In situ field measurement of leaf water potential using thermocouple psychrometers

Thermocouple psychrometers are the only instruments which can measure the in situ water potential of intact leaves, and which can possibly be used to monitor leaf water potential. Unfortunately, their usefulness is limited by a number of difficulties, among them fluctuating temperatures and temperature gradients within the psychrometer, sealing of the psychrometer chamber to the leaf, shading of the leaf by the psychrometer, and resistance to water vapor diffusion by the cuticle when the stomates are closed. Using Citrus jambhiri, we have tested several psychrometer design and operational modifications and showed that in situ psychrometric measurements compared favorably with simultaneous Scholander pressure chamber measurements on neighboring leaves when the latter were corrected for the osmotic potential.     

Chloroplast response to low leaf water potentials: I. Role of turgor

The effect of decreases in turgor on chloroplast activity was studied by measuring the photochemical activity of intact sunflower (Helianthus annuus L. cv. Russian Mammoth) leaves having low water potentials. Leaf turgor, calculated from leaf water potential and osmotic potential, was found to be affected by the dilution of cell contents by water in the cell walls, when osmotic potentials were measured with a thermocouple psychrometer. After the correction of measurements of leaf osmotic potential, both the thermocouple psychrometer and a pressure chamber indicated that turgor became zero in sunflower leaves at leaf water potentials of −10 bars. Since most of the loss in photochemical activity occurred at water potentials below −10 bars, it was concluded that turgor had little effect on the photochemical activity of the leaves.     

Field measurement of leaf water potential with a temperature-corrected in situ thermocouple psychrometer

Abstract. The construction and evaluation of a temperature-corrected in situ thermocouple psychrometer for measurement of leaf water potential (Ψ) is described. The instrument utilized two chromel-constantan thermocouples which allowed for detection of both the psychrometric zero offset and the temperature differential between the sample and the Peltier measuring junction. The psychrometer was subjected to stable temperature gradients while in contact with reference solutions of sodium chloride, and the effects of thermal gradients were quantified. Regression analysis indicated that temperature differentials were responsible for errors in water potential determinations of approximately –7.73 MPa°C⁻¹. When installed on leaves of field-grown cotton ( Gossypium hirsutum L.), corn ( Zea mays L.) and soybean ( Glycine max L. Merr) the instrument detected temperature differentials up to 0.1°C (–6.0 μV) which were associated with relatively small shifts in psychrometric zero offsets (–0.05––0.75 μV). Results indicated that substantial errors in apparent Ψ were caused by non-isothermal conditions between the leaf and the psychrometer measuring junction. The relative magnitude of these errors could be quantified and the corrected results showed good agreement with conventional psychrometric determination of Ψ on excised samples during a diurnal cycle.     

Errors in measuring water potentials of small samples resulting from water adsorption by thermocouple psychrometer chambers

The adsorption of water by thermocouple psychrometer assemblies is known to cause errors in the determination of water potential. Experiments were conducted to evaluate the effect of sample size and psychrometer chamber volume on measured water potentials of leaf discs, leaf segments, and sodium chloride solutions. Reasonable agreement was found between soybean (Glycine max L. Merr.) leaf water potentials measured on 5-millimeter radius leaf discs and large leaf segments. Results indicated that while errors due to adsorption may be significant when using small volumes of tissue, if sufficient tissue is used the errors are negligible. Because of the relationship between water potential and volume in plant tissue, the errors due to adsorption were larger with turgid tissue. Large psychrometers which were sealed into the sample chamber with latex tubing appeared to adsorb more water than those sealed with flexible plastic tubing. Estimates are provided of the amounts of water adsorbed by two different psychrometer assemblies and the amount of tissue sufficient for accurate measurements of leaf water potential with these assemblies. It is also demonstrated that water adsorption problems may have generated low water potential values which in prior studies have been attributed to large cut surface area to volume ratios.     

Psychrometric Field Measurement of Water Potential Changes following Leaf Excision

In situ measurement of sudden leaf water potential changes has not been performed under field conditions. A laboratory investigation involving the measurement of leaf water potential prior to and 2 to 200 minutes after excision of citrus leaves (Citrus jambhiri) showed good linear correlation (r = 0.99) between in situ leaf psychrometer and Scholander pressure chamber measurements. Following this, a field investigation was conducted which involved psychrometric measurement prior to petiole excision and 1 minute after excision. Simultaneous pressure chamber measurements were performed on neighboring leaves prior to the time of excision and then on the psychrometer leaf about 2 minutes after excision. These data indicate that within the first 2 minutes after excision, psychrometer and pressure chamber measurements were linearly correlated (r = 0.97). Under high evaporative demand conditions, the rate of water potential decrease was between 250 and 700 kilopascals in the first minute after excision. These results show that the thermocouple psychrometer can be used as a dynamic and nondestructive field technique for monitoring leaf water potential.     

Effect of Cuticular Abrasion on Thermocouple Psychrometric In Situ Measurement of Leaf Water Potential

Cuticular resistance to water vapour diffusion between the substomatalcavity and the sensing psychrometer junction is a problem uniqueto leaf hygrometry. This resistance is not encountered in soilor solution hygrometry. The cuticular resistance may introduceerror in the measurement of leaf water potential. Using in situleaf hygrometers, we studied the effect of abrading the cuticleof Citrus jambhiri Lushington leaves, to reduce the diffusiveresistance. Field measurements of psychrometer water potentialwere compared with Scholander pressure chamber values for adjacentleaves. Different treatments were compared by sealing pairsof psychrometers on either side of the midrib. The time forwater vapour equilibration between the leaf and the psychrometerchamber was greater than 5 h for no abrasion. For abraded leaves,the true water potential value was obtained within an hour.After equilibration, psychrometer values compared favourablywith pressure chamber values for adjacent leaves (r > 0.97).Measured water potential for unabraded leaves did not correlatewell with corresponding pressure chamber measurements. Scanning electron micrographs indicated that the damage causedby abrading leaves for 60 s using carborundum powder (60 µmdiameter) was surface localized, with numerous scratchings ofthe leaf cuticle. The coarse abrasion treatment (aluminium oxide,75 µm diameter) resulted in fewer but larger cavitiesin the epidermis, which may explain the observed variabilityin the corresponding psychrometric measurements. Key words: Leaf water potential, Cuticular resistance, Leaf abrasion, Thermocouple psychrometer     

Comparison of water potentials measured by in situ psychrometry and pressure chamber in morphologically different species

Leaf water potentials measured by in situ psychrometry were compared with leaf water potentials measured by the pressure chamber technique at various values of water potential in Helianthus annuus, Helianthus nuttallii, Vigna unguiculata, Nerium oleander, Pistacia vera, and Corylus avellana. In V. unguiculata, the leaf water potentials measured by the in situ psychrometer oscillated at the same periodicity as, and proportional to, the leaf conductance. In all species, potentials measured by in situ psychrometers operating in the psychrometric mode were linearly correlated with potentials measured with the pressure chamber. However, the in situ psychrometers underestimated the leaf water potential in the two Helianthus species at low water potentials and overestimated the water potential in P. vera, N. oleander, and C. avellana. The underestimation in the two Helianthus species at low water potentials resulted from differences in water potential across the leaf. The overestimation in P. vera, N. oleander, and C. avellana was considered to arise from low epidermal conductances in these species even after abrasion of the cuticle. Pressure-volume studies with Lycopersicon esculentum showed that less water was expressed from distal than proximal leaflets when the whole leaf was slowly pressurized. The implication of this for water relations characteristics obtained by pressure-volume techniques is discussed. We conclude that in situ psychrometers are suitable for following dynamic changes in leaf water potential, but should be used with caution on leaves with low epidermal conductances.     

Comments on the psychrometric determination of leaf water potentialsin situ

Summary Leaf diffusion resistances may lower the values of leaf water potentials found byin situ measurements with silver-foil psychrometers. With the instrumentation used, the bias ranges from zero to increasing negative deviations as leaf diffusion resistances become larger than 3.5 cm^-1 sec.Water potentials determined with the dewpoint technique are unaffected by diffusion resistances.In situ measurements by this method may also be carried out with the silver-foil sensor.Hence, one and the same sensor may serve to trace both leaf water potentials and leaf diffusion resistance through dewpoint and psychrometric measurements. re]19760720     

Behavior of Corn and Sorghum under Water Stress and during Recovery

Corn (Zea mays L.) and sorghum (Sorghum vulgare, Pers.) plants were grown in a vermiculite-gravel mixture in controlled environment chambers until they were 40 days old. Water was withheld until they were severely wilted, and they were then rewatered. During drying and after rewatering stomatal resistance was measured with a diffusion porometer each morning, and water saturation deficit and water potential were measured on leaf samples. The average resistance of the lower epidermis of well watered plants was lower for corn than for sorghum. When water stress developed, the stomata began to close at a higher water potential in corn than in sorghum. The stomata of both species began to reopen normally soon after the wilted plants were rewatered, and on the 2nd day the leaf resistances were nearly as low as those of the controls. The average leaf water potential of well watered corn was −4.5 bars; that of sorghum, −6.4 bars. The lowest leaf water potential in stressed corn was −12.8 bars at a water saturation deficit of 45%. The lowest leaf water potential in stressed sorghum was −15.7 bars, but the water saturation deficit was only 29%. At these values the leaves of both species were tightly rolled or folded and some injury was apparent. Thus, although the average leaf resistance of corn is little lower than that of sorghum, corn loses much more of its water before the stomata are fully closed than does sorghum. The smaller reduction in water content of sorghum for a given reduction in leaf water potential is characteristic of drought-resistant species.     

Pressure probe and isopiestic psychrometer measure similar turgor

Turgor measured with a miniature pressure probe was compared to that measured with an isopiestic thermocouple psychrometer in mature regions of soybean (Glycine max [L.] Merr.) stems. The probe measured turgor directly in cells of intact stems whereas the psychrometer measured the water potential and osmotic potential of excised stem segments and turgor was calculated by difference. When care was taken to prevent dehydration when working with the pressure probe, and diffusive resistance and dilution errors with the psychrometer, both methods gave similar values of turgor whether the plants were dehydrating or rehydrating. This finding, together with the previously demonstrated similarity in turgor measured with the isopiestic psychrometer and a pressure chamber, indicates that the pressure probe provides accurate measurements of turgor despite the need to penetrate the cell. On the other hand, it suggests that as long as precautions are taken to obtain accurate values for the water potential and osmotic potential, turgor can be determined by isopiestic psychrometry in tissues not accessible to the pressure probe for physical reasons.     

Chloroplast osmotic adjustment and water stress effects on photosynthesis

Previous studies have suggested that chloroplast stromal volume reduction may mediate the inhibition of photosynthesis under water stress. In this study, the effects of spinach (Spinacia oleracea, var `Winter Bloomsdale') plant water deficits on chloroplast photosynthetic capacity, solute concentrations in chloroplasts, and chloroplast volume were studied. In situ (gas exchange) and in vitro measurements indicated that chloroplast photosynthetic capacity was maintained during initial leaf water potential (Ψ_w) and relative water content (RWC) decline. During the latter part of the stress period, photosynthesis dropped precipitously. Chloroplast stromal volume apparently remained constant during the initial period of decline in RWC, but as leaf Ψ_w reached −1.2 megapascals, stromal volume began to decline. The apparent maintenance of stromal volume over the initial RWC decline during a stress cycle suggested that chloroplasts are capable of osmotic adjustment in response to leaf water deficits. This hypothesis was confirmed by measuring chloroplast solute levels, which increased during stress. The results of these experiments suggest that stromal volume reduction in situ may be associated with loss of photosynthetic capacity and that one mechanism of photosynthetic acclimation to low Ψ_w may involve stromal volume maintenance.     

Detrimental effect of rust infection on the water relations of bean

Bean plants (Phaseolus vulgaris L.) infected with the rust Uromyces phaseoli became unusually susceptible to drought as sporulation occurred. Under the conditions used (1,300 ft-c, 27 C, and 55% relative humidity) such plants wilted at soil water potentials greater than −1 bar, whereas healthy plants did not wilt until the soil water potential fell below −3.4 bars. Determinations of leaf water and osmotic potentials showed that an alteration in leaf osmotic potential was not responsible for the wilting of diseased plants. When diffusive resistance was measured as a function of decreasing leaf water content, the resistance of healthy leaves increased to 50 sec cm⁻¹ by the time relative water content decreased to 70%, whereas the resistance of diseased leaves remained less than 8 sec cm⁻¹ down to 50% relative water content. Apparently, water vapor loss through cuticle damaged by the sporulation process, together with the reduction in root to shoot ratio which occurs in diseased plants, upset the water economy of the diseased plant under mild drought conditions.     

The temperature dependence of shoot hydraulic resistance: implications for stomatal behaviour and hydraulic limitation

Recent soil pressurization experiments have shown that stomatal closure in response to high leaf–air humidity gradients can be explained by direct feedback from leaf water potential. The more complex temperature‐by‐humidity interactive effects on stomatal conductance have not yet been explained fully. Measurements of the change in shoot conductance with temperature were made on Phaseolus vulgaris (common bean) to test whether temperature‐induced changes in the liquid‐phase transport capacity could explain these temperature‐ by‐humidity effects. In addition, shoot hydraulic resistances were partitioned within the stem and leaves to determine whether or not leaves exhibit a greater resistance. Changes in hydraulic conductance were calculated based on an Ohm’s law analogy. Whole‐plant gas exchange was used to determine steady‐ state transpiration rates. A combination of in situ psychrometer measurements, Scholander pressure chamber measurements and psychrometric measurements of leaf punches was used to determine water potential differences within the shoot. Hydraulic conductance for each portion of the pathway was estimated as the total flow divided by the water potential difference. Temperature‐induced changes in stomatal conductance were correlated linearly with temperature‐induced changes in hydraulic conductance. The magnitude of the temperature‐induced changes in whole‐plant hydraulic conductance was sufficient to account for the interactive effects of temperature and humidity on stomatal conductance.     

Water relations of the upper and lower surfaces of maize leaves

The objective of this study was to determine if the two surfaces of a leaf had different, or the same, water potentials. Maize (Zea mays L. cv. Cargill 805) was the test plant. During an 11 day period, the water potentials of the upper and lower surfaces of the maize leaves were measured daily with anin situ thermocouple psychrometer under growth-room conditions. Plants were grown in pots with a well-watered, commercial greenhouse soil. Stomatal resistance also was measured. The water potential of the upper surface was less negative (more wet) than that of the lower surface. The overall average of the water potential of the upper and lower surfaces was ?1.48 and ?2.07 MPa, respectively. The stomatal resistance of the upper surface was greater than that of the lower surface. The average stomatal resistance during the experiment was 780 and 600 s m^?1 for the upper and lower surfaces, respectively. Since the upper and lower surfaces were not at the same water potential, the results indicated that the parallel-resistance equation, used to combine resistances on the two surfaces of a leaf, was not strictly valid when applied to the maize leaves, as the law assumes equal potentials on the two surfaces.     

Comparison of the dye method with the thermocouple psychrometer for measuring leaf water potentials

The dye method for measuring water potential was examined and compared with the thermocouple psychrometer method in order to evaluate its usefulness for measuring leaf water potentials of forest trees and common laboratory plants. Psychrometer measurements are assumed to represent the true leaf water potentials. Because of the contamination of test solutions by cell sap and leaf surface residues, dye method values of most species varied about 1 to 5 bars from psychrometer values over the leaf water potential range of 0 to −30 bars. The dye method is useful for measuring changes and relative values in leaf potential. Because of species differences in the relationships of dye method values to true leaf water potentials, dye method values should be interpreted with caution when comparing different species or the same species growing in widely different environments. Despite its limitations the dye method has a usefulness to many workers because it is simple, requires no elaborate equipment, and can be used in both the laboratory and field.     

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.     

Continuous monitoring of plant water potential

Plant water potential was monitored continuously with a Wescor HR-33T dewpoint hygrometer in conjunction with a L51 chamber. This commercial instrument was modified by replacing the AC-DC mains power converter with one stabilized by zener diode controlled transistors. The thermocouple sensor and electrical lead needed to be thermally insulated to prevent spurious signals. For rapid response and faithful tracking a low resistance for water vapor movement between leaf and sensor had to be provided. This could be effected by removing the epidermis either by peeling or abrasion with fine carborundum cloth. A variety of rapid plant water potential responses to external stimuli could be followed in a range of crop plants (sunflower (Helianthus annuus L., var. Hysun 30); safflower (Carthamus tinctorious L., var. Gila); soybean (Glycine max L., var. Clark); wheat (Triticum aestivum L., var. Egret). These included light dark changes, leaf excision, applied pressure to or anaerobiosis of the root system. Water uptake by the plant (safflower, soybean) mirrored that for water potential changes including times when plant water status (soybean) was undergoing cyclical changes.     

The response of foliar gas exchange to exogenously applied ethylene

The responsiveness to ethylene of net photosynthesis and stomatal conductance to water vapor in intact plants was investigated in 13 herbaceous species representing seven plant families. Exposures were conducted in an open, whole-plant exposure system providing controlled levels of irradiance, air temperature, CO₂, relative humidity, and ethylene concentration. Net photosynthesis and stomatal conductance to water vapor in units of moles per square meter per second were measured on recently expanded leaves in control and ethylene-treated plants using a remotely operated single-leaf cuvette. The ethylene concentration was either 0 or 210 micromoles per cubic meter and was maintained for 4 hours. Species varied substantially in the response of their foliar gas exchange to ethylene. In 7 of the 13 species, net photosynthesis was inhibited statistically by 4 hours of ethylene exposure. As a function of the rate in control plants, the responses were most pronounced and statistically significant in Arachis hypogaea (−51.1%), Gossypium hirsutum (−31.7%), Glycine max (−24.8%), Cucurbita pepo (−20.4%), Phaseolus vulgaris (−18.4%), Setaria viridis (−17.5%), and Raphanus sativus (−4.4%). Whereas the responsiveness of net photosynthesis to ethylene among the 13 species showed no specific taxonomic associations, the responsiveness was positively correlated with the intrinsic rate of net photosynthesis. Stomatal conductance to water vapor after 4 hours of ethylene exposure declined statistically in 6 of the 13 species. As a function of control rates, the most marked and statistically significant responses of stomatal conductance were in Glycine max (−53.6%), Gossypium hirsutum (−51.2%), Arachis hypogaea (−42.7%), Phaseolus vulgaris (−38.6%), Raphanus sativus (−26.8%), and Solanum tuberosum (−23.4%). Although ethylene-induced changes in net photosynthesis and stomatal conductance were positively correlated, there were species-specific exceptions in which net photosynthesis declined after 4 hours of exposure without a concurrent change in stomatal conductance, stomatal conductance declined without a change in net photosynthesis, and the decline in stomatal conductance substantially exceeded the corresponding decline in net photosynthesis. Thus, the responsiveness to ethylene of net photosynthesis and stomatal conductance to water vapor were not consistently synchronous or equivalent among the 13 species. It is concluded that foliar gas exchange is responsive to exogenously applied ethylene in many plant species. The sensitivity of foliar gas exchange to ethylene may play a role in general plant response to environmental stress in which one of the physiological sites of action for endogenously produced stress ethylene in the leaf is the plant's photosynthetic capacity and/or stomatal conductance to water vapor.     

Electron Microscope Study of Cuticular Abrasion on Cotton Leaves in Relation to Water Potential Measurements

Wullschleger, S. D. and Oosterhuis, D. M. 1987. Electron microscopestudy of cuticular abrasion on cotton leaves in relation towater potential measurements.—J. exp. Bot 38: 660–667. Accurate determination of plant water potential using thermocouplepsychrometers requires vapour equilibrium between the tissuesample and the sensing psychrometer junction. Failure to achievethis equilibrium due to cuticular resistances to vapour movementmay introduce significant errors into psychrometrically measured. The effect of cuticular abrasion on W equilibration timesfor cotton (Gossypium hirsutum L.) was studied with three typesof thermocouple psychrometers, and the extent of surface andcuticular damage was determined with electron microscopy. Water vapour equilibration between the leaf and psychrometerchamber was achieved in approximately 4 h for unabraded controls,whereas abrasion of the leaf with carborundum powder consistentlyreduced equilibration times to below 2?5 h for all three typesof psychrometers. Microscopic observation of abraded leaf tissueindicated that substantial damage to surface structures occurredduring the cuticular abrasion process. Scanning electron micrographsrevealed localized cellular damage to anastomosing leaf veinsand physical disruption of both the stomatal complex and glandulartrichomes. Transverse sections viewed with a transmission electronmicroscope indicated substantial direct damage to the cuticlewith large sections of cell wall devoid of a cuticular layer.Although the exposed cell walls were intact, the lateral cellwalls were physically compressed and distorted during abrasion.In addition, the cytoplasmic and vacuolar membranes of the epidermalcells were also frequently ruptured. Evaluation of the damagefollowing abrasion indicated that the release of turgor by theaffected cells may contribute to increased sample variabilityand possibly to errors in measurements. Key words: Leaf water potential, cuticular resistance, thermocouple psychrometer     

The origin and theoretical correction of thermal gradients in thermocouple psychrometers

Temperature-compensated psychrometers are explained and a theoretical correction for temperature gradients, with its boundaries, is given. The correction was elaborated for in situ determination of water potential on soybean and tomato stems where it was indispensable. The origin of the thermal gradients in the psychrometric chamber is then discussed. Cooling of the conducting tissue by the circulation of xylem sap seemed to be the major cause for these gradients, while the heating of the psychrometer (thermally insulated) by the surroundings through radiative or conductive transfer was negligible.