Cyclic variations of photosynthetic activity under nitrogen fixing conditions in Synchococcus RF-1

When nitrogen fixing cell cultures of Synechococcus RF-1 were subjected to an alternating lightdark regime (12 h:12 h), a cyclic decrease in the photosynthetic oxygen evolution potential was observed during the dark periods. This rhythm of net photosynthesis rate was maintained for at least two days after transition to continuous light. The decrease in net photosynthesis was accompanied by a stimulation of dark respiration. However, the magnitude of oxygen uptake was considerably smaller than the observed decrease in oxygen evolution. The photosynthetic activity of cells taken from the dark period was characterized by (i) a significantly lower quantum yield and (ii) a strong reduction in the light-saturated rate of photosynthesis. Growing the cultures on nitrate or under continuous light completely suppressed this rhythm. Protein synthesis was not necessary for the recovery of the light-saturated rate of photosynthesis during the light period. The cellular content of chlorophyll a and of phycobiliproteins did not vary between light and dark period, indicating that quantitative changes in the composition of the photosynthetic apparatus are not the basis for the observed oscillations. Regulatory modifications of the photosynthetic efficiency are proposed as an adaptation mechanism to adjust the intracellular oxygen concentration to the needs for nitrogenase activity.Abbreviation Chl chlorophyll     

Effects of Variations in Daylength and Temperature on Net Rates of Photosynthesis, Dark Respiration, and Excretion by Isochrysis galbana Parke

The effects of variable daylength and temperature on net rates of photosynthesis, dark respiration, and excretion of a unicellular marine haptophyte, Isochrysis galbana Parke, were examined and related to division rates. Six combinations of daylength (18:6, 12:12, 6:18 light:dark, LD) and temperature (20, 25 C) were used. Daily rates of net photosynthesis were closely correlated to division rates, suggesting a direct relationship, and were maximal when cells were grown at 12:12 LD at both temperatures and 18:6 LD at 20 C. A daylength of 6 hours decreased daily rates by decreasing the time for carbon uptake. Further, cells grown with this daylength had maximal chlorophyll a contents, suggesting a physiological adaptation by photosynthetic units to short light periods. A photoperiod of 18:6 LD at 25 C decreased daily rates of net photosynthesis by reducing the hourly rate of net photosynthesis via an unidentified mechanism. The importance of rates of net dark respiration in controlling daily net photosynthesis was small, with carbon lost during dark periods varying between 4 and 14% of that gained during light periods. Also, the influence of net excretion was small, varying between 1.0 and 5.5% of daily net photosynthesis.     

A comparison of methods for measuring primary productivity and community respiration in streams

Carbon dioxide and oxygen exchange procedures for measuring community metabolism (two open stream methods and three chamber methods) were compared on the same reach of a third-order stream. Open stream methods were complicated by high diffusion rates and yielded net community primary productivity estimates lower than those obtained with chamber methods. Chamber methods yielded variable productivity and respiration data. However, when normalized for chlorophyll a, productivity estimates from the chamber methods were within an expected range for the system. Balances of photosynthesis and respiration from the chamber methods were similar between methods and indicated that autotrophic or heterotrophic processes could dominate the system. Considerations in applying the various procedures are discussed.     

The fine structure and photosynthetic cost of structural leaf variegation

The leaves of some plants display an optical patchiness on their upper side, displaying light- and dark-green areas with high and low reflectance, respectively. In this investigation, we studied the fine structure of the corresponding sectors and we asked whether the lost reflected light entails a photosynthetic cost to these leaves. Four species, i.e. Arum italicum, Ranunculus ficaria, Cyclamen hederifolium and Cyclamen persicum were investigated. Scanning electron microscope examination revealed that epidermal cells of light-green sectors of all species are more bulgy than corresponding cells of neighboring dark-green leaf sectors. The comparative anatomical study revealed that (i) epidermis thickness of the light-green areas and the number of mesophyll cell layers does not differ from those of the adjacent dark-green leaf sectors and (ii) palisade cells of light-green sectors are slightly larger and more loosely arranged, allowing a much higher percentage of intercellular air spaces. The latter histological feature seems to provide the structural basis for the different optical properties between the two leaf sectors. Contrary to expectations, net photosynthetic rates (expressed on a leaf area basis) were similar in the light-green and the dark-green areas of the two cyclamen species. Yet, in C. persicum net photosynthesis was higher in the light-green areas, if expressed on a dry mass basis. The small size of the light-green spots in the rest of the test plants precluded CO₂ assimilation measurements, yet maximum linear photosynthetic electron transport rates displayed no differences between the two sectors in all plants. Thus, the assumption of a photosynthetic cost in the light-green areas was not confirmed. On the contrary, a higher construction cost was evident in the dark-green areas of three species, displaying a significantly higher specific leaf mass, without any photosynthetic benefit. The results on net photosynthesis were compatible with leaf optical properties and pigment levels. Thus, in spite of the considerably higher reflectance of the light-green areas and their lower (yet normal for a green leaf) chlorophyll levels, corresponding differences in absorptance were slight. In addition, dry mass-based pigment contents in dark-green areas were higher, while chlorophyll a/b (in two species) and carotenoid/chlorophyll ratios (in three species) were lower, pointing to a shade adaptation in these sectors. We conclude that in variegated leaves of this kind, dark-green areas are more costly to build and probably less photosynthetically active. We argue that the high pigment contents of dark-green areas establish steep light gradients in the corresponding mesophyll, rendering deeper chloroplast layers more shade adapted.     

Thermal acclimation of photosynthesis: on the importance of adjusting our definitions and accounting for thermal acclimation of respiration

While interest in photosynthetic thermal acclimation has been stimulated by climate warming, comparing results across studies requires consistent terminology. We identify five types of photosynthetic adjustments in warming experiments: photosynthesis as measured at the high growth temperature, the growth temperature, and the thermal optimum; the photosynthetic thermal optimum; and leaf-level photosynthetic capacity. Adjustments of any one of these variables need not mean a concurrent adjustment in others, which may resolve apparently contradictory results in papers using different indicators of photosynthetic acclimation. We argue that photosynthetic thermal acclimation (i.e., that benefits a plant in its new growth environment) should include adjustments of both the photosynthetic thermal optimum (T _opt) and photosynthetic rates at the growth temperature (A _growth), a combination termed constructive adjustment. However, many species show reduced photosynthesis when grown at elevated temperatures, despite adjustment of some photosynthetic variables, a phenomenon we term detractive adjustment. An analysis of 70 studies on 103 species shows that adjustment of T _opt and A _growth are more common than adjustment of other photosynthetic variables, but only half of the data demonstrate constructive adjustment. No systematic differences in these patterns were found between different plant functional groups. We also discuss the importance of thermal acclimation of respiration for net photosynthesis measurements, as respiratory temperature acclimation can generate apparent acclimation of photosynthetic processes, even if photosynthesis is unaltered. We show that while dark respiration is often used to estimate light respiration, the ratio of light to dark respiration shifts in a non-predictable manner with a change in leaf temperature.     

Importance of canopy structure on photosynthesis in single- and multi-species assemblages of marine macroalgae

Plant communities utilize available irradiance with different efficiency depending not only on their photosynthetic characteristics but also on the canopy structure and density. The importance of canopy structure are well studied in terrestrial plant communities but poorly studied in aquatic plant communities. The objective of this study was to evaluate macroalgal community photosynthesis in artificial constructed communities of one to four species with different morphologies along a range of leaf (i.e.=thallus) area densities. In a laboratory set-up we measured net photosynthesis and dark respiration in constructed assemblages of macroalgae, excluding effects other than photosynthesis of individual tissue and distribution of photons in the canopy from influencing metabolism. We hypothezised that 1) canopy structure determines the actual rates of photosynthesis relative to the optimal rates and 2) multi-species communities attain higher maximum photosynthetic rates than single species communities. We found that differences in canopy structure outweighed large differences in tissue photosynthesis resulting in relatively similar maximum community photosynthetic rates among the different single and multi-species assemblages (20.1–40.5 μmol O₂ m⁻² s⁻¹). Canopy structure influenced community photosynthesis both at low and high leaf area densities because it determines the ability of macroalgae to use the photosynthetic potential of their individual tissues. Due to an averaging effect the photosynthetic rate at high leaf area density was more similar among multi-species community than among single-species communities. Multi-species communities had, on average, a slightly higher photosynthetic production than expected from photosynthesis of single species communities. Moreover multi-species communities were capable of exposing new tissue to irradiance up to high densities thereby avoiding a decrease in net photosynthesis. This finding suggests that multi-species communities may be able to maintain higher biomass per unit ground area than single-species communities.     

ABA and BAP improve the accumulation of carbohydrates and alter carbon allocation in potato plants at elevated CO2

Elevated CO₂ interactions with other factors affects the plant performance. Regarding the differences between cultivars in response to CO₂ concentrations, identifying the cultivars that better respond to such conditions would maximize their potential benefits. Increasing the ability of plants to benefit more from elevated CO₂ levels alleviates the adverse effects of photoassimilate accumulation on photosynthesis and increases the productivity of plants. Despite its agronomic importance, there is no information about the interactive effects of elevated CO₂ concentration and plant growth regulators (PGRs) on potato (Solanum tuberosum L.) plants. Hence, the physiological response and source-sink relationship of potato plants (cvs. Agria and Fontane) to combined application of CO₂ levels (400 vs. 800 µmol mol⁻¹) and plant growth regulators (PGR) [6-benzylaminopurine (BAP) + Abscisic acid (ABA)] were evaluated under a controlled environment. The results revealed a variation between the potato cultivars in response to a combination of PGRs and CO₂ levels. Cultivars were different in leaf chlorophyll content; Agria had higher chlorophyll a, b, and total chlorophyll content by 23, 43, and 23%, respectively, compared with Fontane. The net photosynthetic rate was doubled at the elevated compared with the ambient CO₂. In Agria, the ratio of leaf intercellular to ambient air CO₂ concentrations [C_i:C_a] was declined in elevated-CO₂-grown plants, which indicated the stomata would become more conservative at higher CO₂ levels. On the other hand, the increased C_i:C_a in Fontane showed a stomatal acclimation to higher CO₂ concentration. The higher leaf dark respiration of the elevated CO₂-grown and BAP + ABA-treated plants was associated with a higher leaf soluble carbohydrates and starch content. Elevated CO₂ and BAP + ABA shifted the dry matter partitioning to the belowground more than the above-media organs. The lower leaf soluble carbohydrate content and greater tuber yield in Fontane might indicate a more efficient photoassimilate translocation than Agria. The results highlighted positive synergic effects of the combined BAP + ABA and elevated CO₂ on tuber yield and productivity of the potato plants.     

Chlorophyll, Ribulose-1,5-diphosphate Carboxylase, and Hill Reaction Activity in Developing Leaves of Populus deltoides

The synthesis of chlorophyll and ribulose diphosphate carboxylase as well as the development of Hill reaction activity were followed in expanding Populus deltoides leaves and related to photosynthetic patterns. Total chlorophyll, which was not correlated with photosynthetic rate in expanding leaves, decreased slightly with age in very young leaves, due to a decrease in chlorophyll b, but then increased linearly. The ratio of chlorophyll a to b, which rose sharply in young leaves, was highly correlated with the onset of net photosynthesis. Hill reaction activity was very low in young leaves and did not increase significantly until leaves were about half expanded. Ribulose diphosphate carboxylase activity increased in a sigmoid fashion with leaf ontogenesis and closely paralleled development of the photosynthetic system. The study demonstrates the importance of chlorophyll a and Calvin cycle enzyme synthesis to photosynthetic development in expanding leaves.     

Solar energy conversion efficiencies in photosynthesis: Minimizing the chlorophyll antennae to maximize efficiency

The theoretical maxima of solar energy conversion efficiencies and productivities in oxygenic photosynthesis are evaluated. These are contrasted with actual measurements in a variety of photosynthetic organisms, including green microalgae, cyanobacteria, C4 and C3 plants. Minimizing, or truncating, the chlorophyll antenna size of the photosystems can improve photosynthetic solar energy conversion efficiency and productivity up to 3-fold. Generation of truncated light-harvesting chlorophyll antenna size (tla) strains, in all classes of photosynthetic organisms would help to alleviate excess absorption of sunlight and the ensuing wasteful dissipation of excitation energy, and to maximize solar-to-product energy conversion efficiency and photosynthetic productivity in high-density mass cultivations. The tla concept may find application in the commercial exploitation of microalgae and plants for the generation of biomass, biofuels, chemical feedstocks, as well as nutraceuticals and pharmaceuticals.     

Photosynthetic carbon sources in somepotamogeton species

The ability of somePotamogeton species to use bicarbonate for photosynthesis was assayed by means of relative net photosynthetic rates in alkaline (bicarbonate) and acid (free CO₂) solutions with an equivalent amount of inorganic carbon. The results showedPotamogeton crispus, P. oxyphyllus, P. maackianus, P. perfoliatus, P. malaianus and the submerged leaves ofP. distinctus to be able to use bicarbonate ions for photosynthesis. To the contrary, the submerged leaves ofP. fryeri, which is an inhabitant of soft and acid waters, and the floating leaves ofP. distinctus were proven to be ‘non-users’ of bicarbonate ions. Furthermore, the relative photosynthetic rates were determined using natural waters with various carbon conditions. The dependence of photosynthetic performance on the free CO₂, pH and alkalinity of the waters is discussed in relation to carbon metabolism. Further it is suggested that the carbon conditions are ecologically significant in relation to the productivity and success of macrophyte species in natural habitats.     

Evolutionary Changes in Chlorophyllide a Oxygenase (CAO) Structure Contribute to the Acquisition of a New Light-harvesting Complex in Micromonas

Chlorophyll b is found in photosynthetic prokaryotes and primary and secondary endosymbionts, although their light-harvesting systems are quite different. Chlorophyll b is synthesized from chlorophyll a by chlorophyllide a oxygenase (CAO), which is a Rieske-mononuclear iron oxygenase. Comparison of the amino acid sequences of CAO among photosynthetic organisms elucidated changes in the domain structures of CAO during evolution. However, the evolutionary relationship between the light-harvesting system and the domain structure of CAO remains unclear. To elucidate this relationship, we investigated the CAO structure and the pigment composition of chlorophyll-protein complexes in the prasinophyte Micromonas. The Micromonas CAO is composed of two genes, MpCAO1 and MpCAO2, that possess Rieske and mononuclear iron-binding motifs, respectively. Only when both genes were introduced into the chlorophyll b-less Arabidopsis mutant (ch1-1) was chlorophyll b accumulated, indicating that cooperation between the two subunits is required to synthesize chlorophyll b. Although Micromonas has a characteristic light-harvesting system in which chlorophyll b is incorporated into the core antennas of reaction centers, chlorophyll b was also incorporated into the core antennas of reaction centers of the Arabidopsis transformants that contained the two Micromonas CAO proteins. Based on these results, we discuss the evolutionary relationship between the structures of CAO and light-harvesting systems.     

Inactivation of mitochondrial complex I stimulates chloroplast ATPase in Physcomitrium patens

Light is the ultimate source of energy for photosynthetic organisms, but respiration is fundamental for supporting metabolism during the night or in heterotrophic tissues. In this work, we isolated Physcomitrella (Physcomitrium patens) plants with altered respiration by inactivating Complex I (CI) of the mitochondrial electron transport chain by independently targeting on two essential subunits. Inactivation of CI caused a strong growth impairment even in fully autotrophic conditions in tissues where all cells are photosynthetically active, demonstrating that respiration is essential for photosynthesis. CI mutants showed alterations in the stoichiometry of respiratory complexes while the composition of photosynthetic apparatus was substantially unaffected. CI mutants showed altered photosynthesis with high activity of both Photosystems I and II, likely the result of high chloroplast ATPase activity that led to smaller ΔpH formation across thylakoid membranes, decreasing photosynthetic control on cytochrome b6f in CI mutants. These results demonstrate that alteration of respiratory activity directly impacts photosynthesis in P. patens and that metabolic interaction between organelles is essential in their ability to use light energy for growth.

Mosses lacking mitochondrial Complex I show induced activation of chloroplast ATPase, demonstrating the impact of respiration on the metabolism of plant photosynthetic cells.     

Differential Expression of Proteins in Response to Molybdenum Deficiency in Winter Wheat Leaves Under Low-Temperature Stress

Molybdenum (Mo) is an essential micronutrient for plants. To obtain a better understanding of the molecular mechanisms of cold resistance enhanced by molybdenum application in winter wheat, we applied a proteomic approach to investigate the differential expression of proteins in response to molybdenum deficiency in winter wheat leaves under low-temperature stress. Of 13 protein spots that were identified, five spots were involved in the light reaction of photosynthesis, five were involved in the dark reaction of photosynthesis, and three were highly involved in RNA binding and protein synthesis. Before the application of cold stress, four differentially expressed proteins between the Mo deficiency (?Mo) vs. Mo application (+Mo) comparison are involved in carbon metabolism and photosynthetic electron transport. After 48 h of cold stress, nine differentially expressed proteins between the ?Mo vs. +Mo comparison are involved in carbon metabolism, photosynthetic electron transport, RNA binding, and protein synthesis. Under ?Mo condition, cold stress induced a more than twofold decrease in the accumulation of six differential proteins including ribulose bisphosphate carboxylase large-chain precursor, phosphoglycerate kinase, cp31BHv, chlorophyll a/b-binding protein, ribulose bisphosphate carboxylase small subunit, and ribosomal protein P1, whereas under +Mo condition cold stress only decreased the expression of RuBisCO large subunit, suggesting that Mo application might contribute to the balance or stability of these proteins especially under low-temperature stress and that Mo deficiency has greater influence on differential protein expression in winter wheat after low-temperature stress. Further investigations showed that Mo deficiency decreased the concentrations of chlorophyll a, chlorophyll b, and carotenoids; the maximum net photosynthetic rate; the apparent quantum yield; and carboxylation efficiency, even before the application of the cold stress, although the decrease rates were greater after 48 h of cold treatment, which is consistent with changes in the expressions of differential proteins in winter wheat under low-temperature stress. These findings provide some new evidence that Mo might be involved in the light and dark reaction of photosynthesis and protein synthesis.     

Herbivore-induced changes in plant carbon allocation: assessment of below-ground C fluxes using carbon-14

Effects of above-ground herbivory on short-term plant carbon allocation were studied using maize (Zea mays) and a generalist lubber grasshopper (Romalea guttata). We hypothesized that above-ground herbivory stimulates current net carbon assimilate allocation to below-ground components, such as roots, root exudation and root and soil respiration. Maize plants 24 days old were grazed (c. 25–50% leaf area removed) by caging grasshoppers around individual plants and 18 h later pulse-labelled with¹⁴CO₂. During the next 8 h,¹⁴C assimilates were traced to shoots, roots, root plus soil respiration, root exudates, rhizosphere soil, and bulk soil using carbon-14 techniques. Significant positive relationships were observed between herbivory and carbon allocated to roots, root exudates, and root and soil respiration, and a significant negative relationship between herbivory and carbon allocated to shoots. No relationship was observed between herbivory and¹⁴C recovered from soil. While herbivory increased root and soil respiration, the peak time for¹⁴CO₂ evolved as respiration was not altered, thereby suggesting that herbivory only increases the magnitude of respiration, not patterns of translocation through time. Although there was a trend for lower photosynthetic rates of grazed plants than photosynthetic rates of ungrazed plants, no significant differences were observed among grazed and ungrazed plants. We conclude that above-ground herbivory can increase plant carbon fluxes below ground (roots, root exudates, and rhizosphere respiration), thus increasing resources (e.g., root exudates) available to soil organisms, especially microbial populations.     

Effects of Temperature and Nutrient Supply on Resource Allocation,Photosynthetic Strategy,and Metabolic Rates of Synechococcus sp.

Temperature and nutrient supply are key factors that control phytoplankton ecophysiology, but their role is commonly investigated in isolation. Their combined effect on resource allocation, photosynthetic strategy, and metabolism remains poorly understood. To characterize the photosynthetic strategy and resource allocation under different conditions, we analyzed the responses of a marine cyanobacterium (Synechococcus PCC 7002) to multiple combinations of temperature and nutrient supply. We measured the abundance of proteins involved in the dark (RuBisCO, rbcL) and light (Photosystem II, psbA) photosynthetic reactions, the content of chlorophyll a, carbon and nitrogen, and the rates of photosynthesis, respiration, and growth. We found that rbcL and psbA abundance increased with nutrient supply, whereas a temperature-induced increase in psbA occurred only in nutrient-replete treatments. Low temperature and abundant nutrients caused increased RuBisCO abundance, a pattern we observed also in natural phytoplankton assemblages across a wide latitudinal range. Photosynthesis and respiration increased with temperature only under nutrient-sufficient conditions. These results suggest that nutrient supply exerts a stronger effect than temperature upon both photosynthetic protein abundance and metabolic rates in Synechococcus sp. and that the temperature effect on photosynthetic physiology and metabolism is nutrient dependent. The preferential resource allocation into the light instead of the dark reactions of photosynthesis as temperature rises is likely related to the different temperature dependence of dark-reaction enzymatic rates versus photochemistry. These findings contribute to our understanding of the strategies for photosynthetic energy allocation in phytoplankton inhabiting contrasting environments.     

水分胁迫下烯效唑对百日草幼苗光合特性及叶解剖结构的影响

以百日草‘芳菲1号’为试材,研究不同水分胁迫下烯效唑(S3307)对其幼苗生长、光合特性及叶解剖结构的影响,以明确S3307对百日草的抗旱作用及其机理。结果显示:(1)在水分胁迫下,百日草的生长均受到不同程度的抑制,叶绿素含量显著降低,光合作用受到抑制,叶解剖结构有所变化。(2)S3307处理后,均能够显著降低所对应的不同程度水分胁迫下百日草的株高,显著增加茎粗、叶面积、叶片厚度、栅栏组织厚度和根冠比,显著增加叶绿素含量,提高百日草的净光合速率(Pn)、气孔导度(Gs)、蒸腾速率(Tr)和水分利用效率(WUE)。研究表明,S3307能够提高百日草的抗旱性,而且在轻度和中度水分胁迫下Pn的下降主要是由气孔因素引起,而在重度水分胁迫下光合速率的下降是由非气孔因素引起的。     

Effect of sucrose concentration on photosynthetic activity of in vitro cultures <Emphasis Type="Italic">Gentiana kurroo</Emphasis> (Royle) germlings

In this work, the effect of sucrose on photosynthetic activity during in vitro culture was studied. Experiments were carried out using uniform somatic embryo-derived germlings of Gentiana kurroo (Royle) confirmed by chromosome counting and flow cytometry technique. Photosynthetic activity was measured by chlorophyll a fluorescence and gas exchange method. The efficiency of photosynthetic apparatus as measured by the ratio F _v/F _m, Yield and qP (light phase of photosynthesis) was the highest when the medium was supplemented with 0.3% sucrose which well corresponded with plant gas exchange. Taking all data into consideration for the best development of photosynthetic apparatus and the most efficient of net photosynthesis of studied germlings would be medium supplemented with 0.2–0.4% of sucrose.     

Interactive Effects of Thiourea and Phosphorus on Clusterbean Under Water Stress

Effects of phosphorus and thiourea application (either alone or in combination) were studied on clusterbean (Cyamopsis tetragonoloba Taub.) plants subjected to water stress by withholding irrigation at pre- and post-flowering stages in pot culture trial. Water stress significantly decreased shoot water potential, relative water content of leaves, net photosynthetic rate, contents of total chlorophyll, starch and soluble proteins as well as nitrate reductase activity at both the growth stages. Application of phosphorus and thiourea or combined application increased most of these parameters. Results revealed synergistic effects of P and thiourea in enhancing net photosynthesis, leaf area, chlorophyll content and nitrogen metabolism leading to significant improvement in plant growth and seed yield under water stress condition.