Photosynthesis by carrot tissue cultures

Summary ¹⁴CO₂-fixation rates in green carrot callus cultres (about 35 g chlorophyll/g fresh wt) were determined in gaseous and liquid media using a range of light intensities and CO₂ concentrations. Main products of light-dependent CO₂-fixation were sucrose, alanine, glutamine, serine/glycine and malic acid. In darkness, glutamine and malic acid were formed.Light CO₂-fixation rates were about ten times higher than dark fixation rates and reached 50–90 mol/mg chlorophyll/h in 10000 lux, 1% CO₂ in air. Net O₂-evolution by the tissue was demonstrated polarographically under these conditions. Light CO₂-fixation rates were linearly related to chlorophyll levels while dark fixation was independent of chlorophyll content. Lowered O₂ partial pressures in gaseous conditions increased ¹⁴CO₂-fixation rates. Ribulose diphosphate carboxylase and phosphoenol pyruvate carboxylase activities and their distribution in subcellular fractions were examined.When carrot tissue cultures were grown for two or four weeks on agar media lacking a carbohydrate source, in 10000 lux and 1% CO₂ in either air or N₂, dry weight increases were obtained although chlorophyll levels eventually declined.     

THE RELATIVE SIGNIFICANCE OF CO2-FIXING ENZYMES IN THE METABOLISM OF RAT BRAIN

To evaluate the relative significance of CO₂-fixing enzymes in the metabolism of rat brain, the subcellular distribution of pyruvate carboxylase, phosphoenolpyruvate carboxykinase, NADP-isocitrate dehydrogenase and NADP-malate dehydrogenase, as well as the fixation of H¹⁴CO₃^? by the cytosol and the mitochondria was investigated. Pyruvate carboxylase and phosphoenol-pyruvate carboxykinase are mainly localized in the mitochondria whereas NADP-isocitrate dehydrogenase and NADP-malate dehydrogenase are present in both the cytosol and the mitochondria. In the presence of pyruvate rat brain mitochondria fixed H¹⁴CO₃^? at a rate of about 170 nmol/g of tissue/min whereas these organelles fixed negligible amounts of H¹⁴CO₃^? in the presence of α-ketoglutarate or phosphoenolpyruvate. Rat brain cortex slices fixed H¹⁴CO₃^? at a rate of about 7 nmol/g of tissue/min and it was increased by two-fold when pyruvate was added to the incubation medium. The carboxylation of α-ketoglutarate and pyruvate by the reversal of the cytosolic NADP-isocitrate dehydrogenase and NADP-malate dehydrogenase respectively was very low as compared to that by pyruvate carboxylase. The rate of carboxylation reaction of both NADP-isocitrate dehydrogenase and NADP-malate dehydrogenase was only about 1/10th of that of decarboxylation reaction of the same enzyme. It is suggested that under physiological conditions these two enzymes do not play a significant role in CO₂-fixation in the brain. In rat brain cytosol, citrate is largely metabolized to α-ketoglutarate by a sequential action of aconitate hydratase and NADP-isocitrate dehydrogenase. The operation of the citrate-cleavage pathway in rat brain cytosol is demonstrated. The data show that among four CO₂-fixing enzymes, pyruvate carboxylase, an anaplerotic enzyme, plays the major role in CO₂-fixation in the brain.     

Immunogold Localization of Ribulose-1,5-Bisphosphate Carboxylase with Reference to Pyrenoid Morphology in Chloroplasts of Synchronized Euglena gracilis Cells

Euglena gracilis strain (Z) cells were synchronized under photoautotrophic conditions using a 14 hour light:10 hour dark regimen. The cells grew during the light period (growth phase) and divided during the following 10 hour period either in the dark or in the light (division phase). Changes in morphology of the pyrenoid and in the distribution of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) within the chloroplasts were followed by immunoelectron microscopy during the growth and division phases of Euglena cells. Epon-embedded sections were labeled with an antibody to the holoenzyme followed by protein A-gold. The immunoreactive proteins were concentrated in the pyrenoid, and less densely distributed in the stroma during the growth phase. During the division phase, the pyrenoid could not be detected and the gold particles were dispersed throughout the stroma. Toward the end of the division phase, the pyrenoid began to form in the center of a chloroplast, and the immunoreactive proteins started to concentrate over that rudimentary pyrenoid. During the growth phase, small areas rich in gold particles, called `satellite pyrenoid,' were observed, in addition to the main pyrenoid. From a comparison of photosynthetic CO₂-fixation with the total carboxylase activity of Rubisco extracted from Euglena cells in the growth phase, it is suggested that the carboxylase in the pyrenoid functions in CO₂-fixation in photosynthesis.     

Effects of Arsenite, Sulfite, and Sulfate on Photosynthetic Carbon Metabolism in Isolated Pea (Pisum sativum L., cv Little Marvel) Chloroplasts

Photosynthetic CO₂-fixation in isolated pea (Pisum sativum L., cv Little Marvel) chloroplasts during induction is markedly inhibited by 0.4 millimolar sulfite. Sulfate at the same concentration has almost no effect. The ¹⁴CO₂-fixation pattern indicates that the primary effect of sulfite is inhibition of the reaction catalyzed by ribulose bisphosphate carboxylase and a stimulation of export of intermediates out of the chloroplasts. Inhibition of light modulation of stromal enzyme activity does not appear to account for the toxicity of SO₂ in this Pisum variety. Arsenite at 0.2 millimolar concentrations inhibits light activation and inhibits photosynthetic CO₂ fixation. The ¹⁴CO₂-fixation pattern indicates that the primary effect of arsenite is inhibition of light activation of reductive pentose phosphate pathway enzyme activity.     

Assimilate Partitioning in the Variegated Coleus Leaf

Mature leaves of a variegated cultivar of Coleus blumei Benth. with a green border and central albino region constitute a source-sink system suitable for studies on assimilate partitioning. Leaves treated with ¹⁴CO₂ on a small part of the intact green border export assimilate via the shortest path into the stem. Leaves with all but a small lobe of the green border removed show different partitioning of labeled assimilates when the leaf is exposed to ¹⁴CO₂ (Fisher and Eschrich, 1985): The whole albino region of the leaf is supplied but no tracer is exported into the stem. When the green border is completely removed, ¹⁴CO₂-treatment of the albino region leads to the fixation of CO₂, obviously by PEP carboxylase, as indicated by the occurrence of labeled malate. Results show that the albino region of the variegated leaf constitutes a potential sink when deprived of its green border. In addition, CO₂-fixation by PEP carboxylase in albino tissue seems to indicate a common capacity of leaves which is normally masked by photosynthesis. The difference of assimilate partitioning between leaves with intact and leaves with partly removed green borders demonstrates that the unlabeled assimilates control the movement of labeled assimilates.     

Homotropic effect of CO 2 in ribulose-1,5-diphosphate carboxylase reaction

The concentration effect of aqueous CO₂ on the reaction velocity of spinach leaf ribulose-1,5-diphosphate carboxylase has been reevaluated. The homotropic effect of CO₂ in the enzyme reaction supports the previously reported allosteric nature of the enzyme in the CO₂-fixation process in chloroplasts. The concentration of CO₂ giving the half maximal reaction velocity, S_0.5, has been calculated to be 1.47 × 10⁻⁵M.     

Enzymatic determination of biotin.

An enzymatic method for the quantitative determination of biotin has been developed. The method involves the enzymatic binding of biotin in situ to the pyruvate carboxylase apoprotein of biotin-deficient bakers' yeast and the subsequent estimation of the pyruvate carboxylase activity by a ¹⁴CO₂-fixation method. The method is specific for biotin. Several biotin analogs and precursors were tested, and only biocytin was found to interfere, Biotin amounts of less than 5 pg can be estimated.     

Ribulose-1,5-bisphosphate carboxylase and phosphoenolpyruvate carboxylase activities of photoautotrophic callus of Platycerium coronarium (Koenig ex O.F. Muell.) Desv. under CO2 enrichment

The in vitro activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC) were measured in cell-free extracts of Platycerium coronarium callus cultured for up to 42 days under photoautotrophic conditions with CO₂ enrichment. With an increase in CO₂ in the culture environment to 10% (v/v) at low light, the apparent photoautotrophic fixation of CO₂ by Rubisco declined, whereas the non-photoautotrophic CO₂ fixation by PEPC activity was enhanced. Hence, photosynthesis appears to play a lesser role in providing carbon skeletons and energy with prolonged culture in a CO₂-enriched environment. Instead, the anaplerotic supply of C-skeletons by PEPC may be important under such a situation. Short-term H¹⁴CO₃-fixation experiments indicated that photoautotrophic callus cultured for 3 weeks with 10% CO₂ enrichment assimilated less ¹⁴CO₂ than the control (0.03% CO₂). Analyses of ¹⁴C-metabolites indicated that about 50% of the total soluble ¹⁴CO₂ fixed was in the organic acid fraction and 35% in the amino acid fraction. Despite the changes in the in vitro Rubisco/PEPC activity-ratio, no significant change in the ¹⁴C distribution pattern was apparent in response to increasing sucrose or CO₂ concentrations. The suppression of Rubisco activity and total chlorophyll content in high sucrose or elevated CO₂ concentrations suggests an inhibition of the capacity for photoautotrophic callus growth under these conditions. This revised version was published online in June 2006 with corrections to the Cover Date.     

Carbon assimilation by different developmental stages of Laminaria saccharina

Various stages of the life cycle of the marine brown alga Laminaria saccharina (L.) Lamour. (Laminariales, Phaeophyta) including male and female gametophytes, female gametes, zygotes and young sporophytes of different age were investigated for their potentials of carbon dioxide (¹⁴CO₂) fixation. Rates of photosynthesis attain the same order of magnitude in all stages. Photosynthetic ¹⁴CO₂-fixation is accompanied by a substantial light independent carbon assimilation. This is confirmed by rate determinations of the equivalent carboxylating enzymes present in the plants, ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) and phosphoenolpyruvate carboxokinase (EC 4.1.1.32) as well as by chromatographic analyses of the appropriate [¹⁴C]-assimilate patterns.Abbreviations RuBP-C ribulose-1,5-bisphosphate carboxylase - PEP-CK phosphoenolpyruvate carboxykinase - PEP phosphoenolpyruvate - PS photosynthesis - DF dark fixation     

CO2-Fixation and ATP synthesis in continuous cultures of Chlorella fusca

In continuous cultures of Chlorella fusca under steady state conditions, the CO₂-fixation rate, the ATP-level, the apparent rate of photophosphorylation as calculated from the changes in the ATP-level during light to dark or dark to light transients and the energy charge were measured at various environmental conditions. During growth the energy charge was around 0.64. CO₂-assimilation and the apparent ATP-synthesis were strongly dependant on light intensity, however the ATP-level was independant on it. Since the rates of apparent ATP-synthesis and of the CO₂-fixation do not seem to be strictly correlated in a logic way when environmental factors are changed and furthermore the stoichiometry of 3 ATP necessary per CO₂ fixed was never achieved, the described method frequently used for procaryotes to determine the in vivo rate of phosphorylation does not give valid results in highly compartimented eukaryotic cells.     

Photoautotrophic growth of soybean cells in suspension culture III. Characterization of carbon fixation products under high and low CO2 levels

A photoautotrophic soybean suspension culture (SB-P) was used to study CO₂ assimilation while exposed to elevated or ambient CO₂ levels. These studies showed that under elevated CO₂ (5% v/v) malate is the dominant fixation product, strongly suggesting that phosphoenolpyruvate carboxylase (PEPCase) is the primary enzyme involved in carbon fixation in these cells under their normal growth conditions. Citrate and [aspartate + glutamate] were also significant fixation products during fifteen minutes of exposure to ¹⁴CO₂. During the ten minute unlabeled CO₂ chase however, ¹⁴C-malate continued to increase while citrate and [aspartate + glutamate] declined. Fixation of ¹⁴CO₂ under ambient CO₂ levels (0.037%) showed a very different product pattern as 3-phosphoglycerate was very high in the first one to two minutes followed by increases in [serine + glycine] and [aspartate + glutamate]. Hexose phosphates were also quite high initially but then declined relatively rapidly. Thus, the carbon fixation pattern at ambient CO₂ levels resembles somewhat that seen in C3 leaf cells while that seen at elevated CO₂ levels more closely resembles that of a C₄ plant. The initial fixation product of C₃ plants, 3-PGA, was never detectable under high CO₂ conditions. These data suggest that an in vitro photoautotrophic system would be suitable for studying carbon fixation physiology during photosynthetic and non-photosynthetic growth.Abbreviations SB-P photoautotrophic soybean cells - PEPCase phosphoenol-pyruvate carboxylase - RuBPCase ribulose bisphosphate carboxylase/oxygenase - 3-PGA 3-phosphoglycerate     

Dark CO(2) Fixation and its Role in the Growth of Plant Tissue

Experiments were designed to determine the significance of dark CO₂ fixation in excised maize roots, carrot slices and excised tomato roots grown in tissue culture. Bicarbonate-¹⁴C was used to determine the pathway and amounts of CO₂ fixation, while leucine-¹⁴C was used to estimate protein synthesis in tissues aerated with various levels of CO₂.

Organic acids were labeled from bicarbonate-¹⁴C, with malate being the major labeled acid. Only glutamate and aspartate were labeled in the amino acid fraction and these 2 amino acids comprised over 90% of the ¹⁴C label in the ethanol-water insoluble residue.

Studies with leucine-¹⁴C as an indicator of protein synthesis in carrot slices and tomato roots showed that those tissues aerated with air incorporated 33% more leucine-¹⁴C into protein than those aerated with CO₂-free air. Growth of excised tomato roots aerated with air was 50% more than growth of tissue aerated with CO₂-free air. These studies are consistent with the suggestion that dark fixation of CO₂ is involved in the growth of plant tissues.

     

Light-dependent net CO-evolution by C3 and C4 plants

Light-dependent CO-evolution by the green leaves of C₃ and C₄ plants depends on the CO₂/O₂ ratio in the ambient atmosphere. This and other physiological responses suggest that CO-evolution is a byproduct of photorespiration. At CO₂/O₂ ratios up to 10^-3, the ratio of CO evolved: CO₂ fixed in photosynthesis is significantly higher in C₃ than in C₄ plants. This discrepancy disappears when a correction is made for the CO₂-concentrating mechanism in C₄ photosynthesis, by which CO₂-concentration at the site of ribulose-bis-phosphate carboxylase/oxygenase in the bundle sheaths is raised significantly as compared to the ambient atmosphere. Since the oxygenase function of this enzyme is responsible for glycolate synthesis, i.e., the substrate of photorespiration, this result seems to support the conclusion that CO-evolution is a consequence of photorespiration. CO-evolution may turn out to be a useful and rather straightforward indicator for photorespiration in ecophysiological studies.Abbreviations CAM crassulacean acid metabolism - CO net CO-evolution - CO₂ net CO₂-fixation - PEP-C phosphoenolpyruvate carboxylase - RubP-C ribulose-bisphosphate carboxylase/oxygenase Dedicated to Professor André Pirson on the occasion of his 70th birthday     

The effect of Cd2+ on photosynthetic reactions of mesophyll protoplasts

Photosynthetic CO₂-fixation of mesophyll protoplasts of lambs lettuce [Valerianella locusta (L.) Betcke] was inhibited by short time exposure to Cd⁺. Inhibition was due to uptake of the metal ion into the protoplasts and increased with increasing Cd²⁺ concentrations and the time of preincubation. A 10 min pretreatment at 2 mM Cd²⁺ reduced CO₂-fixation by 40–60%. Inhibition of photosynthesis was independent of the light intensity to which the protoplasts were exposed. Measurement of the lightinduced electrochromic pigment absorption change at 518nm and chlorophyll fluorescence studies revealed that primary photochemical reactions associated with the thylakoid membranes were not affected by the metal ion. Also, light activation of the ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) was not inhibited by Cd²⁺. Under rate-limiting CO₂ concentrations, inhibition of CO₂-fixation was smaller than at V_max of CO₂ reduction indicating that the carboxylation reaction of the Calvin cycle is not susceptible to Cd²⁺. Cd²⁺ treatment of protoplasts significantly extended the lagphase of CO₂-supported O₂-evolution and partly inhibited light activation of the glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.13) and the ribulose-5-phosphate kinase (EC 2.7.1.19). Measurement of relative concentrations of [¹⁴C]-labeled Calvin cycle intermediates showed that Cd²⁺ caused a decrease in the 3-phosphoglycerate/triose phosphate ratio and an increase in the triose phosphate/ribulose-1,5-bisphosphate ratio. It is concluded that in protoplasts Cd²⁺ affects photosynthesis mainly at the level of dark reactions and that the site of inhibition may be localized in the regenerative phase of the Calvin cycle.     

Photosynthesis and Activity of Ribulose Bisphosphate Carboxylase of Wheat and Maize Seedlings during and following Exposure to O(2)-Low, CO(2)-Free N(2)

Seven day old wheat and maize seedlings were exposed to 1300 or 2000 microeinsteins per square meter per second photosynthetically active radiation in CO₂-free air for 3 hours with either 1% O₂ in N₂ or N₂-only and then returned to normal air of 340 microliters per liter CO₂, 21% O₂ in N₂. Activity of the ribulose bisphosphate carboxylase and amount of the substrate, ribulose 1,5-bisphosphate, were measured during and following the CO₂-free treatments as was photosynthetic CO₂ fixation. Photoinhibition of photosynthesis was observed only with wheat seedlings following the N₂ only treatment. During the CO₂-free treatments, the levels of RuBP rose during all experiments except when wheat was photoinhibited. The activity of the ribulose bisphophate carboxylase, measured directly upon grinding the leaves, declined during the CO₂-free conditions. The carboxylase total activity increased in minutes in the leaf during and following the CO₂-free treatments. The specific activities of the wheat carboxylase went from 0.16 to 1.06 micromoles CO₂ fixed per milligram protein per minute while the maize carboxylase varied from 0.05 to 0.36 micromole CO₂ fixed per millogram protein per minute. This suggests that in these seedlings considerable inactive carboxylase must be stored in a form not activatable in extracts by CO₂ and Mg²⁺. Possible mechanisms of regulation of photosynthesis by the ribulose bisphosphate carboxylase must consider not only the amount of active enzyme, but the amount of enzyme which the plant can make activatable upon demand.     

Effect of salicylic acid pretreatment on cadmium toxicity in wheat

Cadmium (100, 400 and 1000 μM CdCl₂) treatments resulted in the inhibition of root dry biomass, root elongation and increased Cd accumulation in wheat (Triticum aestivum L.) roots. Further, these treatments decreased relative water content, chlorophyll content, ¹⁴CO₂-fixation, activities of phosphoenolpyruvate carboxylase and ribulose-1,5-bisphosphate carboxylase and abscisic acid content while increased malondialdehyde, hydrogen peroxide and free proline contents. Chloroplast and root ultrastructure was also changed. Pretreatment of seeds with SA (500 μM) for 20 h resulted in amelioration of these effects.     

Utilization of carbon and nitrogen reserves of alfalfa roots in supporting N2-fixation and shoot regrowth

Perennial legume such as alfalfa have the capacity to sustain shoot regrowth and some nodule N₂-fixation after removal (cutting) of shoots which contain practically all of the plant's photosynthetic capacity. The role of the roots in supporting these processes has not been fully described. Measurements were made of the nodules' responses to removal of shoots from 8-week-old seedlings in terms of N₂-fixation, as nitrogenase activity (NA) measured as acetylene reduction, dark CO₂ fixation, measured as in vitro phosphoenolpyruvate carboxylase (PEPC) activity, and total non-structural carbohydrate (NSC) content. These properties decreased and recovered in that sequence, which suggests that nodule NSC supported the substrate requirements of NA and PEPC immediately after cutting. The utilization and redistribution or root carbon and nitrogen, prelabeled with ¹⁴C and ¹⁵N, were also followed after cutting 8-week-old alfalfa seedlings. In the first 2 weeks of regrowth 12% of root C and 25% of root N were transferred for incorporation into new shoots. Up to 40% of the root C was used for plant respiration to support 28 days of shoot regrowth and N₂-fixation. The decline of N₂-fixation was slower after cutting and its minimum activity rose up 45% of pre-cut activity as root reserves were built up with plant age. Therefore, the stored reserves of nodulated roots play an important role in support of N₂-fixation after cutting.Contribution No. 1265 from Plant Research Center.Contribution No. 1265 from Plant Research Center.     

CO(2) and O(2) Exchanges in the CAM Plant Ananas comosus (L.) Merr: Determination of Total and Malate-Decorboxylation-Dependent CO(2)-Assimilation Rates; Study of Light O(2)-Uptake

Photosynthesis and light O₂-uptake of the aerial portion of the CAM plant Ananas comosus (L.) merr. were studied by CO₂ and O₂ gas exchange measurements. The amount of CO₂ which was fixed during a complete day-night cycle was equal to the amount of total net O₂ evolved. This finding justifies the assumption that in each time interval of the light period, the difference between the rates of net O₂-evolution and of net light atmospheric CO₂-uptake give the rates of malate-decarboxylation-dependent CO₂ assimilation. Based upon this hypothesis, the following photosynthetic characteristics were observed: (a) From the onset of the light to midphase IV of CAM, the photosynthetic quotient (net O₂ evolved/net CO₂ fixed) was higher than 1. This indicates that malate-decarboxylation supplied CO₂ for the photosynthetic carbon reduction cycle during this period. (b) In phase III and early phase IV, the rate of CO₂ assimilation deduced from net O₂-evolution was 3 times higher than the maximum rate of atmospheric CO₂-fixation during phase IV. A conceivable explanation for this stimulation of photosynthesis is that the intracellular CO₂-concentration was high because of malate decarboxylation. (c) During the final hours of the light period, the photosynthetic quotient decreased below 1. This may be the result of CO₂-fixation by phosphoenolpyruvate-carboxylase activity and malate accumulation. Based upon this hypothesis, the gas exchange data indicates that at least 50% of the CO₂ fixed during the last hour of the light period was stored as malate. Light O₂-uptake determined with ¹⁸O₂ showed two remarkable characteristics: from the onset of the light until midphase IV the rate of O₂-uptake increased progressively; during the following part of the light period, the rate of O₂-uptake was 3.5 times higher than the maximum rate of CO₂-uptake. When malate decarboxylation was reduced or suppressed after a night in a CO₂-free atmosphere or in continuous illumination, the rate of O₂-uptake was higher than in the control. This supports the hypothesis that the low rate of O₂-uptake in the first part of the light period is due to the inhibition of photorespiration by increased intracellular CO₂ concentration because of malate decarboxylation. In view of the law of gas diffusion and the kinetic properties of the ribulose-1,5-bisphosphate carboxylase/oxygenase, O₂ and CO₂ gas exchange suggest that at the end of the light period the intracellular CO₂ concentration was very low. We propose that the high ratio of O₂-uptake/CO₂-fixation is principally caused by the stimulation of photorespiration during this period.     

Synthetic biology for CO2 fixation

Recycling of carbon dioxide (CO₂) into fuels and chemicals is a potential approach to reduce CO₂ emission and fossil-fuel consumption. Autotrophic microbes can utilize energy from light, hydrogen, or sulfur to assimilate atmospheric CO₂ into organic compounds at ambient temperature and pressure. This provides a feasible way for biological production of fuels and chemicals from CO₂ under normal conditions. Recently great progress has been made in this research area, and dozens of CO₂-derived fuels and chemicals have been reported to be synthesized by autotrophic microbes. This is accompanied by investigations into natural CO₂-fixation pathways and the rapid development of new technologies in synthetic biology. This review first summarizes the six natural CO₂-fixation pathways reported to date, followed by an overview of recent progress in the design and engineering of CO₂-fixation pathways as well as energy supply patterns using the concept and tools of synthetic biology. Finally, we will discuss future prospects in biological fixation of CO₂.     

The Effect of DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) on Photosynthesis, Glycolate Excretion ('Photorespiration') and Amino Acid Metabolism in the Blue-Green Alga Anacystis

When photosynthesis of the blue-green alga Anacystis nidulans was measured as ¹⁴CO₂-fixation, the inhibitory effect of DCMU at low concentrations was greatest when mainly Photosystem 1 (PS 1) (excitation at 446 or 687 nm) was operative. At concentrations above 10-⁶M the inhibition on ¹⁴CO₂-fixation was greatest when mainly Photosystem 2 (PS 2) was operative (excitation at 619). During excitation of PS 1, the excretion of glycolate was stimulated at low concentrations of DCMU (5 × 10^-8M and lower), while higher concentrations inhibited excretion. All concentrations of DCMU inhibited glycolate excretion when mainly PS 2 was excited. The curves showing the relative effect of DCMU on the two photosystems, measured as PS 1/PS 2, had opposite shapes for ¹⁴CO₂-fixation and glycolate excretion. An increase in ¹⁴CO₂-fixation coincided with a decrease in glycolate excretion and vice versa. It appears that the increased rate of photosynthesis when mainly PS 1 was operative relative to that when mainly PS 2 was excited, increases the consumption of glycolate in an oxidation process associated with the excitation of PS 1, resulting in less excretion of glycolate to the medium. The influence of DCMU inhibition on labelled amino acid pools connected to the glycolate pathway (glycine-serine) is quite similar to that for ¹⁴CO₂-fixation. At concentrations below 10^-6M DCMU, inhibition of ¹⁴CO₂- incorporation into the amino acids was greatest when PS 1 was excited, while at the higher concentrations tested, inhibition was greater when PS 2 was excited. We conclude that the metabolism of glycine and serine is closely connected to the rate of photosynthesis.