The Photosynthetic Cycle. CO2 Dependent Transients

The end game(s) of photosynthetic carbon metabolism

The year 2024 marks 70 years since the general outline of the carbon pathway in photosynthesis was published. Although several alternative pathways are now known, it is remarkable how many organisms use the reaction sequence described 70 yrs ago, which is now known as the Calvin-Benson cycle or variants such as the Calvin-Benson-Bassham cycle or Benson-Calvin cycle. However, once the carbon has entered the Calvin-Benson cycle and is converted to a 3-carbon sugar, it has many potential fates. This review will examine the last stages of photosynthetic metabolism in leaves. In land plants, this process mostly involves the production of sucrose provided by an endosymbiont (the chloroplast) to its host for use and transport to the rest of the plant. Photosynthetic metabolism also usually involves the synthesis of starch, which helps maintain respiration in the dark and enables the symbiont to supply sugars during both the day and night. Other end products made in the chloroplast are closely tied to photosynthetic CO2 assimilation. These include serine from photorespiration and various amino acids, fatty acids, isoprenoids, and shikimate pathway products. I also describe 2 pathways that can short circuit parts of the Calvin-Benson cycle. These final processes of photosynthetic metabolism play many important roles in plants.

The discovery of rubisco

Rubisco is possibly the most important enzyme on Earth, certainly in terms of amount. This review describes the initial reports of ribulose 1,5-bisphosphate carboxylating activity. Discoveries of core concepts are described, including its quaternary structure, the requirement for post-translational modification, and its role as an oxygenase as well as a carboxylase. Finally, the requirement for numerous chaperonins for assembly of rubisco in plants is described.

Late-Stage Carbon-14 Labeling and Isotope Exchange: Emerging Opportunities and Future Challenges

Carbon-14 (¹⁴C) is a gold standard technology routinely utilized in pharmaceutical and agrochemical industries for tracking synthetic organic molecules and providing their metabolic and safety profiles. While the state of the art has been dominated for decades by traditional multistep synthetic approaches, the recent emergence of late-stage carbon isotope labeling has provided new avenues to rapidly access carbon-14-labeled biologically relevant compounds. In particular, the development of carbon isotope exchange has represented a fundamental paradigm change, opening the way to unexplored synthetic transformations. In this Perspective, we discuss the recent developments in the field with a critical assessment of the literature. We subsequently discuss research directions and future challenges within this rapidly evolving field.

Effects of excess ammoniacal nitrogen (NH4+-N) on pigments, photosynthetic rates, chloroplast ultrastructure, proteomics, formation of reactive oxygen species and enzymatic activity in submerged plant Hydrilla verticillata (L.f.) Royle

Although excess ammoniacal-nitrogen (NH₄⁺-N) results in the disturbance of various important biochemical and physiological processes, a detailed study on the effects of NH₄⁺-N stress on the photosynthesis and global changes in protein levels in submerged macrophytes is still lacking. Here, the changes of excess NH₄⁺-N on physiological parameters in Hydrilla verticillata (L.f.) Royle, a submerged macrophyte were investigated, including the contents of photosynthetic pigments, soluble sugars, net photosynthesis and respiration, glutamine synthetase (GS) and glutamate synthase (GOGAT) activities, chloroplast ultrastructure, chloroplast reactive oxygen species (ROS) accumulation and protein levels. Our results showed that the net photosynthetic rate and pigment content reached maximum values when the plants were treated with 1 and 2 mg L^-1 NH₄⁺-N, respectively, and decreased at NH₄⁺-N concentrations at 5, 10, 15 and 20 mg L^-1. This decrease might be caused by ROS accumulation. Compared that in 0.02 mg L^-1 NH₄⁺-N as a control, ROS generation in chloroplasts significantly increased in the presence of more than 2 mg L^-1 NH₄⁺-N. Consistently, the damages caused by over-accumulated ROS were observed in chloroplast ultrastructure, showing a loose thylakoid membranes and swollen grana/stroma lamellae. Furthermore, through proteomic analysis, we identified 91 differentially expressed protein spots. Among them, six proteins involved in photosynthesis decreased in abundance in response to excess NH₄⁺-N. Surprisingly, the abundance of all the identified proteins that were involved in nitrogen assimilation and amino acid metabolism tended to increase under excess NH₄⁺-N compared with the control, suggestive of the imbalanced carbon and nitrogen (C-N) metabolisms. In support, activated GS and GOGAT cycle was observed, evidenced by higher activities of GS and GOGAT enzymes. To our knowledge, this work is the first description that excess NH₄⁺-N results in chloroplast ultrastructural damages and the first proteomic evidence to support that excess NH₄⁺-N can lead to a decline in photosynthesis and imbalance of C-N metabolism in submerged macrophytes.

Systems-level analysis of isotopic labeling in untargeted metabolomic data by X13CMS

Identification of previously unreported metabolites (so-called 'unknowns') in untargeted metabolomic data has become an increasingly active area of research. Considerably less attention, however, has been dedicated to identifying unknown metabolic pathways. Yet, for each unknown metabolite structure, there is potentially a yet-to-be-discovered chemical transformation. Elucidating these biochemical connections is essential to advancing our knowledge of cellular metabolism and can be achieved by tracking an isotopically labeled precursor to an unexpected product. In addition to their role in mapping metabolic fates, isotopic labels also provide critical insight into pathway dynamics (i.e., metabolic fluxes) that cannot be obtained from conventional label-free metabolomic analyses. When labeling is compared quantitatively between conditions, for example, isotopic tracers can enable relative pathway activities to be inferred. To discover unexpected chemical transformations or unanticipated differences in metabolic pathway activities, we have developed X13CMS, a platform for analyzing liquid chromatography/mass spectrometry (LC/MS) data at the systems level. After providing cells, animals, or patients with an isotopically enriched metabolite (e.g., 13C, 15N, or 2H), X13CMS identifies compounds that have incorporated the isotopic tracer and reports the extent of labeling for each. The analysis can be performed with a single condition, or isotopic fates can be compared between multiple conditions. The choice of which metabolite to enrich and which isotopic label to use is highly context dependent, but 13C-glucose and 13C-glutamine are often applied because they feed a large number of metabolic pathways. X13CMS is freely available.

Discovery of the canonical Calvin-Benson cycle

It has been 65 years since the Calvin-Benson cycle was first formulated. In this paper, the development of the concepts that are critical to the cycle is traced and the contributions of Calvin, Benson, and Bassham are discussed. Some simplified views often found in text books such as ascending paper chromatography and the use of the "lollipop" for short labeling are discussed and further details given. Key discoveries that underpinned elucidation of the cycle such as the importance of sedoheptulose phosphate and ribulose 1,5-bisphosphate are described. The interchange of ideas between other researchers working on what is now called the pentose phosphate pathway and the development of the ideas of Calvin and Benson are explored while the gluconeogenic aspects of the cycle are emphasized. Concerns raised about anomalies of label distribution in glucose are considered. Other carbon metabolism pathways associated with the Calvin-Benson cycle are also described. Finally, there is a section describing the rift between Calvin and Benson.

Coupled Carbon, Sulfur, and Nitrogen Cycles Mediated by Microorganisms in the Water Column of a Shallow-Water Hydrothermal Ecosystem

Shallow-water hydrothermal vent ecosystems are distinctly different from deep-sea vents, as other than geothermal, sunlight is one of their primary sources of energy, so their resulting microbial communities differ to some extent. Yet compared with deep-sea systems, less is known about the active microbial community in shallow-water ecosystems. Thus, we studied the community compositions, their metabolic pathways, and possible coupling of microbially driven biogeochemical cycles in a shallow-water hydrothermal vent system off Kueishantao Islet, Taiwan, using high-throughput 16S rRNA sequences and metatranscriptome analyses. Gammaproteobacteria and Epsilonbacteraeota were the major active bacterial groups in the 16S rRNA libraries and the metatranscriptomes, and involved in the carbon, sulfur, and nitrogen metabolic pathways. As core players, Thiomicrospira, Thiomicrorhabdus, Thiothrix, Sulfurovum, and Arcobacter derived energy from the oxidation of reduced sulfur compounds and fixed dissolved inorganic carbon (DIC) by the Calvin-Benson-Bassham (CBB) or reverse tricarboxylic acid cycles. Sox-dependent and reverse sulfate reduction were the main pathways of energy generation, and probably coupled to denitrification by providing electrons to nitrate and nitrite. Sulfur-reducing Nautiliaceae members, accounting for a small proportion in the community, obtained energy by the oxidation of hydrogen, which also supplies metabolic energy for some sulfur-oxidizing bacteria. In addition, ammonia and nitrite oxidation is another type of energy generation in this hydrothermal system, with marker gene sequences belonging to Thaumarchaeota/Crenarchaeota and Nitrospina, respectively, and ammonia and nitrite oxidation was likely coupled to denitrification by providing substrate for nitrate and nitrite reduction to nitric oxide. Moreover, unlike the deep-sea systems, cyanobacteria may also actively participate in major metabolic pathways. This study helps us to better understand biogeochemical processes mediated by microorganisms and possible coupling of the carbon, sulfur, and nitrogen cycles in these unique ecosystems.

A historical perspective on radioisotopic tracers in metabolism and biochemistry

Radioisotopes are used routinely in the modern laboratory to trace and quantify a myriad of biochemical processes. The technique has a captivating history peppered with groundbreaking science and with more than its share of Nobel Prizes. The discovery of radioactivity at the end of the 19th century paved the way to understanding atomic structure and quickly led to the use of radioisotopes to trace the fate of molecules as they flowed through complex organic life. The 1940s saw the first radiotracer studies using homemade instrumentation and analytical techniques such as paper chromatography. This article follows the history of radioisotopic tracers from meager beginnings, through to the most recent applications. The author hopes that those researchers involved in radioisotopic tracer studies today will pause to remember the origins of the technique and those who pioneered this fascinating science.

Advances in synthetic dynamic circuits design: using novel synthetic parts to engineer new generations of gene oscillations

As bioengineering applications expand, the need to design and implement circuits that exhibit dynamic properties increases. In particular, schemes that control precise patterns of gene expression as a function of time are essential for balancing multiple metabolic objectives in natural and synthetic systems. Given that modularity has been an important component of dynamic circuits, recent efforts to improve dynamic circuits have focused on replacing old parts for new components that increase the robustness, stability, and tunability. In this review, we show that incorporation of novel components such as regulatory noncoding RNAs (ncRNAs), promoter-transcription factor pairs, and metabolite sensors have allowed traditional dynamic circuits to obtain more robust functionality and improved dynamic properties.

Discovery of causal mechanisms: Oxidative phosphorylation and the Calvin-Benson cycle

We investigate the context of discovery of two significant achievements of twentieth century biochemistry: the chemiosmotic mechanism of oxidative phosphorylation (proposed in 1961 by Peter Mitchell) and the dark reaction of photosynthesis (elucidated from 1946 to 1954 by Melvin Calvin and Andrew A. Benson). The pursuit of these problems involved discovery strategies such as the transfer, recombination and reversal of previous causal and mechanistic knowledge in biochemistry. We study the operation and scope of these strategies by careful historical analysis, reaching a number of systematic conclusions: (1) even basic strategies can illuminate "hard cases" of scientific discovery that go far beyond simple extrapolation or analogy; (2) the causal-mechanistic approach to discovery permits a middle course between the extremes of a completely substrate-neutral and a completely domain-specific view of scientific discovery; (3) the existing literature on mechanism discovery underemphasizes the role of combinatorial approaches in defining and exploring search spaces of possible problem solutions; (4) there is a subtle interplay between a fine-grained mechanistic and a more coarse-grained causal level of analysis, and both are needed to make discovery processes intelligible.

Graph-theoretic analysis of a model for the coupling between photosynthesis and photorespiration

We develop and analyze a mathematical model based on a previously enunciated hypothesis regarding the origin of rapid, irregular oscillations observed in photosynthetic variables when a leaf is transferred to a low-CO2atmosphere. This model takes the form of a set of differential equations with two delays. We review graph-theoretical methods of analysis based on the bipartite graph representation of mass-action models, including models with delays. We illustrate the use of these methods by showing that our model is capable of delay-induced oscillations. We present some numerical examples confirming this possibility, including the possibility of complex transient oscillations. We then use the structure of the identified oscillophore, the part of the reaction network responsible for the oscillations, along with our knowledge of the plausible range of values for one of the delays, to rule out this hypothetical mechanism.

Calibration and data processing in gas chromatography combustion isotope ratio mass spectrometry

Compound-specific isotope analysis (CSIA) by gas chromatography combustion isotope ratio mass spectrometry (GCC-IRMS) is a powerful technique for the sourcing of substances, such as determination of the geographic or chemical origin of drugs and food adulteration, and it is especially invaluable as a confirmatory tool for detection of the use of synthetic steroids in competitive sport. We review here principles and practices for data processing and calibration of GCC-IRMS data with consideration to anti-doping analyses, with a focus on carbon isotopic analysis ((13)C/(12)C). After a brief review of peak definition, the isotopologue signal reduction methods of summation, curve-fitting, and linear regression are described and reviewed. Principles for isotopic calibration are considered in the context of the Δ(13)C = δ(13)C(M) - δ(13)C(E) difference measurements required for establishing adverse analytical findings for metabolites (M) relative to endogenous (E) reference compounds. Considerations for the anti-doping analyst are reviewed.

Carbon isotope ratio (delta13C) values of urinary steroids for doping control in sport

The detection of steroids originating from synthetic precursors in relation to their chemically identical natural analogues has proven to be a significant challenge for doping control laboratories accredited by the World Anti-Doping Agency (WADA). Endogenous steroid abuse may be confirmed by utilising the atomic specificity of gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) that enables the precise measurement of differences in stable isotope ratios that arise as a result of fractionation patterns inherent in the source of steroids. A comprehensive carbon isotope ratio (delta(13)C) profiling study (n=1262) of urinary ketosteroids is reported that demonstrates the inter-individual variation that can be expected from factors such as diet, ethnicity, gender and age within and between different populations (13 countries). This delta(13)C distribution is shown by principal component analysis (PCA) to provide a statistical comparison to delta(13)C values observed following administration of testosterone enanthate. A limited collection of steroid diol data (n=100; consisting of three countries) is also presented with comparison to delta(13)C values of excreted testosterone to validate criteria for WADA accredited laboratories to prove doping offences.

Discoveries in Rubisco (Ribulose 1,5-bisphosphate carboxylase/oxygenase): a historical perspective

Historic discoveries and key observations related to Rubisco (Ribulose 1,5-bisphosphate carboxylase/oxygenase), from 1947 to 2006, are presented. Currently, around 200 papers describing Rubisco research are published each year and the literature contains more than 5000 manuscripts on the subject. While trying to ensure that all the major events over this period are recorded, this analysis will inevitably be incomplete and will reflect the areas of particular interest to the authors.

Monosaccharide-H2O2 reactions as a source of glycolate and their stimulation by hydroxyl radicals

An analysis of the H(2)O(2)-induced breakdown and transformation of different keto-monosaccharides at physiological concentrations reveals that glycolate and other short-chained carbohydrates and organic acids are produced. Depletion of monosaccharides and glycolate synthesis occurs at increased rates as the length of the carbohydrate chain is decreased, and is significantly increased in the presence of trace amounts of Fe(2+) ions (10 microM). Rates of monosaccharide depletion (initial concentration of 3 mM) observed were up to 1.55 mmol h(-1) in the case of fructose, and 2.59 mmol h(-1) in the case of dihydroxyacetone, depending upon pH, H(2)O(2) concentration, temperature and the presence or absence of catalytic amounts of Fe(2+). Glycolate was produced by dihydroxyacetone cleavage at rates up to 0.45 mmol h(-1) in the absence, and up to 1.88 mmol h(-1) in the presence of Fe(2+) ions (pH 8). Besides glycolate, other sugars (ribose, glyceraldehyde, glucose), glucitol (sorbitol) and organic acids (formic and 2-oxogluconic acid) were produced in such H(2)O(2)-induced reactions with fructose or dihydroxyacetone. EPR measurements demonstrated the participation of the OH radical, especially at higher pH. Presence of metal ions at higher pH values, resulting in increased glycolate synthesis, was accompanied by enhanced hydroxyl radical generation. Observed changes in intensity of DEPMPO-OH signals recorded from dihydroxyacetone and fructose reactions demonstrate a strong correlation with changes in glycolate yield, suggesting that OH radical formation enhances glycolate synthesis. The results presented suggest that different mechanisms are responsible for the cleavage or other reactions (isomerisation, auto- or free-radical-mediated oxidation) of keto-monosaccharides depending of experimental conditions.

New insights into photosynthetic oscillations revealed by two-dimensional microscopic measurements of chlorophyll fluorescence kinetics in intact leaves and isolated protoplasts

Chlorophyll fluorescence kinetic microscopy was used to analyze photosynthetic oscillations in individual cells of leaves and in isolated leaf cell protoplasts. Four Brassicaceae species were used: Arabidopsis halleri (L.) O'Kane & Al-Shehbaz, Thlaspi fendleri (Nels.) Hitchc, Thlaspi caerulescens J.&C. Presl and Thlaspi ochroleucum Boiss et Helder. With the latter two, the measurements were extended also to isolated protoplasts. The oscillations were induced under the microscope by exposing dark-adapted samples to actinic irradiance. Detailed analysis of the induced transients revealed that they consist of several processes oscillating with different frequencies and not only one component as reported earlier. Furthermore, it was found that most of these processes are controlled inside each individual cell. This was shown by differences in oscillations in neighboring cells and protoplasts that share a uniform intercellular environment. The frequency of the dominant oscillation frequency depended neither on irradiance nor on CO2 concentration and is, therefore, not controlled by the photosynthetic rate. Characteristic differences in the frequency spectrum and damping of oscillations have been found among the plant species examined.

Periodicity and chaos in a photosynthetic system

In this paper experimental results of investigation of the oscillations in a photosynthetic system are presented and a model for their interpretation is suggested. Periodicities in photosynthetic systems detected in earlier studies by physical chemical methods can be also detected by means of recording the potential difference between two point electrodes. The observed dependences demonstrate a wide range of various types of behaviour of the system, working, e.g. in periodic, quasiperiodic, chaotic or 'pulse' regimes. Since the until-now-used 2-dimensional theoretical model, based on the existence of two dominant autocatalytical processes, appeared not to be sufficient for explaining such types of the regimes, a generalized 3-dimensional autocatalytical model is suggested, which is able to explain all the above mentioned photosynthetic regimes.

Induction of oscillations in chlorophyll fluorescence by re-illumination of intact isolated pea chloroplasts

Oscillations in chlorophyll fluorescence yield were observed upon re-illumination of intact isolated pea (Pisum sativumL.) chloroplasts that had attained their maximal rate of photosynthesis and had spent a short period in darkness. The oscillations depended on the length of the previous dark period, the length of previous illumination, and the reaction temperature. This finding confirms the presence of an "oscillatory center" in the chloroplasts temselves.

Identification of possible two-reactant sources of oscillations in the Calvin photosynthesis cycle and ancillary pathways

A systematic search for possible sources of experimentally observed oscillations in the photosynthetic reaction system has been performed by application of recent theoretical results characterizing the transient-state rate behaviour of metabolic reactions involving two independent concentration variables. All subsystems involving two independent reactants in metabolically fundamental parts of the Calvin cycle and the ancillary pathways of starch and sucrose synthesis have been examined in order to decide on basis of their kinetic and stoichiometric structure whether or not they may trigger oscillations. The results show that no less than 20 possible oscillators can be identified in the examined reaction system, only three of which have been previously considered as potential sources of experimentally observed oscillations. This illustrates the superiority of the method now applied over those previously used to identify possible two-reactant sources of metabolic oscillations and indicates that there should be no difficulty in complex metabolic pathways to point to a multitude of interactions that may trigger an oscillatory rate behaviour of the system.

Salinity effects on the stomatal behaviour of grapevine

An investigation of the time-course of inhibition of photosynthesis in salt-stressed grapevine (Vitis vinifera L.) leaves revealed two types of stomatal behaviour. Up to tissue concentrations of 165 mM chloride the inhibition was due to a uniform decrease in stomatal conductance, as indicated from autoradiograms of ¹⁴ CO₂ fixation and no change in the relationship of assimilation to calculated intercellular partial pressure of CO₂ (A-C₁ ) compared with control plants. The occurrence of non-stomatal inhibition of photosynthesis at higher levels of leaf chloride, suggested by a decline in the slope of the calculated (A-C₁ ) relationship, was associated with non-uniform ¹⁴ CO₂ uptake over the leaf surface similar to that previously observed for ABA-treated and water-stressed grapevine leaves where non-stomatal inhibition of photosynthesis was shown to be an artifact arising from non-uniform stomatal behaviour. These observations also provide an explanation for the stimulation of photorespiration during salt stress.

Non-cyclic photoreductive carbon fixation in photosynthesis. Light and dark transients of the glycerate-3-P special pair

It is demonstrated that carbon fixation in photosynthesis is regulated in two kinetically coupled pathways involving the specialized pair of non-equivalent, enzyme-bound glycerate-3-P (3-PGA) molecules obtained from ribulose 1,5-bisphosphate (RuBP) carboxylation in the light. A non-cyclic pathway is suggested (reaction 2) for the direct biosynthesis of sucrose from the 3-PGA obtained from C-3, C-4 and C-5 of the six-carbon carboxylation adduct. Concomitant to the appearance of sucrose as the principal product, the Mg2+-bound 3-PGA molecule formed from C-1, C-2 and C-2' of the C6 intermediate is released and subsequently reduced in regenerating the RuBP. It is proposed that the nocturnal inhibitor, 2-carboxyarabinitol-1-phosphate (1-PCA) is obtained from a condensation of 3-PGA and glyceraldehyde.

Photoreductive path of carbon fixation in green plant photosynthesis. Reaction pathway of six-carbon ribulose 1,5-bisphosphate carboxylation adduct intermediate

In this paper we examine the six-carbon intermediate pathway of ribulose 1,5-bisphosphate (RuBP) carboxylation reaction in photosynthesis. Based on the observed reactions of purified RuBP carboxylase, mechanisms are described for carbon dioxide assimilation leading to the hydrolytic splitting of the six-carbon intermediate to two enzyme-bound glycerate-3-P (3-PGA) molecules. It is concluded that, under photosynthetic conditions, the reduction of enzyme-bound NADP+ by the chlorophyll is responsible for the rapid carboxylase turnover rate given by the lifetime, tau L = 0.4 s, which is nearly two orders of magnitude shorter than the corresponding value, tau D = 11 +/- 3 s, for the dark decay of enzyme-bound RuBP. The nocturnal inhibition and photoactivation of RuBP carboxylation are described in terms of the reversible light-dark cycles of the NADP+/NADPH redox couple and endogenous changes that accompany the 2-carboxy-D-arabinitol-1-phosphate binding to the enzyme active site.

Oscillatory response of photosynthesis in leaves to environmental perturbations: a mathematical model

Oscillations in the yield of chlorophyll fluorescence, in oxygen evolution, and in CO2 uptake observed with leaves upon perturbation of steady-state conditions are suggested to be due to the interdependence of turnover of adenylates and Calvin cycle intermediates. This suggestion is quantified in a mathematical model; the behavior of the model system in the neighborhood of the singular point of the system is analyzed. The linearized system is solved analytically, a condition for the occurrence of oscillations is given, and explicit expressions for the oscillation period and the damping constant are derived. The model is shown to be capable of exhibiting oscillations with the period observed with algae or leaves, whereas calculated values of the damping constant are higher than those measured for leaves or algae.

The relationship between light scattering and chlorophyll a fluorescence during oscillations in photosynthetic carbon assimilation

Light scattering, which can be taken as an indicator of the transthylakoid proton gradient, and the 518-nm rise, which can be regarded as a measure of the transthylakoid membrane potential, have been followed during oscillations in chlorophyll a fluorescence, which are known to be associated with corresponding changes in photosynthetic carbon assimilation. Both components oscillated in a manner which was broadly reciprocal to chlorophyll a fluorescence. However, there was a phase shift such that the light-scattering change usually anticipated fluorescence and often also the 518-nm shift. It is concluded that the proton motive force rises and falls slightly in advance of rises and falls in carbon assimilation. The relationship of these changes to a possible underlying mechanism is discussed.

Photosynthesis and increased production of protein

Photosynthesis, the use of light energy in the conversion of CO2 and inorganic nutrients into plant material, is the ultimate source of the food protein necessary to man's existence. Given certain assumptions, the overall maximal theoretical photosynthetic efficiency of agricultural plants can be calculated. Actual measured maximal growth rates of plants are equivalent to efficiency levels well below that theoretical maximum. In air, C4 plants can some closer to the theoretical value than C3 plants, perhaps because C4 plants avoid the occurrence of measurable photorespiration and oxygen inhibition of photosynthesis. Alfalfa, a C3 legume, is an extremely productive protein source. Its protein yield per acre can surpass that of commonly grown C4 crops (corn, sorghum) and C3 seed crops (soybean, wheat, rice). Alfalfa leaf protein is of high nutritional quality and can apparently be used directly in the human diet, eliminating the protein loss involved in animal production. Plant protein productivity can be raised as part of an increase in overall crop yield. The growth of plants in atmospheres with elevated CO2 levels can result in increased yields. In C3 plants this is due, at least in part, to the suppression of photorespiration and oxygen inhibition of photosynthesis. We have investigated the effect of CO2 concentration on alfalfa photosynthetic metabolism. Our results support the contention that alfalfa productivity can be increased by an environment of elevated CO2. A second approach toward increased plant protein productivity is through regulation of carbon flow during photosynthesis so as to increase portein production relative to that of other plant constituents. In particular, we have investigated whether ammonia (the form in which plants first incorporate nitrogen) can act to regulate leaf carbon metabolism. Our results indicate that NH4+, in part through stimulation of pyruvate kinase, brings about increased production of amino acids at the expense of sucrose production in alfalfa. The effect may be of considerable importance in the regulation of green leaf protein synthesis.

Endogenous oscillations in pathways of energy transduction as related to circadian rhythmicity and photoperiodic control

Evidence is presented for endogenous rhythmicity in energy transducing sequences of cellular metabolism which result in a circadian rhythm in adenylate "energy charge" and redox state (NADPH/NADP). From phase dependent photocontrol of enzymatic activity and pyridine nucleotide pool-size levels it is concluded that light - via photoreceptor(s) of photoperiodic control - modulates energy flow under conditions where overall energy transduction displays a circadian rhythm. The results are discussed in relation to temporal organization of development in general.

On the formation of glycolate in photosynthesizing Chlorella using a new gas-liquid chromatography method

Suspensions of Chlorella vulgaris were allowed to photosynthesise with two concentrations of (14)CO2 (101 and 543 ppm) in 80% oxygen, and the incorporation of (14)C into glycolate and 3-phosphoglyceric acid (3-PGA) was followed. The relative specific activity (RSA) of the glycolate formed at both CO2 concentrations decreased initially and then increased slowly. The RSA of glycolate was much lower when the suspension photosynthesised in 101 ppm (14)CO2 compared to 543 ppm. The RSA of 3-PGA was nearly always lower than that of glycolate and the results suggest that refixed dark respiratory CO2 or respiratory 3-PGA, or both, substantially contribute to the total 3-PGA in the algae. It is concluded that glycolate is formed from recent photosynthate as well as from storage material, but the relative contribution of these substrates depends on the conditions under which the algae are grown, as well as those obtaining at the time of glycolate excretion.