Primary Events, Energy Transfer, and Reactions in Photosynthetic Units: Lifetime of the Excited State In Vivo: I. Chlorophyll a in Algae, at Room and at Liquid Nitrogen Temperatures; Rate Constants of Radiationless Deactivation and Trapping

Using a mode-locked laser (λ, 632.8 nm), fluorescence decay of chlorophyll (Chl) a in the green alga Chlorella pyrenoidosa, the red alga Porphyridium cruentum, and the blue-green alga Anacystis nidulans was measured by the phase-shift method under conditions when photosynthesis was not operative (3-(3,4-dichlorophenyl)-1,1-dimethylurea [DCMU] poisoning, or cooling to 77°K). In the presence of 10^-5 M DCMU, the lifetime of Chl a fluorescence (τ) at room temperature is about 1.7 nsec in Chlorella, 1.0 nsec in Porphyridium, and 0.7 nsec in Anacystis. At 77°K, τ is 1.4 nsec (for fluorescence at about 685 nm, F-685) and 2.3 nsec (for F-730) in Chlorella, 0.9 nsec (F-685) and 1.2 nsec (F-730) in Porphyridium, and 0.8 nsec (F-685 and F-730) in Anacystis. From the above measurement, and the assumption that τ₀ (the intrinsic fluorescence lifetime) for Chl a in all three algae is 15.2 nsec, we have calculated the rate constants of radiationless transition (that includes energy transfer to weakly fluorescent system I) processes competing with fluorescence at room temperature to be about 5 × 10⁸ sec^-1 in Chlorella, 9 × 10⁸ sec^-1 in Porphyridium, and 13 × 10⁸ sec^-1 in Anacystis. At 77°K, this rate constant for Chl a that fluoresces at 685 nm remains, in the first approximation, the same as at room temperature. From the τ data, the rate constant for the trapping of excitation energy is calculated to be about 1.2 × 10⁹ sec^-1 for Chlorella, 2 × 10⁹ sec^-1 for Porphyridium, and 2 × 10⁹ sec^-1 for Anacystis. The efficiency of trapping is calculated to be about 66% (Chlorella), 68% (Porphyridium), and 60% (Anacystis). (It is recognized that variations in the above values are to be expected if algae grown under different conditions are used for experimentation.) The maximum quantum yield of Chl a fluorescence for system II (λ, 632.8 nm), calculated from τ measurements, is about 10% in Chlorella, 6-7% in Porhyridium, and 5% in Anacystis under conditions when photosynthesis is not operative; the values at 77°K appear to be very close to those with DCMU added at room temperature. ø for F-730 at 77°K, however, is somewhat higher than for F-685. The predicted quantum yields of fluorescence for Chl a in intact cells (both systems I and II) at low intensities of 632.8 nm light are about 2-3, 1-2, and 1% for Chlorella, Porphyridium, and Anacystis, respectively.     

Measurement of the fluorescence lifetime of chlorophyll a in vivo

New measurements have been made of fluorescence lifetime (τ) of chlorophyll a in the algae Chlorella pyrenoidosa, Porphyridium cruentum, Anacystis nidulans, and in spinach chloroplast. τ-values of 0.6 and 0.7 nsec were obtained with green plants. Anacystis and Porphyridium gave a τ of 0.5 nsec. The previously described two stage decay of fluorescence in vivo in these organisms could not be confirmed. This observation could have been caused by a second wave of light emission from the exciting hydrogen lamp (not detected in earlier work). The lifetimes found in this study (calculated, as before, by the method of convolution integrals) were close to those found by other observers for “low” excitation intensities; the value first reported from this laboratory (1.0-1.7 nsec) may have corresponded to “high” excitation intensity.     

Thermal Characterization,Crystal Field Analysis and In-Band Pumped Laser Performance of Er Doped NaY(WO4)2 Disordered Laser Crystals

Undoped and Er-doped NaY(WO₄)₂ disordered single crystals have been grown by the Czochralski technique. The specific heat and thermal conductivity (κ) of these crystals have been characterized from T = 4 K to 700 K and 360 K, respectively. It is shown that κ exhibits anisotropy characteristic of single crystals as well as a κ(T) behavior observed in glasses, with a saturation mean free phonon path of 3.6 Å and 4.5 Å for propagation along a and c crystal axes, respectively. The relative energy positions and irreducible representations of Stark Er³⁺ levels up to ⁴G_7/2 multiplet have been determined by the combination of experimental low (<10 K) temperature optical absorption and photoluminescence measurements and simulations with a single-electron Hamiltonian including both free-ion and crystal field interactions. Absorption, emission and gain cross sections of the ⁴I_13/2↔⁴I_15/2 laser related transition have been determined at 77 K. The ⁴I_13/2 Er³⁺ lifetime (τ) was measured in the temperature range of 77–300 K, and was found to change from τ (77K) ≈ 4.5 ms to τ (300K) ≈ 3.5 ms. Laser operation is demonstrated at 77 K and 300 K by resonantly pumping the ⁴I_13/2 multiplet at λ≈1500 nm with a broadband (FWHM≈20 nm) diode laser source perfectly matching the 77 K crystal ⁴I_15/2 → ⁴I_13/2 absorption profile. At 77 K as much as 5.5 W of output power were obtained in π-polarized configuration with a slope efficiency versus absorbed pump power of 57%, the free running laser wavelength in air was λ≈1611 nm with the laser output bandwidth of 3.5 nm. The laser emission was tunable over 30.7 nm, from 1590.7 nm to 1621.4 nm, for the same π-polarized configuration.     

Simulation of Interstitial Fluid Flow in Ligaments: Comparison among Stokes,Brinkman and Darcy Models

In this paper, we use Stokes, Brinkman and Darcy equations to approximate the porous continuum media of ligament tissues respectively, simulate the flow field with FLUENT software, and study the shear stress on the cell surface due to the interstitial fluid flow. Since the Brinkman equation approaches Stokes equation well in high hydraulic permeability (k_p) condition (k_p ≥1.0×10^-8 m² in our numerical simulation), and it is an approximation to Darcy model in low k_p condition (k_p ≤5.0×10^-12 m² in our numerical simulation), we used the Brinkman model to simulate the interstitial fluid flow in the ligament where k_p is approximately 1.0×10^-16 m². It shows k_p and anisotropic property have a little effect on the flow field, but have a great effect on the shear stress on the membrane of interstitial cells (τ_cell). There is a linear relationship between τ_cell and , when k_p =1.0×10^-16 m² and the maximum τ_cell (τ_cell,max) is approximately 10 Pa. The anisotropic property will affect τ_cell''s distribution on the cell surface. When k_x/k_y>1, low τ_cell dominates the cell, while when k_x/k_y<1, high τ_cell dominants the cell.     

Early Picosecond Events in the Photocycle of Bacteriorhodopsin

The primary processes of the photochemical cycle of light-adapted bacteriorhodopsin (BR) were studied by various experimental techniques with a time resolution of 5 × 10^-13 s. The following results were obtained. (a) After optical excitation the first excited singlet state S₁ of bacteriorhodopsin is observed via its fluorescence and absorption properties. The population of the excited singlet state decays with a lifetime τ₁ of ~0.7 ps (430 ± 50 fs) (52). (b) With the same time constant the first ground-state intermediate J builds up. Its absorption spectrum is red-shifted relative to the spectrum of BR by ~30 nm. (c) The second photoproduct K, which appears with a time constant of τ₂ = 5 ps shows a red-shift of 20 nm, relative to the peak of BR. Its absorption remains constant for the observation time of 300 ps. (d) Upon suspending bacteriorhodopsin in D₂O and deuterating the retinal Schiff base at its nitrogen (lysine 216), the same photoproducts J and K are observed. The relaxation time constants τ₁ and τ₂ remain unchanged upon deuteration within the experimental accuracy of 20%.     

High Abundances of Aerobic Anoxygenic Photosynthetic Bacteria in the South Pacific Ocean

Little is known about the abundance, distribution, and ecology of aerobic anoxygenic phototrophic (AAP) bacteria, particularly in oligotrophic environments, which represent 60% of the ocean. We investigated the abundance of AAP bacteria across the South Pacific Ocean, including the center of the gyre, the most oligotrophic water body of the world ocean. AAP bacteria, Prochlorococcus, and total prokaryotic abundances, as well as bacteriochlorophyll a (BChl a) and divinyl-chlorophyll a concentrations, were measured at several depths in the photic zone along a gradient of oligotrophic conditions. The abundances of AAP bacteria and Prochlorococcus were high, together accounting for up to 58% of the total prokaryotic community. The abundance of AAP bacteria alone was up to 1.94 × 10⁵ cells ml⁻¹ and as high as 24% of the overall community. These measurements were consistent with the high BChl a concentrations (up to 3.32 × 10⁻³ μg liter⁻¹) found at all stations. However, the BChl a content per AAP bacterial cell was low, suggesting that AAP bacteria are mostly heterotrophic organisms. Interestingly, the biovolume and therefore biomass of AAP bacteria was on average twofold higher than that of other prokaryotic cells. This study demonstrates that AAP bacteria can be abundant in various oligotrophic conditions, including the most oligotrophic regime of the world ocean, and can account for a large part of the bacterioplanktonic carbon stock.     

Somatostatin receptor subtype 4 modulates L-type calcium channels via Gβγ and PKC signaling in rat retinal ganglion cells

Somatostatin subtype-4 receptors (sst₄) inhibit L-type calcium channel currents (I_Ca) in retinal ganglion cells (RGCs). Here we identify the signaling pathways involved in sst₄ stimulation leading to suppression of I_Ca in RGCs. Whole cell patch clamp recordings were made on isolated immunopanned RGCs using barium as a charge carrier to isolate I_Ca. Application of the selective sst₄ agonist, L-803 (10 nM), reduced I_Ca by 41.2%. Pretreatment of cells with pertussis toxin (Gi/o inhibitor) did not prevent the action of L-803, which reduced I_Ca by 34.7%. To determine the involvement of Gβγ subunits after sst₄ activation, depolarizing pre-pulse facilitation paradigms were used to remove voltage-dependent inhibition of calcium channels. Pre-pulse facilitation did not reverse the inhibitory effects of L-803 on I_Ca (8.4 vs. 8.8% reductions, ctrl vs. L-803); however, pharmacologic inhibition of Gβγ reduced I_Ca suppression by L-803 (23.0%, P < 0.05). Inhibition of PKC (GF109203X; GFX) showed a concentration-dependent effect in preventing the action of L-803 on I_Ca (1 μM GFX, 34.3%; 5 μM GFX, 14.6%, P < 0.05). When both PKC and Gβγ were inhibited, the effects of L-803 on I_Ca were blocked (1.8%, P < 0.05). These results suggest that sst₄ stimulation modulates RGC calcium channels via Gβγ and PKC activation. Since reducing intracellular Ca²⁺ is known to be neuroprotective in RGCs, modulating these sst₄ signaling pathways may provide insights to the discovery of unique therapeutic targets to reduce intracellular Ca²⁺ levels in RGCs.     

Stomatal Behavior and Water Status of Maize, Sorghum, and Tobacco under Field Conditions: I. At High Soil Water Potential

Diurnal changes in the vertical profiles of irradiance incident upon the adaxial leaf surface (I), stomatal resistance (r_s), leaf water potential (ψ), osmotic potential (π), and turgor potential (P) were followed concurrently in crops of maize (Zea mays L. var. Pa 602A), sorghum (Sorghum bicolor [L.] Moench var. RS610), and tobacco (Nicotiana tabacum L. var. Havanna Seed 211) on several days in 1968 to 1970 when soil water potentials were high. In all three crops the r_s, measured with a ventilated diffusion porometer, the ψ, measured with the pressure chamber, the π, measured with a vapor pressure osmometer, and the calculated P, decreased from sunrise to reach minimum values near midday and then increased again in the afternoon. The diurnal range of all the variables was greater for leaves in the upper canopy than for those in the lower canopy. P was observed to decrease with decreasing ψ, but never became zero. Sorghum had a higher P at a ψ of, say −10 bars, than did maize, and maize had a higher P than tobacco at the same ψ. Moreover, at the same ψ the upper leaves in all canopies had a higher P than the lower leaves. When compared at high irradiances, r_s did not increase as ψ declined to −13, −15, and −10 bars or as P declined to 0.3, 3.5, and 1.2 bars in maize, sorghum, and tobacco, respectively. The relation between r_s and I in the upper, nonsenescent leaves of all three crops fits a hyperbolic curve, but the response varied with species and leaf senescence. The adaxial and abaxial epidermises had the same response of r_s to I in maize and sorghum, whereas in tobacco the adaxial epidermis had a higher r_s than the abaxial epidermis at all values of I. At equal values of I, tobacco had the lowest leaf resistance (r_l) and maize had the highest r_l. Senescent maize leaves had nonfunctional stomata, whereas the lowermost sorghum leaves had higher stomatal resistances on average than the other leaves.     

Elastic-Mathematical Theory of Cells and Mitochondria in Swelling Process: II. Effect of Temperature upon Modulus of Elasticity of Membranous Material of Egg Cells of Sea Urchin, Strongylocentrotus purpuratus, and of Oyster, Crassostrea virginica

The elastic behavior of the cell wall as a function of the temperature has been studied with particular attention being given to the swelling of egg cells of Strongylocentrotus purpuratus and Crassostrea virginica in different sea water concentrations at different temperatures. It was found that the modulus of elasticity is a nonlinear function of temperature. At about 12-13°C the modulus of elasticity (E) is constant, independent of the stress (σ) and strain (ε_ν) which exist at the cell wall; the membranous material follows Hooke's law, and E ≈ 3 × 10⁷ dyn/cm² for S. purpuratus and C. virginica. When the temperature is higher or lower than 12-13°C, the modulus of elasticity increases, and the membranous material does not follow Hooke's law, but is almost directly proportional to the stresses existing at the cell wall. On increasing the stress, the function E_σ = E(σ) approaches saturation. The corresponding stress-strain diagrams, σ = σ(ε_ν), and the graphs, E_σ = E(σ) and E_σ = E(t) are given. The cyto-elastic phenomena at the membrane are discussed.     

Physiology and Phylogeny of Green Sulfur Bacteria Forming a Monospecific Phototrophic Assemblage at a Depth of 100 Meters in the Black Sea

The biomass, phylogenetic composition, and photoautotrophic metabolism of green sulfur bacteria in the Black Sea was assessed in situ and in laboratory enrichments. In the center of the western basin, bacteriochlorophyll e (BChl e) was detected between depths of 90 and 120 m and reached maxima of 54 and 68 ng liter⁻¹. High-pressure liquid chromatography analysis revealed a dominance of farnesyl esters and the presence of four unusual geranyl ester homologs of BChl e. Only traces of BChl e (8 ng liter⁻¹) were found at the northwestern slope of the Black Sea basin, where the chemocline was positioned at a significantly greater depth of 140 m. Stable carbon isotope fractionation values of farnesol indicated an autotrophic growth mode of the green sulfur bacteria. For the first time, light intensities in the Black Sea chemocline were determined employing an integrating quantum meter, which yielded maximum values between 0.0022 and 0.00075 μmol quanta m⁻² s⁻¹ at the top of the green sulfur bacterial layer around solar noon in December. These values represent by far the lowest values reported for any habitat of photosynthetic organisms. Only one 16S rRNA gene sequence type was detected in the chemocline using PCR primers specific for green sulfur bacteria. This previously unknown phylotype groups with the marine cluster of the Chlorobiaceae and was successfully enriched in a mineral medium containing sulfide, dithionite, and freshly prepared yeast extract. Under precisely controlled laboratory conditions, the enriched green sulfur bacterium proved to be capable of exploiting light intensities as low as 0.015 μmol quanta m⁻² s⁻¹ for photosynthetic ¹⁴CO₂ fixation. Calculated in situ doubling times of the green sulfur bacterium range between 3.1 and 26 years depending on the season, and anoxygenic photosynthesis contributes only 0.002 to 0.01% to total sulfide oxidation in the chemocline. The stable population of green sulfur bacteria in the Black Sea chemocline thus represents the most extremely low-light-adapted and slowest-growing type of phototroph known to date.     

ON THE VARIABILITY OF CRITICAL ILLUMINATION FOR FLICKER FUSION AND INTENSITY DISCRIMINATION

From the data of experiments with bees in which threshold response is employed as a means of recognizing visual discrimination between stripes of equal width alternately illuminated by intensities I ₁ and I ₂, it is shown that the detectable increment of intensity ΔI, where ΔI = I ₂ - I ₁, is directly proportional to σ_I2 (I ₁ being fixed). From tests of visual acuity, where I ₁ = 0 and the width of the stripes is varied, σ_I2 = kI ₂ + const.; here I ₂ = ΔI, and ΔI/I ₂ = 1. When the visual excitability of the bee is changed by dark adaptation, λI ≡ kΔI (= k'' σ_ΔI) = k'''' I + const. For the measurements of critical illumination at threshold response to flicker, σ_I2 (= σ_ΔI) = k I ₂ = k'' ΔI + const. The data for critical illumination producing threshold response to flicker in the sun-fish Lepomis show for the rods σ_I2 = K I ₂ for the cones σ_I2 = K''(I ₂ + const.). The data thus indicate that in all these experiments essentially the same visual function is being examined, and that the recognition of the production of a difference in effect by alternately illuminated stripes takes place in such a way that d (ΔI)/d (σ_I2) = const., and that ΔI is directly proportional to I (or "I ₂," depending on the nature of the experiment). It is pointed out that the curve for each of the cases considered can be gotten equally well if mean I or σ_I is plotted as a function of the independent variable involved in the experiment. Certain consequences of these and related facts are important for the treatment of the general problem of intensity discrimination.     

Energetics of Sodium Transport in Frog Skin : II. The effects of electrical potential on oxygen consumption

Studies were made of the dependence of the rate of oxygen consumption, J_r, on the electrical potential difference, Δψ, across the frog skin. After the abolition of sodium transport by ouabain the basal oxygen consumption was independent of Δψ. In fresh skins J_r was a linear function of Δψ over a range of at least ±70 mv. Treatment with aldosterone stimulated the short-circuit current, I_o, and the associated rate of oxygen consumption, J_ro, and increased their stability; linearity was then demonstrable over a range of ±160 mv. Brief perturbations of Δψ (±30–200 mv) did not alter subsequent values of I_o. Perturbations for 10 min or more produced a "memory" effect both with and without aldosterone: accelerating sodium transport by negative clamping lowered the subsequent value of I_o; positive clamping induced the opposite effect. Changes in J_ro were more readily detectable in the presence of aldosterone; these were in the same direction as the changes in I_o. The linearity of J_r in Δψ indicates the validity of analysis in terms of linear nonequilibrium thermodynamics—brief perturbations of Δψ appear to produce no significant effect on either the phenomenological coefficients or the free energy of the metabolic driving reaction. Hence it is possible to evaluate this free energy.     

Molecular Anatomy of ParA-ParA and ParA-ParB Interactions during Plasmid Partitioning

Firmicutes multidrug resistance inc18 plasmids encode parS sites and two small homodimeric ParA-like (δ₂) and ParB-like (ω₂) proteins to ensure faithful segregation. Protein ω₂ binds to parS DNA, forming a short left-handed helix wrapped around the full parS, and interacts with δ₂. Protein δ₂ interacts with ω₂ and, in the ATP-bound form, binds to nonspecific DNA (nsDNA), forming small clusters. Here, we have mapped the ω₂·δ₂ and δ₂·δ₂ interacting domains in the δ₂ that are adjacent to but distinct from each other. The δ₂ nsDNA binding domain is essential for stimulation of ω₂·parS-mediated ATP hydrolysis. From the data presented here, we propose that δ₂ interacts with ATP, nsDNA, and with ω₂ bound to parS at near equimolar concentrations, facilitating a δ₂ structural transition. This δ₂ “activated” state overcomes its impediment in ATP hydrolysis, with the subsequent release of both of the proteins from nsDNA (plasmid unpairing).     

THEORY AND MEASUREMENT OF VISUAL MECHANISMS : III. ΔI AS A FUNCTION OF AREA,INTENSITY, AND WAVE-LENGTH,FOR MONOCULAR AND BINOCULAR STIMULATION

Measurements of ΔI as a function of retinal area illuminated have been obtained at various levels of standard intensity I ₁, using "white" light and light of three modal wave-lengths (λ465, 525, 680), for monocular stimulation and for simultaneous excitation of the two eyes ("binocular"), using several methods of varying (rectangular) area and retinal location, with control of exposure time. For data homogeneous with respect to method of presentation, log ΔI_m = -Z log A + C, where ΔI = Ĩ ₂ – I ₁, A is area illuminated, and C is a terminal constant (= log ΔI_m for A = 1 unit) depending on the units in which ΔI and A are expressed, and upon I ₁. The equation is readily deduced on dimensional grounds, without reference to specific theories of the nature of ΔI or of retinal area in terms of its excitable units. Z is independent of the units of I and A. Experimentally it is found to be the same for monocular and binocular excitations, as is to be expected. Also as is expected it is not independent of λ, and it is markedly influenced by the scheme according to which A is varied; it depends directly upon the rate at which potentially excitable elements are added when A is made to increase. For simultaneous excitation of the two eyes (when of very nearly equivalent excitability), ΔĪ_B is less than for stimulation of either eye alone, at all levels of I ₁, A, λ. The mean ratio (ΔĪ_L + ΔĪ_R)/2 to ΔI^B was 1.38. For white light, doubling A on one retina reduces ΔI_m in the ratio 1.21, or a little less than for binocular presentation under the same conditions. These facts are consistent with the view that the properties of ΔI are quantitatively determined by events central to the retina. The measure σ_1ΔI of organic variation in discrimination of intensities and ΔI_m are found to be in simple proportion, independent of I ₁, A, λ (and exposure time). Variability (σ_1ΔI) is not a function of the mode of presentation, save that it may be slightly higher when both retinas are excited, and its magnitude (for a given level of ΔI_m) is independent of the law according to which the adjustable intensity I ₂ is instrumentally controlled.     

Quantification of Transmembrane Currents during Action Potential Propagation in the Heart

The measurement, quantitative analysis, theory, and mathematical modeling of transmembrane potential and currents have been an integral part of the field of electrophysiology since its inception. Biophysical modeling of action potential propagation begins with detailed ionic current models for a patch of membrane within a distributed cable model. Voltage-clamp techniques have revolutionized clinical electrophysiology via the characterization of the transmembrane current gating variables; however, this kinetic information alone is insufficient to accurately represent propagation. Other factors, including channel density, membrane area, surface/volume ratio, axial conductivities, etc., are also crucial determinants of transmembrane currents in multicellular tissue but are extremely difficult to measure. Here, we provide, to our knowledge, a novel analytical approach to compute transmembrane currents directly from experimental data, which involves high-temporal (200 kHz) recordings of intra- and extracellular potential with glass microelectrodes from the epicardial surface of isolated rabbit hearts during propagation. We show for the first time, to our knowledge, that during stable planar propagation the biphasic total transmembrane current (I_m) dipole density during depolarization was ∼0.25 ms in duration and asymmetric in amplitude (peak outward current was ∼95 μA/cm² and peak inward current was ∼140 μA/cm²), and the peak inward ionic current (I_ion) during depolarization was ∼260 μA/cm² with duration of ∼1.0 ms. Simulations of stable propagation using the ionic current versus transmembrane potential relationship fit from the experimental data reproduced these values better than traditional ionic models. During ventricular fibrillation, peak I_m was decreased by 50% and peak I_ion was decreased by 70%. Our results provide, to our knowledge, novel quantitative information that complements voltage- and patch-clamp data.     

Stomatal Behavior and Water Status of Maize, Sorghum, and Tobacco under Field Conditions: II. At Low Soil Water Potential

Diurnal changes in the vertical profiles of irradiance incident upon the adaxial leaf surface (I), leaf resistance (r₁), leaf water potential (ψ), osmotic potential (π), and turgor potential (P) were followed concurrently in crops of maize (Zea mays L. cv. Pa602A), sorghum (Sorghum bicolor [L.] Moench cv. RS 610), and tobacco (Nicotiana tabacum L. cv. Havanna Seed 211) on several days in 1968 to 1970 when soil water potentials were low. The r₁, measured with a ventilated diffusion porometer, of the leaves in the upper canopy decreased temporarily after sunrise [~0530 hours Eastern Standard Time] as I increased, but then r₁ increased again between 0700 and 0830 hr Eastern Standard Time as the ψ, measured with a pressure chamber, decreased rapidly from the values of −7, −4 and −6 bars at sunrise to minimal values of −18, −22 and −15 bars near midday in the maize, sorghum, and tobacco, respectively. The π, measured with a vapor pressure osmometer, also decreased after sunrise, but not to the same degree as the decrease in ψ, so that a P of zero was reached in some leaves between 0730 and 0800 hours. The lower (more negative) π of leaves in the upper canopy than those in the lower canopy gave the upper leaves a higher P at a given ψ than the lower leaves in all three species; leaves at intermediate heights had an intermediate P. This difference between leaves at the three heights in the canopy was maintained at all values of ψ. The r₁ remained unchanged over a wide range of P and then increased markedly at a P of 2 bars in maize, −1 bar in sorghum, and near zero P in tobacco: r₁ also remained constant until ψ decreased to −17, −20, and −13 bars in leaves at intermediate heights in maize, sorghum, and tobacco, respectively. In all three species r₁ of leaves in the upper canopy increased at more negative values of ψ than those at the base of the canopy, and in tobacco, leaves in the upper canopy wilted at more negative values of ψ than those in the lower canopy.     

Voltage Gating of Shaker K+ Channels : The Effect of Temperature on Ionic and Gating Currents

Ionic (I_i) and gating currents (I_g) from noninactivating Shaker H4 K⁺ channels were recorded with the cut-open oocyte voltage clamp and macropatch techniques. Steady state and kinetic properties were studied in the temperature range 2–22°C. The time course of I_i elicited by large depolarizations consists of an initial delay followed by an exponential rise with two kinetic components. The main I_i component is highly temperature dependent (Q₁₀ > 4) and mildly voltage dependent, having a valence times the fraction of electric field (z) of 0.2–0.3 e_o. The I_g On response obtained between −60 and 20 mV consists of a rising phase followed by a decay with fast and slow kinetic components. The main I_g component of decay is highly temperature dependent (Q₁₀ > 4) and has a z between 1.6 and 2.8 e_o in the voltage range from −60 to −10 mV, and ∼0.45 e_o at more depolarized potentials. After a pulse to 0 mV, a variable recovery period at −50 mV reactivates the gating charge with a high temperature dependence (Q₁₀ > 4). In contrast, the reactivation occurring between −90 and −50 mV has a Q₁₀ = 1.2. Fluctuation analysis of ionic currents reveals that the open probability decreases 20% between 18 and 8°C and the unitary conductance has a low temperature dependence with a Q₁₀ of 1.44. Plots of conductance and gating charge displacement are displaced to the left along the voltage axis when the temperature is decreased. The temperature data suggests that activation consists of a series of early steps with low enthalpic and negative entropic changes, followed by at least one step with high enthalpic and positive entropic changes, leading to final transition to the open state, which has a negative entropic change.