Minimum pulses of stable and radioactive carbon isotopes to detect belowground carbon transfer between plants

Methodological problems, such as unwanted shifts in plant carbon allocation patterns following large isotopic labeling pulses, have hindered accurate quantification of belowground carbon movement in plant–soil systems. These problems must be addressed before we can understand the factors regulating carbon movement between plants and soils and the importance of this movement to the global carbon cycle. We studied the effects of pulse-label size on carbon allocation and transfer between ectomycorrhizal paper birch (Betula papyrifera Marsh.) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings using increasing pulse levels of either ¹³C or ¹⁴C in two separate laboratory experiments. Our specific objectives were: (1) to determine the minimum pulse of ¹³CO₂ or ¹⁴CO₂ for detecting carbon movement between plants through belowground transfer pathways, (2) to determine whether carbon allocation patterns within these plants change when exposed to short pulses of elevated carbon dioxide and, (3) to determine whether carbon allocation patterns are similar when using two different carbon isotopes. We detected carbon movement between plants at each ¹³C and ¹⁴C pulse level. There was a tendency for the amount of interplant carbon transfer to increase with increasing ¹³C pulse level, but the same amount of transfer occurred at all ¹⁴C pulses between 0.19 and 0.56 MBq. Carbon allocation patterns did not change with pulse level but they were affected by the choice of carbon isotope. We conclude that at least 8 ml of ¹³C or 0.19 MBq of ¹⁴C is sufficient to detect belowground carbon transfer in small seedlings growing in close proximity.     

Photorespiration in Air and High CO(2)-Grown Chlorella pyrenoidosa

Oxygen inhibition of photosynthesis and CO₂ evolution during photorespiration were compared in high CO₂-grown and air-grown Chlorella pyrenoidosa, using the artificial leaf technique at pH 5.0. High CO₂ cells, in contrast to air-grown cells, exhibited a marked inhibition of photosynthesis by O₂, which appeared to be competitive and similar in magnitude to that in higher C₃ plants. With increasing time after transfer to air, the photosynthetic rate in high CO₂ cells increased while the O₂ effect declined. Photorespiration, measured as the difference between ¹⁴CO₂ and ¹²CO₂ uptake, was much greater and sensitive to O₂ in high CO₂ cells. Some CO₂ evolution was also present in air-grown algae; however, it did not appear to be sensitive to O₂. True photosynthesis was not affected by O₂ in either case. The data indicate that the difference between high CO₂ and air-grown algae could be attributed to the magnitude of CO₂ evolution. This conclusion is discussed with reference to the oxygenase reaction and the control of photorespiration in algae.     

Metabolism of Separated Leaf Cells: III. Effects of Calcium and Ammonium on Product Distribution During Photosynthesis with Cotton Cells

Separated mesophyll cells from cotton (Gossypium hirsutum var. Stoneville 1613 Glandless) were isolated with pectinase and mechanical agitation. The separated cells had rates of light-dependent CO₂ fixation between 50 to 100 μmoles CO₂ per mg chlorophyll per hour. The presence of Ca²⁺ in the incubation medium did not significantly affect the type of photosynthetic products formed, but 2 mm Ca²⁺ did cause a 50% decrease in the appearance of photosynthetic products in the incubation medium. The movement of all types of products (sugars, organic, and amino acids) out of the cells was reduced similarly by the Ca²⁺. Light had no affect on the movement of products out of the cells, whereas 1 mm ethylenediaminetetra-acetate greatly increased the movement. The addition of 1.6 mm NH₄Cl to the cell suspensions caused a large increase in the amount of fixed ¹⁴C appearing in the amino acid fraction and a decrease in the sugar fraction. These metabolic changes in the cells were reflected in the movement of products out of the cells so that the incubation medium also contained a larger amount of label in amino acids and a smaller amount in sucrose. Although the cell plasma membrane restricted the movement of soluble products, it did not discriminate significantly between the types of products moved.     

Uptake and metabolism of carbohydrates by epidermal tissue

Isolated epidermis of Commelina communis L. and Tulipa gesneriana L. assimilated ¹⁴CO₂ into malic acid and its metabolites but not into sugars or their phosphates; epidermis could not reduce CO₂ by photosynthesis and therefore must be heterotrophic (Raschke and Dittrich, 1977). If, however, isolated epidermis of Commelina communis was placed on prelabelled mesophyll (obtained by an exposure to ¹⁴CO₂ for 10 min), radioactive sugars appeared in the epidermis, most likely by transfer from the mesophyll. Of the radioactivity in the epidermis, 60% was in sucrose, glucose, fructose, 3-phosphoglyceric acid and sugar phosphates. During a 10-min exposure to ¹⁴CO₂, epidermis in situ incorporated 16 times more radioactivity than isolated epidermal strips. Isolated epidermis of Commelina communis and Tulipa gesneriana took up ¹⁴C-labelled glucose-1-phosphate (without dephosphorylation), glucose, sucrose and maltose. These substances were transformed into other sugars and, simultaneously, into malic acid. Carbons-1 through-3 of malic acid in guard cells can thus be derived from sugars. Radioactivity appeared also in the hydrolysate of the ethanol-insoluble residue and in compounds of the tricarboxylic-acid cycle, including their transamination products. The hydrolysate contained glucose as the only radioactive compound. Radioactivity in the hydrolysate was therefore considered an indication of starch. Starch formation in the epidermis began within 5 min of exposure to glucose-1-phosphate. Autoradiograms of epidermal sections were blackened above the guard cells. Formation of starch from radioactive sugars therefore occurred predominantly in these cells. Epidermis of tulip consistently incorporated more ¹⁴C into malic and aspartic acids than that of Commelina communis (e.g. after a 4-h exposure to [¹⁴C]glucose in the dark, epidermis, with open stomata, of tulip contained 31% of its radioactivity in malate and aspartate, that of Commelina communis only 2%). The results of our experiments allow a merger of the old observations on the involvement of starch metabolism in stomatal movement with the more recent recognition of ion transfer and acid metabolism as causes of stomatal opening and closing.Abbreviation G-1-P glucose-1-phosphate     

Ethoxyzolamide Inhibition of CO(2) Uptake in the Cyanobacterium Synechococcus PCC7942 without Apparent Inhibition of Internal Carbonic Anhydrase Activity

In high inorganic carbon grown (1% CO₂ [volume/volume]) cells of the cyanobacterium Synechococcus PCC7942, the carbonic anhydrase (CA) inhibitor, ethoxyzolamide (EZ), was found to inhibit the rate of CO₂ uptake and to reduce the final internal inorganic carbon (C_i) pool size reached. The relationship between CO₂ fixation rate and internal C_i concentration in high C_i grown cells was little affected by EZ. This suggests that in intact cells internal CA activity was unaffected by EZ. High C_i grown cells readily took up CO₂ but had little or no capacity for HCO₃⁻ uptake. These cells appear to possess a CO₂ utilizing C_i pump that has a CA-like function associated with the transport step such that HCO₃⁻ is the species delivered to the cell interior. This CA-like step may be the site of inhibition by EZ. Low C_i grown cells possess both CO₂ uptake and HCO₃⁻ uptake activities and EZ inhibited both activities to a similar degree, suggesting that a common step in CO₂ and HCO₃⁻ uptake (such as the C_i pump) may have been affected. The inhibitor had no apparent effect on internal CO₂/HCO₃⁻ equilibria (internal CA function) in low C_i grown cells.     

THE SUBCELLULAR LOCALIZATION OF Ach SYNTHESIS IN RAT STRIATAL SYNAPTOSOMES INVESTIGATED WITH THE USE OF TRITON X-100

—The regulation of [¹⁴C]ACh synthesis was studied in rat striatal synaptosomes incubated in presence of various concentrations of Triton X-100, using [2-¹⁴C]pyruvate or [6-¹⁴C]glucose as precursors. The progressive rupture of the cytoplasmic and mitochondrial compartments induced by the non-ionic detergent was followed by studying the release, into the incubating medium, of lactate dehydrogenase and choline acetyltransferase (ChAc) and of fumarate hydratase, respectively. [³H]Choline uptake (1 μm ) was measured to determine the activity of the high affinity choline permease. ¹⁴CO₂ formation from [2-¹⁴C]pyruvate was used as an index of the Krebs cycle activity. The rate of [¹⁴C]ACh synthesis from [2-¹⁴C] pyruvate was dependent on the Triton X-100 concentration; the ester formation decreased between 0·001% (v/v) and 0·010%, but increased again beyond this concentration of detergent. This last phenomenon was interpreted as the result of an extracellular synthesis of ACh involving pyruvate dehydrogenase and ChAc. At 0·002% Triton X-100 the ¹⁴CO₂ formation was not affected, indicating a normal mitochondrial activity. The decrease of [¹⁴C]ACh synthesis observed up to this detergent concentration could be correlated to the decline of the highaffinity choline permease activity. In these experimental conditions, the ester synthesis could not be restored by the addition of large amounts of choline in the incubating medium suggesting that the molecules of choline must cross the high-affinity choline permease system in order to be acetylated. This could indicate a close association between the permease and choline acetyltransferase.     

Application of the Hodgkin-Huxley equations to an electric field model for interaction between excitable cells

We previously described a model for the electrical transfer of excitation from one cell to the next which utilized the electric potential generated in the junctional cleft between the cells. Low-resistance connections between the cells were not used in the model, and it was assumed that the junctional membranes were excitable. This model was analyzed for the static case without capacitances and for the dynamic case in which capacitances were part of the circuit elements. For simplicity, the Na⁺ resistance (R_Na), after a threshold potential was exceeded, was allowed to decrease exponentially (to 1% of its initial value) within 0·25–1·0 ms, and possible changes in the K⁺ resistance were ignored. In this paper, we have incorporated the Hodgkin-Huxley equations into the operation of the lumped membrane units for the electrical equivalent circuit of the cell membrane. The parameters varied are the membrane capacitances, resistances, maximum Na⁺ conductance ($\text{g}{}_{\mathrm{<mtext>Na</mtext>}}$), and the radial cleft resistance (R_jc). We demonstrated that our model worked very well, i.e. the successful transfer of action potentials was achieved, with the membrane units following Hodgkin-Huxley dynamics for changes in g_Na and g_K. The calculations indicate that transmission is facilitated when the junctional units have a higher $\text{g}{}_{\mathrm{<mtext>Na</mtext>}}$ and a lower capacitance and when R_jc is elevated. Lowering the resistance of the junctional membrane units several fold, relative to the surface membrane units, also facilitated transmission; however, the absolute resistance of the junctional membrane was still well above the maximum value that would allow sufficient local-circuit current to flow to effect transmission. Thus, the electric field model provides an alternative means of cell-to-cell propagation between myocardial cells which is electrical in nature but does not require the presence of low-resistance connections between cells.     

CO2 in large-scale and high-density CHO cell perfusion culture

Productivity in a CHO perfusion culture reactor was maximized when pCO₂ was maintained in the range of 30–76 mm Hg. Higher levels of pCO₂ (> 150 mm Hg) resulted in CHO cell growth inhibition and dramatic reduction in productivity. We measured the oxygen utilization and CO₂ production rates for CHO cells in perfusion culture at 5.55×10^-17 mol cell^-1 sec^-1 and 5.36×10^-17 mol cell^-1 sec^-1 respectively. A simple method to directly measure the mass transfer coefficients for oxygen and carbon dioxide was also developed. For a 500 L bioreactor using pure oxygen sparge at 0.002 VVM from a microporous frit sparger, the overall apparent transfer rates (k_La+k_AA) for oxygen and carbon dioxide were 0.07264 min^-1 and 0.002962 min^-1 respectively. Thus, while a very low flow rate of pure oxygen microbubbles would be adequate to meet oxygen supply requirements for up to 2.1×10⁷ cells/mL, the low CO₂ removal efficiency would limit culture density to only 2.4×10⁶ cells/mL. An additional model was developed to predict the effect of bubble size on oxygen and CO₂ transfer rates. If pure oxygen is used in both the headspace and sparge, then the sparging rate can be minimized by the use of bubbles in the size range of 2–3 mm. For bubbles in this size range, the ratio of oxygen supply to carbon dioxide removal rates is matched to the ratio of metabolic oxygen utilization and carbon dioxide generation rates. Using this strategy in the 500 L reactor, we predict that dissolved oxygen and CO₂ levels can be maintained in the range to support maximum productivity (40% DO, 76 mm Hg pCO₂) for a culture at 10⁷ cells/mL, and with a minimum sparge rate of 0.006 vessel volumes per minute.A = volumetric agitated gas-liquid interfacial area at the top of the liquid, 1/mB = cell broth bleeding rate from the vessel, L/minCER = carbon dioxide evolution rate in the bioreactor, mol/min[CO₂] = dissolved CO₂ concentration in liquid, M[CO₂]^* = CO₂ concentration in equilibrium with sparger gas, M[CO₂]^** = CO₂ concentration in equilibrium with headspace gas, MCO₂(1) = dissolved carbon dioxide molecule in water[C_T] = total carbonic species concentration in bioreactor medium, M[C_T]_F = total carbonic species concentration in feed medium, MD = bioreactor diameter, mD_I = impeller diameter, mD_b = the initial delivered bubble diameter, mF = fresh medium feeding rate, L/minH_L = liquid height in the vessel, mk_A = carbon dioxide transfer coefficient at liquid surface, m/mink _infA ^supO = oxygen transfer coefficient at liquid surface, m/minNomenclature     

Enhanced CO2 biofixation and protein production by microalgae biofilm attached on modified surface of nickel foam

In this work, a photobioreactor with microalgae biofilm was proposed to enhance CO₂ biofixation and protein production using nickel foam with the modified surface as the carrier for immobilizing microalgae cells. The results demonstrated that, compared with microalgae suspension, microalgae biofilm lowered mass transfer resistance and promoted mass transfer efficiency of CO₂ from the bubbles into the immobilized microalgae cells, enhancing CO₂ biofixation and protein production. Moreover, parametric studies on the performance of the photobioreactor with microalgae biofilm were also conducted. The results showed that the photobioreactor with microalgae biofilm yielded a good performance with the CO₂ biofixation rate of 4465.6 µmol m⁻³ s⁻¹, the protein concentration of effluent liquid of 0.892 g L⁻¹, and the protein synthesis rate of 43.11 g m⁻³ h⁻¹. This work will be conducive to the optimization design of microalgae culture system for improving the performance of the photobioreactor.

     

Growth factors modulate junctional cell-to-cell communication

Summary The epidermal growth factor (EGF) and the platelet-derived growth factor (PDGF) inhibit gap junctional communication in the mammalian cell lines NRK and BalbC 3T3: cell-to-cell transfer of a 400-dalton tracer molecule is reduced and junctional conductance is reduced. The inhibition of cell-to-cell transfer is reversible and dose dependent; half-maximal effects are obtained at 10^–9 and 10^–11 m concentrations of EGF and PDGF, respectively. The response of junctional conductance is detectable within 2 min of EGF application and reaches a maximum within 10 min. It is among the earliest cellular responses to this growth factor and may be significant in the regulation of growth. The response is lacking in EGF receptor-deficient NIH 3T3 cells. The transforming factor (TGF) enhances junctional communication in BalbC 3T3: cell-to-cell transfer is increased over a period of 8 hr. But in NRK cells, where it upregulates EGF receptors, TGF reduces junctional communication synergistically with EGF.     

Uptake of Choline and Its Conversion to Glycine Betaine by Bacteria in Estuarine Waters

The uptake and degradation of nanomolar levels of [methyl-¹⁴C]choline in estuarine water samples and in seawater filtrate cultures composed mainly of natural free-living bacteria was studied. Uptake of [¹⁴C]choline exhibited Michaelis-Menten kinetics, with K_t + S_n values of 1.7 to 2.9 nM in filtrate cultures and 1.7 to 4.1 nM in estuarine-water samples. V_max values ranged from 0.5 to 3.3 nM · h⁻¹. The uptake system for choline in natural microbial assemblages therefore displays very high affinity and appears able to scavenge this compound at the concentrations expected in seawater. Uptake of choline was inhibited by some natural structural analogs and p-chloromercuribenzoate, indicating that the transporter may be multifunctional and may involve a thiol binding site. When 11 nM [¹⁴C]choline was added to water samples, a significant fraction (>50%) of the methyl carbon was respired to CO₂ in incubations lasting 10 to 53 h. Cells taking up [¹⁴C]choline produced [¹⁴C]glycine betaine ([¹⁴C]GBT), and up to 80% of the radioactivity retained by cells was in the form of GBT, a well-known osmolyte. Alteration of the salinity in filtrate cultures affected the relative proportion of [¹⁴C]choline degraded or converted to [¹⁴C]GBT, without substantially affecting the total metabolism of choline. Increasing the salinity from 14 to 25 or 35 ppt caused more [¹⁴C]GBT to be produced from choline but less ¹⁴CO₂ to be produced than in the controls. Lowering the salinity to 7 ppt decreased [¹⁴C]GBT production and increased ¹⁴CO₂ production slightly. Intracellular accumulations of [¹⁴C]GBT in the salt-stressed cultures were osmotically significant (34 mM). Choline may be used as an energy substrate by estuarine bacteria and may also serve as a precursor of the osmoprotectant GBT, particularly as bacteria are mixed into higher-salinity waters.     

Permeability and structural studies of heart cell gap junctions under normal and altered ionic conditions

Summary The permeability and ultrastructure of communicating junctions of cultured neonatal rat ventricular cells are examined under control conditions and during treatments which raise intracellular Ca²⁺. Lucifer Yellow (487 mol wt) is used to examine junctional permeability. Under normal ionic conditions dye transfer from an injected muscle cell to neighboring muscle cells occurs rapidly (in less than 6 sec) while transfer to neighboring fibroblasts occurs more slowly. Application of monensin, which results in a partial contracture with superimposed asynchrony, or A23187, which results in a partial contracture, do not inhibit the transfer of dye between the muscle cells. A23187 did result in junctional blockade between muscle cells and fibroblasts. Freeze-fractured gap junctions from control and monensin-treated cells exhibit no distinguishable differences. Center-to-center spacing was not significantly different, 9.0 nm±1.4sd versus 9.2 nm±1.3sd, respectively; and particle diameters were virtually unchanged, 8.69 nm±0.9sd versus 8.61 nm±1.07sd, respectively. These results suggest that concentrations of intracellular Ca²⁺ sufficient to support a partial contracture and asynchronous contractile activity do not result in a block of intercellular junctions in cultured myocardial cells. These results are discussed in terms of intracellular Ca²⁺-buffering and junctional sensitivity to Ca²⁺.     

CO2 and intracellular pH

Abstract The experimental determination of cytoplasmic and vacuolar pH values is discussed. Despite variation in these values evidence indicates that intracellular pH values are normally regulated within narrow limits. The regulatory mechanisms proposed involve the metabolic consumption of OH^& and the active efflux of H ⁺. The evidence for intracellular pH modification in response to CO₂ hydration and the production of HCO^?₃ and H⁺ is examined. Theoretical calculations and experimental data indicate that CO₂ concentrations as high as 5% will lower intracellular pH. Conversely, variation in CO₂ levels around atmospheric concentrations is unlikely to perturb intracellular pH. High CO₂ levels are found in bulky tissues, and flooded root systems. Evidence is presented that the slow diffusion of dissolved CO₂ compared to gaseous CO₂ results in its accumulation. It is proposed that the accumulation of respiratory CO₂ may reduce intracellular pH values when plant tissues, cells or protoplasts are maintained in a liquid culture medium. Finally, the possible role of dark CO₂ fixation and organic acid synthesis in the regulation of intracellular pH is examined.     

Effect of Triacontanol on Chlamydomonas: I. Stimulation of Growth and Photosynthetic CO(2) Assimilation

Treatment of Chlamydomonas reinhardtii cells, cultured at 5% CO₂, with 1 to 1000 micrograms triacontanol (TRIA) per liter resulted in 21 to 35% increases in cell density, 7 to 31% increases in total chlorophyll, and 20 to 100% increases in photosynthetic CO₂ assimilation. The increase in CO₂ fixation with TRIA treatment occurred before, and was independent of, increases in total chlorophyll or cell number. Chlamydomonas cells responded to a broad range of TRIA concentrations that were at least one order of magnitude above the optimum concentration established for higher plants. The necessity for larger concentrations of TRIA may be due to destabilizing effects of Ca²⁺ and K⁺ present in the Chlamydomonas growth medium. These ions caused flocculation of the colloidally dispersed TRIA in apparent competition with binding of [¹⁴C]TRIA to Chlamydomonas cells. Octacosanol inhibited the effect of TRIA on photosynthetic CO₂ assimilation. TRIA treatment did not alter the distribution of ¹⁴C-label among photosynthetic products. The effect of TRIA on photosynthetic CO₂ assimilation increased with time after treatment up to 3 days. Chlamydomonas cells that had been grown at low-CO₂ (air) did not respond to TRIA, and transfer of high-CO₂ (5%) grown cells that had responded to TRIA to a low-CO₂ atmosphere resulted in a loss of the effect of TRIA. The effect of pH on photosynthetic CO₂ assimilation indicated that CO₂ is probably the species of inorganic carbon utilized by control and TRIA-treated Chlamydomonas cells.     

Structural and functional features of the leaves of Ranunculus trichophyllus Chaix., a freshwater submerged macrophophyte

AZA, 5-Acetamido-1,3,4-thiadiazole-2-sulphonamide
CA, carbonic anhydrase
DIC, dissolved inorganic carbon
Hepes, A-(2-hydroxyethyl)-1 piperazine-ethane sulfonic acid
IC, inorganic carbon
PAR, photosynthetic active radiation
PATAg, periodic acid-thiosemicarbazide-silver proteinate
Tris, tris (hydroxymethyl)-aminomethane

The structural and physiological strategies developed by the leaves of the freshwater macrophyte Ranunculus trichophyllus to adapt to submersed life were studied. Photosynthesis is carried out mainly by the epidermis cells of the numerous segments into which the leaf is finely dissected. In these cells, containing most of the chloroplasts, a peculiar organization of the wall has been identified by cytochemical tests. A thin compact outer region covers the cell surface and splits up forming large lacunae between adjacent cells. Below it, a thick and loose inner region rich in hydrophilic pectic acids occurs, which grows in along the cell sides giving rise to wide transfer areas. In this latter cell wall region, in which the cell/environment contact and exchanges are amplified, the systems for inorganic carbon supply to photosynthetic cells operate. The leaves of R. trichophyllus can rely on environmental CO₂ and HCO₃^– as sources of inorganic carbon for photosynthesis. A mechanism for bicarbonate utilization seems to involve its conversion to CO₂ by an apoplastic carbonic anhydrase, whose activity gains importance as the availability of environmental CO₂ decreases. Interestingly, it has been demonstrated that in this species CO₂ can also be obtained from HCO₃^– by a photodependent increase in plasmamembrane H⁺-ATPase activity in the transfer areas of the epidermis cells. This is the first time that such a mechanism has been noted in a nonpolar leaf of a submerged macrophyte.     

Stomatal responses to carbon dioxide of isolated epidermis from a C3 plant,the Argenteum mutant of Pisum sativum L., and a crassulacean-acid-metabolism plant Kalanchoë daigremontiana Hamet et Perr

The response of stomata in isolated epidermis to the concentration of CO₂ in the gaseous phase was examined in a C₃ species, the Argenteum mutant of Pisum sativum, and a crassulacean-acid-metabolism (CAM) species, Kalanchoë daigremontiana. Epidermis from leaves of both species was incubated on buffer solutions in the presence of air containing various volume fractions of CO₂ (0 to 10000·10^–6). In both species and in the light and in darkness, the effect of CO₂ was to inhibit stomatal opening, the maximum inhibition of opening occurring in the range 0 to 360·10^–6. The inhibition of opening per unit change in concentration was greatest between volume fractions of 0 and 240·10^–6. There was little further closure above the volume fraction of 360·10^–6, i.e. approximately ambient concentration of CO₂. Thus, although leaves of CAM species may experience much higher internal concentrations of CO₂ in the light than those of C₃ plants, this does not affect the sensitivity of their stomata to CO₂ concentration or the range over which they respond. Stomatal responses to CO₂ were similar in both the light and the dark, indicating that effects of CO₂ on stomata occur via mechanisms which are independent of light. The responses of stomata to CO₂ in the gaseous phase took place without the treatments changing the pH of the buffered solutions. Thus it is unlikely that CO₂ elicited stomatal movement by changing either the pH or the HCO ₃ ^– /CO ₃ ^2- equilibria. It is suggested that the concentration of dissolved unhydrated CO₂ may be the effector of stomatal movement and that its activity is related to its reactivity with amines.     

Aggregative movement of C4 mesophyll chloroplasts is promoted by low CO2 under high intensity blue light

• C₄ plants supply concentrated CO₂ to bundle sheath (BS) cells, improving photosynthetic efficiency by suppressing photorespiration. Mesophyll chloroplasts in C₄ plants are redistributed toward the sides of the BS cells (aggregative movement) in response to environmental stresses under light. Although this chloroplast movement is common in C₄ plants, the significance and mechanisms underlying the aggregative movement remain unknown.
• Under environmental stresses, such as drought and salt, CO₂ uptake from the atmosphere is suppressed by closing stomata to prevent water loss. We hypothesized that CO₂ limitation may induce the chloroplast aggregative movement. In this study, the mesophyll chloroplast arrangement in a leaf of finger millet, an NAD-malic enzyme type C₄ plant, was examined under different CO₂ concentrations and light conditions.
• CO₂ limitation around the leaves promoted the aggregative movement, but the aggregative movement was not suppressed, even at the higher CO₂ concentration than in the atmosphere, under high intensity blue light. In addition, mesophyll chloroplasts did not change their arrangement under darkness or red light.
• From these results, it can be concluded that CO₂ limitation is not a direct inducer of the aggregative movement but would be a promoting factor of the movement under high intensity blue light.

     

The Effect of pH, O(2), and Temperature on the CO(2) Compensation Point of Isolated Asparagus Mesophyll Cells

The effect of pH, O₂ concentration, and temperature on the CO₂ compensation point (Г[CO₂]) of isolated Asparagus sprengeri Regel mesophyll cells has been determined in a closed, aqueous environment by a sensitive gas-chromatographic technique. Measured values range between 10 and 100 microliters per liter CO₂ depending upon experimental conditions. The Г(CO₂) increases with increasing temperature. The rate of increase is dependent upon the O₂ concentration and is more rapid at high (250-300 micromolar), than at low (30-60 micromolar), O₂ concentrations. The differential effect of temperature on Г(CO₂) is more pronounced at pH 6.2 than at pH 8.0, but this pH-dependence is not attributable to a direct, differential effect of pH on the relative rates of photosynthesis and photorespiration, as the O₂-sensitive component of Г(CO₂) remains constant over this range. The Г(CO₂) of Asparagus cells at 25°C decreases by 50 microliters per liter when the pH is raised from 6.2 to 8.0, regardless of the prevailing O₂ concentration. It is suggested that the pH-dependence of Г(CO₂) is related to the ability of the cell to take up CO₂ from the aqueous environment. The correlation between high HCO₃⁻ concentrations and low Г(CO₂) at alkaline pH indicates that extracellular HCO₃⁻ facilitates the uptake of CO₂, possibly by increasing the flux of inorganic carbon from the bulk medium to the cell surface. The strong O₂− and temperature-dependence of Г(CO₂) indicates that isolated Asparagus mesophyll cells lack an efficient means for concentrating intracellular CO₂ to a level sufficient to reduce or suppress photorespiration.     

High‐resolution secondary ion mass spectrometry analysis of carbon dynamics in mycorrhizas formed by an obligately myco‐heterotrophic orchid

Mycorrhiza formation represents a significant carbon (C) acquisition alternative for orchid species, particularly those that remain achlorophyllous through all life stages. As it is known that orchid mycorrhizas facilitate nutrient transfer (most notably of C), it has not been resolved if C transfer occurs only after lysis of mycorrhizal structures (fungal pelotons) or also across the mycorrhizal interface of pre‐lysed pelotons. We used high‐resolution secondary ion mass spectrometry (nanoSIMS) and labelling with enriched ¹³CO₂ to trace C transfers, at subcellular scale, across mycorrhizal interfaces formed by Rhizanthella gardneri, an achlorphyllous orchid. Carbon was successfully traced in to the fungal portion of orchid mycorrhizas. However, we did not detect C movement across intact mycorrhizal interfaces up to 216 h post ¹³CO₂ labelling. Our findings provide support for the hypothesis that C transfer from the mycorrhizal fungus to orchid, at least for R. gardneri, likely occurs after lysis of the fungal peloton.     

Experimental validation of a nonequilibrium model of CO<Subscript>2</Subscript> fluxes between gas,liquid medium,and algae in a flat-panel photobioreactor

Carbon dioxide (CO₂) availability strongly affects the productivity of algal photobioreactors, where it is dynamically exchanged between different compartments, phases, and chemical forms. To understand the underlying processes, we constructed a nonequilibrium mathematical model of CO₂ dynamics in a flat-panel algal photobioreactor. The model includes mass transfer to the algal suspension from a stream of bubbles of CO₂-enriched air and from the photobioreactor headspace. Also included are the hydration of dissolved CO₂ to bicarbonate ion (HCO₃⁻) as well as uptake and/or cycling of these two chemical forms by the cells. The model was validated in experiments using a laboratory-scale flat-panel photobioreactor that controls light, temperature, and pH and where the concentration of dissolved CO₂, and partial pressure of CO₂ in the photobioreactor exhaust are measured. First, the model prediction was compared with measured CO₂ dynamics that occurred in response to a stepwise change in the CO₂ partial pressure in the gas sparger. Furthermore, the model was used to predict CO₂ dynamics in photobioreactors with unicellular, nitrogen-fixing cyanobacterium Cyanothece sp. The metabolism changes dramatically during a day, and the distribution of CO₂ is expected to exhibit a pronounced diurnal modulation that significantly deviates from chemical equilibrium.