Three most common methods used for measuring water status.

Leaf Water Potential

A fully expanded leaf exposed to direct sunlight is chosen for measurement. To measure mid-day leaf water potential, the targeted leaf must first be covered entirely with a small plastic bag that is wrapped tightly around the leaf and secured. Securely bagging the leaf before cutting it from the shoot avoids any further transpiration, which alters the resultant pressure reading. If this critical bagging step is omitted, the data will be inaccurate.

As quickly as possible after bagging, the petiole of the bagged leaf is cut from the shoot with a sharp razor as close to the shoot as possible. The petiole is then quickly placed through the chamber lid and secured tightly, with the cut edge of the petiole facing outside and the bagged leaf blade inside the chamber.

The chamber is sealed and then slowly pressurized with nitrogen gas. When the positive pressure exerted on the leaf in the chamber equals the negative pressure inside the leaf, liquid in the leaf blade will begin to be forced out of the cut edge of the leaf.

During pressurization, the operator carefully watches the exposed edge of the petiole for the appearance of a drop of water (sap). As soon as the drop appears, the user reads the corresponding pressure from the chamber gauge. This pressure value is the leaf water potential, read in negative (–) bars.

In comparison, mid-day stem water potential tests are done during the same time period as mid-day leaf water potential but handling of the leaf is changed. Stem water potential has been considered to be less variable than mid-day LWP, improving the ability to detect small pressure differences among treatments. But until this study was completed, a comprehensive study comparing the two had not been tested in grapes.

Stem Water Potential

The stem is thought to be less susceptible to fluctuations in environmental pressures than the leaf and, therefore, more representative of the actual level of stress in the entire vine. In the mid-day SWP test, a leaf on the shaded side of the canopy is chosen to minimize any possible heating effects.

The leaf is wrapped in a black plastic bag that is covered with aluminum foil to prevent overheating by the sun. The bag is left on the leaf 90 to 120 minutes. This effectively stops the natural transpiration from the leaf, allowing the leaf water potential to come into equilibrium with the stem water potential. After 90 to 120 minutes has elapsed, the leaf is excised and tested in the pressure chamber as stated above.

Pre-dawn Leaf Water Potential

Pre-dawn leaf water potential is determined using the same basic methodology as LWP, but the readings are taken beginning at 3:30 am and ending before sunrise, using fully expanded leaves. It has been assumed that, before sunrise, the vine is in equilibrium with the soil’s water potential, therefore making PDLWP a more sensitive indicator of soil water availability. But the obvious difficulty with the method is timing: readings must be done prior to sunrise, making its practicality questionable.

Comparison of methods

For any measure of plant water status to be a sensitive indicator of water stress, it must be responsive to differences in soil moisture status and/or the resulting growth differences due to water application. The measure should also be closely related to short- and medium-term plant stress responses and less dependent on changes in environmental conditions.

For winegrapes, it would seem that LWP, SWP, and PDLWP each meet these criteria. The best indicator of which method is the most effective and yet most practical might be as simple as the ease of operation if the data from all three plant-based measures of vine water stress can be proven to be highly correlated.

Additionally, the value of that plant-based stress data would be even greater if it could also be shown to be highly correlated with other indicators of vine water status. In the Williams and Araujo study, other indicators of vine water status used for further correlation with vine water potential are net CO2 assimilation rates (A) and stomatal conductance to water vapor (gs), both measured at solar noon, and soil water content (SWC), measured with a neutron probe.

The three indicators of vine water potential in this study were measured on both Chardonnay and Cabernet Sauvignon vines grown in Napa Valley in the 1999 growing season. Because both vineyards were part of a study on the effects of deficit irrigation, all vines had been irrigated weekly at various fractions of estimated vineyard evapotranspiration from berry set until the dates of measurements.

Vine water status and leaf gas exchange were measured on two dates in the Chardonnay vineyard and one date in the Cabernet Sauvignon vineyard.

Individual leaf replicates numbered six for each scion-rootstock combination and irrigation treatment in the Chardonnay vineyard on the first date, August 24, 1999, and five for each treatment in the Chardonnay on September 21, 1999.

Individual leaf replicates for the Cabernet Sauvignon on the only date measured (August 24, 1999) was also five. This produced 86 total data points.

Use of irrigation treatments at both locations resulted in a wide range of vine water status. The lowest values of PDLWP, LWP, and SWP recorded for an individual leaf were –0.85, –1.85, and –1.65 Mpa, (–8.5, –18.5, and –16.5 bars) respectively. The highest values of PDLWP, LWP, and SWP were –0.02, –0.75, and –0.55 Mpa, (–0.2, –7.5, and –5.5 bars) respectively. In most cases, significant differences among irrigation treatments for one measure of vine water status were also similarly different for the other two.

Test results showed that all three methods of estimating vine water status were highly correlated with one another. The best correlation was between mid-day LWP and mid-day SWP (r2 = 0.92).

All three methods were significantly (r2 = 0.69) correlated with soil water status in the Chardonnay vineyard and also significantly correlated with net CO2 assimilation (r2 = 0.67, 0.50, 0.48) and stomatal conductance at mid-day (r2 = 0.69. 0.58, 0.54) in both vineyards.

All three measures of vine leaf water potential were linearly correlated (r2 = 0.93) with berry weight and vine yield when measured the first week of October 1999. These data would indicate that either measurement of mid-day leaf water potential would give a good estimate of the water status of grapevines.

Pre-dawn leaf water potential has been used in many studies as the standard to which other measures of vine water status are compared. It is assumed that this is the period when the vine is in equilibrium with soil water potential.

However, the authors cite references showing that PDLWP of some non-grape species come into equilibrium with the wettest portion of the soil in the plant’s root zone. Therefore, the soil moisture a vine responds to at mid-day may differ from that at pre-dawn due to the flux of water that is occurring when a vine is actively transpiring. If this is correct, differences at pre-dawn may not necessarily reflect the water status of the vine later in the day, as was observed in the Williams and Araujo study.

It has also been demonstrated that season-long measurements of mid-day LWP have been shown to be highly correlated with the seasonal changes in soil water content of treatments irrigated with differing amounts of water. That data and the data from this study in Chardonnay indicated that mid-day LWP was reflective of the amount of water in the soil profile.

All three methods of estimating vine water status were similarly correlated with SWC, applied amounts of water, and with one another, and were also significantly correlated with leaf gas exchange. Therefore, under the conditions of the Williams’ and Araujo study, the criterion that measurements of plant water status should reflect: 1) the availability of soil moisture and/or, 2) applied water amounts, or 3) short- and medium-term plant-stress responses, were tested and met for all three measures of leaf water potential.

For practical use, critical values of mid-day leaf water potential, stem water potential, and pre-dawn leaf water potential could be established and utilized to make decisions such as when to begin irrigating each season and the interval between irrigation events. This would allow a grower/ manager to maintain a specific degree of vine water stress to produce winegrapes that are appropriate for the wine style.

However, from a purely practical standpoint, measurement of mid-day leaf water potential would be most convenient. The main limitation is the time frame allowable to assure consistency. In this study, that time was one half hour before and after solar noon.

The short time limits the acreage or the number of vines that can be measured in one day. The time can be lengthened, however, in a practical field situation, to one hour before and one hour after solar noon. This allows two hours for data collection and is certainly acceptable as long as the other factors affecting consistency (using the same vines each time, well-trained users, bagged samples, replicates) are carefully observed.

There is one other critical factor in using a pressure chamber to ascertain vine water status. It has been demonstrated that the individual making measurements of plant water status is a significant source of variation. It is, therefore, imperative that technicians be well-trained in use of the pressure chamber, and the choice of leaves to sample, and data discrepancy recognition. Trainees should be monitored closely for awhile to ensure they are using the equipment properly and their technique is appropriate and consistent.

Conclusions

In the above study, it was shown that mid-day leaf water potential, mid-day stem water potential, and pre-dawn leaf water potential values from two vineyards on three dates were linearly correlated with each other and with measurements of net CO2 assimilation and stomatal conductance.

There is another technique that is the pressure probe and goes straight into the cell. You can learn about this technique in http://www.plantphysiol.org/content/61/2/158.short

Hi Michael,

Here you can take a glance on: http://6e.plantphys.net/topic03.06.html

There are good information for measuring plant water potential.

Good Luck!

Ali

Hi Michael

That's a nice reference, Ali. In addition to the techniques mentioned in that, there is also the Dixon psychrometer that can be attached to a stem and connected to a datalogger to measure Water potential with e.g. 15 min. intervals. See: Plant, Cell and Environment (1992) 15, 947-953. 

There are several heterogeneities in a canopy under light. A point potential would balance that frictional loss due to stem conductance with transpiration that depends on leaf number and conductance of stomata, and some elastic property that goes in between. Frequently embolism is created and vapour releases any tension. At the very end I suppose that Simonneau  and Habib use of root suckers covered with alu foil  in PCE 1991 is justified, and current evolution on a canopy would in any way have to cope with heterogeneity. Dendrometers can mesaure better wood growth.

We have recently proposed a new method which is adequate for flat leaves based on air-coupled ultrasonic resonance. During this last summer we have used this method in Vitis vinifera and the results indicate that we are able of estimate very small changes in turgor pressure through the day. You can find several papers describin the method in my list of publications.

What I was trying to stress is that pre-down water potential can be measured in attached twigs  tp the trunk base kept in darkness, and that any attempt to go on during a day on lightened portions would find heterogeneities depending on the balance between friction and transpiration. A passing cloud would change the result. This would not diminish the value of in vivo potential or new instruments but drives back to Slatyer intuition of plant wall pressures as drivng forces for growth, and the entire history is then going to hormons, cavitation and wall porosity issues. in brief the wall tension is a driver or an effect? 

This is a matter of many discussions. We have found, in Vitis, that maintaining turgor pressure ensures a good stomatal conductance and thus CO2 income.

The practical benefit can be seen in comparative studies of sorghum leaves made by De Roo(1969) using equilibrium methods and Blum et al. (1973) using similar

methods but not operated at equilibrium. De Roo (1969) compared the

water potential measured with an isopiestic psychrometer and the

xylem pressure measured at balance in a pressure chamber and found

them to be virtually equivalent, i.e., close to the line of equivalency. De Roo (1969) also determined the osmotic potential of the xylem solution and found it to be on average -0.05 MPa. Adding this potential to the xylem pressure in Fig. 5.1A gives the leaf water potential (see Chap. 2) and an even closer correspondence to the

equivalency line. Note that there was little variation in the data.

Because the data fall on the line of equivalency, there is no uncertainty

about which method is correct and no need to calibrate. The pressure

chamber compares so well with the psychrometer, which has already

been shown to give absolute values of the water potential (see Chap. 3),

that both methods can be considered to give absolute values of the

water potential of sorghum leaves.

Blum et al. (1973) also studied sorghum leaves but in contrast

to De Roo (1969) used the pressure chamber as a nonequilibrium

method by noting the first appearance of xylem solution when the

sample was pressurized at a constant rate . A comparison was made with thermocouple psychrometer readings by the Peltier method which also is not an equilibrium technique. It is clear that the data do not match the line of equivalency and that the relation depends on the rate of pressure application .

Also, the data show a large variability. Blum et al. (1973) indicate that

if one uses nonequilibrium methods, careful calibration is essential.

They also point out that it is not clear whether the pressure chamber or

psychrometer gives the more accurate values. Therefore, one may

consider the nonequilibrium data to be only relative approximations.

Clearly, the equilibrium techniques used by De Roo (1969) are

preferred. Their freedom from calibration and lack of variability are

desirable features, and the ability to interpret the measurements is

simplified by having absolute values of the potential. 

Hello:

I think the most useful way to measure plant water potential is with the Scholander pump. I do use it always, and I like to measure the plant water status during midday, this technique is known as stem water potential.

Pressure bomb and psychrometer are standard methods for measuring water potential in plants. However, when I was working on banana plants, the exuding latex created a real problem in determining the correct water potential. As we pressurise, it is a mixture of latex and xylem sap that comes out. In psychrometer also, the leaking latex creates a problem. So we devised another method to determine the banana plant water potential. This is by allowing the plant to exude continuously, and finally when it stops it is expected to equilibrate with the tissues. At that time collect a drop of the latex and determine its osmotic potential. This will be equal to the water potential of the leaf. You can read the details from the following publication:

https://www.researchgate.net/publication/259494443_Water_relations_of_the_banana._I._Predicting_the_water_relations_of_the_Field-grown_banana_using_the_exuding_latex

Article Water Relations of the Banana. I. Predicting the Water Relat...

For trees, I still recommend the Scholander pressure chamber. We also tried stem psychrometers with trunks of large forest trees. The advantage of this method is continuous measurement below the canopy level. However, this method is very sensitive to temperature changes (so it has to be insulated very thoroughly) and also to desiccation of the tissue. After approximately 3 days, we obtained implausibly negative values of water potential due to desiccation, which cannot be avoided completely. The problem is that the point of time at which the data become unplausible cannot be determined exactly. Therefore, I only recommend psychrometers if you have to record the water potential continuously, but for only a short period of time,

I guess that time for the term 'potential' is less than adequate today, for the many reasons given above and the complex literature about water conduction in trees and in leaves. Is not clear if it is a cause or an effect of water motion. This can now be measured directly. Anyway ICT and some other sells Psi instrumentation and they would have their good reasons.

I aslo recommend the Pressure chamber technique for field measurements of leaf water potentials.

Please note that the group of Abraham Stroock at Cornell has designed a new device that looks very promising for measuring water potentials in stems:

Pagay, V., Santiago, M., Sessoms, D.A., Huber, E.J., Vincent, O., Pharkya, A., Corso, T.N., Lakso, A.N. & Stroock, A.D. A microtensiometer capable of measuring water potentials below -10 MPa. Lab-on-a-Chip, 14, 2806-2817 (2014). 

http://www.stroockgroup.org/pubs_file_cabinet/2014_Pagay_microtensiometer_LoC.pdf?attredirects=0&d=1

Three most common methods used for measuring water status.

Leaf Water Potential

A fully expanded leaf exposed to direct sunlight is chosen for measurement. To measure mid-day leaf water potential, the targeted leaf must first be covered entirely with a small plastic bag that is wrapped tightly around the leaf and secured. Securely bagging the leaf before cutting it from the shoot avoids any further transpiration, which alters the resultant pressure reading. If this critical bagging step is omitted, the data will be inaccurate.

As quickly as possible after bagging, the petiole of the bagged leaf is cut from the shoot with a sharp razor as close to the shoot as possible. The petiole is then quickly placed through the chamber lid and secured tightly, with the cut edge of the petiole facing outside and the bagged leaf blade inside the chamber.

The chamber is sealed and then slowly pressurized with nitrogen gas. When the positive pressure exerted on the leaf in the chamber equals the negative pressure inside the leaf, liquid in the leaf blade will begin to be forced out of the cut edge of the leaf.

During pressurization, the operator carefully watches the exposed edge of the petiole for the appearance of a drop of water (sap). As soon as the drop appears, the user reads the corresponding pressure from the chamber gauge. This pressure value is the leaf water potential, read in negative (–) bars.

In comparison, mid-day stem water potential tests are done during the same time period as mid-day leaf water potential but handling of the leaf is changed. Stem water potential has been considered to be less variable than mid-day LWP, improving the ability to detect small pressure differences among treatments. But until this study was completed, a comprehensive study comparing the two had not been tested in grapes.

Stem Water Potential

The stem is thought to be less susceptible to fluctuations in environmental pressures than the leaf and, therefore, more representative of the actual level of stress in the entire vine. In the mid-day SWP test, a leaf on the shaded side of the canopy is chosen to minimize any possible heating effects.

The leaf is wrapped in a black plastic bag that is covered with aluminum foil to prevent overheating by the sun. The bag is left on the leaf 90 to 120 minutes. This effectively stops the natural transpiration from the leaf, allowing the leaf water potential to come into equilibrium with the stem water potential. After 90 to 120 minutes has elapsed, the leaf is excised and tested in the pressure chamber as stated above.

Pre-dawn Leaf Water Potential

Pre-dawn leaf water potential is determined using the same basic methodology as LWP, but the readings are taken beginning at 3:30 am and ending before sunrise, using fully expanded leaves. It has been assumed that, before sunrise, the vine is in equilibrium with the soil’s water potential, therefore making PDLWP a more sensitive indicator of soil water availability. But the obvious difficulty with the method is timing: readings must be done prior to sunrise, making its practicality questionable.

Comparison of methods

For any measure of plant water status to be a sensitive indicator of water stress, it must be responsive to differences in soil moisture status and/or the resulting growth differences due to water application. The measure should also be closely related to short- and medium-term plant stress responses and less dependent on changes in environmental conditions.

For winegrapes, it would seem that LWP, SWP, and PDLWP each meet these criteria. The best indicator of which method is the most effective and yet most practical might be as simple as the ease of operation if the data from all three plant-based measures of vine water stress can be proven to be highly correlated.

Additionally, the value of that plant-based stress data would be even greater if it could also be shown to be highly correlated with other indicators of vine water status. In the Williams and Araujo study, other indicators of vine water status used for further correlation with vine water potential are net CO2 assimilation rates (A) and stomatal conductance to water vapor (gs), both measured at solar noon, and soil water content (SWC), measured with a neutron probe.

The three indicators of vine water potential in this study were measured on both Chardonnay and Cabernet Sauvignon vines grown in Napa Valley in the 1999 growing season. Because both vineyards were part of a study on the effects of deficit irrigation, all vines had been irrigated weekly at various fractions of estimated vineyard evapotranspiration from berry set until the dates of measurements.

Vine water status and leaf gas exchange were measured on two dates in the Chardonnay vineyard and one date in the Cabernet Sauvignon vineyard.

Individual leaf replicates numbered six for each scion-rootstock combination and irrigation treatment in the Chardonnay vineyard on the first date, August 24, 1999, and five for each treatment in the Chardonnay on September 21, 1999.

Individual leaf replicates for the Cabernet Sauvignon on the only date measured (August 24, 1999) was also five. This produced 86 total data points.

Use of irrigation treatments at both locations resulted in a wide range of vine water status. The lowest values of PDLWP, LWP, and SWP recorded for an individual leaf were –0.85, –1.85, and –1.65 Mpa, (–8.5, –18.5, and –16.5 bars) respectively. The highest values of PDLWP, LWP, and SWP were –0.02, –0.75, and –0.55 Mpa, (–0.2, –7.5, and –5.5 bars) respectively. In most cases, significant differences among irrigation treatments for one measure of vine water status were also similarly different for the other two.

Test results showed that all three methods of estimating vine water status were highly correlated with one another. The best correlation was between mid-day LWP and mid-day SWP (r2 = 0.92).

All three methods were significantly (r2 = 0.69) correlated with soil water status in the Chardonnay vineyard and also significantly correlated with net CO2 assimilation (r2 = 0.67, 0.50, 0.48) and stomatal conductance at mid-day (r2 = 0.69. 0.58, 0.54) in both vineyards.

All three measures of vine leaf water potential were linearly correlated (r2 = 0.93) with berry weight and vine yield when measured the first week of October 1999. These data would indicate that either measurement of mid-day leaf water potential would give a good estimate of the water status of grapevines.

Pre-dawn leaf water potential has been used in many studies as the standard to which other measures of vine water status are compared. It is assumed that this is the period when the vine is in equilibrium with soil water potential.

However, the authors cite references showing that PDLWP of some non-grape species come into equilibrium with the wettest portion of the soil in the plant’s root zone. Therefore, the soil moisture a vine responds to at mid-day may differ from that at pre-dawn due to the flux of water that is occurring when a vine is actively transpiring. If this is correct, differences at pre-dawn may not necessarily reflect the water status of the vine later in the day, as was observed in the Williams and Araujo study.

It has also been demonstrated that season-long measurements of mid-day LWP have been shown to be highly correlated with the seasonal changes in soil water content of treatments irrigated with differing amounts of water. That data and the data from this study in Chardonnay indicated that mid-day LWP was reflective of the amount of water in the soil profile.

All three methods of estimating vine water status were similarly correlated with SWC, applied amounts of water, and with one another, and were also significantly correlated with leaf gas exchange. Therefore, under the conditions of the Williams’ and Araujo study, the criterion that measurements of plant water status should reflect: 1) the availability of soil moisture and/or, 2) applied water amounts, or 3) short- and medium-term plant-stress responses, were tested and met for all three measures of leaf water potential.

For practical use, critical values of mid-day leaf water potential, stem water potential, and pre-dawn leaf water potential could be established and utilized to make decisions such as when to begin irrigating each season and the interval between irrigation events. This would allow a grower/ manager to maintain a specific degree of vine water stress to produce winegrapes that are appropriate for the wine style.

However, from a purely practical standpoint, measurement of mid-day leaf water potential would be most convenient. The main limitation is the time frame allowable to assure consistency. In this study, that time was one half hour before and after solar noon.

The short time limits the acreage or the number of vines that can be measured in one day. The time can be lengthened, however, in a practical field situation, to one hour before and one hour after solar noon. This allows two hours for data collection and is certainly acceptable as long as the other factors affecting consistency (using the same vines each time, well-trained users, bagged samples, replicates) are carefully observed.

There is one other critical factor in using a pressure chamber to ascertain vine water status. It has been demonstrated that the individual making measurements of plant water status is a significant source of variation. It is, therefore, imperative that technicians be well-trained in use of the pressure chamber, and the choice of leaves to sample, and data discrepancy recognition. Trainees should be monitored closely for awhile to ensure they are using the equipment properly and their technique is appropriate and consistent.

Conclusions

In the above study, it was shown that mid-day leaf water potential, mid-day stem water potential, and pre-dawn leaf water potential values from two vineyards on three dates were linearly correlated with each other and with measurements of net CO2 assimilation and stomatal conductance.