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.     

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     

Theoretical and experimental errors for in situ measurements of plant water potential

Errors in psychrometrically determined values of leaf water potential caused by tissue resistance to water vapor exchange and by lack of thermal equilibrium were evaluated using commercial in situ psychrometers (Wescor Inc., Logan, UT) on leaves of Tradescantia virginiana (L.). Theoretical errors in the dewpoint method of operation for these sensors were demonstrated. After correction for these errors, in situ measurements of leaf water potential indicated substantial errors caused by tissue resistance to water vapor exchange (4 to 6% reduction in apparent water potential per second of cooling time used) resulting from humidity depletions in the psychrometer chamber during the Peltier condensation process. These errors were avoided by use of a modified procedure for dewpoint measurement. Large changes in apparent water potential were caused by leaf and psychrometer exposure to moderate levels of irradiance. These changes were correlated with relatively small shifts in psychrometer zero offsets (−0.6 to −1.0 megapascals per microvolt), indicating substantial errors caused by nonisothermal conditions between the leaf and the psychrometer. Explicit correction for these errors is not possible with the current psychrometer design.     

Leaf water potentials measured with a pressure chamber

Leaf water potentials were estimated from the sum of the balancing pressure measured with a pressure chamber and the osmotic potential of the xylem sap in leafy shoots or leaves. When leaf water potentials in yew, rhododendron, and sunflower were compared with those measured with a thermocouple psychrometer known to indicate accurate values of leaf water potential, determinations were within ± 2 bars of the psychrometer measurements with sunflower and yew. In rhododendron. water potentials measured with the pressure chamber plus xylem sap were 2.5 bars less negative to 4 bars more negative than psychrometer measurements.

The discrepancies in the rhododendron measurements could be attributed, at least in part, to the filling of tissues other than xylem with xylem sap during measurements with the pressure chamber. It was concluded that, although stem characteristics may affect the measurements, pressure chamber determinations were sufficiently close to psychrometer measurements that the pressure chamber may be used for relative measurements of leaf water potentials, especially in sunflower and yew. For accurate determinations of leaf water potential, however, pressure chamber measurements must be calibrated with a 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.     

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.     

Short communication. Comparative measurements of xylem pressure in transpiring and non-transpiring leaves by means of the pressure chamber and the xylem pressure probe

Simultaneous measurements were made with the xylem pressure probe on exposed, transpiring leaves and with the Scholander pressure chamber on both transpiring and covered, non-transpiring leaves of sugarcane and maize plants. Xylem tensions inferred from pressure chamber balancing pressures on non-transpiring leaves were similar to those measured directly with the xylem pressure probe in transpiring leaves. However, tensions inferred with the pressure chamber on transpiring leaves that were placed in plastics bags just prior to excision were up to 0.6 MPa greater than those measured concurrently with the xylem pressure probe. These findings suggest that relatively large differences in water potential between the xylem and bulk leaf tissue can exist during periods of rapid transpiration, and they confirm that the balance pressure of an excised, previously transpiring leaf is only a measure of the bulk average equilibrium leaf water potential and not of the true xylem pressure that existed prior to excision.Key words: Cohesion-Tension theory, xylem pressure probe, pressure chamber, xylem tension.      

Effect of Excision on Leaf Water Potential

Improvements of thermocouple hygrometric techniques for in situleaf water potential measurement in the field now allows forcontinuous monitoring of water potential in response to an externalperturbation, such as leaf excision. Using Citrus jambhiri plants,measured leaf water potentials of completely excised leaf portionsimmediately increased when the petiole was excised or incisionswere made either transverse or parallel to the midrib. Incisionsparallel to the midrib were on the side nearest the hygrometeror opposite it if preceded by a petiole excision. Midrib incisionswere 100–150 mm long with the nearest cut edge being 20–50mm from the hygrometer cavity. All excisions were such thatleaf tissue was removed from the leaf with water potential onone of the leaf portions being measured continuously prior toand after excising. The peak increase in measured water potentialof the excised leaf portions ranged between 20 kPa and 80 kPabut averaged 50 kPa. In uncovered leaves, particularly underfield conditions with the associated high evaporative demand,measured leaf water potential declined rapidly after the initialincrease. The increase in measured water potential immediatelyfollowing various types of excision was confirmed for dark andlight conditions (laboratory and field respectively) using bothpsychrometric and dewpoint modes and occurred for secondaryexcisions, but to a lesser extent. Discovery of this phenomenonimplies that water potential measured on detached leaves maynot always represent accurately in situ leaf water potential. Key words: Leaf water potential, Thermocouple hygrometers, Leaf excision effects     

A rapid leaf-disc sampler for psychrometric water potential measurements

An instrument was designed which facilitates faster and more accurate sampling of leaf discs for psychrometric water potential measurements. The instrument consists of an aluminum housing, a spring-loaded plunger, and a modified brass-plated cork borer. The leaf-disc sampler was compared with the conventional method of sampling discs for measurement of leaf water potential with thermocouple psychrometers on a range of plant material including Gossypium hirsutum L., Zea mays L., and Begonia rex-cultorum L. The new sampler permitted a leaf disc to be excised and inserted into the psychrometer sample chamber in less than 7 seconds, which was more than twice as fast as the conventional method. This resulted in more accurate determinations of leaf water potential due to reduced evaporative water losses. The leaf-disc sampler also significantly reduced sample variability between individual measurements. This instrument can be used for many other laboratory and field measurements that necessitate leaf disc sampling.     

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.     

Correction of flow resistances of plants measured from covered and exposed leaves

The difference in water potential between an enclosed nontranspiring leaf and an adjacent exposed transpiring leaf, and the transpiration rate of a similarly exposed leaf, were used to calculate the change in hydraulic resistance of sorghum (Sorghum bicolor [L.] Moench) and sunflower (Helianthus annuus L.) leaves throughout the day and at various rates of transpiration. Since cotton (Gossypium hirsutum L.) leaves enclosed in aluminum foil alone had enclosed leaf water potentials about 0.06 megapascals lower than similar leaves enclosed in a polyethylene bag shielded with aluminum foil, the sorghum and sunflower leaves were enclosed in polyethylene bags shielded with aluminum foil. Enclosing the exposed leaf in a plastic sheath just prior to excision led to the water potential measured by the pressure chamber technique being 0.3 to 0.4 megapascals higher at rapid transpiration rates than in exposed leaves not sheathed just prior to excision. This error, previously shown to arise from rapid water loss after excision, led to an overestimation of the leaf hydraulic resistance in both species. Correction of the error reduced the resistance by 40 to 90% in irrigated sorghum and by about 40% in irrigated and unirrigated sunflower. After correction, the hydraulic resistances were still flow-dependent, but the dependency was markedly reduced in sorghum.     

The Relationship Between Leaf Thickness and Plant Water Potential

Leaf thickness was continuously measured in a wide range ofenvironments using a new type of displacement transducer whichis easy to set-up and automatically compensates for the effectsof temperature. Simultaneous measurements were made of waterpotential using either a psychrometer attached to the leaf petioleor a leaf pressure chamber. Thickness of leaves was a sensitiveindicator of plant water status but calibrations against anindependent method were necessary in every plant for accurateestimates of water potential. The relationship between leafthickness changes and water potential, measured in detachedleaves, was usually curvilinear and was strongly influencedby leaf age, stress history and, in young leaves, by the effectsof leaf growth. Leaf thickness growth was absent in mature cabbageleaves. Key words: Leaf thickness, plant water potential, psychrometer     

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.     

The Use of the Pressure Chamber Technique for Measurement of the Water Potential of Transpiring Plant Organs

The existence of water potential gradients in flowering shoots and leaves of roses (Rosa sp., cv. Baccara) and along flag leaves of wheat (Triticum aestivum L.) were studied by means of the Scholander pressure chamber. In roses grown in greenhouse, the water potential measured in transpiring shoots was higher than in leaves detached from these shoots, whereas the potential differences between leaf and shoot after equilibration in the dark were small or negligible. A progressive decrease in water potential was found upon repeated measurement on the same organ; this decline was steeper in leaves than in shoots. Extrapolating this decline to excision time resulted in water potential values which, in transpiring shoots, were 3 to 5 bars higher than in leaves. Detopping the flower bud did not alter this pattern, indicating that the highest water potential in the shoot was in the stem. In field-grown wheat, the water potential measured in a whole flag leaf was about 6 bars higher than that measured in the apical one-third of the leaf, and this difference disappeared after equilibrating the detached leaf for 1 h in the dark. These potential differences indicate the presence of resistances along the water path in the organ. The results obtained by the pressure chamber represent the highest water potential in the organ, rather than the average water potential.     

Comparison of pressure chamber and psychrometer estimates of leaf water potential in rice

Summary The need to compare pressure-chamber estimates of leaf water potential with a psychrometric method has been established for several crop species. We investigated this relationship for rice (Oryza sativa L.) as well as the need to protect leaves from water loss during sampling and measuring period in the pressure chamber. Two rice cultivars grown in containers on a clay-loam soil were stressed to varying degrees by withholding water. Fully expanded leaves were sampled for estimation of leaf water potential by the dew point hygrometer and pressure-chamber techniques. The same leaf was used in both methods allowing direct comparison. Additionally, two alternative methods of leaf handling for measurement by the pressure chamber technique were compared. Protection of leaf samples against water loss during excision, transport and handling was found to be more important at higher leaf water potentials (>−1.0 MPa). The two cultivars used appeared to differ in their response to protection of the leaf sample. These results serve to further caution pressure chamber users on extrapolating comparisons between the two measurement methods and between tissue handling techniques even within a crop species.     

Measured and Calculated Transpiration in Trifolium repens under Different Water Potentials

The relationship between transpiration measured gravimetrically,a generalized transpiration equation, and the ratio VPD/r_leafwas investigated in Trifolium repens plants subjected to varyingwater potentials. Dawn leaf water potential was measured witha pressure chamber, leaf diffusion resistance with a diffusionporometer, leaf temperatures with a thermistor, and relativehumidity with an aspirated psychrometer. During drought transpirationrates determined by both methods were quite similar particularlyat the lowest water potentials. After rewatering calculatedrates were somewhat higher than measured ones. It is concludedthat transpiration calculated by the indirect method is a usefuland reasonable estimate of transpiration for single plants undervarying water potentials.     

Measurement of Leaf Water Potential Using the J14 Press

The diurnal variation in leaf water potential of wheat underfield conditions is determined by means of the Scholander pressurechamber and the J14 press. Experimental work was carried outon several days during both the vegetative and reproductivegrowth stages. An exponential relationship (v = 4.09 e^0.05x),with coefficient of determination r² = 0.81, is found to existbetween the J14 press and Scholander pressure chamber measurements.Both the Scholander pressure chamber and J14 press appear capableof following hourly variations in leaf water potential provided8 and 16 leaves per sample are used for the two instrumentsrespectively.     

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.     

Synthesis and movement of abscisic Acid in water-stressed cotton leaves

Synthesis and movement of abscisic acid (ABA) into the apoplast of water-stressed cotton (Gossypium hirsutum L.) leaves were examined using pressure dehydration techniques. The exudates of leaves dehydrated in a pressure chamber contained ABA. The level of ABA in the exudates was insensitive to the leaf water potential when dehydration occurred over a 3-hour period. When leaves were rapidly dehydrated in the pressure chamber and held at a balance pressure coincident with the point of zero turgor, ABA accumulated in the leaf tissue and then in the apoplast, but only after 2 to 3 hours of zero turgor. Slow dehydration of leaves by equilibration over varying mannitol concentrations resulted in some accumulation of ABA prior to the point of zero turgor, but ABA accumulated in the tissue and apoplast most rapidly after the onset of zero turgor.     

Water potential in excised leaf tissue: comparison of a commercial dew point hygrometer and a thermocouple psychrometer on soybean, wheat, and barley

Leaf water potential (Ψ_leaf) determinations were made on excised leaf samples using a commercial dew point hygrometer (Wescor Inc., Logan, Utah) and a thermocouple psychrometer operated in the isopiestic mode. With soybean leaves (Glycine max L.), there was good agreement between instruments; equilibration times were 2 to 3 hours. With cereals (Triticum aestivum L. and Hordeum vulgare L.), agreement between instruments was poor for moderately wilted leaves when 7-mm-diameter punches were used in the hygrometer and 20-mm slices were used in the psychrometer, because the Ψ_leaf values from the dew point hygrometer were too high. Agreement was improved by replacing the 7-mm punch samples in the hygrometer by 13-mm slices, which had a lower cut edge to volume ratio. Equilibration times for cereals were normally 6 to 8 hours. Spuriously high Ψ_leaf values obtained with 7-mm leaf punches may be associated with the ion release and reabsorption that occur upon tissue excision; such errors evidently depend both on the species and on tissue water status.