Water Balance in Cucumis Plants, Measured by Nuclear Magnetic Resonance, II

Nuclear magnetic resonance (NMR) was used to investigate theeffects of changes in root temperature, of changes in the areaof root in contact with culture solution and of day/night rhythmon the water balance of a cucumber and a gherkin plant. Resultsare discussed in terms of water potential, flow rate and resistanceusing a previously presented model of water balance. As longas water uptake alone is varied, flow rate and water content(or potential) will change in the same direction. In contrast,from that model it is predicted that changes in transpirationwill affect flow rate and water content in opposite ways. Anexperimental verification of this prediction was given in theprevious paper. Results obtained by the NMR method are comparedto those determined using a dendrometer. The results demonstratethat the NMR method is a valuable tool to study plant waterbalance and that it can serve as a technique for discriminatingbetween changes in plant water balance that are due to changesin water uptake by roots and those due to changes in transpiration. Key words: Water balance model, Cucumis satious L., flow, water content, NMR, water balance measurement     

In situ plant water balance studies using a portable NMR spectrometer

A portable ¹H NMR spectrometer has been applied to whole plantsin situ, i.e. in climate rooms and in a greenhouse, to studyplant water relations under these conditions. The spectrometerconsists of a 30 kg permanent magnet system of 0.235 T, modifiedBruker Minispec electronics, and a standard XT pc for spectrometercontrol. Unattended, the system automatically measures the xylemsap stream and tissue water content in a well-defined sectionof the plant stem. Because of the small size of the magnet thesemeasurements can be made at different positions on the plantstem or on different plants in succession. Magnetic field driftdue to the varying climates that occur in greenhouses was correctedby field locking. Results are presented for a single plant ina climate room, demonstrating a method for the study of plantwater hydraulics. In addition, a single plant in a greenhousecrop was measured, and the NMR results were compared with thewater uptake and transpiration rates under (natural) variationof light intensity and relative humidity, demonstrating thereliability of the portable NMR under realistic greenhouse conditions.Finally, the application for the measurement of the root wateruptake efficiency is demonstrated for a number of grafted cucumberplants under constant climatic conditions in a phytotron. Key words: Portable NMR system, plant water balance sensor, greenhouse applications, flow, water content     

Frequency and Time-Domain Dielectric Measurements of Stem Water Content in the Arborescent Palm, Sabal palmetto

Two methods for monitoring stem water content in the arborescentpalm, Sabal palmetto by determining its dielectric constantwerecompared. The first approach used an oscillating circuit whosefrequency (40 to 70 kHz) was determined by a parallel-platecapacitor that sandwiched a portion of the stem. The secondtechnique was based on measurement of the velocity of an electromagneticpulse (frequency range of 500 kHz and 1 GHz) propagating withina wave-guide embedded in the stem (Time-Domain Reflectometry,TDR). There was basic agreement in the apparent dielectric constantas determined by the two techniques; both resulted in valuesof approximately 90 when the plant was fully hydrated, fallingto values near 50 when water was withheld for one week. The capacitance technique was non-invasive, but was influencedby temperature fluctuations, and we were unable to calibrateit accurately against stem volumetric water content. Insertionof TDR probes did not lead to tissue damage and determinationof an empirical relationship to volumetric water content allowedquantitative estimates of stem water content. Sensitivity ofTDR to small changes in stem water content was restricted bythe fact that attenuation of the pulse within the stem necessitatedthe use of short (0·125 m) wave guides. Despite this,during periods of high transpiration (>10kg plant^–1d^–1) bi-hourly changes in stemmoisture content were detectable. Key words: Dielectric constant, Sabal palmetto, stem capacitance, time-domain reflectometry, water storage     

Responses of a CAM Plant to Drought and Rainfall: Capacitance and Osmotic Pressure Influences on Water Movement

Daily and seasonal patterns in water flow and water potentialwere investigated for the Crassulacean acid metabolism succulentAgave deserti during an extended summer drought and for a periodfollowing rainfall. Field measurements of transpiration andof osmotic pressure changes over selected 24 h periods wereused as input variables for a computer model of water flow thatwas based on an electrical circuit analog of the whole plant.Parameters such as root resistance and tissue capacitance werealso varied to reflect the effects of changing plant or soilwater status. The model predicted internal water flow and waterpotential during the drought cycle and was used to assess therole of tissue osmotic properties in water uptake from the soiland in internal water redistribution. For plants under wet soil conditions, 55% of the night-timetranspiration was derived from water storage, this storage beingrecharged during the day. As drought progressed, transpirationand the nocturnal increase in osmotic pressure declined, althoughthe osmotic pressure itself increased. The difference in osmoticpressure between the water storage tissue and the chlorenchymacaused a net flow of water into the chlorenchyma after 3 weeksof drought, thereby increasing chlorenchyma turgor pressure.Simulations also indicated that a large increase in root resistancemust occur to prevent substantial water loss from the plantto the dry soil. After rainfall, recharge of plant water storagewas complete within one week, although full recovery in theamplitude of daily osmotic pressure variations took longer. Key words: Agave deserti, transpiration, water potential, water storage     

Quantification of water transport in plants with NMR imaging

A new nuclear magnetic resonance imaging (NMRi) method is described to calculate the characteristics of water transport in plant stems. Here, dynamic NMRi is used as a non-invasive technique to record the distribution of displacements of protons for each pixel in the NMR image. Using the NMR-signal of the stationary water in a reference tube for calibration, the following characteristics can be calculated per pixel without advance knowledge of the flow-profile in that pixel: the amount of stationary water, the amount of flowing water, the cross-sectional area of flow, the average linear flow velocity of the flowing water, and the volume flow. The accuracy of the method is demonstrated with a stem segment of a chrysanthemum flower by comparing the volume flow, measured with NMR, with the actual volumetric uptake, measured with a balance. NMR measurements corresponded to the balance uptake measurements with a rms error of 0.11 mg s(-1) in a range of 0 to 1.8 mg s(-1). Local changes in flow characteristics of individual voxels of a sample (e.g. intact plant) can be studied as a function of time and of any conceivable changes the sample experiences on a time-scale, longer than the measurement time of a complete set of pixel-propagators (17 min).     

Influence of insulating dead leaves and low temperatures on water balance in an Andean giant rosette plant

Abstract. The influence of insulating dead leaves on water balance in Espeletia timotensis Cuatr., an Andean caulescent giant rosette plant, was studied under field and laboratory conditions. Removal of the dead leaf layer surrounding the stem changed the pattern of diurnal stem temperature variation and produced transient and permanent effects on water balance. The pattern of liquid water flow resistance increase at low stem temperatures suggested that much of the water flow in the stem was through living membranes, probably those of the pith cells. The pith was determined to be an important source of stored water for daily transpirational needs. The lethal effects of dead leaf removal were attributed to one or more of the following causes: (1) inhibition of pith recharge by subfreezing stem temperatures; (2) embolisms in stem xylem; (3) freezing injury to pith tissue. The results suggested that an insulating layer of marcescent leaves and the presence of an internal water reservoir closer to the rosette than the soil water are important adaptations for maintenance of a favourable water balance in tropical alpine habitats where freezing temperatures occur regularly but last only a few hours.     

Diurnal Changes in Stem Diameter Depend Upon Variations in Water Content: Direct Evidence in Peach Trees

Three to five-year-old peach trees (Prunus persica (L.) Batschcv. ‘Maycrest’) grafted on P. ‘Damas 1869’grown in a sand trench were removed in the spring and grownhydroponically for several months. The system comprised twobalances continuously recording the mass of the nutrient solutionand that of the tree, so as to estimate transpiration and wateruptake rates separately. Diurnal variation in plant water content(transpiration minus water uptake) was observed, with rapidlydecreasing values when the solar radiation increased, whilethe reverse occurred when radiation decreased. Changes in stemdiameter were continuously recorded using linear variable differentialtransducers. Data collected over several days of contrastingclimatic conditions revealed that rapid changes in the stemdiameter occurred throughout the day and were closely relatedto plant water content. A lag-time not exceeding 10 min wasfound between changes in stem diameter and plant water content.These results are discussed in relation to the use of micromorphometricmethods to control irrigation in fruit trees. Moreover, we givevalues for the water stored in the shoots which may contributeto the transpiration stream. Key words: Prunus persica, stem shrinkage, plant water storage, water uptake, transpiration     

Plant and soil resistance to water flow in faba bean (Vicia faba L. major Harz.)

An experiment was conducted to determine soil and plant resistance to water flow in faba bean under field conditions during the growing season. During each sampling period transpiration flux and leaf water potential measured hourly were used with daily measurements of root and soil water potential to calculate total resistance using Ohm's law analogy. Plant growth, root density and soil water content distributions with depth were measured. Leaf area and root length per plant reached their maximum value during flowering and pod setting (0.31 m² and 2200 m, respectively), then decreasing until the end of the growing period. Root distribution decreased with depth ranging, on average, between 34.2% (in the 0–0.25 m soil layer) and 18.1% (in the 0.75–1.0 m soil layer). Mean root diameter was 0.6 mm but most of the roots were less than 0.7 mm in diameter. Changes in plant and soil water potentials reflected plant growth characteristics and climatic patterns. The overall relationship between the difference in water potential between soil and leaf and transpiration was linear, with the slope equal to average plant resistance (0.0165 MPa/(cm³ m^-1 h^-1 10^-3). Different regression parameters were obtained for the various measurement days. The water potential difference was inversely related to transpiration at high leaf stomatal resistance and at high values of VPD. Total resistance decreased with transpiration flux in a linear relationship (r=−0.68). Different slope values were obtained for the different measurement days. Estimated soil resistance was much lower than the observed total resistance to water flow. The change from vegetative growth to pod filling was accompanied by an increase in plant resistance. The experimental results support previous findings that resistance to water flow through plants is not constant but is influenced by plant age, growth stage and environmental conditions. A more complex model than Ohm's law analogy may be necessary for describing the dynamic flow system under field conditions. This revised version was published online in June 2006 with corrections to the Cover Date.     

Water storage and osmotic pressure influences on the water relations of a dicotyledonous desert succulent

Abstract Water storage and nocturnal increases in osmotic pressure affect the water relations of the desert succulent Ferocactus acanthodes, which was studied using an electrical circuit analog based on the anatomy and morphology of a representative individual. Transpiration rates and osmotic pressures over a 24-h period were used as input variables. The model predicted water potential, turgor pressure and water flow for various tissues. Plant capacitances, storage resistances and nocturnal increases in osmotic pressure were varied to determine their role in the water relations of this dicotyledonous succulent. Water coming from storage tissues contributed about one-third of the water transpired at night: the majority of this water came from the nonphotosynthetic, water storage parenchyma of the stem. Time lags of 4 h were predicted between maximum transpiration and maximum water uptake from the soil. Varying the capacitance of the plant caused proportional changes in osmotically driven water movement but changes in storage resistance had only minor effects. Turgor pressure in the chlorenchyma depended on osmotic pressure, but was fairly insensitive to doubling or halving of the capacitance or storage resistance of the plant. Water uptake from the soil was only slightly affected by osmotic pressure changes in the chlorenchyma. For this stem succulent, the movement of water from the chlorenchyma to the xylem and the internal redistribution of water among stem tissues were dominated by nocturnal changes in chlorenchyma osmotic pressure, not by transpiration.     

Field Estimates of Root and Xylem Resistances in Pinus contorta using Root Excision

A root excision technique was used to estimate the proportionof total resistance to water flux residing in the soil, theroot, and the xylem of lodgepole pine (Pinus contorta Douglex. Loud.) trees in the field. Root excision at mid-day alwaysresulted in rapid recovery of leaf water potential when waterwas supplied to the cut stem, suggesting a high soil-root resistance.Transpiration was unaffected if leaf water potential beforecutting was not limiting leaf conductance. By mid-June wateruptake by the excised stem always exceeded calculated crowntranspiration indicating recharge of internal sapwood storage.Predawn leaf water potential before root excision was highlycorrelated with total soil-plant resistance (r² = 0·89)and calculated root water uptake (r² = 0·92).     

Effects of Tissue-type and Development on Dark Respiration in Two Herbaceous Perennials

Perennial plants go through a number of developmental stagesduring the growing season. Changes in metabolism during thesephases have been documented in laboratory-grown plants but neverin native plants growing in natural habitats. The purpose ofthis study was to describe the seasonal pattern of dark respirationin the above-ground tissues of two herbaceous perennials, Bistortabistortoides(Pursh) Small and Campanula rotundifolia L., growingin the Rocky Mountains (USA). The effect of biomass accumulationon respiration rate and differences in respiration rate amongtissues were measured. Respiration rate differed significantlyamong the above-ground tissues. Reproductive structures hadthe highest respiration rates, followed by leaves, then stems.Respiration rate decreased by 10–90% over the growingseason in these tissues but was generally not correlated witha decrease in biomass accumulation. The seasonal pattern ofrespiration rate varied significantly among tissues. Total tissuerespiratory flux was calculated at 15 °C for each tissue.In both species, total above-ground respiratory flux was eitherrelatively constant during the growing season with a markeddecrease at seed dispersal or a maximum rate was reached atmid-season. In B. bistortoides, leaves had the highest totalrespiratory fluxes, and the respiratory fluxes of the stem andreproductive structures were similar to one another. In C. rotundifolia,leaf and stem respiratory fluxes were similar, while the respiratoryflux of the reproductive structures was considerably lower thanthat of leaves and stems. This study emphasizes the importanceof developmental processes and tissue-type on respiration rateand highlights the importance of including all plant tissuesin predictive models of plant carbon balance.Copyright 2001Annals of Botany Company Respiration, development, growth, maintenance, tissue, Bistorta bistortoides, Campanula rotundifolia, harebell     

Seasonal Water Acquisition and Redistribution in the Australian Woody Phreatophyte, Banksia prionotes

Sap flows in the xylem of plant roots in response to gradientsin water potential, either between soil and atmosphere (transpiration)or soil layers of different moisture content (termed hydraulicredistribution). The latter has the potential to influence waterbudgets and species interactions, but we lack information forall but a few plant communities. We combined heat pulse measurementsof sap flow with dye and isotope tracing techniques to gaugethe movement of xylem sap within, and exudation from, rootsof Banksia prionotes (Lindley). We demonstrated ‘ hydrauliclift’ during the dry season and provide some evidencethat extremely dry soils limit hydraulic lift. In addition wereport difficulties posed by spiralled xylem tissue in rootsfor the application of heat pulse techniques. Copyright 2000Annals of Botany Company Banksia prionotes, sap flow, hydraulic lift, heat ratio method, deuterium, stable isotopes, root architecture.     

Functional Imaging of Plants: A Nuclear Magnetic Resonance Study of a Cucumber Plant

Functional magnetic resonance imaging was used to study transients of biophysical parameters in a cucumber plant in response to environmental changes. Detailed flow imaging experiments showed the location of xylem and phloem in the stem and the response of the following flow characteristics to the imposed environmental changes: the total amount of water, the amount of stationary and flowing water, the linear velocity of the flowing water, and the volume flow. The total measured volume flow through the plant stem was in good agreement with the independently measured water uptake by the roots. A separate analysis of the flow characteristics for two vascular bundles revealed that changes in volume flow of the xylem sap were accounted for by a change in linear-flow velocities in the xylem vessels. Multiple-spin echo experiments revealed two water fractions for different tissues in the plant stem; the spin-spin relaxation time of the larger fraction of parenchyma tissue in the center of the stem and the vascular tissue was down by 17% in the period after cooling the roots of the plant. This could point to an increased water permeability of the tonoplast membrane of the observed cells in this period of quick recovery from severe water loss.     

In Situ Estimates of Variable Plant Resistance to Water Flux in Ilex opaca Ait

Xylem pressure potentials and stomatal diffusion resistances were measured in the field in Ilex opaca Ait. during days which differed in temperature and vapor pressure deficit. Water flux into leaves was calculated by combining the field data with laboratory determinations of the relation between tissue water deficit and water potential. Estimates of apparent plant resistance were then calculated from fluxes and differences between soil water potential and xylem tension. The resistance depended strongly on water flux, dropping by a factor of over 7 from low to high water flow rates. This extends the generality of variable plant resistances measured in controlled environment studies to I. opaca as it occurs naturally in the field. The relation of apparent plant resistance to water flux as estimated in this study can be useful in simulation models which calculate water uptake to leaves as a flux driven by a difference in soil and leaf water potentials across a resistance between the bulk soil and the leaf.     

Temperature and Water Relations for Sun and Shade Leaves of a Desert Broadleaf, Hyptis emoryi

The temperature and water relations of sun versus shade leavesof Hyptis emoryi Torr. were evaluated from field measurementsmade in late summer. Throughout most of the day sun leaves hadhigher temperatures and higher resistances to water vapour diffusion,but lower transpiration rates and lower stem water potentials,than did shade leaves. Leaf absorptivity to solar irradiationwas less for 1.5-cm-long sun leaves (0.44) than for 4.0-cm shadeleaves (0.56). For both leaf types the stomatal resistance increasedas the water vapour concentration drop from the leaf to theair increased. Energy balance equations were used together with the measuredtemperature dependence of photosynthesis to predict the effectof variations in leaf absorptivity, length, and resistance onnet photosynthesis. The influence of leaf dimorphism on wholeplants was determined by calculating daily photosynthesis andtranspiration for plants with various percentages of sun andshade leaves. A hypothetical plant with all sun leaves in thesun had about twice the photosynthesis and half the transpirationratio as did plants with sun leaves in the shade or shade leavesin the sun or shade. Plants with both sun and shade leaves hadthe highest predicted photosynthesis per unit ground area. Thepossible adaptive significance of the seasonal variation insun and shade leaf percentages observed for individual H. emoryibushes is discussed in terms of water economy and photosynthesi     

The NMR Microscope: a Unique and Promising Tool for Plant Science

An outline is given of nuclear magnetic resonance (NMR) microscopyand its application to plant science. An NMR microscope non-destructivelydetects free water in tissues and creates anatomical imagesof the tissues. Since the quantity and mobility of cell-associatedwater is closely related to the condition of the cells,¹H-NMRimages represent physiological maps of the tissue. In addition,the technique locates soluble organic compounds accumulatedin the tissues, such as sugars in vacuoles or fatty acids storedas oil droplets in vesicles.²³Na-NMR imaging is suitable forstudying the physiology of salt-tolerant plants. Diffusion measurementsprovide information about the transport of substances and ionsaccompanied by water movement. The recently developed techniquesof three-dimensional imaging, flow-encoded imaging and spectroscopicimaging open up new opportunities for plant biologists. TheNMR microscope is thus a unique and promising tool for the studyof living plant systems in relation to morphology, the truefeatures of which are often lost during preparation for moreconventional tissue analysis. Copyright 2000 Annals of BotanyCompany Review, NMR microscope,¹H-NMR imaging, non-destructive analysis, anatomy, cell-associated water, relaxation times, soluble compound mapping,²³Na-NMR imaging, physiological mapping, diffusion measurement, flow-encoded imaging     

Modelling of the Hydraulic Architecture of Root Systems: An Integrated Approach to Water Absorption--Distribution of Axial and Radial Conductances in Maize

The ‘Hydraulic Tree Model’ of the root system simulateswater uptake through root systems by coupling a root architecturemodel with laws for water flow into and along roots (Doussan,Pagès and Vercambre,Annals of Botany81: 213–223,1998). A detailed picture of water absorption in all roots comprisingthe root system is thus provided. Moreover, the influence ofdifferent distributions of radial and axial hydraulic conductancesin the root system on the patterns of water uptake can be analysed.Use of the model with Varney and Canny's data (1993) for flowalong maize roots demonstrated that a constant conductance inthe root system cannot reproduce the observed water flux profiles.Taking into account the existing data on hydraulic conductancesin maize roots, we fitted the distribution of conductances inthe root system to the observed flux data. The result is that,during root tissue maturation, the radial conductivity decreasesby one order of magnitude while the axial conductance increasesby about three orders of magnitude. Both types of conductanceexhibit abrupt changes in their evolution. Due to the conductancedistribution in the root system, appreciable water potentialgradients may develop in the roots, in both the branch rootsand main axes. An important point is that the conductance distributionin the branch roots described by the model should be relatedto the age of the tissue (and not the distance from the branchroot tip) and is therefore closely related to the developmentprocess. Thus for branch roots, which represent about 90% ofthe calculated total water uptake in 43-d-old maize, water absorptionwill depend on the opening of the metaxylem in the axes, andon the time dependent variation of the conductances in the branchroots.Copyright 1998 Annals of Botany Company Water; absorption; root system; architecture; model; hydraulic conductance;Zea maysL.     

Improved Thermocouple Psychrometer for the Measurement of Plant and Soil Water Potential: I. THERMOCOUPLE PSYCHROMETRY AND AN IMPROVED INSTRUMENT DESIGN

This paper is the first of a series which describes: (a) thedesign and operation of a thermocouple psychrometer which providesimproved range of measurement, accuracy, and equilibration rateof water potential determinations of plant and soil samples,and which avoids or minimizes six sources of large error, oneor more of which occur with previous psychrometers and (b) studiesthat provide a better understanding of the thermocouple psychrometricmethod. The present paper describes the main difficulties encounteredwith the method, examines those due to absorption phenomena,and describes an improved thermocouple psychrometer which overcomesor reduces some of these difficulties. Improvements include: (i) chamber walls of stainless steel type316 and samples arranged in shielding geometries to reduce delaysin equilibration due to adsorption, (ii) sample holders of geometriesthat avoid or reduce errors of leaf resistance, adsorption andtissue damage when appropriate cooling periods and tissue segmentsof adequate size are used, (iii) shielding arrangement of respiringtissue samples to permit thermal equilibration in the regionsensed by the ‘active’ thermojunction, a new thermocoupleassembly for increased range of measurement of water potential,(v) sample holders that permit plant or soil samples to be accommodatedin the one chamber.     

Nuclear magnetic resonanceimaging of membrane permeability changes in plants during osmoticstress

The cell water balance of maize (Zea mays L., cv LG 11) andpearl millet (Pennisetum americanum L., cv MH 179) duringosmotic stress was studied non‐invasively using ¹H nuclearmagnetic resonance (NMR) microscopy. Single NMR parameter imagesof (i) the water content (ii) the transverse relaxation time (T₂)and (iii) the apparent diffusion coefficient (D_app)were used to follow the water status of the stem apical region duringosmotic stress. During stress there are hardly any changes in watercontent or T₂ of the stem region of maize. Incontrast, the apical tissue of pearl millet showed a ～ 30% decreaseof T₂ within 48 h of stress, whereasthe water content and D_app did not change. Thesechanges can be explained by an increase of the membrane permeabilityfor water. This conclusion is supported by results from scanningelectron microscopy, relaxation measurements of sugar solutionsand numerical simulations of the relaxation and (apparent) diffusionbehaviour of water in a plant cell.     

Effect of Interruption of Flow Path on Stomatal Conductance of Abies amabilis

Wounding of root or stem water conduction systems or coolingof roots in Abies amabilis produced rapid stomatal closure independentof evaporative demand or leaf water potential. The responsealso occurred in a branch if its xylem was only partially cut,but did not occur if the branch was completely severed. Removingpart of the root system or cooling the roots produced the sameeffect as partial severing of the stem. The speed and uniformityof stomatal closure indicated that the stimulus was physical,linked to water flux in the xylem, and not caused by releaseof a chemical stimulus at the point of xylem flow disruption.The results suggested that stomatal closure could be rapidlyinduced with a change in the flux of water through the soil-plant-atmospherecontinuum. Key words: Capacitance, Stomata, Xylem water flux, Xylem wounding