Use of the Pressure Bomb for Hydraulic Conductance Studies

Stem sections of alfalfa (Medicago sativa L.) were subjectedto forced flows of both water and high molecular weight dextransolutions. In either case, stems responded in accordance withthe Ohm's law analogy for flow, i.e., flow was a linear functionof pressure, at pressures ranging from 7.0–500 kPa andvacuum from 25.69 kPa. Since conductance values were the samefor both pressure and vacuum powered flows, it is possible toextrapolate conductance values determined with a pressure bombto negative pressure gradients experienced by plants. Key words: Pressure bomb, Conductance, Medicago sativa     

Soil water deficits decrease the internal conductance to CO2 transfer but atmospheric water deficits do not

The internal conductance to CO₂ supply from substomatal cavitiesto sites of carboxylation poses a large limitation to photosynthesis.It is known that internal conductance is decreased by soil waterdeficits, but it is not known if it is affected by atmosphericwater deficits (i.e. leaf to air vapour pressure deficit, VPD).The aim of this paper was to examine the responses of internalconductance to atmospheric and soil water deficits in seedlingsof the evergreen perennial Eucalyptus regnans F. Muell and theherbaceous plants Solanum lycopersicum (formerly Lycopersiconesculentum) Mill. and Phaseolus vulgaris L. Internal conductancewas estimated with the variable J method from concurrent measurementsof gas exchange and fluorescence. In all three species steady-statestomatal conductance decreased by 30% as VPD increased from1 kPa to 2 kPa. In no species was internal conductance affectedby VPD despite large effects on stomatal conductance. In contrast,soil water deficits decreased stomatal conductance and internalconductance of all three species. Decreases in stomatal andinternal conductance under water deficit were proportional,but this proportionality differed among species, and thus therelationship between stomatal and internal conductance differedamong species. These findings indicate that soil water deficitsaffect internal conductance while atmospheric water deficitsdo not. The reasons for this distinction are unknown but areconsistent with soil and atmospheric water deficits having differingeffects on leaf physiology and/or root–shoot communication. Key words: Carbon dioxide, drought, internal conductance, mesophyll conductance, photosynthesis, stomatal conductance, transfer conductance, vapour pressure deficit, water deficit Received 11 October 2007; Revised 9 November 2007 Accepted 15 November 2007     

The Partitioning of Hydraulic Conductances within Mature Orange Trees

Sap flow (F) and leaf water potential (LWP) were followed diurnallyin mature Valencia and Shamouti orange trees in an orchard.The hydraulic conductance of these trees was computed from thediurnal relationship between the LWP and F. The driving forcefor water movement was estimated from a weighted average ofsunlit and shaded LWP, assuming that leaves in the shade transpireto some extent. LWP of covered, non-transpiring leaves was alsomeasured hourly. It was assumed to represent the xylem waterpotential within the axial conduit of the trunk. Relating coveredLWP to F on an hourly basis enables the computation of the hydraulicconductance of the root system, including axial conductances.The hydraulic conductance of the transpiring crown was computed.Its magnitude was comparable to the root system hydraulic conductance. Key words: Orange trees, hydraulic conductance, sap flow, leaf water potential     

Dynamic measurements of roots hydraulic conductance using a high-pressure flowmeter in the laboratory and field

A new high-pressure flowmeter(HPFM)is described which is capableof rapid water-flow measurements. The HPFM permits dynamic determinationof hydraulic conductance of roots, K_r, and can be used in tehlaboratory or field. The base of a root is connected to theHPFM and water is perfused into the root system opposite tothe normal direction of flow during trnaspiration. The perfusionpressure is changed at a constant rate of 3–7 kPa s^–1while measuring the flow into the root every 2–4 s. Theslope of the plot of flow versus applied pressure is K_r. This paper describes the HPFM, presnents the theory of dynamicflow measurements, discusses sources of error, presnets evidencethat dynamic measurements of K_r in Ficus maclellandi (and sixother tropical species from Panama) yield the correct result,and demonstrates the use of the method under field conditionsin Panama on Cecropia obtusifolia and Palicourea guianensis. Key words: High-pressure flowmeter, root and shoot hydraulic conductance, Ficus maclellandi, Cecropia obtusifolia, Palicourea guianensis     

Effects of cytochrome c on the mitochondrial apoptosis-induced channel MAC

Recent studies indicate that cytochrome c is released early in apoptosis without loss of integrity of the mitochondrial outer membrane in some cell types. The high-conductance mitochondrial apoptosis-induced channel (MAC) forms in the outer membrane early in apoptosis of FL5.12 cells. Physiological (micromolar) levels of cytochrome c alter MAC activity, and these effects are referred to as types 1 and 2. Type 1 effects are consistent with a partitioning of cytochrome c into the pore of MAC and include a modest decrease in conductance that is dose and voltage dependent, reversible, and has an increase in noise. Type 2 effects may correspond to "plugging" of the pore or destabilization of the open state. Type 2 effects are a dose-dependent, voltage-independent, and irreversible decrease in conductance. MAC is a heterogeneous channel with variable conductance. Cytochrome c affects MAC in a pore size-dependent manner, with maximal effects of cytochrome c on MAC with conductance of 1.9–5.4 nS. The effects of cytochrome c, RNase A, and high salt on MAC indicate that size, rather than charge, is crucial. The effects of dextran molecules of various sizes indicate that the pore diameter of MAC is slightly larger than that of 17-kDa dextran, which should be sufficient to allow the passage of 12-kDa cytochrome c. These findings are consistent with the notion that MAC is the pore through which cytochrome c is released from mitochondria during apoptosis. patch clamp; ion channels     

Growth Rate and Water Relations of Citrus Leaf Flushes

Growth rates of seasonal leaf flushes of ‘Valencia’orange [Citrus sinensis (L.) Osbeck] were measured and waterrelations characteristics of young (new) and over-wintered (old)citrus leaves were compared. New flush leaves had lower specificleaf weights and lower midday leaf water potentials than comparablyexposed old leaves. Spring and summer flush new leaves had higherosmotic potentials than old leaves. These differences becamenon-significant as the new leaves matured. During summer conditions,water-stressed new leaves reached zero turgor and stomatal conductancealso began to decrease in them at higher leaf water potentialsthan in old leaves. Old leaves were capable of maintaining openstomata at lower leaf water potentials. Opened flowers and newflush leaves lost more water, on a dry weight basis, than flowerbuds, fruit or mature leaves. The results illustrate differencesin leaf water potential and stomatal conductance which can beattributed to the maintenance of leaf turgor by decreases inleaf osmotic potentials as leaves mature. These changes in citrusleaf water relations are especially important since water stressresulting from high water loss rates of new tissues could reduceflowering and fruit set. Citrus sinensis (L.) Osbeck, orange, Citrus paradisi Macf., grapefruit, growth rate, leaf water relations, osmotic potential, water potential, stomatal conductance     

Modeling action potential generation and propagation in NRK fibroblasts

Normal rat kidney (NRK) fibroblasts change their excitability properties through the various stages of cell proliferation. The present mathematical model has been developed to explain excitability of quiescent (serum deprived) NRK cells. It includes as cell membrane components, on the basis of patch-clamp experiments, an inwardly rectifying potassium conductance (G_Kir), an L-type calcium conductance (G_CaL), a leak conductance (G_leak), an intracellular calcium-activated chloride conductance [G_Cl(Ca)], and a gap junctional conductance (G_gj), coupling neighboring cells in a hexagonal pattern. This membrane model has been extended with simple intracellular calcium dynamics resulting from calcium entry via G_CaL channels, intracellular buffering, and calcium extrusion. It reproduces excitability of single NRK cells and cell clusters and intercellular action potential (AP) propagation in NRK cell monolayers. Excitation can be evoked by electrical stimulation, external potassium-induced depolarization, or hormone-induced intracellular calcium release. Analysis shows the roles of the various ion channels in the ultralong (30 s) NRK cell AP and reveals the particular role of intracellular calcium dynamics in this AP. We support our earlier conclusion (De Roos A, Willems PH, van Zoelen EJ, and Theuvenet AP. Am J Physiol Cell Physiol 273: C1900–C1907, 1997) that AP generation and propagation may act as a rapid mechanism for the propagation of intracellular calcium waves, thus contributing to fast intercellular calcium signaling. The present model serves as a starting point to further analyze excitability changes during contact inhibition and cell transformation. Hodgkin-Huxley model; intracellular calcium dynamics; L-type calcium conductance; inward rectifier; calcium-activated chloride conductance; gap junctional coupling     

Vapour Pressure Deficit Response of Cuticular Conductance in Intact Leaves of Fagus sylvatica L.

The cuticular conductance (g_c) of the astomatous adaxial surfaceof Fagus sylvatica L. leaves was determined under varying vapourpressure deficits at a constant temperature of 20 °C. Cuticularconductance was determined from the weight loss of detachedleaves after both the stomatous abaxial surface and the cutend of the petiole had been sealed using low melting-point paraffinwax. Cuticular conductance was found to decrease as the watervapour pressure was increased in steps. No response was observedwhen vapour pressure deficit was decreased from an initiallyhigh value. It is concluded that these results are consistentwith the hypothesis that cuticular conductance is influencedby the water content of the cuticle and that the apparent hysteresisis likely to be a result of the long time-constant for the processof cuticle rehydration in intact leaves. Key words: Cuticle, cuticular conductance, Fagus sylvatica L., Fagaceae, vapour pressure deficit, VPD     

The Effect of Darkness on Active and Passive Transport in Chara corallina

In Chara corallina, the membrane potential may stay much morenegative than the equilibrium potential for potassium in thedark, indicating that the proton pump is operative. The highproton conductance which occurs at high external pH, as indicatedby a high membrane conductance and a membrane potential nearthe equilibrium potential for protons, is not seen in the darkat pH 11. This effect is likely to be related to inhibitionof photosynthesis since DCMU has the same effect. The effectis similar but not identical to the effect of a decreased internalpH. Key words: Light, dark, membrane potential, Chara     

Electromotive force of Nitella membrane during excitation

Excitation of the Nitella membrane is analysed by assuming themembrane to be an electromotive force in series with a resistance,both being variables of time and of membrane potential. Duringstep depolarization beyond a threshold, conductance and electromotiveforce increase transiently, finally reaching their respectivesteady state levels. The conductance increase peak is attainedearlier than the peak for electromotive force increase. Wheneverelectromotive force increases beyond the level of clamped membranepotential, the ionic current flows inward. This is consideredto be the origin of the apparent negative resistance characteristicof the excitable membrane. Anodal break response and spontaneousfiring of Nitella membrane are also caused by transient increasesin electromotive force and conductance irrespective of whetherthe membrane potential is being held at its resting level. Thetransient increase in electromotive force reflects changes,like a phase transition, occurring during excitation. (Received May 6, 1968; )     

The role of abscisic acid and water relations in drought responses of subterranean clover

The role of water relations and abscisic acid (ABA) in the responsesto drought were studied in a mediterranean forage crop, Trifoliumsubterraneum L. under field conditions. Soil and plant waterstatus, leaf gas exchange parameters, and xylem sap ABA contentwere determined at different times during a long-term soil dryingepisode in irrigated and droughted plants. The diurnal time-coursesof these parameters were also measured at the end of a droughtperiod. In response to soil drying stomatal conductance (g) was reducedearly to 50% that of irrigated plants before any substantialchange in water potential was detected. A close logarithmicregression between photosynthesis rate (A) and g was present.For the first weeks of drought the decline in A was less pronouncedthan in g, thus increasing water use efficiency. Stomatal conductanceduring diurnal time-courses showed no consistent relationshipswith respect to etther ABA or leaf water potential. Throughoutthe experimental period dependence of g on leaf water statuswas evident from the tight correlation (r²=0.88, P<0.01)achieved between stomatal conductance and midday water potential,but the correlation was also high when comparing g with respectto ABA content in xylem sap (r=0.83, P<0.001). However, thestomata from drought acclimated plants were apparently moresensitive to xylem ABA content. For similar xylem ABA concentrationsstomatal conductance was significantly higher in irrigated thanin waterstressed plants. Key words: Drought, stomatal conductance, water potential, abscisic acid     

Some Photosynthetic Characteristics of Tropical Timber Trees as Affected by the Light Regime During Growth

The stomatal conductance and rate of photosynthesis of tropicaltree seedlings are reported (Terminalia ivorensis, T. superba,Triplochiton scleroxylon and Khaya senegalensis). The seedlingswere grown in various light regimes, defined by the photon fluxdensity and red: far-red ratio. Mesophyll conductance and apparentquantum efficiency were calculated. The maximum rates of photosynthesison an area or weight basis depended on the photon flux densityduring growth, being increased by growth at high photon fluxdensity except in the shade tolerant seedlings of K. senegalensis.The stomatal conductance was also highest in leaves which hadbeen grown at high photon flux density. Using plants which hadbeen grown at low photon flux density and contrasting red: far-redratio, it was found that the highest rates of photosynthesiswere attained when the red: far-red ratio was low. The resultsare discussed in relation to what is known of the natural lightclimate, and evaluated in relation to the carbon balance ofseedlings. Terminalia ivorensis, Terminalia superba, Triplochiton scleroxylon, Khaya senegalensis, tropical tree, photosynthesis, stomata, shade, red: far-red     

Mechanism of activation of Xenopus CFTR by stimulation of PKC

PKA-mediated phosphorylation of the regulatory (R) domain plays a major role in the activation of the human cystic fibrosis transmembrane conductance regulator (hCFTR). In contrast, the effect of PKC-mediated phosphorylation is controversial, smaller than that of PKA, and dependent on the cell type. In the present study, we expressed Xenopus CFTR (XCFTR) and hCFTR in Xenopus oocytes and examined their responses (i.e., macroscopic membrane conductance) to maximal stimulation by PKC and PKA agonists. With XCFTR, the average response to PKC was approximately sixfold that of PKA stimulation. In contrast, with hCFTR, the response to PKC was 90% of the response to PKA stimulation. The reason for these differences was the small response of XCFTR to PKA stimulation. Using the substituted cysteine accessibility method, we found no evidence for insertion of functional CFTR channels in the plasma membrane in response to PKC stimulation. The increase in macroscopic conductance in response to PKC stimulation of XCFTR was due to an approximately fivefold increase in single-channel open probability, with a minor (30%) increase in single-channel conductance. The responses of XCFTR to PKC stimulation and of hCFTR to PKA stimulation were mediated by similar increases in P_o. In both instances, there were no changes in the number of channels in the membrane. We speculate that in animals other than humans, PKC stimulation may be the dominant mechanism for activation of CFTR. chloride channel; channel regulation; cystic fibrosis transmembrane conductance regulator gating; cystic fibrosis; phosphorylation; protein kinase A     

Assessment of the Response to Selection for Leaf Water Conductance in Lolium perenne

Measurements of leaf water conductance (LWC) were made in agrowth room on plants previously selected for either high orlow LWC as seedlings from two Lolium perenne L. populations.Differences in LWC were maintained between selection groupswhen measured as older plants, but there were considerable changesin LWC of individual selection groups on remeasurement. TheLWCs and the rankings of LWCs of individual genotypes were correlatedbetween experiments, thus demonstrating that selection of genotypeswas worthwhile. Thus the possibility of including LWC in selectionprogrammes was established. Investigations conducted to optimize sampling for LWC are described.It was concluded that little further reduction in the standarderror of LWC measurements was obtained by making more than eightmeasurements per genotype, and it was important to sample onat least four tillers. A new measure for the phenotypic stabilityof a selection group is proposed, named realized ‘selectivity’.The realized selectivity can be used to estimate the upper valuefor realized heritability and is about 0.5 for LWC in L. perenne.The values of phenotypic repeatability of LWC are compared withother similar measurements. Lolium perenne L, leaf water conductance, selection, components of variance, scale of measurement     

A leaf disc method for measuring cuticular conductance

This paper describes a new method for the measurement of cuticularconductance (g_o;) using a leaf disc sealed in a specially-designedenvelope. Conductances for astomatous (adaxial) and stomatous(abaxial) surfaces of beech {Fagus sylvatica L.) were determinedfrom measurements of water flux. Leaf discs were punched outfrom attached leaves and placed inside individual envelopesthat provided a water supply. Water flux from an exposed epidermalsurface of the leaf discs was measured gravimetrically. Allmeasurements were made under darkness. Conductance of the adaxialsurface was referred to as g_c, whereas conductance of the abaxialsurface was considered as a minimum leaf surface conductance()- The main advantage of this method is that it enables measurement of g_c and from leaf samples with intact cuticles and a highrelative water content [RWC) for periods of up to 12 d. Conductancesof leaf discs in envelopes were compared with those of wholeleaves and leaf discs without envelopes. Data demonstratinga strong positive relationship between conductance and RWC ispresented. Key words: Cuticular conductance, leaf disc, relative water content     

Loss of Stomatal Function in Ageing Hybrid Poplar Leaves

Under a variety of conditions old, non-senescent hybrid poplar(Populus sp.) leaves exhibited less stomatal control than young,mature leaves. Stomata of older leaves displayed oscillatorybehaviour more frequently, and oscillations were more random,than in younger leaves. Also, diffusive conductance of olderleaves changed less following sudden shifts from either darkto light, or vice versa, than in younger leaves, and temporalpatterns of diffusive conductance in older leaves appeared tobe relatively independent of microenvironmental conditions.Levels of conductance of older leaves were higher both in thedark and following excision than in younger leaves, while inthe light the situation was reversed. Total range of responseand stability of diffusive conductance were also lower in olderrather than in younger leaves. All of the observed age-relateddifferences suggest a loss of stomatal control with increasingleaf age. Leaf age, Populus sp., stomatal cycling, stomatal function, hybrid poplar     

Characterization of a wilty sunflower (Helianthus annuus L.) mutant: III. Phenotypic interaction in reciprocal grafts from wilty mutant and wild-type plants

The present study was conducted to evaluate phenotypic interactionin reciprocal grafts between wilty (w-1) sunflower mutant andnormal (W-1) plants. The w-1 genotype is a ‘leaky’ABA-deficient mutant, characterized by high stomatal conductance,in both light and dark conditions, and high transpiration rate. In well-watered conditions, mutant scions grafted on to normalrootstock (w-1/W-1) showed higher leaf relative water content,leaf water potential and ABA levels than those of control grafts(w-1/w-1). In addition, detached leaves of w-1/W-1 exhibitedlower water loss than w-1/w-1 grafts, while mutant rootstockdid not affect the transpiration rate of detached W-1 leaves.When drought stress was imposed to potted plants by withholdingwater, the mutant scions grafted on to normal roots showed apartial phenotypic reversion. A rapid stomatal closure and arise in ABA levels in response to a small decrease in leaf waterpotential was observed. By contrast, in w-1/w-1 grafts significantreductions in stomatal conductance and ABA accumulation weredetected only in conjunction with a severe water deficit. W-1scions on mutant stocks (W-1/w-1) maintained the normal phenotypeof control wild-type grafts (W1/W-1). Key words: ABA, grafting, Helianthus annuus, stomatal conductance, water relations, wilty mutant     

Water Use Characteristics of Dactylis glomerata L., Lolium perenne L. and L. multiflorum Lam. Plants

Cocksfoot (Dactylis glomerata L.). perennial ryegrass (Loliumperenne L.) and Italian ryegrass (L. multiflorum Lam.) plantswere grown on deep (75–95 cm) columns of soil in glasshousesand growth rooms with and without irrigation. The species inwhich growth declined least rapidly after water had been withheldwere those which transpired most slowly. During early establishmentin the glasshouse cocksfoot transpired least because of slowroot growth. In the growth room, when root systems were deeperand denser, perennial ryegrass transpired least because of lowleaf water conductance. Results are discussed in relation to(a) drought resistance in the three species; (b) breeding forincreased drought resistance through modifying root distributionand leaf water conductance; and (c) the use of isolated soilcolumns in water relations studies. Dactylis glomerata L., Lolium perenne L., Lolium multiflorum Lam., cocksfoot, perennial ryegrass, Italian ryegrass, transpiration, roots, leaf water conductance