Showing papers on "Photosynthesis published in 1997"

More Efficient Plants : a consequence of rising atmospheric CO2

Abstract: ▪ Abstract The primary effect of the response of plants to rising atmospheric CO2 (Ca) is to increase resource use efficiency. Elevated Ca reduces stomatal conductance and transpiration and improves water use efficiency, and at the same time it stimulates higher rates of photosynthesis and increases light-use efficiency. Acclimation of photosynthesis during long-term exposure to elevated Ca reduces key enzymes of the photosynthetic carbon reduction cycle, and this increases nutrient use efficiency. Improved soil–water balance, increased carbon uptake in the shade, greater carbon to nitrogen ratio, and reduced nutrient quality for insect and animal grazers are all possibilities that have been observed in field studies of the effects of elevated Ca. These effects have major consequences for agriculture and native ecosystems in a world of rising atmospheric Ca and climate change.

A model separating leaf structural and physiological effects on carbon gain along light gradients for the shade‐tolerant species Acer saccharum

Abstract: A process-based leaf gas exchange model for C3 plants was developed which specifically describes the effects observed along light gradients of shifting nitrogen investment in carboxylation and bioenergetics and modified leaf thickness due to altered stacking of photosynthetic units. The model was parametrized for the late-successional, shadetolerant deciduous species Acer saccharum Marsh. The specific activity of ribulose-1,5-bisphosphate carboxylase (Rubisco) and the maximum photosynthetic electron transport rate per unit cytochrome f (cyt f) were used as indices that vary proportionally with nitrogen investment in the capacities for carboxylation and electron transport. Rubisco and cyt f per unit leaf area are related in the model to leaf dry mass per area (MA), leaf nitrogen content per unit leaf dry mass (Nm), and partitioning coefficients for leaf nitrogen in Rubisco (PR) and in bioenergetics (PB). These partitioning coefficients are estimated from characteristic response curves of photosynthesis along with information on leaf structure and composition. While PR and PB determine the light-saturated value of photosynthesis, the fraction of leaf nitrogen in thylakoid light-harvesting components (PL) and the ratio of leaf chlorophyll to leaf nitrogen invested in light harvesting ( CB), which is dependent on thylakoid stoichiometry, determine the initial photosynthetic light utilization efficiency in the model. Carbon loss due to mitochondrial respiration, which also changes along light gradients, was considered to vary in proportion with carboxylation capacity. Key model parameters ‐ Nm, PR, PB, PL, CB and stomatal sensitivity with respect to changes in net photosynthesis ( Gf) ‐ were examined as a function of MA, which is linearly related to irradiance during growth of the leaves. The results of the analysis applied to A. saccharum indicate that PB and PR increase, and Gf, PL and CB decrease with increasing MA. As a result of these effects of irradiance on nitrogen partitioning, the slope of the light-saturated net photosynthesis rate per unit leaf dry mass (A m max ) versus Nm relationship

The roles of specific xanthophylls in photoprotection

Abstract: Xanthophyll pigments have critical structural and functional roles in the photosynthetic light-harvesting complexes of algae and vascular plants. Genetic dissection of xanthophyll metabolism in the green alga Chlamydomonas reinhardtii revealed functions for specific xanthophylls in the nonradiative dissipation of excess absorbed light energy, measured as nonphotochemical quenching of chlorophyll fluorescence. Mutants with a defect in either the α- or β-branch of carotenoid biosynthesis exhibited less nonphotochemical quenching but were still able to tolerate high light. In contrast, a double mutant that was defective in the synthesis of lutein, loroxanthin (α-carotene branch), zeaxanthin, and antheraxanthin (β-carotene branch) had almost no nonphotochemical quenching and was extremely sensitive to high light. These results strongly suggest that in addition to the xanthophyll cycle pigments (zeaxanthin and antheraxanthin), α-carotene-derived xanthophylls such as lutein, which are structural components of the subunits of the light-harvesting complexes, contribute to the dissipation of excess absorbed light energy and the protection of plants from photo-oxidative damage.

A metalloradical mechanism for the generation of oxygen from water in photosynthesis

Abstract: In plants and algae, photosystem II uses light energy to oxidize water to oxygen at a metalloradical site that comprises a tetranuclear manganese cluster and a tyrosyl radical. A model is proposed whereby the tyrosyl radical functions by abstracting hydrogen atoms from substrate water bound as terminal ligands to two of the four manganese ions. Molecular oxygen is produced in the final step in which hydrogen atom transfer and oxygen-oxygen bond formation occur together in a concerted reaction. This mechanism establishes clear analogies between photosynthetic water oxidation and amino acid radical function in other enzymatic reactions.

N2 Fixation by non-heterocystous cyanobacteria

Abstract: Many, though not all, non-heterocystous cyanobacteria can fix N2. However, very few strains can fix N2 aerobically. Nevertheless, these organisms may make a substantial contribution to the global nitrogen cycle. In this general review, N2 fixation by laboratory cultures and natural populations of non-heterocystous cyanobacteria is considered. The properties and subcellular location of nitrogenase in these organisms is described, as is the response of N2 fixation to environmental factors such as fixed nitrogen, O2 and the pattern of illumination. The integration of N2 fixation with other aspects of cell metabolism (in particular photosynthesis) is also discussed. Similarities and differences between different individual strains of non-heterocystous cyanobacteria are highlighted.

Fermentation in cyanobacteria

Abstract: Although cyanobacteria are oxygenic phototrophic organisms, they often thrive in environments that become periodically anoxic. This is particularly the case in the dark when photosynthetic oxygen evolution does not take place. Whereas cyanobacteria generally utilize endogenous storage carbohydrate by aerobic respiration, they must use alternative ways for energy generation under dark anoxic conditions. This aspect of metabolism of cyanobacteria has received little attention but nevertheless in recent years a steadily increasing number of publications have reported the capacity of fermentation in cyanobacteria. This review summarizes these reports and gives a critical consideration of the energetics of dark fermentation in a number of species. There are a variety of different fermentation pathways in cyanobacteria. These include homo- and heterolactic acid fermentation, mixed acid fermentation and homoacetate fermentation. Products of fermentation include CO2, H2, formate, acetate, lactate and ethanol. In all species investigated, fermentation is constitutive. All enzymes of the fermentative pathways are present in photoautotrophically grown cells. Many cyanobacteria are also capable of using elemental sulfur as electron acceptor. In most cases it seems unlikely that sulfur respiration occurs. The main advantage of sulfur reduction seems to be the higher yield of ATP which can be achieved during fermentation. Besides oxygen and elemental sulfur no other electron acceptors for chemotrophic metabolism are known so far in cyanobacteria. Calculations show that the yield of ATP during fermentation, although it is low relative to aerobic respiration, exceeds the amount that is likely to be required for maintenance, which appears to be very low in these cyanobacteria. The possibility of a limited amount of biosynthesis during anaerobic dark metabolism is discussed.

Leaf Form and Photosynthesis

Abstract: M orphological and anatomical features of plant leaves are commonly associated with metabolic type (e.g., Kranz anatomy of C4 species), amount of sun exposure (e.g., sun and shade leaves), or water stress (e.g., xeromorphism). However, although the primary function of the leaf is to absorb and process sunlight and carbon dioxide for photosynthesis, few structural features of leaves have been related mechanistically to these tasks. For example, it has been known for over a century that the internal anatomy of leaves is characterized by different cell layers (e.g., the palisade and spongy mesophyll) and that stomatal pores can be located on one or both sides of a leaf. Yet, only recently has any functional relationship between leaf form and photosynthetic performance been suggested. A variety of ecological studies have correlated numerous leaf structural parameters with photosynthetic performance (e.g., Abrams and Kubiske 1990, 1994, Hinckley et al. 1989,

Sugar repression of photosynthesis: the role of carbohydrates in signalling nitrogen deficiency through source:sink imbalance

Abstract: The aim of this work was to examine whether carbohydrates are involved in signalling N deficiency through source:sink imbalance. Photosynthetic metabolism in tobacco was studied over 8 d during the withdrawal of N from previously N-sufficient plants in which the source:sink ratio was manipulated by shading leaves on some of the plants. In N-sufficient plants over this time-scale, there was a small decline in photosynthetic rate, Rubisco protein and amino acid content, with a larger decrease in carbohydrate content. Withdrawal of N from the growing medium induced a large decrease in the rate of photosynthesis (35% reduction after 8 d under the growing conditions, with a reduction also apparent at high and low measuring CO 2 ), which was caused by a large decrease in the amount of Rubisco protein (62% after 8 d) and Rubisco activity. Higher amounts of hexoses preceded the loss of photosynthetic activity and sucrose and starch accumulation. Reduction of the source:sink ratio by shading prevented the loss of photosynthetic activity and the increase in hexoses and other carbohydrates. These data indicate that the reduction of photosynthesis that accompanies N deficiency in intact plants has the characteristics of sugar repression of photosynthesis observed in model systems, but that the accumulation of hexose prior to the decline in photosynthesis is small. The possibility that sugar repression of photosynthesis under physiological conditions depends more crucially on the C:N status of leaves than the carbohydrate status alone is discussed.

Acclimation of photosynthesis to irradiance and spectral quality in British plant species: chlorophyll content, photosynthetic capacity and habitat preference

Abstract: Twenty-two common British angiosperms were examined for their ability to acclimate photosynthetically to sun and shade conditions Plants were grown under low irradiance, far-red enriched light (50 μmol m−2 s−1), selected to mimic as closely as possible natural canopy shade, and moderately high light of insufficient irradiance to induce photoinhibitory or photoprotective responses (300 μmol m−2 s−1) Light-and CO2-saturated photosynthetic rates of oxygen evolution (Pmax) and chlorophyll content were measured Large variation was found in both parameters, and two ‘strategies’ for long-term acclimation were identified: firstly a change in chlorophyll per unit leaf area which was found to correlate positively with photosynthetic capacity, and secondly changes in chlorophyll alb ratio and Pmax, indicative of alterations at the chloroplast level, which were not associated with a change in chlorophyll content per unit leaf area Combinations of these two strategies may occur, giving rise to the observed diversity in photosynthetic acclimation The extent and nature of photosynthetic acclimation were compared with an index of shade association, calculated from the association each species has with woodland It was found that the greatest flexibility for change at the chloroplast level was found in those species possessing an intermediate shade association, whilst acclimation in ‘sun’ species proceeded by a change in chlorophyll content; obligate shade species showed little capacity for acclimation at either the chloroplast or leaf level A framework for explaining the variation between plant species in leaf-level photosynthetic capacity, in relation to the natural light environment, is presented This is the first time the potential for light acclimation of photosynthesis in different plant species has been satisfactorily linked to habitat distribution

STRUCTURE AND MEMBRANE ORGANlZ4TION OF PHOTOSYSTEM 11 IN GREEN PLANTS

Abstract: Photosystem I1 (PSII) is the pigment protein complex embedded in the thylakoid membrane of higher plants, algae, and cyanobacteria that uses solar energy to drive the photosynthetic water-splitting reaction. This chapter reviews the primary, secondary, tertiary, and quaternary structures of PSII as well as the function of its constituent subunits. The understanding of in vivo organization of PSI1 is based in part on freeze-etched and freeze-fracture images of thylakoid membranes. These images show a resolution of about 40-50 A and so provide information mainly on the localization, heterogeneity, dimensions, and shapes of membrane-embedded PSII complexes. Higher resolution of about 15-40 A has been obtained from single particle images of isolated PSII complexes of defined and differing subunit composition and from electron crystallography of 2-D crystals. Observations are discussed in terms of the oligomeric state and subunit organization of PSI1 and its antenna components.

Physiological nitrogen efficiency in rice: Nitrogen utilization, photosynthesis, and yield potential

Abstract: Characteristics of rice (Oryza sativa) as a crop plant are briefly introduced, and the relationship between formation of yield potential and nitrogen (N) nutrition is described on the basis of studies using 15N as a tracer In addition, the relationship between the leaf photosynthetic capacity and leaf N, and the factors limiting leaf photosynthesis under different growth conditions are reviewed Finally, targets for improving rice yield potential are discussed with a focus on the role of increased photosynthesis efficiency in relation to leaf N status and the photosynthetic components in the leaves

The Effect of Elevated Partial Pressures of CO2 on the Relationship between Photosynthetic Capacity and N Content in Rice Leaves

Abstract: The effects of growth CO2 levels on the photosynthetic rates; the amounts of ribulose-1,5-bisphosphate carboxylase (Rubisco), chlorophyll (Chl), and cytochrome f; sucrose phosphate synthase activity; and total N content were examined in young, fully expanded leaves of rice (Oryza sativa L.). The plants were grown hydroponically under two CO2 partial pressures of 36 and 100 Pa at three N concentrations. The light-saturated photosynthesis at 36 Pa CO2 was lower in the plants grown in 100 Pa CO2 than those grown in 36 Pa CO2. Similarly, the amounts of Rubisco, Chl, and total N were decreased in the leaves of the plants grown in 100 Pa CO2. However, regression analysis showed no differences between the two CO2 treatments in the relationship between photosynthesis and total N or in the relationship between Rubisco and Chl and total N. Although a relative decrease in Rubisco to cytochrome f or sucrose phosphate synthase was found in the plants grown in 100 Pa CO2, this was the result of a decrease in total N content by CO2 enrichment. The activation state of Rubisco was also unaffected by growth CO2 levels. Thus, decreases in the photosynthetic capacity of the plants grown in 100 Pa CO2 could be simply accounted for by a decrease in the absolute amount of leaf N.

Enhanced Employment of the Xanthophyll Cycle and Thermal Energy Dissipation in Spinach Exposed to High Light and N Stress

Abstract: The involvement of the xanthophyll cycle in photoprotection of N-deficient spinach (Spinacia oleracea L. cv Nobel) was investigated. Spinach plants were fertilized with 14 mM nitrate (control, high N) versus 0.5 mM (low N) fertilizer, and grown under both high- and low-light conditions. Plants were characterized from measurements of photosynthetic oxygen exchange and chlorophyll fluorescence, as well as carotenoid and cholorophyll analysis. Compared with the high-N plants, the low-N plants showed a lower capacity for photosynthesis and a lower chlorophyll content, as well as a lower rate of photosystem II photosynthetic electron transport and a corresponding increase in thermal energy dissipation activity measured as nonphotochemical fluorescence quenching. The low-N plants displayed a greater fraction of the total xanthophyll cycle pool as zeaxanthin and antheraxanthin at midday, and an increase in the ratio of xanthophyll cycle pigments to total chlorophyll. These results indicate that under N limitation both the light-collecting system and the photosynthetic rate decrease. However, the increased dissipation of excess energy shows that there is excess light absorbed at midday. We conclude that spinach responds to N limitation by a combination of decreased light collection and increased thermal dissipation involving the xanthophyll cycle.

Development of Arabidopsis thaliana leaves at low temperatures releases the suppression of photosynthesis and photosynthetic gene expression despite the accumulation of soluble carbohydrates

Abstract: Arabidopsis thaliana plants were grown at 23 degrees C end changes in carbohydrate metabolism, photosynthesis and photosynthetic gene expression were studied after the plants were shifted to 5 degr ...

Plant biochemistry and molecular biology

Abstract: Contents : 1. A leaf cell consists of several metabolic compartments -2. The use of energy from sunlight by photosynthesis -3. Photosynthesis is an electron transport process -4. ATP generation by photosynthesis -5. Mitochondria, the power stations of cell -6. Photosynthetic CO2 assimilation by the Calvin cycle -7. Photorespiration -8. Photosynthesis and water consumption -9. Polysaccharides -10. Nitrate assimilation -11. Nitrogen fixation -12. Sulfate assimilation -13. Phloem transport -14. Plant storage proteins -15. Glycerolipids -16. The function of secondary metabolites in plants -17. Isoprenoids -18. Phenylpropanoids -19. Signals regulating the growth and development of plant organs -20. The genomes of plant cells -21. Protein biosynthesis -22. Gene technology in plants.

Impacts of CO2 Enrichment on Productivity and Light Requirements of Eelgrass.

Abstract: Seagrasses, although well adapted for submerged existence, are CO2-limited and photosynthetically inefficient in seawater. This leads to high light requirements for growth and survival and makes seagrasses vulnerable to light limitation. We explored the long-term impact of increased CO2 availability on light requirements, productivity, and C allocation in eelgrass (Zostera marina L.). Enrichment of seawater CO2 increased photosynthesis 3-fold, but had no long-term impact on respiration. By tripling the rate of light-saturated photosynthesis, CO2 enrichment reduced the daily period of irradiance-saturated photosynthesis (Hsat) that is required for the maintenance of positive whole-plant C balance from 7 to 2.7 h, allowing plants maintained under 4 h of Hsat to perform like plants growing in unenriched seawater with 12 h of Hsat. Eelgrass grown under 4 h of Hsat without added CO2 consumed internal C reserves as photosynthesis rates and chlorophyll levels dropped. Growth ceased after 30 d. Leaf photosynthesis, respiration, chlorophyll, and sucrose-phosphate synthase activity of CO2-enriched plants showed no acclimation to prolonged enrichment. Thus, the CO2-stimulated improvement in photosynthesis reduced light requirements in the long term, suggesting that globally increasing CO2 may enhance seagrass survival in eutrophic coastal waters, where populations have been devastated by algal proliferation and reduced water-column light transparency.

Effect of water restriction on carbohydrate metabolism and photosynthesis in mature maize leaves

Abstract: The time course of the modifications induced by a mild water stress has been examined for photosynthesis and several traits of carbohydrate metabolism in adult leaves of two inbred maize lines of North American and European origins, respectively. An early response was a sharp increase of the acid soluble invertase activity in adult leaves, 3–4 d after initiation of water shortage. Accordingly, correlated accumulations of fructose, glucose and to a lesser extent sucrose were observed. In the most responsive genotype, invertase activity finally reached a value > 3 times larger than the control value. By contrast, sucrose phosphate synthase activity, measured either under saturating or limiting substrate conditions, was progressively reduced by 20–40% on the 5th day and by 50–80% on the 7th day, depending on the genotype. Leaf photosynthetic rate was affected at approximately the same time as carbohydrate metabolism and stomatal conductance. Leaf water status, as measured by relative water content, declined afterwards. For all the observed responses, the two genotypes behaved very differently.

How Nature Harvests Sunlight

Abstract: It is through photosynthesis that Earth's biosphere derives its energy from sunlight. Photosynthetic organisms—plants, algae and photosynthetic bacteria—have developed efficient systems to harvest the light of the Sun and to use its energy to drive their metabolic reactions, such as the reduction of carbon dioxide to sugar. The ubiquitous green color of plants is testimony to the key molecular participant in the light harvesting of plants, chlorophyll. More hidden in this respect, but no less widespread, is a second participating molecule, carotenoid. In green leaves, the color of the carotenoids is masked by the much more abundant chlorophylls, whereas in ripe tomatoes or the petals of yellow flowers, the carotenoids predominate. Chlorophyll molecules exist in slightly different chemical structures in various photosynthetic organisms, as chlorophyll a or b in plants or algae, and as bacteriochlorophyll a or b in photosynthetic bacteria. Molecules such as chlorophyll and carotenoid that absorb light and i...

Does Decrease in Ribulose-1,5-Bisphosphate Carboxylase by Antisense RbcS Lead to a Higher N-Use Efficiency of Photosynthesis under Conditions of Saturating CO2 and Light in Rice Plants?

Abstract: Rice (Oryza sativa L.) plants with decreased ribulose-l,5bisphosphate carboxylase (Rubisco) were obtained, by transformation with the rice rbcS antisense gene under the control of the rice r6cS promoter. lhe primary transformants were screened for the Rubisco to leaf N ratio, and the transformant with 65% wild-type Rubisco was selected as a plant set with optimal Rubisco content at saturating CO, partial pressures for photosynthesis under conditions of high irradiance and 25°C. This optimal Rubisco content was estimated from the amounts and kinetic constants of Rubisco and the gas-exchange data. lhe R, selfed progeny of the selected transformant were grown hydroponically with different N concentrations. Rubisco content in the R, population was distributed into two groups: 56 plants had about 65% wild-type Rubisco, whereas 23 plants were very similar to the wild type. Although the plants with decreased Rubisco showed 20% lower rates of light-saturated photosynthesis in normal air (36 Pa CO,), they had 5 to 15% higher rates of photosynthesis in elevated partial pressures of CO, (100115 Pa CO,) than the wild-type plants for a given leaf N content. We conclude that the rice plants with 65% wild-type Rubisco show a higher N-use efficiency of photosynthesis under conditions of saturating CO, and high irradiance.

Analysis of limitations to CO2 assimilation on exposure of leaves of two Brassica napus cultivars to UV‐B

Abstract: Apex and Bristol cultivars of oilseed rape (Brassica napus) were irradiated with 0.63 W m -2 of UV-B over 5 d. Analyses of the response of net leaf carbon assimilation to intercellular C0 2 concentration were used to examine the potential limitations imposed by stomata, carboxylation velocity and capacity for regeneration of ribulose 1,5-hisphosphate on leaf photosynthesis. Simultaneous measurements of chlorophyll fluorescence were used to estimate the maximum quantum efficiency of photosystem II (PSII) photochemistry, the quantum efficiency of linear electron transport at steady-state photosynthesis, and the light and CO 2 -saturated rate of linear electron transport. Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) content and activities were assayed in vitro. In both cultivars the UV-B treatment resulted in decreases in the light-saturated rate of CO 2 assimilation, which were accompanied by decreases in carboxylation velocity and Rubisco content and activity. No major effects of UV-B were observed on end-product inhibition and stomatal limitation of photosynthesis or the rate of photorespiration relative to CO 2 assimilation. In the Bristol cultivar, photoinhibition of PSII and loss of linear electron transport activity were observed when CO 2 assimilation was severely inhibited. However, the Apex cultivar exhibited no major inhibition of PSII photochemistry or linear electron transport as the rate of CO 2 assimilation decreased. It is concluded that loss of Rubisco is a primary factor in UV-B inhibition of CO 2 assimilation.

Effects of elevated CO2 and temperature on photosynthesis and Rubisco in rice and soybean

Abstract: Rice (Oryza sativa L. cv. IR-72) and soybean (Glycine max L. Merr. cv. Bragg), which have been reported to differ in acclimation to elevated CO2, were grown for a season in sunlight at ambient and twice-ambient [CO2], and under daytime temperature regimes ranging from 28 to 40°C. The objectives of the study were to test whether CO2 enrichment could compensate for adverse effects of high growth temperatures on photosynthesis, and whether these two C3 species differed in this regard. Leaf photosynthetic assimilation rates (A) of both species, when measured at the growth [CO2], were increased by CO2 enrichment, but decreased by supraoptimal temperatures. However, CO2 enrichment more than compensated for the temperature-induced decline in A. For soybean, this CO2 enhancement of A increased in a linear manner by 32–95% with increasing growth temperatures from 28 to 40°C, whereas with rice the degree of enhancement was relatively constant at about 60%, from 32 to 38°C. Both elevated CO2 and temperature exerted coarse control on the Rubisco protein content, but the two species differed in the degree of responsiveness. CO2 enrichment and high growth temperatures reduced the Rubisco content of rice by 22 and 23%, respectively, but only by 8 and 17% for soybean. The maximum degree of Rubisco down-regulation appeared to be limited, as in rice the substantial individual effects of these two variables, when combined, were less than additive. Fine control of Rubisco activation was also influenced by both elevated [CO2] and temperature. In rice, total activity and activation were reduced, but in soybean only activation was lowered. The apparent catalytic turnover rate (Kcat) of rice Rubisco was unaffected by these variables, but in soybean elevated [CO2] and temperature increased the apparent Kcat by 8 and 22%, respectively. Post-sunset declines in Rubisco activities were accelerated by elevated [CO2] in rice, but by high temperature in soybean, suggesting that [CO2] and growth temperature influenced the metabolism of 2-carboxyarabinitol-1-phosphate, and that the effects might be species-specific. The greater capacity of soybean for CO2 enhancement of A at supraoptimal temperatures was probably not due to changes in stomatal conductance, but may be partially attributed to less down-regulation of Rubisco by elevated [CO2] in soybean than in rice. However, unidentified species differences in the temperature optimum for photosynthesis also appeared to be important. The responses of photosynthesis and Rubisco in rice and soybean suggest that among C3 plants species-specific differences will be encountered as a result of future increases in global [CO2] and air temperatures.

Influence of increasing carbon dioxide concentration on the photosynthetic and growth stimulation of selected C4 crops and weeds

Abstract: Plants of six weedy species (Amaranthus retroflexus, Echinochloa crus-galli, Panicum dichotomiflorum, Setaria faberi, Setaria viridis, Sorghum halapense) and 4 crop species (Amaranthus hypochondriacus, Saccharum officinarum, Sorghum bicolor and Zea mays) possessing the C4type of photosynthesis were grown at ambient (38 Pa) and elevated (69 Pa) carbon dioxide during early development (i.e. up to 60 days after sowing) to determine: (a) if plants possessing the C4photosynthetic pathway could respond photosynthetically or in biomass production to future increases in global carbon dioxide and (b) whether differences exist between weeds and crops in the degree of response. Based on observations in the response of photosynthesis (measured as A, CO2assimilation rate) to the growth CO2condition as well as to a range of internal CO2(Ci) concentrations, eight of ten C4species showed a significant increase in photosynthesis. The largest and smallest increases observed were for A. retroflexus (+30%) and Z. mays (+5%), respectively. Weed species (+19%) showed approximately twice the degree of photosynthetic stimulation as that of crop species (+10%) at the higher CO2concentration. Elevated carbon dioxide also resulted in significant increases in whole plant biomass for four C4weeds (A. retroflexus, E. crus-galli, P. dichotomiflorum, S. viridis) relative to the ambient CO2condition. Leaf water potentials for three selected species (A. retroflexus, A. hypochondriacus, Z. mays) indicated that differences in photosynthetic stimulation were not due solely to improved leaf water status. Data from this study indicate that C4plants may respond directly to increasing CO2concentration, and in the case of some C4weeds (e.g. A. retroflexus) may show photosynthetic increases similar to those published for C3species.

The c2 oxidative photosynthetic carbon cycle.

Abstract: The C2 oxidative photosynthetic carbon cycle plus the C3 reductive photosynthetic carbon cycle coexist. Both are initiated by Rubisco, use about equal amounts of energy, must regenerate RuBP, and result in exchanges of CO2 and O2 to establish rates of net photosynthesis, CO2 and O2 compensation points, and the ratio of CO2 and O2 in the atmosphere. These concepts evolved from research on O2 inhibition, glycolate metabolism, leaf peroxisomes, photorespiration, 18O2/16O2 exchange, CO2 concentrating processes, and a requirement for the oxygenase activity of Rubisco. Nearly 80 years of research on these topics are unified under the one process of photosynthetic carbon metabolism and its self-regulation.

ELIPs – Light‐induced stress proteins

Abstract: Exposure of plants to light intensities higher than those required to saturate photosynthesis leads to a reduction in photosynthetic capacity. This effect is known as photoinhibition. Photoinhibition is followed by destruction of carotenoids, bleaching of chlorophylls and increased lipid peroxidation due to damage by oxygen-derivatives. The oxygen concentration in chloroplasts in the light is high because of oxygen production by photosystem II (PSII). This can result in the release of reactive intermediates of reduced dioxygen such as superoxide radicals, hydroxyl radicals, hydrogen peroxide or singlet oxygen. In order to maintain their normal function under light stress conditions, chloroplasts have developed multiple repair and protection systems. The induction of specific light stress proteins, the ELIPs (for early light-induced proteins) can be considered to be part of these protective responses. The accumulation of ELIPs under light stress conditions is correlated with the photoinactivation of PSII, degradation of the D 1 -protein of PSII reaction centre and changes in the level of pigments. Futher-more, the accumulation of ELIPs in the thylakoids is strictly controlled by the pigment content, especially by chlorophylls. Isolation of ELIPs in a native form and analysis of pigments bound to these proteins revealed that ELIPs can bind chlorophyll a and lutein. These data indicate that ELIPs might represent unique chlorophyll-binding proteins which have a transient function(s) during light stress. A transient 'pigment-carrier function is postulated for ELIPs.

Mechanisms of photoinduced toxicity of photomodified anthracene to plants: inhibition of photosynthesis in the aquatic higher plant Lemna gibba (duckweed)

Abstract: Polycyclic aromatic hydrocarbon (PAH) toxicity is enhanced by light, especially ultraviolet (UV) radiation. To examine a potential mechanism(s) of photoinduced toxicity of PAHs to plants, the effects of anthracene and its photoproducts on photosynthesis were investigated using the aquatic higher plant Lemna gibba L. G-3 (duckweed). Photosynthetic activity was monitored both in vivo and in vitro by measuring chlorophyll a (Chl a) fluorescence, carbon fixation, and electron transport. In simulated solar radiation (a light source with a visible light: UV-A: UV-B ratio similar to sunlight), inhibition of photosynthesis was more rapid with photomodified anthracene than with intact anthracene, and intact anthracene appeared to only inhibit photosynthesis following its photomodification. The primary site of action of photomodified anthracene was found to be electron transport at or near photosystem I (PSI). This was followed by inhibition of photosystem II (PSII), probably due to excitation pressure on PSII once the downstream electron transport through PSI was blocked. Accordingly, higher chemical concentrations and/or longer exposures were required to inhibit PSII than PSI. Net photosynthesis (carbon fixation) was also inhibited, implying that the inhibition of electron transport in PSI by photomodified anthracene can lead to diminished primary productivity. A linkage between inhibition of photosynthesis and inhibition of plant growth was established in terms of the initial site of action (PSI) and primary productivity (carbon fixation), which suggested that Chl a fluorescence can be used as a bioindicator of PAH impacts on plants.

Chloroplast redox regulation of nuclear gene transcription during photoacclimation

Abstract: The role of the redox state of ferredoxin/thioredoxin within the chloroplast is well established for the feedback regulation of enzyme activity in the Calvin cycle. However, evidence has emerged also suggesting that chloroplast electron transport components regulate plastid and nuclear gene expression. Using the unicellular green alga, Dunaliella tertiolecta, as a model organism, we have shown that the cell acclimates to changes in growth irradiance by altering the abundance and activities of photosynthetic components, in particular the light harvesting complexes (LHC). Pharmacological data suggests that light intensity is sensed through the redox status of the plastoquinone pool leading to the regulation of nuclear encoded genes, such as Lhcb. This signal may be transduced through a redox regulated protein kinase that (in)directly interacts with the nuclear transcription apparatus. The redox state of the plastoquinone pool seems to play a pivotal role in sensing cellular energy status and in regulating photosynthetic capacity. Other cellular pathways, including carbon fixation, carbohydrate metabolism and nutrient assimilation have been shown to have feedback influences on photosynthesis, that may be sensed by the redox state of the plastoquinone pool.

Effect of Chilling on Carbon Assimilation, Enzyme Activation, and Photosynthetic Electron Transport in the Absence of Photoinhibition in Maize Leaves

Abstract: The relationships between electron transport and photosynthetic carbon metabolism were measured in maize (Zea mays L.) leaves following exposure to suboptimal temperatures. The quantum efficiency for electron transport in unchilled leaves was similar to that previously observed in C3 plants, although maize has two types of chloroplasts, mesophyll and bundle sheath, with PSII being largely absent from the latter. The index of noncyclic electron transport was proportional to the CO2 assimilation rate. Chilled leaves showed decreased rates of CO2 assimilation relative to unchilled leaves, but the integral relationships between the quantum efficiency for electron transport or the index of noncyclic electron transport and CO2 fixation were unchanged and there was no photoinhibition. The maximum catalytic activities of the Benson-Calvin cycle enzymes, fructose-1,6-bisphosphatase and ribulose-1,5-bisphosphate carboxylase, were decreased following chilling, but activation was unaffected. Measurements of thiol-regulated enzymes, particularly NADP-malate dehydrogenase, indicated that chilling induced changes in the stromal redox state so that reducing equivalents were more plentiful. We conclude that chilling produces a decrease in photosynthetic capacity without changing the internal operational, regulatory or stoichiometric relationships between photosynthetic electron transport and carbon assimilation. The enzymes of carbon assimilation are particularly sensitive to chilling, but enhanced activation may compensate for decreases in maximal catalytic activity.

Horizontal transfer of genes coding for the photosynthetic reaction centers of purple bacteria

Abstract: Phylogenetic trees were drawn and analyzed based on the nucleotide sequences of the 1.5-kb gene fragment coding for the L and M subunits of the photochemical reaction center of various purple photo-synthetic bacteria. These trees are mostly consistent with phylogenetic trees based on 16S rRNA and soluble cy-tochrome c, but differ in some significant details. This inconsistency implies horizontal transfer of the genes that code for the photosynthetic apparatus in purple bacteria. Possibilities of similar transfers of photosynthesis genes during the evolution of photosynthesis are discussed especially for the establishment of oxygenic photosynthesis.

Leaf photosynthesis, plant growth and nitrogen allocation in rice under different irradiances

Abstract: The photosynthetic rates and various components of photosynthesis including ribulose-1,5-bisphosphate carboxylase (Rubisco; EC 4.1.1.39), chlorophyll (Chl), cytochrome (Cyt) f, and coupling factor 1 (CF1) contents, and sucrose-phosphate synthase (SPS; EC 2.4.1.14) activity were examined in young, fully expanded leaves of rice (Oryza sativa L.) grown hydroponically under two irradiances, namely, 1000 and 350 μmol quanta · m−2 · s−1, at three N concentrations. The light-saturated rate of photosynthesis measured at 1800 μmol · m−2 · s−1 was almost the same for a given leaf N content irrespective of growth irradiance. Similarly, Rubisco content and SPS activity were not different for the same leaf N content between irradiance treatments. In contrast, Chl content was significantly greater in the plants grown at 350 μmol · m−2 · s−1, whereas Cyt f and CF1 contents tended to be slightly smaller. However, these changes were not substantial, as shown by the fact that the light-limited rate of photosynthesis measured at 350 μmol · m−2 · s−1 was the same or only a little higher in the plants grown at 350 μmol · m−2 · s−1 and that CO2-saturated photosynthesis did not differ between irradiance treatments. These results indicate that growth-irradiance-dependent changes in N partitioning in a leaf were far from optimal with respect to N-use efficiency of photosynthesis. In spite of the difference in growth irradiance, the relative growth rate of the whole plant did not differ between the treatments because there was an increase in the leaf area ratio in the low-irradiance-grown plants. This increase was associated with the preferential N-investment in leaf blades and the extremely low accumulation of starch and sucrose in leaf blades and sheaths, allowing a more efficient use of the fixed carbon. Thus, morphogenic responses at the whole-plant level may be more important for plants as an adaptation strategy to light environments than a response of N partitioning at the level of a single leaf.