grown in the growth cabinet under high daily irradiance were thicker (63%), had a greater Smes (41%) due to the development of thicker palisade tissue, had higher photosynthetic capacity (27%) and contained more chlorophyll (58%) and Rubisco (77%), than leaves from plants grown in the glasshouse. Irrespective of genotype or growth environment, CO2 transfer conductance varied in proportion to surface area of chloroplasts exposed to intercellular airspaces. While the method for calculating CO2 transfer conductance could not distinguish between limitations due to the gas or liquid phases, there was no reduction in CO2 transfer conductance associated with more closely packed cells, thicker leaves, nor with increasing chloroplast thickness in tobacco.

The kinetics of ribulose-1,5-bisphosphate carboxylase/oxygenase in vivo inferred from measurements of photosynthesis in leaves of transgenic tobacco

Abstract: Transgenic tobacco (Nicotiana tabacum L. cv. W38) with an antisense gene directed against the mRNA of the ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) small subunit was used to determine the kinetic properties of Rubisco in vivo. The leaves of these plants contained only 34% as much Rubisco as those of the wild type, but other photosynthetic components were not significantly affected. Consequently, the rate of CO2 assimilation by the antisense plants was limited by Rubisco activity over a wide range of CO2 partial pressures. Unlike in the wild-type leaves, where the rate of regeneration of ribulose bisphosphate limited CO2 assimilation at intercellular partial pressures above 400 ubar, photosynthesis in the leaves of the antisense plants responded hyperbolically to CO2, allowing the kinetic parameters of Rubisco in vivo to be inferred. We calculated a maximal catalytic turnover rate, kcat, of 3.5+0.2 mol CO2·(mol sites)−1·s−1 at 25° C in vivo. By comparison, we measured a value of 2.9 mol CO2·(mol sites)−1·−1 in vitro with leaf extracts. To estimate the Michaelis-Menten constants for CO2 and O2, the rate of CO2 assimilation was measured at 25° C at different intercellular partial pressures of CO2 and O2. These measurements were combined with carbon-isotope analysis (13C/12C) of CO2 in the air passing over the leaf to estimate the conductance for transfer of CO2 from the substomatal cavities to the sites of carboxylation (0.3 mol·m−2·s−1·bar−1) and thus the partial pressure of CO2 at the sites of carboxylation. The calculated Michaelis-Menten constants for CO2 and O2 were 259 ±57 μbar (8.6±1.9μM) and 179 mbar (226 μM), respectively, and the effective Michaelis-Menten constant for CO2 in 200 mbar O2 was 549 μbar (18.3 μM). From measurements of the photocompensation point (Γ* = 38.6 ubar) we estimated Rubisco's relative specificity for CO2, as opposed to O2 to be 97.5 in vivo. These values were dependent on the size of the estimated CO2-transfer conductance.

Photosynthetic acclimation in trees to rising atmospheric CO2: A broader perspective.

Abstract: Analysis of leaf-level photosynthetic responses of 39 tree species grown in elevated concentrations of atmospheric CO2 indicated an average photosynthetic enhancement of 44% when measured at the growth [CO2]. When photosynthesis was measured at a common ambient [CO2], photosynthesis of plants grown at elevated [CO2] was reduced, on average, 21% relative to ambient-grown trees, but variability was high. The evidence linking photosynthetic acclimation in trees with changes at the biochemical level is examined, along with anatomical and morphological changes in trees that impact leaf- and canopy-level photosynthetic response to CO2 enrichment. Nutrient limitations and variations in sink strength appear to influence photosynthetic acclimation, but the evidence in trees for one predominant factor controlling acclimation is lacking. Regardless of the mechanisms that underlie photosynthetic acclimation, it is doubtful that this response will be complete. A new focus on adjustments to rising [CO2] at canopy, stand, and forest scales is needed to predict ecosystem response to a changing environment.

Diurnal Regulation of Photosynthetic Carbon Metabolism in C3 Plants

Abstract: ORGANIZA nON AND REGULA nON OF THE CAL YIN CYCLE 236 Organization of the Cycle 236 The Calvin Cycle-A Responsive Carbon Conveyor 237 Levels of Regulation ...... . ...... ... ... 239 Biochemical Conductance and Potential ........ ........ 240

Regulation of Light Harvesting in Green Plants (Indication by Nonphotochemical Quenching of Chlorophyll Fluorescence).

Abstract: ce has become one of the most powerful methods for assessing photosynthetic performance in plant physiological experiments (Horton and Bowyer, 1990; Krause and Weis, 1991). This has resulted almost entirely from the development of methods to distinguish photochemical and nonphotochemical quenching of fluorescence. Moreover, it is now clear that the process of nonphotochemical quenching itself indicates important regulatory adjustments in the photosynthetic membrane in response to altered external and internal conditions (Demmig-Adams and Adams, 1992; Horton and Ruban, 1992). In particular, the dissipation of excess absorbed excitation that is monitored by the main component of nonphotochemical quenching is a process that is necessary if plants are to avoid photoinhibition and photodestruction under conditions of light stress. When light is absorbed by the Chl molecules in the thylakoid membrane, the excited state has several alternative and competing fates: a small proportion is emitted as fluorescence, but, under light-limiting conditions, the major pathway of de-excitation is through photosynthetic electron transfer. The effect of photochemical utilization of energy is to quench fluorescence, and it is well known that when photosynthetic electron flow is saturated the yield of fluorescence rises. This photochemical quenching has been termed qP and, using the light-doubling principle as applied with modulated fluorimetry, it is easy to calculate it in leaves, chloroplasts, and cells (Schreiber et al., 1986; Horton and Bowyer, 1990; van Kooten and Snel, 1990). However, qP does not account for all of the quenching observed. Indeed, in light saturating for electron transport, qP tends to zero, yet there can be large amounts of quenching. Such quenching is therefore called nonphotochemical quenching and refers to the difference between the initial, dark-adapted maximum level of fluorescence and that recorded after a period of illumination. This quenching can be calculated in a number of ways, leading to it being termed variously as qN (Schreiber et al., 1986; van Kooten and Snel, 1990), NPQ (Bilger and Bjorkman, 1994), or SV, (Gilmore and Bjorkman, 1994); these all refer to the

Assimilatory Nitrogen Metabolism and Its Regulation

Abstract: The element nitrogen (N) constitutes about 5–10% of the dry weight of a cyanobacterial cell. The purpose of this chapter is to review the assimilatory pathways which in free-living cyanobacteria lead from different extracellular N-sources to cellular N-containing components. Inorganic nitrogen in the form of ammonium is incorporated into glutamine and glutamate via the glutamine synthetase/glutamate synthase cycle. The glnA gene, encoding glutamine synthetase, has been characterized in a number of cyanobacteria. Glutamate (and glutamine) distribute N to other organic compounds by means of transaminases, and glutamate is itself a precursor of some other nitrogenous metabolites. Ammonium can be taken up from the external medium by the cyanobacterial cell, but it can also be derived from other nutrients, essentially N2, nitrate and urea. Many cyanobacteria are able to fix N2 under aerobic conditions. Strategies for protecting nitrogenase from O2 in cyanobacteria include the temporal separation of nitrogenase activity and photosynthetic O2 evolution, and in some filamentous cyanobacteria, the differentiation of heterocysts (cells specialized in N2 fixation). A detailed characterization of nif genes has only been performed in a heterocyst-forming cyanobacterium. Nitrate reduction has been found to use photosynthetically reduced ferredoxin as an electron donor, and genes encoding nitrate transport and reduction proteins have been identified and shown to constitute an operon. Some amino acids like arginine and glutamine can also contribute N to some cyanobacteria; however, urea and amino acid utilization have been poorly investigated thus far. Pathways of N assimilation in cyanobacteria are induced upon ammonium deprivation, ammonium being the preferred N source. A gene, ntcA, encoding a transcriptional regulator required for expression of proteins subjected to nitrogen control has been identified. A major theme for future research is how information about the N status of the cell is sensed and transduced to the protein(s) effecting regulation of gene expression.

Light-emitting diodes as a light source for photosynthesis research

Abstract: Light-emitting diodes (LED) can provide large fluxes of red photons and so could be used to make lightweight, efficient lighting systems for photosynthetic research. We compared photosynthesis, stomatal conductance and isoprene emission (a sensitive indicator of ATP status) from leaves of kudzu (Pueraria lobata (Willd) Ohwi.) enclosed in a leaf chamber illuminated by LEDs versus by a xenon arc lamp. Stomatal conductance was measured to determine if red LED light could sufficiently open stomata. The LEDs produced an even field of red light (peak emission 656±5 nm) over the range of 0–1500 μmol m-2 s-1. Under ambient CO2 the photosynthetic response to red light deviated slightly from the response measured in white light and stomatal conductance followed a similar pattern. Isoprene emission also increased with light similar to photosynthesis in white light and red light. The response of photosynthesis to CO2 was similar under the LED and xenon arc lamps at equal photosynthetic irradiance of 1000 μmol m-2 s-1. There was no statistical difference between the white light and red light measurements in high CO2. Some leaves exhibited feedback inhibition of photosynthesis which was equally evident under irradiation of either lamp type. Photosynthesis research including electron transport, carbon metabolism and trace gas emission studies should benefit greatly from the increased reliability, repeatability and portability of a photosynthesis lamp based on light-emitting diodes.

Carotenoid composition and down regulation of photosystem II in three conifer species during the winter

Abstract: The carotenoid composition of the needles of three conifer species. Ponderosa pine (Pinus ponderosa). Douglas fir (Pseudotsuga menziesu) and blue spruce (Picea pun—gens). growing in full sunlight was found to differ between the summer and the winter, with higher levels of lutein and the carotenoids of the xanthophyll cycle and lower levels of α-carotene being present during the winter. In addition, the extent of the de-epoxidation of violaxunthin to antheraxanthin and zeaxanthin at midday was greater during the winter than during the summer. The latter two carotenoids were also found to be retained at high levels overnight on cold days during the winter. Associated with the retention of antheraxanlhin and zeaxanthin were sustained depressions of photosystem II (PSII) efficiency. These decreases in photo system II efficiency were accompanied by changes in chlorophyll fluorescence characteristics that are indicative of increased levels of energy dissipation in the chlorophyll pigment bed. Sustained depressions in PSII efficiency are commonly interpreted as “photoinhibitton”. We therefore suggest that low temperature-induced photo inhibition in these conifers during the winter was due to a down regulation of photosystem II that involved sustained xanthophyll cycle-associated energy dissipation. Furthermore, the predawn conversion state of the xanthophyll cycle responded rapidly to day to day variation in temperature. Being less epoxidized on colder days and more epoxidized on both previous and subsequent warmer days. Such flexibility in the response of the santhophyll cycle to changes in temperature may be important in the regulation and protection of the photosynthetic apparatus of such evergreen plants in a climate that experiences relatively rapid changes in temperature.

Acclimation of photosynthetic proteins to rising atmospheric CO2.

Abstract: In this review we discuss how the photosynthetic apparatus, particularly Rubisco, acclimates to rising atmospheric CO2 concentrations (ca). Elevated ca alters the control exerted by different enzymes of the Calvin cycle on the overall rate of photosynthetic CO2 assimilation, so altering the requirement for different functional proteins. A decreased flux of carbon through the photorespiratory pathway will decrease requirements for these enzymes. From modeling of the response of CO2 uptake (A) to intracellular CO2 concentration (ci) it is shown that the requirement for Rubisco is decreased at elevated ca, whilst that for proteins limiting ribulose 1,5 bisphosphate regeneration may be increased. This balance may be altered by other interactions, in particular plasticity of sinks for photoassimilate and nitrogen supply; hypotheses on these interactions are presented. It is speculated that increased accumulation of carbohydrate in leaves developed at elevated ca may signal the ‘down regulation’ of Rubisco. The molecular basis of this ‘down regulation’ is discussed in terms of the repression of photosynthetic gene expression by the elevated carbohydrate concentrations. This molecular model is then used to predict patterns of acclimation of perennials to long term growth in elevated ca.

Specific reduction of chloroplast carbonic anhydrase activity by antisense RNA in transgenic tobacco plants has a minor effect on photosynthetic CO2 assimilation

Abstract: As an approach to understanding the physiological role of chloroplast carbonic anhydrase (CA), this study reports on the production and preliminary physiological characterisation of transgenic tobacco (Nicotiana tabacum L.) plants where chloroplast CA levels have been specifically suppressed with an antisense construct directed against chloroplast CA mRNA. Primary transformants with CA levels as low as 2% of wild-type levels were recovered, together with intermediate plants with CA activities of about 20–50% of wild-type levels. Plants with even the lowest CA levels were not morphologically distinct from the wild-type plants. Segregation analysis of the low-CA character in plants grown from T1 selfed seed indicated that at least one of the low-CA plants appears to have two active inserts and that at least two of the intermediate-CA plants have one active insert. Analysis of CO2 gas exchange of a group of low-CA plants with around 2% levels of CA indicated that this large reduction in chloroplastic CA did not appear to cause a measurable alteration in net CO2 fixation at 350 μbar CO2 and an irradiance of 1000 μmol quanta·m−2·s−1. In addition, no significant differences in Rubisco activity, chlorophyll content, dry weight per unit leaf area, stomatal conductance or the ratio of intercellular to ambient CO2 partial pressure could be detected. However, the carbon isotope compositions of leaf dry matter were significantly lower (0.85%o) for low-CA plants than for wildtype plants. This corresponds to a 15-μbar reduction in the CO2 partial pressure at the sites of carboxylation. The difference, which was confirmed by concurrent measurement of discrimination with gas exchange, would reduce the CO2 assimilation rate by 4.4%, a difference that could not be readily determined by gas-exchange techniques given the inherent variability found in tobacco. A 98% reduction in CA activity dramatically reduced the 18O discrimination in CO2 passing over the leaf, consistent with a marked reduction in the ratio of hydrations to carboxylations. We conclude that a reduction in chloroplastic CA activity of two orders of magnitude does not produce a major limitation on photosynthesis at atmospheric CO2 levels, but that normal activities of the enzyme appear to play a role in facilitated transfer of CO2 within the chloroplast, producing a marginal improvement in the efficiency of photosynthesis in C3 plants.

Effects of cobalt on plants

Abstract: Cobalt, a transition element, is an essential component of several enzymes and co-enzymes. It has been shown to affect growth and metabolism of plants, in different degrees, depending on the concentration and status of cobalt in rhizosphere and soil. Cobalt interacts with other elements to form complexes. The cytotoxic and phytotoxic activities of cobalt and its compounds depend on the physico-chemical properties of these complexes, including their electronic structure, ion parameters (charge-size relations) and coordination. Thus, the competitive absorption and mutual activation of associated metals influence the action of cobalt on various phytochemical reactions. The distribution of cobalt in plants is entirely species-dependent. The uptake is controlled by different mechanisms in different species. Biosorption involves ion-exchange mechanism in algae, but in fungi both metabolism-independent and -dependent processes are operative. Physical conditions like salinity, temperature, pH of the medium, and presence of other metals influence the process of uptake and accumulation in algae, fungi, and mosses. Toxic concentrations inhibit active ion transport. In higher plants, absorption of Co2+ by roots involves active transport. Transport through the cortical cells is operated by both passive diffusion and active process. In the xylem, the metal is mainly transported by the transpirational flow. Distribution through the sieve tubes is acropetal by complexing with organic compounds. The lower mobility of Co2+ in plants restricts its transport to leaves from stems. Cobalt is not found at the active site of any respiratory chain enzymes. Two sites of action of Co2+ are found in mitochondrial respiration since it induces different responses toward different substrates like α-keto glutarate and succinate. In lower organisms, Co2+ inhibits tetraphyrrole biosynthesis, but in higher plants it probably participates in chlorophyll b formation. Exogenously added metal causes morphological damage in plastids and changes in the chlorophyll contents. It also inhibits starch grain differentiation and alters the structure and number of chloroplasts per unit area of leaf. The role of cobalt in photosynthesis is controversial. Its toxic effect takes place by inhibition of PS2 activity and hence Hill reaction. It inhibits either the reaction centre or component of PS2 acceptor by modifying secondary quinone electron acceptor Qb site. Co2+ reduces the export of photoassimilates and dark fixation of CO2. In C4 and CAM plants, it hinders fixation of CO2 by inhibiting the activity of enzymes involved. Cobalt acts as a preprophase poison and thus retards the process of karyokinesis and cytokinesis. The action of cobalt on plant cells is mainly turbagenic. Cobalt compounds act on the mitotic spindle, leading to the formation of chromatin bridges, fragmentation, and sticky bridges at anaphase and binucleate cells. High concentrations of cobalt hamper RNA synthesis, and decrease the amounts of the DNA and RNA probably by modifying the activity of a large number of endo- and exonucleases. The mutagenic action of cobalt salts results in mitochondrial respiratory deficiency in yeasts. In cytokinesis-deficient mutant of Chlamydomonas it increases the amount of sulfhydryl compounds. Cobalt has been shown to alter the sex of plants like Cannabis sativa, Lemna acquinoclatis, and melon cultivars. It decreases the photoreversible absorbance of phytochrome in pea epicotyl and interferes with heme biosynthesis in fungi. Low concentration of Co2+ in medium stimulates growth from simple algae to complex higher plants. Relatively higher concentrations are toxic. A similar relationship is seen with crop yield when the metal is used in the form of fertilizer, pre-seeding, and pre-sowing chemicals. Toxic effect of cobalt on morphology include leaf fall, inhibition of greening, discolored veins, premature leaf closure, and reduced shoot weight. Being a component of vitamin B12 and cobamide coenzyme, Co2+ helps in the fixation of molecular nitrogen in root nodules of leguminous plants. But in cyanobacteria, CoCl2 inhibits the formation of heterocyst, ammonia uptake, and nitrate reductase activity. The interaction of cobalt with other metals mainly depends on the concentration of the metals used. For example, high levels of Co2+ induce iron deficiency in plants and suppress uptake of Cd by roots. It also interacts synergistically with Zn, Cr, and Sn. Ni overcomes the inhibitory effect of cobalt on protonemal growth of moss, thus indicating an antagonistic relationship. The beneficial effects of cobalt include retardation of senescence of leaf, increase in drought resistance in seeds, regulation of alkaloid accumulation in medicinal plants, and inhibition of ethylene biosynthesis. In lower plants, cobalt tolerance involves a cotolerance mechanism. The mechanism of resistance to toxic concentration of cobalt may be due to intracellular detoxification rather than defective transport. In higher plants, only a few advanced copper-tolerant families showed cotolerance to Co2+. Tolerance toward Co2+ may sometimes determine the taxonomic shifting of several members of Nyssaceae. Due to the high cobalt content in serpentine soil, essential element uptake by plants is reduced, a phenomenon known as “serpentine problem,” for New Caledonian families like Flacourtiaceae. Large amounts of calcium in soil may compensate for the toxic effects of heavy metals in adaptable genera grown in this type of soil. The biomagnification of potentially toxic elements, such as cobalt from coal ash or water into food webs, needs additional study for effective biological filtering.

Responses of ribulose-1, 5-bisphosphate carboxylase, cytochrome f, and sucrose synthesis enzymes in rice leaves to leaf nitrogen and their relationships to photosynthesis

Abstract: The photosynthetic gas-exchange rates and various biochemical components of photosynthesis, including ribulose-1,5-bisphosphate carboxylase (Rubisco) content, cytochrome (Cyt) f content, and the activities of two sucrose synthesis enzymes, were examined in young, fully expanded leaves of rice (Oryza sativa L.) grown hydroponically in different nitrogen concentrations. The light-saturated rate of photosynthesis at an intercellular CO2 pressure of 20 Pa (CO2-limited photosynthesis) was linearly dependent on leaf nitrogen content, but curvilinearly correlated with Rubisco content. This difference was due to a greater than proportional increase in Rubisco content relative to leaf nitrogen content and the presence of a CO2 transfer resistance between the intercellular air spaces and the carboxylation sites. CO2-limited photosynthesis was proportional to Cyt f content, one of the key components of electron transport, but was not proportional to the activities of cytosolic fructose-1,6-bisphosphatase and sucrose phosphate synthase, the two regulatory enzymes of sucrose synthesis. Light-saturated photosynthesis above an intercellular CO2 pressure of 60 Pa (CO2-saturated photosynthesis) was curvilinearly dependent on leaf nitrogen content. This CO2-saturated photosynthesis was proportional to Cyt f content in the low- and normal-nitrogen leaves, and correlated better with the activities of cytosolic fructose-1,6-bisphosphatase and sucrose phosphate synthase in the high-nitrogen leaves. The increase in the activities of these two enzymes with increasing leaf nitrogen was not as great as the increase in Cyt f content. Thus, as leaf nitrogen increased, the limitation caused by the activities of sucrose synthesis enzymes came into play, which resulted in the curvilinear relationship. However, this limitation by sucrose synthesis enzymes did not affect photosynthesis under normal ambient air.

A comparison of leaf physiology and anatomy of Quercus (section Erythrobalanus-Fagaceae) species in different light environments

Abstract: Physiological and anatomical attribut-es of leaves were examined of three species of Quercus section Erythrobalanus All three species occur in moist temperate deciduous forests of eastern North America Seedlings of each species were grown in different light conditions for comparison The attributes measured were net photosynthesis, stomatal conductivity, blade and cuticle thickness, stomatal density, thickness of upper and lower epidermis, and thickness of palisade mesophyll The results generally demonstrate the close association between anatomical adaptations and efficiency of physiological processes; they also elucidate the distribution patterns of the three Quercus species across the forest topography in southern New England The most drought-tolerant and light-demanding species, Q velutina (Lam), exhibited the greatest leaf anatomical plasticity, the highest net photosynthesis in the different light conditions, and the lowest stomatal area per unit area of leaf The most drought-intolerant species, Q rubra (L), exhibited the least leaf anatomical plasticity, the lowest net photosynthesis in the different light conditions, and the highest stomatal area per unit area of leaf Quercus coccinea (Muenchh) usually exhibited values that were intermediate between Q rubra and Q velutina

Evidence for a common origin of chloroplasts with light-harvesting complexes of different pigmentation

Abstract: THE red algae (Rhodophyta), which like cyanobacteria have only chlorophyll a and use phycobilisomes for light-harvesting1,2, are often considered to have originated independently of other photosynthetic eukaryotes, namely the chlorophyll a/b-containing Chlorophyta and the chlorophyll a/c-containing Chromophyta3. Here we report that the red alga Porphyridium cruentum has a chlorophyll a-containing antenna complex functionally associated with photosystem I, and that polypeptides of this antenna complex are immunologically related to those of higher-plant chlorophyll a/b complexes and to those of chromophyte fucoxanthin–chlorophyll a/c antenna complexes. This establishes a clear link between organisms containing phycobilisomes and those containing chlorophyll-based light-harvesting complexes and shows that these antennae can coexist in the same organism. Furthermore, it suggests that the light-harvesting proteins of all photosynthetic eukaryotes had a common origin and supports the idea that chloroplasts had a common ancestor4–6.

Turbulence in mass algal cultures and the role of light/dark fluctuations

Abstract: In mass algal cultures, some form of agitation is usually provided; among other effects, this moves the organisms though an optically dense profile and provides mixing. During this transport, medium frequency fluctuations in the light energy supply are perceived by the algae, which are of the order of 1 Hz and less. It has been suggested that turbulence with the resultant light/dark cycles of medium frequency enhances productivity. However, turbulence has two major influences in a well mixed system: it facilitates fluctuating light regimes and increases the transfer rates between the growth medium and the cultured organism. An estimation of productivity as oxygen liberation was measured under laminar and turbulent flow rates, and varying light/dark ratios. Increased turbulence, which increased exchange rates of nutrients and metabolites between the cells and their growth medium, together with increased light/dark frequencies, increased productivity and photosynthetic efficiency.

Measurements of mesophyll conductance, photosynthetic electron transport and alternative electron sinks of field grown wheat leaves.

Abstract: Photosynthetic electron transport drives the carbon reduction cycle, the carbon oxidation cycle, and any alternative electron sinks such as nitrogen reduction. A chlorophyll fluorescence— based method allows estimation of the total electron transport rate while a gas-exchange-based method can provide estimates of the electron transport needed for the carbon reduction cycle and, if the CO2 partial pressure inside the chloroplast is accurately known, for the carbon oxidation cycle. The gas-exchange method cannot provide estimates of alternative electron sinks. Photosynthetic electron transport in flag leaves of wheat was estimated by the fluorescence method and gasexchange method to determine the possible magnitude of alternative electron sinks. Under non-photorespiratory conditions the two measures of electron transport were the same, ruling out substantial alternative electron sinks. Under photorespiratory conditions the fluorescence-based electron transport rate could be accounted for by the carbon reduction and carbon oxidation cycle only if we assumed the CO2 partial pressure inside the chloroplasts to be lower than that in the intercellular spaces of the leaves. To further test for the presence of alternative electron sinks, carbon metabolism was inhibited by feeding glyceraldehyde. As carbon metabolism was inhibited, the electron transport was inhibited to the same degree. A small residual rate of electron transport was measured when carbon metabolism was completely inhibited which we take to be the maximum capacity of alternative electron sinks. Since the alternative sinks were small enough to ignore, the comparison of fluorescence and gas-exchange based methods for measuring the rate of electron transport could be used to estimate the mesophyll conductance to CO2 diffusion. The mesophyll conductance estimated this way fell as wheat flag leaves senesced. The age-related decline in photosynthesis may be attributed in part to the reduction of mesophyll conductance to CO2 diffusion and in part to the estimated decline of ribulose 1,5-bisphosphate carboxylase amount.

Measurement of CO2 and HCO3− fluxes in cyanobacteria and microalgae during steady‐state photosynthesis

Abstract: A mass spectrometric procedure is described which allows the simultaneous estimation of both CO2 and HCO−3 fluxes associated with cyanobacteria and green algae during steady-stale photosynthesis. This technique utilizes the chemical disequilibrium which exists between CO2 and HCO−3 during photosynthesis in cell suspensions which lack carbonic anhydrase activity. The kinetic equations which are derived for flux determinations are based on models of photosynthesis in both cyanobacteria and green algae which seem most reasonable given our present level of understanding, together with direct measurement of [CO2] estimation of [HCO−3] and application of the kinetic rate constants for the interconversion of CO2 and HCO−3 From measurements made in the light, net uptake of both CO2 and HCO−3 can be readily determined. In addition, analysis of the dark phase immediately following light-off provides the possibility of also determining the CO2 evolution which is occurring during photosynthesis, and thus also the gross CO2 uptake rates in the light. Results are presented for the response of dissolved inorganic carbon (C1) flux rates to external C, in low-C1 grown cells of both Synechococcus PCC7942 and Chlamydomonas reinhardtii and these are consistent with previous studies showing that such cells possess capacities to utilize both CO2 and HCO−3 for photosynthesis. The advantages and potential errors which are inherent in this technique are discussed together with its potential for future studies on C1 transport under various experimental conditions.

Nitrogen Source Regulation of Growth and Photosynthesis in Beta vulgaris L.

Abstract: Sugar beets (Beta vulgaris L cv F58-554H1) were grown hydroponically in a 16-h light, 8-h dark period at a photosynthetic photon flux density of 05 mmol m-2 s-1 for 4 weeks in half-Hoagland culture solution containing only nitrate-nitrogen Half of the plants were then transferred to half-Hoagland solution with ammonium-nitrogen (735mM), while the other half continued on 75 mM nitrate Growth analysis was carried out by sampling the plants at 3-d intervals over a period of 21 d Compared to plants supplied with nitrate, ammonium initially slowed the growth of shoots more than roots Ammonium reduced both the area expansion of individual leaves and the relative water content of these leaves, but increased the amount of dry matter/area The increase in specific leaf weight in ammonium-grown leaves was associated with a doubling of chloroplast volume, as much as a 62% rise in chlorophyll content, and a 43-fold higher accumulation of soluble protein Ammonium nutrition substantially decreased the rate of expansion of photosynthetic (leaf) surface but did not decrease the rate of photosynthesis per area; in fact, net photosynthetic CO2 exchange rates were slightly higher than in nitrate plants, due to the build-up in stromal enzymes of the Calvin cycle, several of which increased in total extractable activity on a leaf area basis, eg ribulose-1,5-biphosphate carboxylase oxygenase, sedoheptulose-1,7-biphosphatase Nitrogen source had no effect on stomatal conductance Rates of photosynthesis per chlorophyll were decreased slightly in ammonium-grown leaves, possibly due to an increased CO2-diffusion resistance associated with the enlarged chloroplasts

Acclimation of Arabidopsis thaliana to the light environment: changes in composition of the photosynthetic apparatus

Abstract: Acclimation to changes in the light environment was investigated in Arabidopsis thaliana (L.) Heynh. cv. Landsberg erecta. Plants grown under four light regimes showed differences in their development, morphology, photosynthetic performance and in the composition of the photosynthetic apparatus. Plants grown under high light showed higher maximum rates of oxygen evolution and lower levels of light-harvesting complexes than their low light-grown counterparts; plants transferred to low light showed rapid changes in maximum photosynthetic rate and chlorophyll-a/b ratio as they became acclimated to the new environment. In contrast, plants grown under lights of differing spectral quality showed significant differences in the ratio of photosystem II to photosystem I. These changes are consistent with a model in which photosynthetic metabolism provides signals which regulate the composition of the thylakoid membrane.

Regulation of the expression of photosynthetic nuclear genes by CO2 is mimicked by regulation by carbohydrates: a mechanism for the acclimation of photosynthesis to high CO2?

Abstract: The abundance of transcripts of cab-7 and cab-3C, which code for the chlorophyll a/b binding proteins of the light-harvesting complexes I and II, respectively, and the abundance of transcripts of Rca, which encodes Rubisco activase, were reduced in tomato plants exposed to high CO2 for up to 9d, whereas the abundance of mRNA from psa A–psa B and psb A, which encode the proteins of the core complex of PSI and the D1 protein of PSII, respectively, and the abundance of glycolate oxidase, which is involved in photorespiration, were not affected. However, the abundance of the transcript for the B subunit of ADP-glucose pyrophosphorylase was increased after 1 d at elevated CO2. The chlorophyll a/b ratio decreased significantly over 9 d of exposure to elevated CO2. The responses of the nuclear genes to high CO2 were enhanced when leaves were detached so as to deprive them of any major sink. The responses of these transcripts to high CO2 were mimicked when sucrose or glucose was supplied to the leaf tissue, whereas acetate or sorbitol had no effect. Carbohydrate analyses of leaves grown in high CO2 or supplied with sucrose revealed that major increases occurred in the amount of glucose and fructose. Based on these and other published data, a molecular model involving the repression or activation of the transcription of nuclear genes coding for chloroplast proteins by photosynthetic end-products is proposed to account for photosynthetic acclimation to high CO2 in tomato plants and other species.

Invertase: understanding changes in the photosynthetic and carbohydrate metabolism of barley leaves infected with powdery mildew

Abstract: summary Infection of barley leaves with powdery mildew results in an increase in the activity of acid invertase, concomitant with an accumulation of glucose, fructose and sucrose in the infected leaf; this increase is confined to the mesophyll cells. The rate of photosynthesis is controlled by different factors depending upon the experimental conditions under which it is measured. In saturating light and ambient CO2, photosynthesis is determined to a large extent by Rubisco whereas, in saturating light and saturating CO2. it is mainly determined by the rate of end-product synthesis (Pi-limitation). The rate of photosynthesis was measured under these conditions to reveal which of the partial processes was most affected in mildewed leaves. Under conditions of saturating light and ambient CO2, the rate of photosynthesis declined in mildewed leaves from 3 d after inoculation, suggesting that carboxylation had been affected. However, the maximum capacity for photosynthesis, measured at saturating CO2 and irradiance, increased in mildewed leaves for the first 3 d after infection and then decreased to below control values on days 5 and 7, suggesting that Pi was not limiting photosynthesis. This hypothesis was investigated by measuring changes in photosynthetic intermediates and in the activity and amount of key enzymes of the Calvin cycle as infection progressed. There was a decline in the activity of the stromal fru-1, 6-bisPase, Rubisco and NADP-GAPDH in mildewed leaves. These decreases were consistent with changes in the ratio of metabolites. As infection progressed, there was an increase in the ratio of Rul,5P2: PGA and triose-P: Rul,5P2, the first indicating a restriction in the carboxylation of CO2 and the second a restriction in the regeneration of Rul,5P2, The PGA: triose-P ratio was similar in control and mildewed leaves until day 7 when it decreased, suggesting that the reduction of PGA to triose-P was not affected by the disease. There was little evidence of Pi-limitation in the mildewed leaves; the amount of Pi in infected leaves was similar to controls, infected leaves showed no secondary oscillations during a transition from dark to light and there was a reduction in the amount of PGA in infected leaves. We suggest that the high concentration of carbohydrates, resulting from the increase in invertase activity, causes directly or indirectly a gradual down-regulation of the Calvin cycle leading to aft inhibition of photosynthesis.

Photoinhibition of photosynthesis in chilled potato leaves is not correlated with a loss of Photosystem-II activity : Preferential inactivation of Photosystem I.

Abstract: When 23 °C-grown potato leaves (Solanum tuberosum L.) were irradiated at 23 °C with a strong white light, photosynthetic electron transport and Photosystem-II (PS II) activity were inhibited in parallel. When the light treatment was given at a low temperature of 3 °C, the photoinhibition of photosynthesis was considerably enhanced, as expected. Surprisingly, no such stimulation of photoinhibition was observed with respect to the PS II function. A detailed functional analysis of the photosynthetic apparatus, using in-vivo fluorescence, absorbance, oxygen and photoacoustic measurements, and artificial electron donors/acceptors, showed a pronounced alteration of PS I activity during light stress at low temperature. More precisely, it was observed that both the pool of photooxidizeable reaction center pigment (P700) of PS I and the efficiency of PS I to oxidize P700 were dramatically reduced. Loss of P700 activity was shown to be essentially dependent on atmospheric O2 and to require a continued flow of electrons from PS II, suggesting the involvement of the superoxide anion radical which is produced by the interaction of O2 and the photosynthetic electron-transfer chain through the Mehler reaction. Mass spectrometric measurements of O2 exchange by potato leaves under strong illumination did not reveal, however, any stimulation of the Mehler reaction at low temperature, thus leading to the conclusion that O2 toxicity mainly resulted from a chilling-induced inhibition of the scavenging system for O2-radicals. Support for this interpretation was provided by the light response of potato leaves infiltrated with an inhibitor (diethyldithiocarbamate) of the chloroplastic Cu-Zn superoxide dismutase. It was indeed possible to simulate the differential inhibition of the PS II photochemical activity and the linear electron transport observed during light stress at low temperature by illuminating at 23 °C diethyldithiocarbamate-poisoned leaves. The experimental data presented here suggests that (i) the previously reported resistance of PS I to photoinhibition damage in-vivo is not an intrinsic property of PS I but results from efficient protective systems against O2 toxicity, (ii) PS I is photoinhibited in chilled potato leaf due to the inactivation of this PS I defence system and (iii) PS I is more sensitive to superoxide anion radicals than PS II.

The unsaturation of membrane lipids stabilizes photosynthesis against heat stress

Abstract: The effect of the unsaturation of glycerolipids of thylakoid membranes on the heat tolerance of the photosynthetic evolution of oxygen was studied in vivo by mutation and transformation of fatty-acid desaturases in the cyanobacterium Synechocystis PCC6803. The experimental results indicate that elimination of dienoic lipid molecules decreases, to a small but distinct extent, the heat tolerance of photosynthetic oxygen evolution, but that elimination of trienoic lipid molecules has no effect on the heat tolerance. This conclusion contrasts with the previous hypothesis that the heat tolerance of photosynthesis is enhanced upon an increase in the level of saturation of membrane lipids. It is also shown that light does not affect the nature of the effect of lipid unsaturation on the heat tolerance of photosynthesis.