Photosynthetic Induction in a C(4) Dicot, Flaveria trinervia: II. Metabolism of Products of CO(2) Fixation after Different Illumination Times

The metabolism of fixed ¹⁴CO₂ and the utilization of the C-4 carboxyl of malate and aspartate were examined during photosynthetic induction in Flaveria trinervia, a C₄ dicot of the NADP-malic enzyme subgroup. Pulse/chase experiments indicated that both malate and aspartate appeared to function directly in the C₄ cycle at all times during the induction period (examined after 30 seconds, 5 minutes and 20 minutes illumination). However, the rate of loss of ¹⁴C-label from the C-4 position of malate plus aspartate was relatively slow after 30 seconds of illumination, compared to treatments after 5 or 20 minutes of illumination. Similarly, the appearance of label in other photosynthetic products (e.g. 3-phosphoglycerate, sugar phosphates, alanine) during the chase periods was generally slower after only 30 seconds of leaf illumination, compared to that after 5 of 20 minutes illumination. This may be due to the lower rate of photosynthesis after 30 seconds illumination. The appearance of label in carbons 1→3 of each C₄ acid during the chase periods was relatively slow after either 30 seconds or 5 minutes illumination, while there was a relatively rapid accumulation of label in carbons 1→3 of both C₄ acids after 20 minutes illumination. Thus, while the turnover rate of the ¹⁴C-4 label in both C₄ acids increased only during the first 5 minutes of the induction period, only later during induction is there an increased rate of appearance of label in other carbon atoms of the C₄ acids. The implied source of ¹⁴C for labeling of the 1→3 positions of the C₄ acids is an apparent carbon flux from 3-phosphoglycerate of the reductive pentose phosphate pathway to phosphoenolpyruvate of the C₄ cycle.     

The intracellular distribution of free nucleotides in the tobacco leaf. Formation of adenosine 5′-phosphate from adenosine 5′-triphosphate in the chloroplasts

1. A method of studying the free nucleotides of leaf tissue is outlined and the need for a metal ion-binding agent for the complete extraction of certain nucleotides by aqueous ethanol is established. 2. The method was used to study the effect of illumination or darkening of tobacco plants on the free nucleotides present in the chloroplast and non-chloroplast tissue components. 3. When plants that had been in the dark for a prolonged period were given 30sec. of bright light there was a rapid phosphorylation of ADP to ATP in both chloroplast and non-chloroplast components of the tissue. 4. Where plants were moved into the dark from conditions suitable for rapid photosynthesis there was a rapid conversion of ATP into AMP and the AMP was formed only in the chloroplast fraction. In continuing darkness the AMP remained restricted mainly to the chloroplast fraction for at least 2min., but eventually its concentration fell to the low value that is typical of tobacco leaves during conditions of constant illumination. If the plants were returned to the light for 30sec. the AMP was rapidly rephosphorylated.     

Effects of glyoxylate on photosynthesis by intact chloroplasts

Because glyoxylate inhibits CO₂ fixation by intact chloroplasts and purified ribulose bisphosphate carboxylase/oxygenase, glyoxylate might be expected to exert some regulatory effect on photosynthesis. However, ribulose bisphosphate carboxylase activity and activation in intact chloroplasts from Spinacia oleracea L. leaves were not substantially inhibited by 10 millimolar glyoxylate. In the light, the ribulose bisphosphate pool decreased to half when 10 millimolar glyoxylate was present, whereas this pool doubled in the control. When 10 millimolar glyoxylate or formate was present during photosynthesis, the fructose bisphosphate pool in the chloroplasts doubled. Thus, glyoxylate appeared to inhibit the regeneration of ribulose bisphosphate, but not its utilization.

The fixation of CO₂ by intact chloroplasts was inhibited by salts of several weak acids, and the inhibition was more severe at pH 6.0 than at pH 8.0. At pH 6.0, glyoxylate inhibited CO₂ fixation by 50% at 50 micromolar, and glycolate caused 50% inhibition at 150 micromolar. This inhibition of CO₂ fixation seems to be a general effect of salts of weak acids.

Radioactive glyoxylate was reduced to glycolate by chloroplasts more rapidly in the light than in the dark. Glyoxylate reductase (NADP⁺) from intact chloroplast preparations had an apparent K_m (glyoxylate) of 140 micromolar and a V_max of 3 micromoles per minute per milligram chlorophyll.

     

The redox state of the NADP system in illuminated chloroplasts

Pyridine nucleotide levels were measured in intact spinach chloroplasts. The NADPH/NADP ratio was close to unity in darkened chloroplasts. On illumination, chloroplast NADP levels decreased rapidly. The decrease was more prominent at low than at high light intensities. In the presence of bicarbonate, NADP subsequently increased to reach a steady-state level. The kinetics of the increase were related in general, but not in detail, to the lag phase of photosynthesis. In the steady state, chloroplast NADP was sometimes, particularly during photosynthesis at high light intensities, less reduced in the light than in the dark. In the dark-light transition, phosphoglycerate reduction is driven by increases in the ratios NADPH/NADP and ATP/ADP. When photosynthesis accelerates after the initial lag phase, the NADPH/NADP ratio decreases and a high ratio of phosphoglycerate to triose phosphate becomes an important factor in driving carbon reduction. Under photosynthetic flux conditions, the redox state of the chloroplast NADP system appeared to be governed largely by the chloroplast ratio of phosphoglycerate to dihydroxyacetone phosphate and by the phosphorylation potential [ATP]/[ADP] [P_i]. The inhibitor of cyclic electron transport, antimycin A, increased reduction of the chloroplast NADP system. Even when reduction was almost complete in the presence of 5 μM antimycin A, photosynthesis was still significant at low light intensities. Electrons appeared to be effectively distributed between the cyclic electron-transport pathway and the noncyclic route to NADP at NADPH/NADP ratios as low as about 1. When bicarbonate was absent, the NADP system remained largely reduced in the light. The energy-transfer inhibitor, Dio-9, and uncouplers and agents which interfered with pH regulation of the Calvin cycle increased reduction of the NADP system while decreasing photosynthesis.     

Intermediates Of Photosynthesis in Acetabularia mediterranie Chloroplasts

The chloroplast fraction isolated from Acetabularia mediterranie was exposed to ¹⁴CO₂ as NaH¹⁴CO₃ in light and darkness, and soluble radioactive compounds were analyzed at frequent intervals. The behavior of Calvin cycle intermediates indicates that this cycle was responsible for much of the carbon fixation in the chloroplasts. However, a substantial part of recently fixed carbon was metabolized via glycolic and glyceric acids. Possible pathways for their metabolism are discussed. Some carboxylation of C₃ acids was suggested by the behavior of phosphoenolpyruvate and malate. A number of amino acids were formed. Small amounts of such compounds as citrate, succinate, and fumarate not usually associated with photosynthesis might have been derived from a low level of mitochondrial contamination. About one-third of the carbon fixed in light was present in acid-labile insoluble compounds other than polysaccharides or proteins. Dark fixation of CO₂ was very small compared with photosynthesis.     

Photosynthesis, Photorespiration and Respiration of Chloroplasts From Acetabularia mediterrania

A chloroplast fraction isolated from Acetabularia mediterrania carries on photosynthesis at rates essentially equal to those of whole cells. Electron and phase contrast microscopy reveals that the chloroplasts are intact and well preserved. Preparations contain no identifiable peroxisomes, but some cytoplasmic and mitochondrial contamination is present. Photosynthesis and CO₂ production in light by chloroplast preparations are in many respects similar to that of bean leaves, although the measured rates are somewhat lower. Respiration and photosynthesis of chloroplast preparations and whole cells of Acetabularia is essentially similar except that cells have a strong dark-type respiration which continues in light and is CO₂ dependent, the substrate being mainly recent photosynthate. The data suggest that chloroplasts are the site of photorespiration.     

Enhanced Dark CO(2) Fixation by Preilluminated Chlorella pyrenoidosa and Anacystis nidulans

The products of short time photosynthesis and of enhanced dark ¹⁴CO₂ fixation (illumination in helium prior to addition of ¹⁴CO₂ in dark) by Chlorella pyrenoidosa and Anacystis nidulans were compared. Glycerate 3-phosphate, phosphoenolpyruvate, alanine, and aspartate accounted for the bulk of the ¹⁴C assimilated during enhanced dark fixation while hexose and pentose phosphates accounted for the largest fraction of isotope assimilated during photosynthesis. During the enhanced dark fixation period, glycerate 3-phosphate is carboxyl labeled and glucose 6-phosphate is predominantly labeled in carbon atom 4 with lesser amounts in the upper half of the C₆ chain and traces in carbon atoms 5 and 6. Tracer spread throughout all the carbon atoms of photosynthetically synthesized glycerate 3-phosphate and glucose 6-phosphate. During the enhanced dark fixation period, there was a slow formation of sugar phosphates which subsequently continued at 5 times the initial rate long after the cessation of ¹⁴CO₂ uptake. To explain the kinetics of changes in the labelling patterns and in the limited formation of the sugar phosphates during enhanced dark CO₂ fixation, the suggestion is made that most of the reductant mediating these effects did not have its origin in the preillumination phase.

It is concluded that a complete photosynthetic carbon reduction cycle operates to a limited extent, if at all, in the dark period subsequent to preillumination.

     

光照下菜豆叶片抗氰呼吸与光合作用关系的分析北大核心CSCD

为了进一步了解光照下植物呼吸作用的内在机理以及呼吸作用和光合作用的关系,该文研究了在光照下菜豆(Phaseolus vulgaris)叶片抗氰呼吸与光合作用的关系。研究发现,将黑暗下生长的菜豆幼苗叶片转到光照下10 h,总呼吸、抗氰呼吸以及抗氰呼吸在总呼吸中的比例均逐步上升;光照也导致了叶片叶绿体光合放氧和CO2固定的出现及其速率的增加,但光合放氧和CO2固定速率的增加均滞后于抗氰呼吸的增加。将黑暗下生长的叶片转到光照下之前用抗氰呼吸的抑制剂水杨基氧肟酸(SHAM)处理叶片,发现用SHAM处理并没有导致叶片在光照下光合放氧和CO2固定速率的明显变化,这也提示了黑暗下生长的叶片转至光照的过程中,抗氰呼吸和光合作用没有产生偶联。进一步研究发现,在黑暗中对叶片施加短时间的光照能够增加抗氰呼吸在总呼吸中的比例,但短时间的光照对叶片光合CO2固定速率没有影响。这些结果表明了光照对抗氰呼吸的诱导可以不依赖于光合作用,光照可能是作为一种直接的信号去诱导抗氰呼吸。     

Photosynthetic carbon metabolism in seagrasses C-labeling evidence for the c(3) pathway

The δ¹³C values of several seagrasses were considerably less negative than those of terrestrial C₃ plants and tended toward those of terrestrial C₄ plants. However, for Thalassia hemprichii (Ehrenb.) Aschers and Halophila spinulosa (R. Br.) Aschers, phosphoglycerate and other C₃ cycle intermediates predominated among the early labeled products of photosynthesis in ¹⁴C-labeled seawater (more than 90% at the earliest times) and the labeling pattern at longer times was brought about by the operation of the C₃ pathway. Malate and aspartate together accounted for only a minor fraction of the total fixed label at all times and the kinetic data of this labeling were not at all consistent with these compounds being early intermediates in seagrass photosynthesis. Pulse-chase ¹⁴C-labeling studies further substantiated these conclusions. Significant labeling of photorespiratory intermediates was observed in all experiments. The kinetics of total fixation of label during some steady-state and pulse-chase experiments suggested that there may be an intermediate pool of inorganic carbon of variable size closely associated with the leaves, either externally or internally. Such a pool may be one cause for the C₄-like carbon isotope ratios of seagrasses.     

Isolation of Intact Chloroplasts from Spinach Leaf by Centrifugation in Gradients of the Modified Silica “Percoll”

The isolation of the photosynthetically competent chloroplast preparations was undertaken by means of the density gradient centrifugation on the modified silica sol “Percoll.” A clear separation of the intact chloroplast sustaining the high photosynthetic activities (light dependent CO₂ fixation ca. 130μmol/mg Chl·hr) was established. The contamination of mitochondria and peroxisomes was estimated to be less than 3% by measuring the activities of their marker enzymes. The chloroplasts were proved to be free from endoplasmic reticulum and cytosol. The photosynthetic CO₂ fixation of the isolated chloroplast preparations was saturated by illumination of the light intensity of 20,000 Lux (12 mW/cm², 400~750 nm).     

Limitation of Photosynthesis by Carbon Metabolism : I. Evidence for Excess Electron Transport Capacity in Leaves Carrying Out Photosynthesis in Saturating Light and CO(2)

It has been investigated how far electron transport or carbon metabolism limit the maximal rates of photosynthesis achieved by spinach leaves in saturating light and CO₂. Leaf discs were illuminated with high light until a steady state rate of O₂ evolution was attained, and then subjected to a 30 second interruption in low light, to generate an increased demand for the products of electron transport. Upon returning to high light there is a temporary enhancement of photosynthesis which lasts 15 to 30 seconds, and can be up to 50% above the steady state rate of O₂ evolution. This temporary enhancement is only found when saturating light intensities are used for the steady state illumination, is increased when low light rather than darkness is used during the interruption, and is maximal following a 30 to 60 seconds interruption in low light. Decreasing the temperature over the 10 to 30°C range led to the transient enhancement becoming larger. The temporary enhancement is associated with an increased ATP/ADP ratio, a decreased level of 3-phosphoglycerate, and increased levels of triose phosphate and ribulose 1,5-bisphosphate. Since electron transport can occur at higher rates than in steady state conditions, and generate a higher energy status, it is concluded that leaves have a surplus electron transport capacity in saturating light and CO₂. From the alterations of metabolites, it can be calculated that the enhanced O₂ evolution must be accompanied by an increased rate of ribulose 1,5-bisphosphate regeneration and carboxylation. It is suggested that the capacity for sucrose synthesis ultimately limits the maximal rates of photosynthesis, by restricting the rate at which inorganic phosphate can be recycled to support electron transport and carbon fixation in the chloroplast.     

Nonphotosynthetic retardation of chloroplast senescence by light

Excised apical portions of green wheat leaf sections were treated with aminotriazole to prevent formation of new chloroplasts. Illumination retarded the decline in chlorophyll content per leaf section, the disintegration of chloroplast ultrastructure, and the loss of capacity for photosynthetic carbon fixation. We interpret these 3 effects of illumination as facets of a single light effect in retarding chloroplast senescence. This light effect in retarding chloroplast senescence has features differing from characteristics of photosynthetic carbon fixation. For example, A) application of the photosynthetic inhibitor 3-(3,4-dichlorophenyl)-1, 1-dimethylurea did not decrease, and may have even slightly increased, the effectiveness of light; B) although the action spectrum contains peaks in the blue and red regions, it differs from the action spectrum for photosynthetic CO₂ assimilation in wheat; C) in nonphotosynthesizing tissue, application of sugars did not retard chloroplast senescence; D) light saturation was achieved by only a few hundred microwatts/cm². Considered together with the well-known light requirement for chloroplast formation, our results indicate that light has a dual, photomorphogenetic control in maintaining the green status of the plant by also exerting a second effect: retarding of senescence of chloroplasts already present.     

Analysis of steady state photosynthesis in alfalfa leaves

A method for carrying out kinetic tracer studies of steady state photosynthesis in whole leaves has been developed. An apparatus that exposes whole leaves to ¹⁴CO₂ under steady state conditions, while allowing individual leaf samples to be removed as a function of time, has been constructed. Labeling data on the incorporation of ¹⁴C into Medicago sativa L. metabolite pools are reported. A carbon dioxide uptake rate of 79 micromoles ¹⁴CO₂ per milligram chlorophyll per hour was observed at a CO₂ level slightly below that of air. Several actively turning over pools of early and intermediate metabolites, including 3-phosphoglyceric acid, glycerate, citrate, and uridine diphosphoglucose, showed label saturation after approximately 10 to 20 minutes of photosynthesis with ¹⁴CO₂ under steady state conditions. Alanine labeling increased more rapidly at first, and then at a lower rate as saturation was approached. Sucrose was a major product of photosynthesis and label saturation of the sucrose pool was not observed. Labeled carbon appeared rapidly in secondary metabolites. The steady state apparatus used has numerous advantages, including leaf temperature control, protection against leaf dehydration, high illumination, known ¹⁴CO₂ specific radioactivity, and provision for control and adjustment of ¹⁴CO₂ concentration. The apparatus allows for experiments of long duration and for sufficient sample points to define clearly the metabolic steady state.     

Photosynthesis at low water potentials in sunflower: lack of photoinhibitory effects

The losses in chloroplast capacity to fix CO₂ when photosynthesis is reduced at low leaf water potential (ψ₁) have been proposed to result from photoinhibition. We investigated this possibility in soil-grown sunflower (Helianthus annuus L. cv IS894) using gas exchange techniques to measure directly the influence of light during dehydration on the in situ chloroplast capacity to fix CO₂. The quantum yield for CO₂ fixation as well as the rate of light- and CO₂-saturated photosynthesis were strongly inhibited at low ψ₁. The extent of inhibition was the same whether the leaves were exposed to high or to low light during dehydration. When intercellular partial pressures of CO₂ were decreased to the compensation point, which was lower than the partial pressures resulting from stomatal closure, the inhibition of the quantum yield was also unaffected. Photoinhibition could be observed only after high light exposures were imposed under nonphysiological low CO₂ and O₂ where both photosynthesis and photorespiration were suppressed. The experiments are the first to test whether gas exchange at low ψ₁ is affected by potentially photoinhibitory conditions and show that the loss in chloroplast capacity to fix CO₂ was entirely the result of a direct effect of water availability on chloroplast function and not photoinhibition.     

Dependence of the membrane potential on intracellular ATP concentration in tonoplast-free cells of Nitellopsis obtusa

Intact chloroplasts were obtained from mesophyll protoplasts isolated from Mesembryanthemum crystallinum in the C₃ or Crassulacean acid metabolism (CAM) photosynthetic mode, and examined for the influence of inorganic phosphate (Pi) on aspects of bicarbonate-dependent O₂ evolution and CO₂ fixation. While the chloroplasts from both modes responded similarly to varying Pi, some features appear typical of chloroplasts from species capable of CAM, including a relatively high capacity for photosynthesis in the absence of Pi, a short induction period, and resistance to inhibition of photosynthesis by high levels of Pi. In the absence of Pi the chloroplasts retained 75–85% of the ¹⁴CO₂ fixed and the total export of dihydroxyacetone phosphate was low compared with the rate of photosynthesis. In CAM plants the ability to conduct photosynthesis and retain most of the fixed carbon in the chloroplasts at low external Pi concentrations may enable storage of carbohydrates which are essential for providing a carbon source for the nocturnal synthesis of malic acid. At high external Pi concentrations (e.g. 10 25 mM), the amount of total dihydroxyacetone phosphate exported to the assay medium relative to the rate of photosynthesis was high while the products of ¹⁴CO₂ fixation were largely retained in the chloroplasts which indicates starch degradation is occurring at high Pi levels. Starch degradation normally occurs in CAM plants in the dark; high levels of Pi may induce starch degradation in the light which has the effect of limiting export of the immediate products of photosynthesis and thus the degree of Pi inhibition of photosynthesis with the isolated chloroplast.     

An investigation into the roles of photosynthesis and respiration in h efflux from aerated suspensions of asparagus mesophyll cells

Aerated and stirred suspensions of mechanically isolated Asparagus sprengeri Regel mesophyll cells were used to investigate the roles of respiration and photosynthesis in net H⁺ efflux. Rates varied between 0.12 and 1.99 nanomoles H⁺ per 10⁶ cells per minute or 3 and 40 nanomoles H⁺ per milligram chlorophyll per minute. The mean rate of H⁺ efflux was 10% greater in the dark. 3-(3,4-Dichlorophenyl)-l,l-dimethylurea, an inhibitor of noncyclic photophosphorylation, did not inhibit H⁺ efflux from illuminated cells. Bubbling with N₂ or addition of oligomycin, an inhibitor of mitochondrial ATP production, resulted in rapid and virtually complete inhibition of H⁺ efflux in light or dark. In the absence of aeration, H⁺ efflux came to a halt but resumed with aeration or illumination. When aeration was switched to CO₂-free air, rates of H⁺ efflux were reduced 43% in the dark and 57% in the light. Oligomycin eliminated dark CO₂ fixation but not photosynthetic CO₂ fixation. It is suggested that H⁺ efflux is dependent on respiration and dark CO₂ fixation, but independent of photosynthesis.     

Mass-spectrometric evidence for the double-carboxylation pathway of malate synthesis by Crassulacean acid metabolism plants in light

Phyllodia of the Crassulacean acid metabolism (CAM) plant Kalanchoë tubiflora were allowed to fix ¹³CO₂ in light and darkness during phase IV of the diurnal CAM cycle, and during prolongation of the regular light period. After ¹³CO₂ fixation in darkness, only singly labelled [¹³C]malate molecules were found. Fixation of ¹³CO₂ under illumination, however, produced singly labelled malate as well as malate molecules which carried label in two, three or four carbon atoms. When the irradiance during ¹³CO₂ fixation was increased, the proportion of singly labelled malate decreased in favour of plurally labelled malate. The irradiance, however, did not change either the ratio of labelled to unlabelled malate molecules found in the tissue after the ¹³CO₂ application, or the magnitude of malate accumulation during the treatment with label. The ability of the tissue to store malate and the labelling pattern changed throughout the duration of the prolonged light period. The results indicate that malate synthesis by CAM plants in light can proceed via a pathway containing two carboxylation steps, namely ribulose-1,5-bisphosphate-carboxylase/oxygenase (EC 4.1.1.39) and phosphoenolpyruvate carboxylase (EC 4.1.1.31) which operate in series and share common intermediates. It can be concluded that, in light, phosphoenolpyruvate carboxylase can also synthesize malate independently of the proceeding carboxylation step by ribulose-1,5-bisphosphate carboxylase/oxygenase.Abbreviations CAM Crassulacean acid metabolism - PEP phosphoenolpyruvate - PEPCase phosphoenolpyruvate carboxylase (EC 4.1.1.31) - RuBPCase ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) - TMS trimethylsilyl     

RNA metabolism during light-induced chloroplast development in euglena

Methods are described which provide good recoveries of non-degraded chloroplast and non-chloroplast RNAs from Euglena gracilis var. bacillaris. These have been characterized by comparing the RNA from W₃BUL (an aplastidic mutant of Euglena), with that of wild-type cells which have been resolved into chloroplast and non-chloroplast fractions. Using E. coli RNA as a standard, the RNAs from W₃BUL and from the non-chloroplast fraction of green cells exhibit optical density peaks, upon sucrose gradient centrifugation, at 4S, 10S, and 19S. The chloroplast fraction exhibits optical density peaks at 19S and 14S with the 19S component predominating. Application of various techniques for the separation of RNAs to the problem of separating the chloroplast and non-chloroplast RNAs, without prior separation of the organelle, have not proven successful.

³²P_i is readily incorporated into RNA by cells undergoing light-induced chloroplast development, and fractionation at the end of development reveals that although chloroplast RNAs have a higher specific activity, the other RNAs of the cells are significantly labeled as well. The succession of labeling patterns of total cellular RNA as light-induced chloroplast development proceeds are displayed and reveal that all RNA species mentioned above eventually become labeled. In contrast, cells kept in darkness during this period incorporate little ³²P_i into any RNA fraction. In addition, a heavy RNA component, designated as 28S, while representing a negligible fraction of the total RNA, becomes significantly labeled during the first 24 hours of illumination. While there is light stimulated uptake of ³²P_i into the cells, this uptake is never limiting in the light or dark, for RNA labeling.

On the basis of these findings, we suggest that extensive activation of non-chloroplast RNA labeling during chloroplast development is the result of the activation of the cellular synthetic machinery external to the chloroplast necessary to provide metabolic precursors for plastid development. Thus the plastid is viewed as an auxotrophic resident within the cell during development. Other possibilities for interaction at this and other levels are also discussed.

     

Structural and metabolic properties of green tissue cultures from a CAM plant, Kalanchoë blossfeldiana hybr. Montezuma

Abstract Crassulacean acid metabolism (CAM) was studied in mixotrophic callus tissue cultures of Kalanchoë blossfeldiana hybr. Montezuma and compared with plants propagated from the calli. The ultrastructural properties of the green callus cells are similar to mesophyll cells of CAM plants except that occasionally abnormal mitochondria were observed. There was permanent net CO₂ output by the calli in light and darkness, which was lower in darkness than in light. The calli exhibited a diurnal rhythm of malic acid, with accumulation during the night and depletion during the day. ¹⁴C previously incorporated by dark CO₂ fixation into malate was transferred upon subsequent illumination into end products of photosynthesis. All these data indicate that CAM operates in the calli tissue. The results revealed that the capacity for CAM is obviously lower in the calli compared with plantlets developing from the calli, or with ‘adult’ plants. The data suggest also that CAM in the calli was not limited by the activities of CAM enzymes.     

Photosynthetic products of division synchronized cultures of euglena

Rates and products of photosynthetic ¹⁴CO₂ fixation by division synchronized cultures of Euglena gracilis strain Z were determined over the cycle. Rate of ¹⁴CO₂ fixation doubled in a continuous manner throughout the light phase followed by a slight reduction of photosynthetic capacity in the dark phase. Greater ¹⁴C incorporation into the nucleic acid-polysaccharide fraction occurred with mature cells. Products of ¹⁴CO₂ fixation varied markedly over the cycle: although with mature cells ¹⁴C-labeled sucrose was not detected, with dividing cells this was the main sugar labeled; in young cells ¹⁴C maltose was formed. Cells removed at end of dark phase accumulated ¹⁴C in glycolate, whereas at other stages over the cycle less ¹⁴C was present in glycolate, and this was accompanied by a rapid incorporation of ¹⁴C into glycine and serine. Glycerate was an early and major product of photosynthesis with cells at the mature stage of the cycle.