Electrophysiological investigation of pathways in the middle cervical sympathetic ganglion of the turtle

Single unit responses in the middle cervical sympathetic ganglion ofEmys orbicularis to stimulation of other nerves and changes in these responses during the action of sympathetic blocking agents on the ganglion were investigated. The results showed that some fibers of the cervical sympathetic trunk of the turtle are interrupted in this ganglion. Postganglionic fibers pass out of the ganglion and enter the lateral branch and the sympathetic trunk. Other fibers pass through the ganglion without interruption and, together with postganglionic fibers, leave the ganglion in the cervical sympathetic trunk in a cranial direction. The velocity of conduction of excitation along the preganglionic fibers is between 4–3 and 2–1.5 m/sec and along the postganglionic fibers between 4–2.6 and 0.7–0.5 m/sec (fibers of types B₂ and C). Synaptic delay in the fast-conducting fibers averages 6.6 msec. Preganglionic fast-conducting fibers form synaptic contacts on neurons with type B₂ axons, while preganglionic slow-conducting fibers form contacts on neurons with type C axons. Terminals of two preganglionic fibers differing very slightly in their threshold of excitability, and probably constituting the same group, converge on some neurons.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukranian SSR, Kiev. Translated from Neirofiziologiya, Vol. 4, No. 1, pp. 83–89, January–February, 1972.     

A study of pre- and postganglionic fibres in the intestinal nerve (Remak's nerve) of the chicken

Summary The mean peak CV's of two electrophysiologically defined groups of fibres in the intestinal nerve of the chicken have been determined.One group of fibres is constituted by the processes of enteric cholinergic neurones which project along the side branches of the intestinal nerve and synapse within the nerve trunk. These preganglionic fibres have a mean peak CV (at 40 °C) of 0.31 m·s^–1.The other group is made up of fibres of postganglionic neurones which project orally along the nerve trunk. The results suggest that some postganglionic neurones project only as far as the next ganglion whilst others project beyond the next two ganglia for distances greater than 5 mm. The postganglionic fibres have a mean peak CV (at 40 °C) of 0.71 m·s^–1.These figures demonstrate that both pre- and postganglionic fibres are unmyelinated. The temperature coefficient (Q₁₀) for the CV of unmyelinated fibres in the intestinal nerve was 1.57.Abbreviations CAP compound action potential - CV conduction velocity - Q ₁₀ temperature coefficient     

Effect of Temperature on the Potential and Current Thresholds of Axon Membrane

The effect of temperature on the potential and current thresholds of the squid giant axon membrane was measured with gross external electrodes. A central segment of the axon, 0.8 mm long and in sea water, was isolated by flowing low conductance, isoosmotic sucrose solution on each side; both ends were depolarized in isoosmotic KCl. Measured biphasic square wave currents at five cycles per second were applied between one end of the nerve and the membrane of the central segment. The membrane potential was recorded between the central sea water and the other depolarized end. The recorded potentials are developed only across the membrane impedance. Threshold current values ranged from 3.2 µa at 267deg;C to 1 µa at 7.5°C. Threshold potential values ranged from 50 mv at 26°C to 6 mv at 7.5°C. The mean Q₁₀ of threshold current was 2.3 (SD = 0.2), while the Q₁₀ for threshold potentials was 2.0 (SD = 0.1).     

Activation of a Reserve Pool of Photosystem II in Chlamydomonas reinhardtii Counteracts Photoinhibition

The effect of strong irradiance (2000 micromole photons per square meter per second) on PSII heterogeneity in intact cells of Chlamydomonas reinhardtii was investigated. Low light (LL, 15 micromole photons per square meter per second) grown C. reinhardtii are photoinhibited upon exposure to strong irradiance, and the loss of photosynthetic functioning is due to damage to PSII. Under physiological growth conditions, PSII is distributed into two pools. The large antenna size (PSII_α) centers account for about 70% of all PSII in the thylakoid membrane and are responsible for plastoquinone reduction (Q_b-reducing centers). The smaller antenna (PSII_β) account for the remainder of PSII and exist in a state not yet able to photoreduce plastoquinone (Q_b-nonreducing centers). The exposure of C. reinhardtii cells to 60 minutes of strong irradiance disabled about half of the primary charge separation between P680 and pheophytin. The PSII_β content remained the same or slightly increased during strong-irradiance treatment, whereas the photochemical activity of PSII_α decreased by 80%. Analysis of fluorescence induction transients displayed by intact cells indicated that strong irradiance led to a conversion of PSII_β from a Q_b-nonreducing to a Q_b-reducing state. Parallel measurements of the rate of oxygen evolution revealed that photosynthetic electron transport was maintained at high rates, despite the loss of activity by a majority of PSII_α. The results suggest that PSII_β in C. reinhardtii may serve as a reserve pool of PSII that augments photosynthetic electron-transport rates during exposure to strong irradiance and partially compensates for the adverse effect of photoinhibition on PSII_α.     

Propagation Speed in Myelinated Nerve: I. Experimental Dependence on External Na+ and on Temperature

Conduction speed (θ) in single myelinated Rana pipiens sciatic nerve fibers has been precisely measured using intracellular recording and on-line digital computer techniques. The dependence of relative speed on external Na concentration at 15°C has been found to be ln(θ₁/θ₂) = 0.524 (±0.018) ln ([Na⁺]₁/[Na⁺]₂) + 0.003. Thus θ has very close to a square root dependence on [Na⁺]₀ for these fibers. This experimental finding is not in complete agreement with a theoretical prediction based on a solution of the Hodgkin-Huxley (H.H.) equations. The effect of small temperature variations around 15°C on θ has also been measured for Rana fibers in Ringer's solution. θ has close to an exponential dependence on T and a Q₁₀ of 2.95 has been estimated.     

Neuronal organization of ganglia in the lumbar sympathetic chain in cats

The study of the neuronal organization of ganglia L₃–L₆ of the sympathetic chain in cats by intracellular recording showed that neurons of the ganglion can be divided into three main groups on the principle of sympathetic preganglionic fibers of different types converging on them. The most numerous group (66%) consists of neurons on which sympathetic preganglionic fibers of the B₁, B₂, and C groups (with conduction velocities of 12.0±0.7, 4.4±0.3, and 1.0±0.1 m/sec respectively) simultaneously converge, while the least numerous group (10%) is formed by neurons with only sympathetic preganglionic fibers of the C-group converging on them; an intermediate group (24%) consists of neurons activated by sympathetic preganglionic fibers of the B₁ and B₂ groups. The preganglionic fibers to the ganglionic neurons can mainly be traced from the rostral segments of the spinal cord through the white rami communicantes. Sympathetic preganglionic fibers activating the neurons also enter the ganglion through their own and caudally situated white rami communicantes. Neurons of the ganglion were found to receive a preganglionic (C input) run in the composition of the gray ramus communicans and caudal commissure; the remaining neurons send their axons evidently into visceral branches.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 9, No. 5, pp. 518–526, September–October, 1977.     

Temperature Dependence of Voltage-gated H+ Currents in Human Neutrophils,Rat Alveolar Epithelial Cells,and Mammalian Phagocytes

H⁺ currents in human neutrophils, rat alveolar epithelial cells, and several mammalian phagocyte cell lines were studied using whole-cell and excised-patch tight-seal voltage clamp techniques at temperatures between 6 and 42°C. Effects of temperature on gating kinetics were distinguished from effects on the H⁺ current amplitude. The activation and deactivation of H⁺ currents were both highly temperature sensitive, with a Q ₁₀ of 6–9 (activation energy, E _a, ≈ 30–38 kcal/mol), greater than for most other ion channels. The similarity of E _a for channel opening and closing suggests that the same step may be rate determining. In addition, when the turn-on of H⁺ currents with depolarization was fitted by a delay and single exponential, both the delay and the time constant (τ_act) had similarly high Q ₁₀. These results could be explained if H⁺ channels were composed of several subunits, each of which undergoes a single rate-determining gating transition. H⁺ current gating in all mammalian cells studied had similarly strong temperature dependences. The H⁺ conductance increased markedly with temperature, with Q ₁₀ ≥ 2 in whole-cell experiments. In excised patches where depletion would affect the measurement less, the Q ₁₀ was 2.8 at >20°C and 5.3 at <20°C. This temperature sensitivity is much greater than for most other ion channels and for H⁺ conduction in aqueous solution, but is in the range reported for H⁺ transport mechanisms other than channels; e.g., carriers and pumps. Evidently, under the conditions employed, the rate-determining step in H⁺ permeation occurs not in the diffusional approach but during permeation through the channel itself. The large E _a of permeation intrinsically limits the conductance of this channel, and appears inconsistent with the channel being a water-filled pore. At physiological temperature, H⁺ channels provide mammalian cells with an enormous capacity for proton extrusion.     

THE LOCOMOTION OF LIMAX : I. TEMPERATURE COEFFICIENT OF PEDAL ACTIVITY.

Pedal progression of the slug Limax maximus was studied to obtain relations between wave velocity on the sole of the foot, wave frequency, the advance due to a single wave, and the velocity of vertically upward creeping. Each of the first three quantities is directly proportional to the simultaneous velocity of progression. Under comparable conditions, that is when work is done at a constant rate, the frequency of pedal waves is influenced by the temperature according to the equation of Arrhenius, with µ = 10,700 (Q ₁₀ for 11° to 21° = 2.1). The velocity of a single wave must have very nearly the same "temperature characteristic," which is found also in another case of nerve net transmission (in Renilla).     

Physiology of Root-Associated Nitrogenase Activity in Oryza sativa

An intact method for measuring immediately linear rates of acetylene reduction was used to investigate the relationship between temperature, pH, O₂ concentration, and light intensity with the rate of root-associated nitrogenase activity in rice (Oryza sativa L.). Nitrogenase activity varied over a temperature range of 10 to 50°C and optimal rates of acetylene reduction were recorded at 35°C. Nitrogenase activity was also influenced by the pH of the liquid surrounding the roots prior to assay. Maximal rates of acetylene reduction were recorded over a pH range from 5.8 to 7.5. Nitrogenase activity was significantly reduced by concentrations of O₂ 0.5% (v/v) or more when the intact plant assay method was used, and no optimum was detected. However, when the plant tops were removed and the cut ends sealed from the atmosphere for 4 hours, acetylene reduction rates were maximal at 0.25% O₂ (v/v). When plants were moved from sunlight (1,400 microeinsteins per square meter per second) to shade (9.6) root-associated nitrogenase activity at 35° C significantly decreased 15 min later to one-fourth the rate and recovered upon return to sunlight. When the light intensity reaching the leaf canopy was progressively reduced from 1,050 to 54 microeinsteins per square meter per second the rate of root-associated nitrogenase activity decreased from 550 ± 135 to 192 ± 55 nanomoles ethylene per gram dry root per hour. The study suggests that the rate of root-associated nitrogenase activity in rice at constant temperature may well be mediated by variations in the concentration of O₂ resulting from changes in the rate of photosynthesis as well as variations in the rate of transport of photosynthate.     

Water Transport in the Liana Bauhinia fassoglensis (Fabaceae)

To determine the efficiency of xylem conductance in the liana (woody vine) Bauhinia fassoglensis Kotschy ex Schweinf., we measured hydraulic conductance per unit stem length (measured K_h), leaf-specific conductivity (LSC = K_h/distal leaf area), transpiration rate (E), xylem water potential (ε), vessel number, and vessel diameter. The measured K_h was 49% (se = 7%) of the predicted K_h from Poiseuille's law. The mean LSC for unbranched stem segments was 1.10 × 10⁻⁸ square meters per megapascal per second (se = 0.07). LSCs were much lower (about 0.2) at branch junctions. At midday, with E at 7 × 10⁻⁸ meters per second, the measured drop in ε was about 0.08 megapascal per meter along the stems and branches and about 0.27 megapascal in going from stem to leaf. In addition, there was a drop of about 0.20 megapascal at branch junctions as predicted by E/LSC. In diurnal measurements leaf ε never dropped below about −1.2 megapascal. For long (e.g. 16 meters) stems, the predicted mid-day drop in ε through the xylem transport system might be great enough to have substantial physiological impact.     

Cytoplasm Resistivity of Mammalian Atrial Myocardium Determined by Dielectrophoresis and Impedance Methods

Many cardiac arrhythmias are caused by slowed conduction of action potentials, which in turn can be due to an abnormal increase of intracellular myocardial resistance. Intracellular resistivity is a linear sum of that offered by gap junctions between contiguous cells and the cytoplasm of the myocytes themselves. However, the relative contribution of the two components is unclear, especially in atrial myocardium, as there are no precise measurements of cytoplasmic resistivity, R_c. In this study, R_c was measured in atrial tissue using several methods: a dielectrophoresis technique with isolated cells and impedance measurements with both isolated cells and multicellular preparations. All methods yielded similar values for R_c, with a mean of 138 ± 5 Ω·cm at 23°C, and a Q₁₀ value of 1.20. This value is about half that of total intracellular resistivity and thus will be a significant determinant of the actual value of action potential conduction velocity. The dielectrophoresis experiments demonstrated the importance of including divalent cations (Ca²⁺ and Mg²⁺) in the suspension medium, as their omission reduced cell integrity by lowering membrane resistivity and increasing cytoplasm resistivity. Accurate measurement of R_c is essential to develop quantitative computational models that determine the key factors contributing to the development of cardiac arrhythmias.     

Tonic impulsation in preganglionic and postganglionic sympathetic fibers of mammals

A study of the tonic electrical activity of nerves containing preganglionic and postganglionic fibers in the superior cervical and stellate sympathetic ganglia of cats and rabbits has shown that this activity consists of groups of spikes synchronous with the pulse or respiration, and occurs on a background of irregular low-amplitude impulses. The frequency of spikes is higher (250/sec) in nerves containing preganglionic fibers than in those containing postganglionic fibers (100/sec). Groups of spikes in a nerve containing preganglionic fibers correspond in some preparations to groups of spikes of lower frequency in a nerve containing postganglionic fibers of the same ganglion; in other preparations, this correspondence was lacking, apparently due to the absence of synaptic contacts between those groups of pre- and postganglionic neurons whose activity was recorded. Neurons send axons to different nerves (cardiac and vertebral) of the stellate ganglion discharged synchronously in some preparations, and asynchronously in others. Where synchronization was observed, the neurons discharged in rhythm with cardiac contractions.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 1, No. 3, pp. 303–308, November–December, 1969.     

Cation exchange between cells and plasma of mammalian blood; methods and application to potassium exchange in human blood

The exchange of potassium between cells and plasma of heparinized human blood has been studied in vitro using the radioactive isotope K⁴². The changes in cell and plasma specific activity are characteristic of a simple two-compartment system. The mean of seven determinations of the exchange rate at 38°C. is 1.8 per cent of the cellular potassium per hour. The results indicate that at 38°C. the rate is relatively insensitive to oxygenation or reduction of the hemoglobin, and to 1200 r of gamma radiation. With varying temperature the rate follows pseudo first order kinetics with a Q₁₀ of 2.35. Below 15°C. the rate of loss of potassium exceeds the rate of uptake.     

Water transport in the midrib tissue of maize leaves : direct measurement of the propagation of changes in cell turgor across a plant tissue

Water movement across plant tissues occurs along two paths: from cell-to-cell and in the apoplasm. We examined the contribution of these two paths to the kinetics of water transport across the parenchymatous midrib tissue of the maize (Zea mays L.) leaf. Water relations parameters (hydraulic conductivity, Lp; cell elastic coefficient, ε; half-time of water exchange for individual cells, T_½) of individual parenchyma cells determined with the pressure probe varied in different regions of the midrib. In the adaxial region, Lp = (0.3 ± 0.3)·10⁻⁵ centimeters per second per bar, ε = 103 ± 72 bar, and T_½ = 7.9 ± 4.8 seconds (n = seven cells); whereas, in the abaxial region, Lp = (2.5 ± 0.9)·10⁻⁵ centimeters per second per bar, ε = 41 ± 9 bar, and T_½ = 1.3 ± 0.5 seconds (n = 7). This zonal variation in Lp, ε, and T_½ indicates that tissue inhomogeneities exist for these parameters and could have an effect on the kinetics of water transport across the tissue.

The diffusivity of the tissue to water (D_t) obtained from the sorption kinetics of rehydrating tissue was D_t = (1.1 ± 0.4)·10⁻⁶ square centimeters per second (n = 6). The diffusivity of the cell-to-cell path (D_c) calculated from pressure probe data ranged from D_c = 0.4·10⁻⁶ square centimeters per second in the adaxial region to D_c = 6.1·10⁻⁶ square centimeters per second in the abaxial region of the tissue. D_t D_c suggests substantial cell-to-cell transport of water occurred during rehydration. However, the tissue diffusivity calculated from the kinetics of pressure-propagation across the tissue (D_t′) was D_t′ = (33.1 ± 8.0)·10⁻⁶ square centimeters per second (n = 8) and more than 1 order of magnitude larger than D_t. Also, the hydraulic conductance of the midrib tissue (Lp_m per square centimeter of surface) estimated from pressure-induced flows across several parenchyma cell layers was Lp_m = (8.9 ± 5.6)·10⁻⁵ centimeters per second per bar (n = 5) and much larger than Lp.

These results indicate that the preferential path for water transport across the midrib tissue depends on the nature of the driving forces present within the tissue. Under osmotic conditions, the cell-to-cell path dominates, whereas under hydrostatic conditions water moves primarily in the apoplasm.

     

Simultaneous Measurements of Steady State Chlorophyll a Fluorescence and CO(2) Assimilation in Leaves: The Relationship between Fluorescence and Photosynthesis in C(3) and C(4) Plants

Rates of CO₂ assimilation and steady state chlorophyll a fluorescence were measured simultaneously at different intercellular partial pressures of CO₂ in attached cotton (Gossypium hirsutum L. cv Deltapine 16) leaves at 25°C. Electron transport activity for CO₂ assimilation plus photorespiration was calculated for these experiments. Under light saturating (1750 microeinsteins per square meter per second) and light limiting (700 microeinsteins per square meter per second) conditions there was a good correlation between fluorescence and the calculated electron transport activity at 19 and 200 millibars O₂, and between fluorescence and rates of CO₂ assimilation at 19 millibars but not 200 millibars O₂. The values of fluorescence measured at about 220 microbars intercellular CO₂ were not greatly affected by increasing O₂ from 19 to 800 millibars. Fluorescence increased with light intensity at any one intercellular CO₂ partial pressure. But the values obtained for fluorescence, expressed as a ratio of the maximum fluorescence obtained in DCMU-treated tissue, over the same range of CO₂ partial pressure at 500 microeinsteins per square meter per second were similar to those obtained at 1000 and 2000 microeinsteins per square meter per second. There were two phases in the observed correlation between fluorescence and calculated electron transport activity: an initial inverse relationship at low CO₂ partial pressures which reversed to a positive correlation at higher values of CO₂ partial pressures. Similar results were observed in the C₃ species Helianthus annuus L., Phaseolus vulgaris L., and Brassica chinensis. In all C₄ species (Zea mays L., Sorghum bicolor L., Panicum maximum Jacq., Amaranthus edulis Speg., and Echinochloa frumentacea [Roxb.] Link) examined changes in fluorescence were directly correlated with changes in CO₂ assimilation rates. The nature and the extent to which Q (primary quencher) and high-energy state (q_E) quenching function in determining the steady state fluorescence obtained during photosynthesis in leaves is discussed.     

Temperature dependence of proton permeation through a voltage-gated proton channel

Voltage-gated proton channels are found in many different types of cells, where they facilitate proton movement through the membrane. The mechanism of proton permeation through the channel is an issue of long-term interest, but it remains an open question. To address this issue, we examined the temperature dependence of proton permeation. Under whole cell recordings, rapid temperature changes within a few milliseconds were imposed. This method allowed for the measurement of current amplitudes immediately before and after a temperature jump, from which the ratios of these currents (I_ratio) were determined. The use of I_ratio for evaluating the temperature dependence minimized the contributions of factors other than permeation. Temperature jumps of various degrees (ΔT, −15 to 15°C) were applied over a wide temperature range (4–49°C), and the Q₁₀s for the proton currents were evaluated from the I_ratios. Q₁₀ exhibited a high temperature dependence, varying from 2.2 at 10°C to 1.3 at 40°C. This implies that processes with different temperature dependencies underlie the observed Q₁₀. A novel resistivity pulse method revealed that the access resistance with its low temperature dependence predominated in high temperature ranges. The measured temperature dependence of Q₁₀ was decomposed into Q₁₀ of the channel and of the access resistances. Finally, the Q₁₀ for proton permeation through the voltage-gated proton channel itself was calculated and found to vary from 2.8 at 5°C to 2.2 at 45°C, as expected for an activation enthalpy of 64 kJ/mol. The thermodynamic features for proton permeation through proton-selective channels were discussed for the underlying mechanism.     

Developmental history affects the susceptibility of spinach leaves to in vivo low temperature photoinhibition

Room temperature chlorophyll a fluorescence was used to determine the effects of developmental history, developmental stage, and leaf age on susceptibility of spinach to in vivo low temperature (5°C) induced photoinhibition. Spinach (Spinacia oleracea cv Savoy) leaves expanded at cold hardening temperatures (5°C day/night), an irradiance of 250 micromoles per square meter per second of photosynthetic proton flux density, and a photoperiod of 16 hours were less sensitive than leaves expanded at nonhardening temperatures (16 or 25°C day/night) and the same irradiance and photoperiod. This differential sensitivity to low-temperature photoinhibition was observed at high (1200) but not lower (500 or 800 micromoles per square meter per second) irradiance treatment. In spite of a differential sensitivity to photoinhibition, both cold-hardened and nonhardened spinach exhibited similar recovery kinetics at either 20 or 5°C. Shifting plants grown at 16°C (day/night) to 5°C (day/night) for 12 days after full leaf expansion did not alter the sensitivity to photoinhibition at 5°C. Conversely, shifting plants grown at 5°C (day/night) to 16°C (day/night) for 12 days produced a sensitivity to photoinhibition at 5°C similar to control plants grown at 16°C. Thus, any resistance to low-temperature photoinhibition acquired during growth at 5°C was lost in 12 days at 16°C. We conclude that leaf developmental history, developmental stage, and leaf age contribute significantly to the in vivo photoinhibitory response of spinach. Thus, these characteristics must be defined clearly in studies of plant susceptibility to photoinhibition.     

Variations in Temperature Sensitivity (Q10) of CH4 Emission from a Subtropical Estuarine Marsh in Southeast China

Understanding the functional relationship between greenhouse gas fluxes and environmental variables is crucial for predicting the impacts of wetlands on future climate change in response to various perturbations. We examined the relationships between methane (CH₄) emission and temperature in two marsh stands dominated by the Phragmites australis and Cyperus malaccensis, respectively, in a subtropical estuarine wetland in southeast China based on three years of measurement data (2007–2009). We found that the Q₁₀ coefficient of CH₄ emission to soil temperature (Q_s10) from the two marsh stands varied slightly over the three years (P > 0.05), with a mean value of 3.38 ± 0.46 and 3.89 ± 0.41 for the P. australis and C. malaccensis stands, respectively. On the other hand, the three-year mean Q_a10 values (Q₁₀ coefficients of CH₄ emission to air temperature) were 3.39 ± 0.59 and 4.68 ± 1.10 for the P. australis and C. malaccensis stands, respectively, with a significantly higher Q_a10 value for the C. malaccensis stand in 2008 (P < 0.05). The seasonal variations of Q₁₀ (Q_s10 and Q_a10) differed among years, with generally higher values in the cold months than those in the warm months in 2007 and 2009. We found that the Q_s10 values of both stands were negatively correlated with soil conductivity, but did not obtain any conclusive results about the difference in Q₁₀ of CH₄ emission between the two tidal stages (before flooding and after ebbing). There were no significant differences in both Q_s10 and Q_a10 values of CH₄ emission between the P. australis stand and the C. malaccensis stands (P > 0.05). Our results show that the Q₁₀ values of CH₄ emission in this estuarine marsh are highly variable across space and time. Given that the overall CH₄ flux is governed by a suite of environmental factors, the Q₁₀ values derived from field measurements should only be considered as a semi-empirical parameter for simulating CH₄ emissions.     

Regulation of Ribulose-1,5-Bisphosphate Carboxylase Activity in Response to Light Intensity and CO(2) in the C(3) Annuals Chenopodium album L. and Phaseolus vulgaris L

The light and CO₂ response of (a) photosynthesis, (b) the activation state and total catalytic efficiency (k_cat) of ribulose-1,5-bisphosphate carboxylase (rubisco), and (c) the pool sizes of ribulose 1,5-bisphosphate, (RuBP), ATP, and ADP were studied in the C₃ annuals Chenopodium album and Phaseolus vulgaris at 25°C. The initial slope of the photosynthetic CO₂ response curve was dependent on light intensity at reduced light levels only (less than 450 micromoles per square meter per second in C. album and below 200 micromoles per square meter per second in P. vulgaris). Modeled simulations indicated that the initial slope of the CO₂ response of photosynthesis exhibited light dependency when the rate of RuBP regeneration limited photosynthesis, but not when rubisco capacity limited photosynthesis. Measured observations closely matched modeled simulations. The activation state of rubisco was measured at three light intensities in C. album (1750, 550, and 150 micromoles per square meter per second) and at intercellular CO₂ partial pressures (C₁) between the CO₂ compensation point and 500 microbars. Above a C₁ of 120 microbars, the activation state of rubisco was light dependent. At light intensities of 550 and 1750 micromoles per square meter per second, it was also dependent on C₁, decreasing as the C₁ was elevated above 120 microbars at 550 micromoles per square meter per second and above 300 microbars at 1750 micromoles per square meter per second. The pool size of RuBP was independent of C₁ only under conditions when the activation state of rubisco was dependent on C₁. Otherwise, RuBP pool sizes increased as C₁ was reduced. ATP pools in C. album tended to increase as C₁ was reduced. In P. vulgaris, decreasing C₁ at a subsaturating light intensity of 190 micromoles per square meter per second increased the activation state of rubisco but had little effect on the k_cat. These results support modelled simulations of the rubisco response to light and CO₂, where rubisco is assumed to be down-regulated when photosynthesis is limited by the rate of RuBP regeneration.     

Studies of the Uptake of Nitrate in Barley : II. Energetics

Q₁₀ values for ¹³NO₃⁻ influx were determined in `uninduced' (NO<sub>3</sub><sup>−</sup>-starved) and `induced' (NO₃⁻-pretreated) roots of barley (Hordeum vulgare L.) plants at various concentrations of external NO₃⁻ ([NO₃⁻]₀). At 0.02 mole per cubic meter [NO₃⁻]₀, Q₁₀ values for influx were from 3 to 4 between 5 and 10°C. As [NO₃⁻]₀ increased Q₁₀ values decreased, reaching values of 1.2 and 2.0, respectively, at 20 moles per cubic meter in uninduced and induced plants. The metabolic dependence of ¹³NO₃⁻ influx at low and high [NO₃⁻]₀ (0.1 and 20.0 moles per cubic meter, respectively) in uninduced and induced plants was probed by the use of various inhibitors. These experiments confirmed the findings of the Q₁₀ studies, demonstrating that at low [NO₃⁻]₀¹³NO₃⁻ influx was extremely sensitive to metabolic inhibition. By contrast, at high [NO₃⁻]₀, influx was relatively insensitive to the presence of inhibitors.