Design calibration and field use of a stomatal diffusion porometer

Modifications of the design and calibration procedure of a diffusion porometer permit determinations of stomatal resistance which agree well with results obtained by leaf energy balance. The energy balance and the diffusion porometer measurements indicate that the boundary layer resistances of leaves in the field are substantially less than those predicted from heat transport formulas based on wind flow and leaf size.     

Photosynthesis, Transpiration, Leaf Stomatal and Mesophyll Resistance Measurements by the Use of a Ventilated Diffusion Porometer

A ventilated diffusion porometer was modified and adapted for simultaneous measurements of leaf resistance and photosynthesis (using ¹⁴C). The system enables measurements to be made under field and laboratory conditions with different concentrations of CO₂ and vapor pressure gradients between the evaporating surfaces inside the leaf and the external atmosphere. The leaf is subjected to the porometer's atmosphere only for short periods (up to 30 seconds) and it is assumed that stomata are not affected. Establishing the linear regression of the effect of CO₂ concentration on net photosynthesis makes it possible to extrapolate for CO₂ compensation point, to calculate the overall resistance to CO₂ and the mesophyll resistance to CO₂.     

The effect of temperature differences between porometer head and leaf surface on stomatal conductance measurements

It is generally known that instantaneous values of leaf conductance as measured with a dynamic porometer need to be corrected for the temperature difference, ΔT, between the porometer cup and the sampled leaf. Leaf conductances, obtained with a Delta-T AP4 dynamic porometer, with and without correction for ΔT are compared for a bush species (Guiera senegalensis) and two forb species (Jacquemontia tamnifolia and Mitracarpus scaber). With temperature differences predominantly varying within the ±2.5 °C recommended by the manufacturer, it appears that the differences between uncorrected and corrected conductances are very large, up to 100% on average, especially for the two forbs. Furthermore, it is shown that, using the Mitracarpus data, a relatively small error of ±0.5 °C in ΔT can cause a difference of 25–50% in the final conductance value, in particular for the high conductance range. An error of ±0.5 °C may easily occur: the accuracy of ΔT as measured by the thermistors in the porometer is 0.2 °C and the temperature variation within a leaf can be much larger. This result will have implications for upscaling of leaf conductances to canopy values or may explain why upscaled values appear not to correspond with down-scaled values, obtained from eddy correlation measurements and an inverted canopy transpiration model.     

A condensation type porometer for field use

A double cup transpiration porometer is described which simultaneously measures the resistance to water vapor diffusion from the upper and lower epidermises of a leaf. In use the cups are dried, the porometer is clamped on the leaf, and the transit time required for water vapor to condense on a cold (0 C) reflective surface in each cup is recorded. Since the sensing element remains at constant temperature, only transit time and leaf temperature are required to calculate leaf resistance.     

A New Porometer Based upon the Electrical Current Produced by Guard Cells

Stomatal guard cells extrude protons when the stomata open.This gives rise to an electrical current which is proportionalto the degree of stomatal opening. An instrument has been developedto measure this leaf surface current which is, in effect, anew type of porometer. The performance of the new porometerhas been compared with that of a commercially available diffusionporometer and a close relationship between leaf surface currentand stomatal conductance was observed for all the species investigated.It is concluded that the instrument has several advantages overthe diffusion porometer, in particular, its small size and simplicityof operation, making it especially suitable for use in the field. Key words: Leaves, stomata, electrical currents, porometry     

A Krypton Diffusion Porometer for the Direct Field Measurement of Stomatal Resistance

A stomatal diffusion porometer is described which measures directlythe diffusion of radioactive krypton through amphistomatousleaves. The porometer is relatively small and portable and iseasily used under field conditions. It consists of a miniaturediffusion chamber above an acrylic plastic reservoir which contains1200 cm³ of air enriched with ⁸⁵Kr. Geiger tubes in the diffusionchamber and in the reservoir monitor the relative concentrationsof 85Kr. Krypton is allowed to diffuse from the larger reservoirthrough the leaf into the diffusion chamber and the time forits concentration in relation to that in the reservoir to changebetween two fixed values is recorded. When this time lapse wascalibrated against known resistances a linear relationship,independent of temperature was found. Sources of error are analysedand some experiments are described in which the porometer wasused to measure diurnal changes in stomatal resistance. Resistancesof potted sunflower plants (Helianthus annuus L.) grown in agreenhouse were measured with both the krypton diffusion porometerand a condensation-type porometer and the results were usedto calculate both cuticular and stomatal resistances. Demonstrationof field measurements with the porometer include data from eucalyptustrees {Eucalyptus camaldulensis Dehn) and from an unirrigatedcotton crop {Gossypium hirsutum var. SJ 2) growing under semi-aridconditions. Stomatal conductance of the cotton crop during theopening phase was linearly related to solar radiation.     

Responses of apple leaf stomata to environmental factors

Abstract. Stomatal conductances ( g _s) were measured on the leaves of 3–4 year old Golden Delicious trees and of seedlings of two other cultivars. Measurements were made on container grown trees in the field with a diffusion porometer in 1975 and 1976, and in controlled conditions in a leaf chamber in the laboratory in 1976. Stomatal densities in the Golden Delicious leaves were assessed from scanning electron micrographs. Stomatal density on extension shoot leaves was higher than on other leaf types after June.
The response to irradiance shown by both the porometer and the leaf chamber results could be described by a rectangular hyperbola: where g _max is maximum conductance and β indicates the sensitivity of g_s to photon influx density ( Q _p). The values of β were in the range 60–90 μmol m⁻² s⁻¹.
There was no evidence that apple stomata are sensitive to temperature per se, but g _s was reduced by increasing leaf to air vapour pressure deficits ( D ). There was a linear relationship between g _s and D which was not attributable to feed-back to leaf water potential (ψ_L) as the latter did not affect g _s until a threshold of about −2.0 to −2.5 MPa was reached. Conductance generally declined with increasing ambient CO₂ concentration.     

A Null Balance Carbon Dioxide and Water Vapour Porometer

The Binos IRGA is suitable for portable use in the field aftermodification and can be added to the null-balance water vapourdiffusion porometer in a closed loop with the porometer chamberand used as a null-balance indicator for measurements of CO²flux, without the need for substantial structural or electricalmodification. The null-balance concept works satisfactorily in the measurementof both CO² influx and efflux and enables the CO² flux to bemeasured at either the ambient CO² concentration or at otherknown CO² concentrations. The combined CO² and water vapour porometer enables measurementsof the rate of photosynthesis, and estimates of stomatal andmesophyll conductance to be made extensively in the field withoutthe need for an elaborate mobile laboratory.     

Das Seifenblasenporometer (Zur Messung der Stomaweite an Amphistomatischen Blättern)

Zusammenfassung Luft wird aus einer Porometerkammer durch ein amphistomatisches Blatt hindurchgedrückt, auf der anderen Seite des Blattes von einer zweiten Porometerkammer aufgefangen und einem Seifenblasenströmungsmesser zugeleitet. Es wird die Zeit gestoppt, die eine vom Gasstrom vorangetriebene Flüssigkeitslamelle braucht, um ein bestimmtes Rohrvolumen zu durchlaufen. Es handelt sich somit um eine absolute Meßmethode. Neue Untersuchungen Levys bestätigen die hohe Genauigkeit dieses Verfahrens zur Messung des Volumflusses von Gasen.Die Messung kann innerhalb einiger Sekunden erfolgen; sie erfordert keinen hohen applizierten Druck. So werden Fehler durch CO₂-Verarmung im Porometer, Stomareaktionen und elastische Dehnung des Blattes so klein wie möglich gehalten. Falschluft ist ausgeschlossen.Die Methode eignet sich auch fürs Freiland, mit Luft aus einer beschwerten Luftmatratze und einer rasch anzusetzenden Porometerklammer. Als Beispiel wird eine Durchflußtopographie eines Blattes gegeben

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The soap-bubble porometer (measuring stomatal width in amphistomatic leaves)

Summary Two identical porometer cups are attached in opposition to both sides of an amphistomatic leaf. Air pressure is supplied to one of them. The resulting flow of air through the leaf is fed into a soap-bubble flowmeter. The time needed by a lamella of soap solution to move through a known volume of the meter is taken. Recent investigations made by Levy confirm the high accuracy obtainable by this absolute method of measurement of volume flow.Measurements are possible within some seconds, no high pressure need be applied. Thus, errors are minimized, caused by CO₂ depletion within the porometer cups, by reactions of the stomata, and by elastic stretch of the leaf. Interference of false air is excluded.The method can be used in the field with air supplied by a weighted air-bed, and using a quick-fastening clip-type porometer. The topography of stomatal conductance of a leaf, obtained with the method described, is presented as an example.

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Mit 5 Textabbildungen     

Use of thermography for quantitative studies of spatial and temporal variation of stomatal conductance over leaf surfaces

This paper describes a new approach to the calibration of thermal infrared measurements of leaf temperature for the estimation of stomatal conductance and illustrates its application to thermal imaging of plant leaves. The approach is based on a simple reformulation of the leaf energy balance equation that makes use of temperature measurements on reference surfaces of known conductance to water vapour. The use of reference surfaces is an alternative to the accurate measurement of all components of the leaf energy balance and is of potentially wide application in studies of stomatal behaviour. The resolution of the technique when applied to thermal images is evaluated and some results of using the approach in the laboratory for the study of stomatal behaviour in leaves of Phaseolus vulgaris L. are presented. Conductances calculated from infrared measurements were well correlated with estimates obtained using a diffusion porometer.     

A portable steady-state porometer for measuring the carbon dioxide and water vapour exchanges of leaves under natural conditions

A portable porometer is described for measuring the steady-state CO₂ and H₂O exchange rates of leaves under natural conditions. The porometer has an open gas exchange system which monitors the differences in concentrations of CO₂ and H₂O entering and leaving a cuvette which is clamped on or around leaves. The cuvette is designed to maintain ambient air temperature and humidity around the leaf. This instrument may also be used to determine CO₂ response curves in the field. Examples of diurnal courses are presented for attached leaves of different species having high and low rates of CO₂ exchange.     

An Ideal Viscous Flow Porometer

A new design of viscous flow porometer is described which isbased on the pronciple of air suction into a syringe which thenmeasures the volume of air atmosphere pressure. This porometerallows accurate readings to be made on the 0-100 scale on thesyringe barrel when the piston has returned following its release.Each reading takes only seconds to make so that data can becollected for statistical analysis before stomata change inaperture and with minimal interference with the leaf envirnionment.Calibiration procedures are described: examples ofthe new porometersand also silicone rubber imprints support the validity of measurementsmade by the new porometer.     

Studies in Stomatal Behaviour: V. THE ROLE OF CARBON DIOXIDE IN THE LIGHT RESPONSE OF STOMATA

It was found that stomata on illuminated leaves, both of Pelargoniumand wheat, opened much wider where the leaf surface was enclosedin a small volume of air, as in a normal porometer cup, thanelsewhere. This was shown for both species by the infiltrationmethod, and for Pelargonium by Lloyd's method and direct microscopicalobservation also. The effect was shown not to be due to pressure of the porometercup or glass plate on the leaf, or to temperature differences,nor directly to the lack of movement or high humidity of theenclosed air. A considerable body of data was collected which appeared tosupport the hypothesis that the wide opening was due to accumulationof some volatile substance produced by the leaf, but all theresults were also consistent with the view that it was causedby reduction in the carbon dioxide content of the enclosed airbelow the normal 0·03 per cent. owing to photosynthesis.Further crucial experiments with both the porometer and infiltrationmethods left virtually no doubt that the latter hypothesis wascorrect. This extreme sensitivity of stomata to carbon dioxide concentrationwithin the range 0·03 per cent. to zero is discussedin relation to their operation in nature, and a possible biologicaladvantage is suggested. The bearing of the effect upon porometer investigations is alsodiscussed and it is concluded that for all quantitative or semi-quantitativeexperimentation it is essential to use a cup detached betweenreadings, or at least swept with air such as surrounds the restof the leaf, and to have the upper leaf surface above the cuparea freely exposed or similarly swept. For qualitative investigationof the light response of stomata the traditional form of cupmay be used. The importance is stressed of allowing porometer readings toreach equilibrium under one set of conditions before changingto another, when investigating the ‘closing’ or‘opening’ effects of external factors. Several subsidiary effects, observed in the course of the investigation,are discussed; in particular an effect of humidity upon therate of response to other factors.     

A simple porometer for precise recording of leaf resistance

A porometer, which can be easily constructed with a common photometricunit and yet records the air flow through a leaf blade accuratelyand sensitively, was developed and used to record the flow changesthrough Brassica chinensis leaves under several light, air andwater conditions. (Received May 17, 1973; )     

A digital diffusion porometer circuit

An automatic digital diffusion porometer circuit is described. The circuit is highly stable with respect to temperature and supply voltage. The device is capable of high timing accuracy over very short measurement intervals, so that measurements may be made rapidly without the danger of stomatal changes occurring during the measurement period.     

The Measurement of Stomatal Responses to Stimuli in Leaves and Leaf Discs

A comparison has been made of stomatal responses in intact leaves,leaf discs supplied with water via their cut edges and leafdiscs floating on water. Xanthium pennsylvanicum leaf discswatered via their cut edges appeared to be more turgid thanintact leaves; this considerably slowed down the rate of stomatalopening but it slightly increased the final steady-state stomatalopening. When the water potential of such leaf discs was loweredby pre-treatment with mannitol solutions rates of stomatal openingincreased whereas maximum steady-state openings decreased. In tobacco leaf discs floating on water the stomata in contactwith water were wider open than those in contact with normalair and they did not respond to treatment with carbon dioxide-freeair. The rate of photosynthesis was severely reduced in tobaccoleaf discs floating with the lower epidermis on water, mostprobably owing to the slow rate of diffusion of carbon dioxidein water. By floating such discs on osmotica the degree of stomatalopening was increased, however, a response to treatment withcarbon dioxide-free air was still not measurable. It is postulatedthat, on account of the relative unavailability of carbon dioxidefrom the water, the carbon dioxide concentration in the substomatalcavities of the lower surface is abnormally low, irrespectiveof whether ordinary air or carbon dioxide-free air is availableto the upper surface. A comparison between porometer readings and measurements ofsiliconerubber impressions of stomatal pores taken from insidethe porometer cup confirmed that the silicone-rubber impressionmethod of assessing stomatal responses to stimuli has severelimitations, especially at small stomatal apertures.     

Developments in Mass Flow Porometry

A ventilated leaf attachment clamp is described with which lateralair flow can be measured separately from through flow. Stomatalconductances are expressed with dimensions: mm s^–1. Basedon parallel measurements with a diffusion porometer the relationbetween mass flow and diffusive flow conductances has been experimentallydetermined for three species. Records of stomatal movementsin air of different compositions are presented. Key words: Stomata, porometry, diffusive and mass flow conductance     

Dark opening of stomata in successional trees

Summary Dark opening of stomata was found in shade intolerant tree species in three different sets of experiments. In the field, leaves of eight successional species were darkened in light-proof bags and leaf resistance measured with a diffusion porometer. White ash, quaking aspen and American beech were sampled more intensively using darkened cuvettes and a dew point hygrometer. In the lab, white ash seedlings were kept in constant darkness and temperature, and their leaves also monitored with a cuvette and hygrometer. The stomata of all sampled shade intolerant species showed a dark opening response, often to leaf resistance levels typical of illuminated leaves. Shade tolerant species did not. Evidence suggests that this response is endogenous and could contribute to the morning opening of stomata of species under moderate water stress growing in well illuminated habitats.     

A comparative study of three leaf wetness sensors

The laboratory and field performance of two electrical resistance (ER) sensors of leaf surface wetness were compared with that of a beta-ray gauge (BRG). The BRG provided the most accurate measurements of wetness duration, which were in agreement with visual observations. A Campbell and a cotton cloth ER sensor consistently underestimated the duration of leaf surface wetness compared to the value obtained with the BRG in a dew chamber. However, the response of the Campbell sensor improved considerably with increase in the severity of dewfall. A superior performance of one of the two ER sensors could not be decisively established on the basis of the field experiments of 1989 and 1990 on soybean and tobacco crops, respectively. For studies where accurate measurements of surface wetness are critical, it is suggested that a beta-ray gauge should be used.     

Water stress in field-grown perennial ryegrass.

Two sets of irrigated and water-stressed swards of perennial ryegrass (Lolium perenne) growing in the field were compared. One set of swards was grown normally (field swards) and the other was established in boxes of low water-holding capacity (simulated swards). Rain covers were used to exclude rain from half of both sets of swards: the others were kept within 20 mm of field capacity. Daily minimum values of leaf water potential fell to —12 bars (—1.2 MPa) in the irrigated swards, —16 bars in the stressed field swards and —20 bars in the stressed simulated swards. Dry-matter production was reduced in the stressed field swards and leaf extension declined markedly in both the field and simulated swards. Canopy photosynthesis at saturating light intensity was reduced by about half in the stressed field swards and by more than 80% in the stressed simulated swards. In the former case a proportion of this reduction was due to the lower leaf area but the mean rate of leaf photosynthesis at saturating light intensity (P_max/LAI) was reduced by about 40% and this was attributable to increased stomatal resistance calculated from canopy transpiration rates or measured with a porometer. The more marked decline in photosynthesis in the stressed simulated swards was not only the consequence of more complete stomatal closure but also a decrease in the quantum efficiency of photosynthesis. When water stress occurs naturally in the field it appears that changes in the leaves take place as stress is developing and these changes result in a less severe effect of water stress on leaf photosynthesis. When stress is imposed with unnatural rapidity, as in the simulated swards, there is no opportunity for these changes in the leaf to occur and stress leads to a rapid and severe decline in leaf photosynthesis. The importance of these findings for the grass sward under water stress is discussed.