Photosynthetic Carbon Fixation Pathway Research Articles

Integrative physiology and transcriptome reveal salt-tolerance differences between two licorice species: Ion transport, Casparian strip formation and flavonoids biosynthesis

BackgroundGlycyrrhiza inflata Bat. and Glycyrrhiza uralensis Fisch. are both original plants of ‘Gan Cao’ in the Chinese Pharmacopoeia, and G. uralensis is currently the mainstream variety of licorice and has a long history of use in traditional Chinese medicine. Both of these species have shown some degree of tolerance to salinity, G. inflata exhibits higher salt tolerance than G. uralensis and can grow on saline meadow soils and crusty saline soils. However, the regulatory mechanism responsible for the differences in salt tolerance between different licorice species is unclear. Due to land area-related limitations, the excavation and cultivation of licorice varieties in saline-alkaline areas that both exhibit tolerance to salt and contain highly efficient active substances are needed. The systematic identification of the key genes and pathways associated with the differences in salt tolerance between these two licorice species will be beneficial for cultivating high-quality salt-tolerant licorice G. uralensis plant varieties and for the long-term development of the licorice industry. In this research, the differences in growth response indicators, ion accumulation, and transcription expression between the two licorice species were analyzed.ResultsThis research included a comprehensive comparison of growth response indicators, including biomass, malondialdehyde (MDA) levels, and total flavonoids content, between two distinct licorice species and an analysis of their ion content and transcriptome expression. In contrast to the result found for G. uralensis, the salt treatment of G. inflata ensured the stable accumulation of biomass and total flavonoids at 0.5 d, 15 d, and 30 d and the restriction of Na+ to the roots while allowing for more K+ and Ca2+ accumulation. Notably, despite the increase in the Na+ concentration in the roots, the MDA concentration remained low. Transcriptome analysis revealed that the regulatory effects of growth and ion transport on the two licorice species were strongly correlated with the following pathways and relevant DEGs: the TCA cycle, the pentose phosphate pathway, and the photosynthetic carbon fixation pathway involved in carbon metabolism; Casparian strip formation (lignin oxidation and translocation, suberin formation) in response to Na+; K+ and Ca2+ translocation, organic solute synthesis (arginine, polyamines, GABA) in response to osmotic stresses; and the biosynthesis of the nonenzymatic antioxidants carotenoids and flavonoids in response to antioxidant stress. Furthermore, the differential expression of the DEGs related to ABA signaling in hormone transduction and the regulation of transcription factors such as the HSF and GRAS families may be associated with the remarkable salt tolerance of G. inflata.ConclusionCompared with G. uralensis, G. inflata exhibits greater salt tolerance, which is primarily attributable to factors related to carbon metabolism, endodermal barrier formation and development, K+ and Ca2+ transport, biosynthesis of carotenoids and flavonoids, and regulation of signal transduction pathways and salt-responsive transcription factors. The formation of the Casparian strip, especially the transport and oxidation of lignin precursors, is likely the primary reason for the markedly higher amount of Na+ in the roots of G. inflata than in those of G. uralensis. The tendency of G. inflata to maintain low MDA levels in its roots under such conditions is closely related to the biosynthesis of flavonoids and carotenoids and the maintenance of the osmotic balance in roots by the absorption of more K+ and Ca2+ to meet growth needs. These findings may provide new insights for developing and cultivating G. uralensis plant species selected for cultivation in saline environments or soils managed through agronomic practices that involve the use of water with a high salt content.

Root exudation under maize/soybean intercropping system mediates the arbuscular mycorrhizal fungi diversity and improves the plant growth.

Maize/soybean intercropping is a common cropping practice in Chinese agriculture, known to boost crop yield and enhance soil fertility. However, the role of below-ground interactions, particularly root exudates, in maintaining intercropping advantages in soybean/maize intercropping systems remains unclear. This study aimed to investigate the differences in root exudates between intercropping and monocropping systems through two pot experiments using metabolomics methods. Multiple omics analyses were conducted to explore correlations between differential metabolites and the community of Arbuscular Mycorrhizal Fungi (AMF), shedding light on the mechanisms underlying the dominance of intercropping from the perspective of root exudates-soil microorganism interactions. The study revealed that intercropping significantly increased the types and contents of root exudates, lowered soil pH, increased the availability of nutrients like available nitrogen (AN) and available phosphorus (AP), and enhanced AMF colonization, resulting in improving the community composition of AMF. Besides, root exudates in intercropping systems differed significantly from those in monocropping, with 41 and 39 differential metabolites identified in the root exudates of soybean/maize, predominantly amino acids and organic acids. The total amount of amino acids in the root exudates of soybean intercropping was 3.61 times higher than in monocropping. Additionally, the addition of root exudates significantly improved the growth of soybean/maize and AMF colonization, with the mycorrhizal colonization rate in intercropping increased by 105.99% and 111.18% compared to monocropping, respectively. The identified metabolic pathways associated with root exudates were closely linked to plant growth, soil fertility improvement, and the formation of AMF. Correlation analysis revealed a significant relationship (P < 0.05) between certain metabolites such as tartaric acid, oxalic acid, malic acid, aspartic acid, alanine, and the AMF community. Notably, the photosynthetic carbon fixation pathway involving aspartic acid showed a strong association with the function of Glomus_f_Glomerace, the dominant genus of AMF. A combined analysis of metabolomics and high throughput sequencing revealed that the root exudates of soybean/maize intercropping have direct or indirect connections with AMF and soil nutrients. This suggests that the increased root exudates of the soybean/maize intercropping system mediate an improvement in AMF community composition, thereby influencing soil fertility and maintaining the advantage of intercropping.

Molecular Mechanism of Organic Pollutant-Induced Reduction of Carbon Fixation and Biomass Yield in Oryza sativa L.

Photosynthetic carbon fixation is fundamental for plant growth and is a key process driving the global carbon cycle. This study explored the mechanism of disturbed carbon fixation in Oryza sativa L. by organic pollutants 2,3,4,5-tetrachlorobiphenyl (CB 61), 4'-hydroxy-2,3,4,5-tetrachlorobiphenyl (4'-OH-CB 61), 2,2',4,4'-tetrabromo diphenyl ether (BDE 47), tricyclazole (TRI), and pyrene. The biomass of rice exposed to 4'-OH-CB 61, TRI, and BDE 47 was on average 80.63% of that of the control (p < 0.05), and the inhibition of net photosynthetic rate was 59.15% by 4'-OH-CB 61. Proteomics confirmed that 4'-OH-CB 61 significantly downregulated the enzymes in the photosynthetic carbon fixation pathway, which was attributed to the decrease in ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), the rate-limiting enzyme in the Calvin cycle. In detail, decreased Rubisco activity (6.96-33.44%) and downregulated OsRBCS2-5 encoding small Rubisco subunits (-6.80 < log2FC < -2.13) by 4'-OH-CB 61, TRI, and BDE 47 were in line with biomass yield reduction. Molecular docking and dynamic simulation suggested that the three pollutants potentially competed with CO2 for binding to the active sites in Rubisco, leading to reduced CO2 capture efficiency. These results revealed the molecular mechanism of organic pollution-induced rice yield reduction, contributing to improving the understanding of crop growth and carbon sequestration capacity of organics-contaminated soils globally.

Third-generation sequencing and metabolome analysis reveal candidate genes and metabolites with altered levels in albino jackfruit seedlings

BackgroundMost plants rely on photosynthesis; therefore, albinism in plants with leaves that are white instead of green causes slow growth, dwarfing, and even death. Although albinism has been characterized in annual model plants, little is known about albino trees. Jackfruit (Artocarpus heterophyllus) is an important tropical fruit tree species. To gain insight into the mechanisms underlying the differential growth and development between albino jackfruit mutants and green seedlings, we analyzed root, stem, and leaf tissues by combining PacBio single-molecule real-time (SMRT) sequencing, high-throughput RNA-sequencing (RNA-seq), and metabolomic analysis.ResultsWe identified 8,202 differentially expressed genes (DEGs), including 225 genes encoding transcription factors (TFs), from 82,572 full-length transcripts. We also identified 298 significantly changed metabolites (SCMs) in albino A. heterophyllus seedlings from a set of 692 metabolites in A. heterophyllus seedlings. Pathway analysis revealed that these DEGs were highly enriched in metabolic pathways such as ‘photosynthesis’, ‘carbon fixation in photosynthetic organisms’, ‘glycolysis/gluconeogenesis’, and ‘TCA cycle’. Analysis of the metabolites revealed 76 SCMs associated with metabolic pathways in the albino mutants, including L-aspartic acid, citric acid, succinic acid, and fumaric acid. We selected 225 differentially expressed TF genes, 333 differentially expressed metabolic pathway genes, and 76 SCMs to construct two correlation networks. Analysis of the TF–DEG network suggested that basic helix-loop-helix (bHLH) and MYB-related TFs regulate the expression of genes involved in carbon fixation and energy metabolism to affect light responses or photomorphogenesis and normal growth. Further analysis of the DEG–SCM correlation network and the photosynthetic carbon fixation pathway suggested that NAD-ME2 (encoding a malic enzyme) and L-aspartic acid jointly inhibit carbon fixation in the albino mutants, resulting in reduced photosynthetic efficiency and inhibited plant growth.ConclusionsOur preliminarily screening identified candidate genes and metabolites specifically affected in albino A. heterophyllus seedlings, laying the foundation for further study of the regulatory mechanism of carbon fixation during photosynthesis and energy metabolism. In addition, our findings elucidate the way genes and metabolites respond in albino trees.

Geological Factors Impacted Cadmium Availability and use as an Alternative Cofactor for Zinc in the Carbon Fixation Pathways of Marine Diatoms

AbstractTransition metal cofactors are crucial for many biological processes. Despite being primarily considered to be toxic, the transition metal cadmium (Cd) was discovered to be a substitute cofactor for zinc (Zn) in photosynthetic carbon fixation pathways of marine diatoms. However, it is not known how conditions in the geosphere impacted Cd availability and its incorporation as an alternative metal cofactor for phytoplankton. We employed mineral chemistry network analysis to investigate which geochemical factors may have influenced the availability of Cd and Zn during the putative time period that the alternative Cd‐based pathway evolved. Our results show that Zn minerals are more chemically diverse than are Cd minerals, but Zn‐ and Cd‐containing minerals have similar network centrality values when specifically considering sulfur (S)‐containing species. Cadmium and Zn sulfides are the most common Cd‐ and Zn‐containing mineral species over the past 500 million years. In particular, the Cd and Zn sulfides, respectively greenockite and sphalerite, were highly abundant during this time period. Furthermore, S‐containing Cd and Zn minerals are commonly co‐located in geologic time, allowing them to be weathered and transported to the ocean in tandem, rather than from separate sources. We suggest that the simultaneous weathering of Cd and Zn sulfides allowed for Cd to be a bioavailable direct substitute for Zn in protein complexes during periods of Zn depletion. The biogeochemical cycles of Zn and Cd exemplify the importance of the coevolution of the geosphere and biosphere in shaping primary production in the modern ocean.

Transcriptome Analysis of Differentially Expressed Genes in Wild Jujube Seedlings under Salt Stress

Wild jujube (Ziziphus acidojujuba) and cultivated jujube (Ziziphus jujuba) belong to the family Rhamnaceae. Jujubes have marked drought- and salt-tolerant properties. After salt stress, wild jujube seedling growth was inhibited and photosynthetic efficiency was reduced. A bioinformatics approach was used to analyze the transcriptomics data from wild jujube seedlings grown under salt stress, and the genes differentially expressed under the salt stress were identified to provide a theoretical basis for the development and use of wild jujube plantations in salinized soil. The transcriptome sequencing from leaves of wild jujube seedlings was carried out using second-generation sequencing technology. The effects of salt stress on the differential expression of photosynthesis-related genes in wild jujube seedlings were analyzed. Transcriptome sequencing revealed a total of 5269 differentially expressed genes (DEGs), of which 2729 were up-regulated and 2540 were down-regulated. DEGs were mainly enriched with respect to photosynthesis, photosynthetic antenna proteins, glyoxylic acid and dicarboxylic acid metabolism, linolenic acid metabolism, cysteine and methionine metabolism, and porphyrin and chlorophyll metabolism. Among them, the photosynthesis pathway-related DEGs were most highly enriched. Further analysis of porphyrin and chlorophyll synthesis and photosynthesis-related pathways revealed that they were significantly enriched by 97 photosynthesis-related DEGs. The DEGs in the photosynthesis and photosynthetic antenna protein pathways were down-regulated, whereas the DEGs glutamyl-tRNA reductase (HEMA), ferrochelatase (HEMH), and pheophorbide a oxygenase (PAO) in the porphyrin and chlorophyll synthesis pathways were up-regulated, with the remainder being down-regulated. The nuclear gene encoding Rubisco, the key enzyme in the photosynthetic carbon fixation pathway, was also down-regulated. The results showed that the photosynthetic rate of wild jujube seedlings decreased following exposure to salinity stress, an effect that was related to the increased synthesis of 5-aminolevulinic acid and heme, and the up-regulation of expression of a gene encoding a chlorophyll-degrading enzyme, and was related to the down-regulation of gene expression in photosynthesis-related pathways such as light energy capture and carbon fixation. Selection of nine DEGs related to photosynthesis and chlorophyll biosynthesis by quantitative real-time-PCR confirmed that expression changes of these nine DEGs were consistent with the transcriptome sequencing results.

Ultrastructural evolution, physiological traits, and carbon and nitrogen assimilation–related gene expression are compatible with the developmental transitions of parthenogenesis in Pyropia haitanensis (Bangiales, Rhodophyta)

Pyropia haitanensis has a typical parthenogenetic behavior under unisexual culture condition. In this study, ultrastructural evolution and physiological traits of photosynthesis and the expression characteristics of key enzyme genes related to carbon and nitrogen assimilation pathways at different developmental stages of parthenogenesis were analyzed. Ultrastructural observations showed that the gametophytic vegetative cells had cell wall, pyrenoids, abundant chloroplasts, and rarely scattered floridean starch grains. However, the appearance of gathered chloroplasts, plastoglobuli, large lipid droplets, pyrenoids encircled by numerous plastoglobuli, and an increased number of floridean starch grains in the cells during parthenogenesis provide neutral lipid and energy resources that are compatible with the developmental transitions. Physiologically, vegetative gametophytes possessed the highest Fv/Fm and Chl. a content and the lowest C/N ratio. However, compared with vegetative cell proliferation stage, the activities of RubisCO (ribulose bisphosphate carboxylase) and external CA (carbonic anhydrase) increased at early stages of parthenogenesis, and Fv/Fm and activities of PEPC (phosphoenolpyruvate carboxylase) and PEPCK (phosphoenolpyruvate carboxykinase) remained higher at later stages of parthenogenesis, suggesting that dynamic photosynthetic carbon fixation occurs in parthenogenesis. Quantitative real-time PCR analysis showed that selective regulation of key enzyme genes related to carbon and nitrogen assimilation pathways maintained stable carbon and nitrogen metabolism during developmental transitions of parthenogenesis. Moreover, during parthenogenesis, photosynthetic carbon fixation pathway potentially changed from C3 pathway at female gametophyte stage to C4 pathway at parthenosporophyte stage, which is similar to that of gametophytes and sporophytes in sexual reproduction. Meanwhile, the ammonia assimilation involved GDH pathway (glutamate dehydrogenase pathway) and GS/GOGAT cycle (glutamine synthetase/glutamate synthase cycle) in female gametophyte stage, while GS/GOGAT cycle was dominant in parthenosporophyte stage, suggesting that GS/GOGAT cycle may play an important role in the carbon and nitrogen balance during the developmental transitions of parthenogenesis. These results provide important information for revealing the adaptation strategy of parthenogenesis in Pyropia.

Stable isotope (S, C) chemostratigraphy and hydrocarbon biomarkers in the Ediacaran upper section of Sierras Bayas Group, Argentina

The Neoproterozoic sedimentary successions of the Sierras Bayas Group (SBG) are situated within the Tandilia Belt, Buenos Aires Province, Argentina. The Loma Negra Formation (LNF) from the upper part of the SBG has been investigated using a combination of inorganic, organic and isotopic geochemical methods in order to determine the stable isotope composition of carbonates (δ13Ccar, δ18Ocar), kerogen (δ13Cker), pyrite and carbonate associated sulfur (δ34Spy, δ34SCAS). Concentrations of major, trace and rare earth elements and hydrocarbon distributions were also investigated. This data set was used to further the understanding of the interactions between paleo-biodiversity and paleoenvironmental conditions for terminal Ediacaran (post-Gaskiers) shelf deposits. The high δ34Spy values exceeding the coeval δ34SCAS values in green micritic limestones of the lower LNF may be explained by a combination of different events such as globally low seawater sulfate concentrations, an increased rate of bacterial sulfate reduction, and a decrease in sulfate levels in the near-bottom deepest water layer inherited from glaciations. Negative Ce anomalies in the dark grey micritic limestones of the upper LNF suggest they were deposited under oxidizing conditions. However, higher concentrations of Fe, Mo, Zn and REE, and the occurrence of authigenic 34S-enriched pyrite suggest that the sediments were maintained under reducing conditions. The distributions of hydrocarbon biomarkers (e.g. hopanes maximizing at C29) are in line with this hypothesis and indicate a diverse microbial community including primary producers such as cyanobacteria (e.g. terminally-branched monomethyl alkanes, hopanoid distribution), phototrophic bacteria (e.g. acyclic isoprenoids C<21) and green bacteria (e.g. n-C18 ≫ Phytane). The unsteady Δ13Ccar-ker values reflect changes in primary biomass due to relative contributions of bacterial microorganisms using different photosynthetic carbon-fixation pathways. The lowest Δ13Ccar-ker values coincided with the latest biomarker signal interpreted as the signature of green non-sulfur bacteria using a less 13C fractionation pathway. The combined biogeochemical features of the Loma Negra Formation, indicated a well-stratified water column with oxygenated surface waters, oxygen-poor bottom waters and anoxic sediments which helped to refine a correlation with the Polanco Formation from the Arroyo del Soldado Group in Uruguay.

Comparison of salinity tolerance of three Atriplex spp. in well-watered and drying soils

Abstract Members of the Chenopodiaceae are well adapted to both salt and drought stress and can serve as model species to understand the mechanisms of tolerance in plants. We grew Atriplex hortensis (ATHO), A. canescens (ATCA), and A. lentiformis (ATLE) along a NaCL salinity gradient under non-water-limited conditions and in drying soils in greenhouse experiments. The species differed in photosynthetic carbon fixation pathway, capacity for sodium uptake, and habitat preferences. Under non-water-limited conditions, ATLE (C4) maintained high growth rates up to 30 g L −1 NaCl. ATHO (C3) had lower growth than ATLE at high salinities, while ATCA (C4) grew more slowly than either ATLE or ATHO and showed no net growth above 20 g L −1 NaCl. ATHO and ATLE accumulated twice as much sodium in their shoots as ATCA, but all three species had increasing sodium levels at higher salinities. Potassium, magnesium and calcium levels were relatively constant over the salinity gradient. All three species showed marked accumulation of chloride across the salinity gradient, whereas nitrate, phosphorous and sulfate decreased with salinity. The effect of drought was simulated by growing plants in sealed pots with an initial charge of water plus NaCl, and allowing them to grow to the end point at which they no longer were able to extract water from the soil solution. Drought and salinity were not additive stress factors for Atriplex spp. in this experiment. NaCl increased their ability to extract water from the soil solution compared to fresh water controls. ATLE showed increased shoot dry matter production and increased water use efficiency (WUE) as initial salinity levels increased from 0 to 30 g L −1 NaCl, whereas dry matter production and WUE peaked at 5 g L −1 for ATHO and ATCA. Final soil moisture salinities tolerated by species were 85 g L −1 , 55 g L −1 and 160 g L −1 NaCl for ATHO, ATCA and ATLE, respectively. C4 photosynthesis and sodium accumulation in shoots were associated with high drought and salt tolerance.

The Complete Genome Sequence of Roseobacter denitrificans Reveals a Mixotrophic Rather than Photosynthetic Metabolism

Purple aerobic anoxygenic phototrophs (AAPs) are the only organisms known to capture light energy to enhance growth only in the presence of oxygen but do not produce oxygen. The highly adaptive AAPs compose more than 10% of the microbial community in some euphotic upper ocean waters and are potentially major contributors to the fixation of the greenhouse gas CO2. We present the complete genomic sequence and feature analysis of the AAP Roseobacter denitrificans, which reveal clues to its physiology. The genome lacks genes that code for known photosynthetic carbon fixation pathways, and most notably missing are genes for the Calvin cycle enzymes ribulose bisphosphate carboxylase (RuBisCO) and phosphoribulokinase. Phylogenetic evidence implies that this absence could be due to a gene loss from a RuBisCO-containing alpha-proteobacterial ancestor. We describe the potential importance of mixotrophic rather than autotrophic CO2 fixation pathways in these organisms and suggest that these pathways function to fix CO2 for the formation of cellular components but do not permit autotrophic growth. While some genes that code for the redox-dependent regulation of photosynthetic machinery are present, many light sensors and transcriptional regulatory motifs found in purple photosynthetic bacteria are absent.

Carboxylic acid distribution patterns of temperate C 3 and C 4 crops

Agricultural grasses cover-a major part of the land surface in temperate agro-ecosystems and contribute significantly to the formation of soil organic matter. Crop-derived lipids are assumed to be responsible for fast carbon turnover in soils. Differences in lipid distribution patterns between crops following C 3 and C 4 photosynthesis pathways have rarely been described, but could be useful for source apportionment of crop-derived input into soils or sediments. The distribution of long chain n-carboxylic acids (C 22 , C 24 , C 26 ) reveals significant differences between crop plants following either the C 3 or the C 4 photosynthetic carbon fixation pathway. The plant compartments leaves, stems and roots of C 4 plants contain relatively large proportions (> 40%) of n-C 24 carboxylic acid when compared to C 3 plants. These reveal larger relative proportions of n-C 22 and n-C 26 acids, whose relative abundance is subject to change between different plant compartments and during the growing season. The carboxylic acid ratio [CAR = n-C 24 /(n-C 22 + n-C 26 ) carboxylic acids] provides distinct fatios for C 4 (> 0.67) and C 3 crops (< 0.67) and can thus be used as a molecular marker for the differentiation of crop plant biomass. In combination with the bulk stable carbon isotopic composition (δ 13 C) the CAR can be used as a tool for the estimation of the C 4 derived carbon proportion in soils or sediments.

Le vie fotosintetiche C3, C4e CAM nel contesto ecologico

Abstract Ecological aspects of C3, C4 and CAM photosynthetic pathways. - Three different photosynthetic CO2 fixation pathways are known to occur in higher plants. However all three pathways ultimately depend on the Calvin-Benson cycle for carbon reduction. The oxygenase activity of RuBP carboxilase is responsible for photorespiratory CO2 release. Both C4 and CAM pathways behave as a CO2 concentrating mechanism which prevent photorespiration. The CO2-concentrating mechanism in C4 plants is based on intracellular symplastic transport of C4 dicarboxylic acids from mesophyll-cells to the adjacent bundle-sheath cells. On the contrary in CAM plants the CO2-concentrating mechanism is based on the intracellular transport of malic acid into and out of the vacuole. The C4 photosynthetic pathway as compared to the C3 pathway permits higher rates of CO2 fixation in high light and high temperature environments at low costs in terms of water loss, given the stability of the photosynthetic apparatus under such conditions. CAM is interpreted as an adaptation to arid environments because it enables carbon assimilation to take place at very low water costs during the night when the evaporative demand is low. Nevertheless many aquatic species of Isoetes and some relatives are CAM, suggesting the adaptive role of CAM to environments which become depleted in CO2. The photosynthetic carbon fixation pathway certainly contributes to the ecological success of plants in different environments. However the distribution of plants may also reflect their biological history. On the other hand plants with different photosynthetic pathways coexist in many communities and tend to share resources in time. In any case some generalizations are possible: C4 plants enjoy an ecological advantage in hot, moist, high light regions while the majority of species in desert environments are C3; CAM plants are more frequent in semiarid regions with seasonal rainfall, coastal fog deserts, and in epiphytic habitats in tropical rain forests.

Photosynthesis in Submersed Macrophytes of a Temperate Lake

The photosynthetic carbon fixation pathways and levels of carbon-fixing enzymes of four dominant submersed macrophytes of Lawrence Lake, southern Michigan, were investigated during the main growth season (May to November). All four species (Scirpus subterminalis Torr., Najas flexilis (Willd.) Rostk. and Schmidt, Potamogeton praelongus Wulf., and Myriophyllum heterophyllum Michx.) were C(3) plants based on their patterns of (14)C pulse-chase incorporation. High levels of phosphoenolpyruvate carboxylase were also found in these species. These levels, as well as the ribulose 1,5-biphosphate carboxylase/phosphoenolpyruvate carboxylase ratio of the leaves, varied throughout the growing season and exhibited highest values in July. No shift in carbon fixation pathways, however, could be detected from July to October. The possible functions of phosphoenolypyruvate carboxylase in these plants, as well as the significance of C(3) metabolism in submersed plants of temperate lakes, are delineated.

Photosynthetic carbon fixation pathways in Zostera marina and three Florida seagrasses

Abstract The photosynthetic carbon fixation pathways of four seagrass species, Zostera marina L. from Alaska and Thalassia testudinum Banks ex Konig, Syringodium filiforme Kutz. and Halodule wrightii Aschers. from the Gulf of Mexico, were investigated with a 14 C pulse-chase technique. All species were found to be principally of the C 3 type. However, Thalassia and Halodule had higher initial incorporation rates into organic acids than is typical for terrestrial C 3 plants. Of 11 seagrass species investigated thus far for C 3 or C 4 metabolism using this technique, 10 were found to be principally of the C 3 type while only one exhibited C 4 metabolism.

Leaf anatomy of species in some dicotyledon families as related to the C3 and C4 pathways of carbon fixation

Species in the families Amaranthaceae, Aizoaceae, Chenopodiaceae, Convolvulaceae, Euphorbiaceae, Nyctaginaceae, Portulacaceae, and Zygophyllaceae were examined for leaf anatomy typical of plants having the C4 type photosynthetic carbon fixation pathway. They are assembled by families into three groups: genera in which all species possess the specialization; genera in which some but not all species possess the specialization; and genera in which no species possess the specialization. The specialization in leaf anatomy was noted in species of 24 genera. Its presence is highly correlated with a habitat of limited water availability and (or) with a tropical origin. The carbon dioxide compensation points of nine species in the Chenopodiaceae family were measured. Low values were obtained only for those species that possess a form of specialized leaf anatomy (Atriplex confertifolia (Torr. &amp; Frem.) Wats., A. falcata (M. E. Jones) Standl., Halogeton glomeratus (Bieb.) Meyer, Salsola kali L. var. tenuiflora Tausch.). The latter two species, having centric leaves, do not possess the more typical differential bundle sheath chlorenchyma but do possess two, individually distinct, single-cell layers of chlorenchyma, adjacent and external to the peripheral veins.