Plant Water Relations - Science topic

thanks for the document I.V.Srinivasa Reddy

Thanks David W. Lawlor , Unfortunately frequent measurement is very difficult+expensive as our field sites are far away from where we are based.

Siddarth Machado I am seriously considering a psychrometer, but am also vary about how comparable it would be. Just trying to see if there is a way to work with pressure chambers since we already have that equipment.

Andrew Paul McKenzie Pegman

Hi, Andrew, thank you for your answer. The spatially arithmetic and weighted mean of soil water isotopes are usually different owing to heterogeneity in soil water content, for example, if water content is taken as a weight. Meanwhile, the soil water content is a ratio scale variable, which is further related to the soil texture at the interest site, but water isotopes are not. Therefore, there might be some problems if soil water content is taken as a weight and the weighted mean is thus meaningless. I suppose this might be the reason why most people used arithmetic mean (±1SD) as the input of stable isotope models.

Best,

Qin

An interesting question . Synthesis of salicyclic acid and mycorhization has very strong inter-relationship. The process of plant defense as systemic acquired resistance operates through elevated synthesis of salicyclic acid , which is further elevated upon mycorrhization , thereby , prepares the mycorrhized plants physiologically to fight against any possible biotic stress.

see link below ↓↓↓↓

Thanks , very useful artcile^.^

If you are using water from kitchen, it would probably also has a lot of grease, in this case, it is not recommended to used it directly over plants. First you would have to install a grease filter.

You also need to investigate which species can tolerate or even storage pollutants, and be careful not to eat them because you do not know which are the pollutants the water has.

Therefore, if you are planning to recycle water for plants it would be better to build containers to avoid filtration to phreatic aquifers

In my opinion this sensor measures the sap flow in the petiole, which is correlated to the stomatal conductance, however, they are not the same thing.

I would compare it to measuring sap flow and transpiration, both are different but correlated...

Dear Heather,

when I was a student during my MSc I found "Plant Physiology and Development" by Taiz and Zieger a very good book (http://www.sinauer.com/plant-physiology-and-development.html). A key point in the learning process is the availability of internet resources which are related to each chapter of the book. You may find topics, essays and study questions for almost each chapter at the following link:

I hope this helps. I'm quite confident it will help your students.

Kind regards

Hello Juliana,

Water potential is a state of energy and not a plant trait per se. Water potential is a measurement, like temperature, so you could also ask: Can changes in plant temperature be characterized as phenotypic plasticity? It would be difficult to argue that plant temperature is a phenotypic trait; rather a series of co-ordinated plant traits can act to regulate plant temperature. Similarly, a series of plant traits can co-ordinate to regulate plant water potential.

Polyethylene glycol molecules with a molecular weight

larger than 6,000 (PEG 6000) are inert, non-ionic and cell impermeable.

They are small enough to influence the osmotic

pressure, but large enough unabsorbed by plants. Therefore, they are frequently used

to simulate osmotic stress.

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.

Thank you very much for the answer sir. We have ThetaProbe.

Dear Spencer Muthoka

The use of certain types of basils to adsorb water to form mucilages has been explored by several local communities. In India, seeds of Ocimum basilicum chemotype rich in methylcinnamate (the essential oil recovered from the herb is rich in this compound and not the seeds) are traditionally added to water and after the seeds swell, the water is consumed. This seed-water is believed to cool the body during hot summer months (when temperatures cross 40^oC). I have not come across any reference where this property of basil seeds has been explored to make available the water adsorbed by them to plants. There are synthetic chemical water adsorbents that have been used as soil conditioners for supplying water to plants under water-stress conditions. CSIR-National Chemical Laboratory, Pune, India has done research on such chemical compounds. You may check with the Institute by contacting the concerned authorities.

Best wishes

would you mind list some latest papers concering this regard?

Now I am working on subtropical forest in China, it seems to me that the difference of Carbon contents in three part of tree is not as much as N contents and other traits. I guess you are just start to make a research plan, maybe we can discuss on it.

This is a good resource: http://prometheuswiki.publish.csiro.au/tiki-index.php?page=Quantifying+leaf+vein+traits

Hello Michael,

I have custom made heat pulse sensors used in various species of tropical trees.

Hi Peter, the slow rate of flow may be because the safranin is a positively charged dye and is adhering to the negatively charged xylem walls. Perhaps try a dye with a negative charge, e.g. the acid fuchsin that Josef suggested. Good luck - it's frustrating because these experiments are time-consuming.

I am quite agree with you. As precipitations are getting more erratic and sparse in humid regions (due to global warming and climate change), the need for supplementary irrigation increase rapidly. But in the view of farmers in these regions (who didn't use to apply irrigation in their fields) it may seem odd that they need irrigation for their crop growth and so should pay for irrigation systems and their related equipments. These projects may face some culturally-related problems at the beginning. We have such experience in the north of Iran.