Hello,

Ricardo is correct that the soil-plant-atmosphere  system is very dynamic.  As `reductionist scientists' we seek to have a single number to describe a complex phenomenon. So to analyse plant responses to a deficient water supply, a single value (e.g. 30% soil water content) seems to answer our need.  However, in seeking simplicity there is a strong tendency to ignore the scientific analyses (largely made in the 1950-60s but now apparently forgotten)  which show that the simple does not provide what we need.  It is not possible to identify the point in the soil profile at which to measure soil water potential which correlates strongly with the plants responses because it depends on rooting depth and root length/area per unit soil volume (root density). To illustrate this consider what happens when, for example,  roots in moist soil at the bottom of the root zone provide water for growth etc even if a large part of the root system is in dry soil. And the response depends strongly on the rate of transpiration. That is why it became  obvious that it is essential to measure the status of water in the plant as well as in the soil at different points. 

The application of water to sub-saturated soil results in a zone of wet soil which is used by the plant  (or may be - depending on the root distribution and amount of water added and the volume of soil and its water content). Experimentally it is often used as it is a simple method and (taking proper care as Ricardo says) will give results - causing changes in growth etc - but it would be simply incorrect to say that a " uniform 30% drought stress" was achieved and then to relate this measurement to the growth etc . Note also that comparisons, say of different varieties/genotypes and genetically modified plants, based on the method is extremely doubtful, and should not be accepted at face-value.   

Sorry to keep repeating the same message. But I am sure that much of the work on plant responses to water deficits would be greatly improved if the lessons already learned were appreciated and applied.  The simple approach my not always be the best.

Regards

David

Dear Moaed Almeselmani

Pls. find the attached files, may help in your research, good luck

http://link.springer.com/article/10.1007/s00344-015-9500-2

http://www.sciencedirect.com/science/article/pii/S0570178314000402

https://www.researchgate.net/post/How_can_I_generate_drought_stress_in_wheat_grown_in_potting_soil

http://www.fao.org/docrep/006/y4011e/y4011e06.htm

https://hal.archives-ouvertes.fr/hal-00886451/document

http://www.bio-protocol.org/e867

Regards

Prof. Houda Kawas

Thank you Huda

In post you may add enologhe

Yuo may add to pots some water daily, in order to jeep 70% of a saturated control  a tree digit pan is necessary

Thanks Guido 

Dear Moaed,

It is actually not possible to "exposed wheat plants to 30% drought stress of saturated soil and maintain it for 20 days"  if the plants are actively transpiring.  The idea is very common that plants growing in pots of soil (but the same applies in the field) can be allowed to dry the soil to a particular mass and then water is added at intervals to maintain that mass. So, you think of  saturating pots  with water (but note  that such a treatment will rapidly damage wheat) and then weighing to give 100% and then allowing them to dry down to a particular weight? When the weight is achieved the pots are weighed every day perhaps and then water applied to the soil surface to maintain the target. 

What actually happens when water is added to a partially dry  (subsaturated0 soil  is that the  water more or less saturates a small volume of soil. The water is not distributed evenly throughout the whole volume of  dry soil in the pot.  So the plant adapts (particularly over  a period as long as  20 days) to the smaller  supply of water at close to zero water potential.  The supposed response is not to 30% water content but to a smaller supply of water close to `pot capacity' of the volume of the rewetted soil.

The physics of water in soils cannot be ignored if any reasonable studies of plant responses to water are to be achieved.

See the following on Research Gate where the basics are considered

J Exp Bot. 2013 Jan;64(1):83-108. doi: 10.1093/jxb/ers326. Epub 2012 Nov 16.

Genetic engineering to improve plant performance under drought: physiological evaluation of achievements, limitations, and possibilities.

Lawlor DW

Hope this helps

David

you can not sustain you plant to a given stress for instance 30%

you can exposed your plants to  ranges of water availabilities you can say 

Irrigation was applied whenever 10, 20, 30 .....100% AWC depleted  

Dear David and Caser

thanks a lot that was really helpful

Anyhow Dear Moaed Almeselmani

you can put a pup to your pots connected to S.M sensor which operate with in given S.m and to replace the evaporated water within 29-30%, however, from 0 t0 30% this was recommended  as adequate watering accompanied with no biomass  reduction in most mesophytes plants 

Hello Everyone

The point is that adding water to a dry or partially dry soil, by manual application  or by means of a soil moisture sensor, to achieve a given pot weight does not achieve a uniform sub-saturated water content or water potential throughout the soil volume. What results is a small volume of  soil with a water content/potential close to saturation.  This causes effects on growth etc which are very  difficult to relate to water status of the plant or soil.

Regards

David

Thank you all

I believe what Dr. Lawlor argues is true. The plant-soil-water system is very dynamic and complex. In nature plants do not have a uniform 30% drought stress. And yet because of the difficulties involved I think it is acceptable to use these types of tests (gravimetric control of moisture) as long as all your plants are properly replicated and you clarify your procedure.   Best of luck with your research.

Thank you Ricardo

Hello,

Ricardo is correct that the soil-plant-atmosphere  system is very dynamic.  As `reductionist scientists' we seek to have a single number to describe a complex phenomenon. So to analyse plant responses to a deficient water supply, a single value (e.g. 30% soil water content) seems to answer our need.  However, in seeking simplicity there is a strong tendency to ignore the scientific analyses (largely made in the 1950-60s but now apparently forgotten)  which show that the simple does not provide what we need.  It is not possible to identify the point in the soil profile at which to measure soil water potential which correlates strongly with the plants responses because it depends on rooting depth and root length/area per unit soil volume (root density). To illustrate this consider what happens when, for example,  roots in moist soil at the bottom of the root zone provide water for growth etc even if a large part of the root system is in dry soil. And the response depends strongly on the rate of transpiration. That is why it became  obvious that it is essential to measure the status of water in the plant as well as in the soil at different points. 

The application of water to sub-saturated soil results in a zone of wet soil which is used by the plant  (or may be - depending on the root distribution and amount of water added and the volume of soil and its water content). Experimentally it is often used as it is a simple method and (taking proper care as Ricardo says) will give results - causing changes in growth etc - but it would be simply incorrect to say that a " uniform 30% drought stress" was achieved and then to relate this measurement to the growth etc . Note also that comparisons, say of different varieties/genotypes and genetically modified plants, based on the method is extremely doubtful, and should not be accepted at face-value.   

Sorry to keep repeating the same message. But I am sure that much of the work on plant responses to water deficits would be greatly improved if the lessons already learned were appreciated and applied.  The simple approach my not always be the best.

Regards

David

It is very difficult because you need a lot of place, a lot of plant and a gravimetric methods and specially a place aconditioned where you can control temperature and humidity. Sorry but for this approximation you need a controlled conditions difficult to get in fields.

Dr. Lawlor, thanks for the answer, I appreciate the message and I understand it. What do you think it is the best way to proceed with these types of studies that have the goal of comparing some level of drought? My concern would be the practicality of a more sophisticated approach. Greetings.    

Very interesting discussion! I did this type of research last year, I measured the field capacity (FC) of soil by pressure membrane apparatus then maintained the different water levels (40, 60, 80 and 100% of FC) by gravimetric method. I used the term "water deficit stress" instead of "drought". But, Editor of Acta Physiologiae Plantarum denied our paper for publication by saying authors are confused by "drought stress" and "osmotic stress". I have seen in many research articles, different authors have used the term "water deficit stress" instead of "drought" because we cannot maintain drought in general. I believe, we can not say 30% drought but, alternative to drought "water deficit stress" may be correct because we actually maintain different levels of moisture in soil. However, I am still wondering while revising my manuscript what is write either "drought" or "water deficit stress"?

Since soil water is in  dynamic nature, we consider average soil water content in the root-zone depth let say 30% of field capacity. However, to maintain constant drought stress of 30% for 20 days seems not possible under field conditions.  

Hello,

With regard to the correspondence, a quote taken from of P J Kramers’s book Plant and Soil Relationships: A modern synthesis, 1969. McGraw-Hill. Page 93 serves to emphasise the point that maintaining a constant soil water content cannot be achieved. “Maintenance of definite levels of soil water stress.     One of the most troublesome problems in plant water relations research is that of maintaining plants growing in soil at uniform levels of water potential lower than field capacity. …... The impossibility of doing this should have been realized by all …” There is a newer Edition P J Kramer and  J S Boyer which is an excellent text dealing with so many aspects of plants and water: I recommend it. Note also that the same applies to automated phenotyping and watering systems now fashionable for testing `drought  resistance’ of plants. The terms used in water relations can be very unclear and confusing: `drought resistance' or `tolerance' require careful definition and application.  Muhamad and Attila - please note that it is possible to say that the water content of the soil varied between x% and y% , if these were measured  and give an average  but the plants will not have been at a constant, particular %. 

Responding to the very valid question – what can be done experimentally to study the response of plants to the water supply or to the state of water in the soil? It is such a large topic I cannot do more here than suggest a general approach and then consider some aspects which are important.

If the aim is to establish the best irrigation treatments or to test which crop varieties produce the greatest yields or best quality – i.e. has a strong practical aim – then field studies will give the most useful information: pot studies alter conditions so substantially that results require confirmation in the field.  However, measurements made in the field on plants and soil are invaluable for understanding the results obtained.  Irregular rainfall in many (most?) regions is a major problem for such trials. Repetition in different seasons often results in considerable variation in the data, and is expensive. Studies are occasionally done with protective covers – rain-out shelters – which move across the crop to exclude rain. These are expensive and technically demanding.  Permanent covers alter the environment substantially and affect plant/crop responses.    

Pot studies are much easier for analysing mechanisms and for advancing scientific understanding, but as they are generally made in very artificial conditions (controlled environments, glass houses) they need confirmation in the field.   The greater the size (volume and depth) of the pot and the more field-like soil/rooting conditions, together with  a greater volume of soil relative to the size of the plant (leaf area particularly), the better. However, it is difficult to specify the best experimental approaches: the options require careful evaluation.

A very important point to realise is that, what-ever the growth conditions,  plant water content (PWC) and water potential (PWP) often do not follow the soil water content (SWC) and soil water potential (SWP) closely. This is because there are many more roots, usually, in a volume of soil (root density) in the uppermost layers of soil than lower down in the profile: so water has to move over a smaller distance from the bulk soil into the root and plant in the upper soil layers than lower soil layers.  Also the movement of water from lower soil layers through the longer roots to the top of the plant encounters greater resistance than for water movement from upper soil through shorter roots. Both factors cause the upper soil to dry more rapidly as the plant transpires.  This means for studies of the effects of soil water on plants it is essential to measure SWC & SWP at different depths in the profile over time. 

It is actually the state of water in the plant (PWC & PWP) which is important for plant growth and metabolism.  So for useful (and particularly for analytical studies) they should be measured. PWC can be measured as water content (g water/g dry mass x 100 = water content %) but relative water content (RWC = (fresh wt sample – dry wt at 80oC of sample)/ (saturated wt sample – dry wt at 80) x 100%) usually of leaves but of other parts, is a better measure.  Both PWC and RWC require destructive sampling. So sufficient replication is essential to avoid working with damaged plants.

Measurements of the above soil and plant attributes are possible with relatively simple methods, balances for weighing and ovens for drying samples. SWC requires samples to be taken with an auger, rapid weighing of the fresh sample and then drying to constant weight.  SWP is not so easy the measure but the literature gives methods.    PWC is a routine method, but care should be taken with sampling, and measurements on particular organs – leaves especially – may be more informative than on the whole-plant.  RWC – of leaves but also other organs - requires a reasonably sensitive balance, rapid sampling and care with saturating and drying after saturation.   PWP is relatively easy to measure with a pressure chamber but other techniques are available.   Investing time in finding practical methods and trying them out will pay-off.

How to apply the soil drying? This depends on what the question to be answered is.   As already stated, for short term studies of processes such a photosynthesis or leaf growth, plants are often grown in small pots of soil. This allows the whole pot to be weighed and so water loss can be assessed with a suitable balance.  However the transition from unstressed to very stressed plants can (depending on conditions) be remarkably rapid, and this applies even in large pots.  If longer-term growth and/or yield are to be studied then very large pots are needed (so difficult to weigh) or plants should be grown in the field requiring specialised methods of soil sampling to estimate water use or expensive methods to estimate water loss (e.g. weighing lysimeters).  The sampling of soil water is often been done at different depths down the profile at frequently intervals. Plant water is measured at the same time. Indeed for all systems measurement of soil and plant during the drying-down process is very important as these data are correlated statistically with the measurements made on the plant, e.g. photosynthesis, growth and provide the basis for a sound comparison.

Drying the soli-plant system is achieved by transpiration from the plant.  Plants are grown to a particular stage with full watering. Then watering is stopped. As the soil dries measurements of SWC/SWP and PWC (RWC) and PWP are made frequently along with the measurements of plant response – photosynthesis or growth etc. Measurements are also made on well-watered controls, usually kept at freely drained pot or field capacity.

A simple study, for example, is of the response of single species to drought to analyse the mechanisms of processes. More complex experimental studies may compare plant varieties: metabolism, photosynthesis, gene expression etc (see  Habash DZ, Baudo M, Hindle M, Powers SJ, Defoin-Platel M, Mitchell R, et al. (2014) Systems Responses to Progressive Water Stress in Durum Wheat. PLoS ONE 9(9): e108431. doi:10.1371/journal.pone.0108431  which is available on Research Gate).  It may be of scientific interest to analyse the effects on the plant of interactions between environmental factors, such as a study of growth as affected by the interaction between soil water and light intensity.  Such conditions greatly affect the rate of transpiration and then rate of soil drying making comparison very difficult: the approach I suggest allows a proper comparison. Another type of study which is important practically is to understand the difference between genotypes. They may differ in leaf area and/or stomatal conductance and so rate of water loss and therefore soil drying. But by measuring the water status of soil and plant and correlating with plant performance, genotypes can be compared and valuable information obtained about how they differ.  The methods are general and applicable to many situations.  Details for the case of genetically modified plants has been given (see Research Gate - Lawlor DW 2013  Exp Bot. 2013 Jan;64(1):83-108. doi: 10.1093/jxb/ers326  Genetic engineering to improve plant performance under drought: physiological evaluation of achievements, limitations, and possibilities). 

The methods outlined have also been used in field studies on crops protected from rain.  (An example is Lawlor et al, 1981 Growth of spring barley under drought: Crop development, photosynthesis, dry-matter accumulation and nutrient content, The Journal of Agricultural Science 96(01):167 - 186 · and details are to be found in references there and on Research Gate). There, soil drying was applied at different growth stages and after drying plots were fully re-watered to allow the effects of drought at specific periods to  be assessed.  The same approach can be used for pot studies.  Full re-watering avoids the difficulties of interpretation which arise if plants are partially re-watered.

The aim here was to indicate what can be done to understand and assess plant/crop responses to variations in water supply.  So, even if it is impossible to maintain transpiring plants at constant sub-saturated water content/potential it is possible with relatively simple techniques to make meaningful studies.

Thank you David again

Dear David,

Thanks for such a detailed bunch of suggestions.

I am following this discussion and NOW I found a very important point of managing or studying the water deficiency effects on plant water, osmotic and photosynthetic relations. However, I want to re-confirm that:

1) Keep one set of replicates (pots) at well watered conditions (through out the study) in control treatment.

2) After a certain growth stage (met with well watering, as in control), allow the pots to dry down to e.g first wilting symptoms on plants and then re-water the pots (well watering not specific amount) in limited water supply treatments.

Am I right or ?

I did similar work see the link.

https://www.researchgate.net/publication/267298082_Biochar_but_not_humic_acid_product_amendment_affected_maize_yields_via_improving_plant-soil_moisture_relations

However, managing the daily weight gain by plants is difficult, therefore it is necessary to have extra replicates for destructive sampling. But this is much time consuming, space consuming in glasshouse, measuring specific physiological measurements in one go and expensive indeed!  

Article Biochar but not humic acid product amendment affected maize ...

@ Muhammad yahya Khan.

Why not to use the word "limited water supply" instead of "water deficit stress" ? I think it will be more easy to explain your objective and to avoid confusions.

Do you mean soil water holding capacity? You may refer to this webpage. http://www.sugarresearch.com.au/icms_docs/188695_Soil_water_holding_capacity_IS13107.pdf. You will need to make sure that the pots in the drought treatment are gradually reduced to 30% soil water . Record the weight of each pot at the starting point. Weigh the pots every day and add relevant water to maintain a constant pot weight. Hope it helps.

Hello

A brief response to the comments from:

Muhammad Yahya Khan    ·   

I am sorry, but it is not possible as you wrote "to  maintained the different water levels (40, 60, 80 and 100% of FC) by gravimetric method " nor ""water deficit stress" may be correct because we actually maintain different levels of moisture in soil"  for the reasons explained at (considerable!) length. The term  "water deficit " seems better to me than water deficit stress" - as it is a simple statement  and  "drought" has a more meteorological meaning. I do not know why an editor would there should be  confusion between "drought stress" and "osmotic stress"..

Ghulam Haider:

You are correct, in my view, for both 1 and 2 1) Keep one set of replicates (pots as control treatment.

2) After  the pots try down to e.g first wilting symptoms then re-water fully.

Your paper is interesting and does not claim to maintain the pots at a specific water content.  It is, of course, possible to re-water with different  amounts but that does not mean the whole pot was at a given water content.

Regards

;

Hello,

A final comment to Zhong-Hua Chen  and I hope you will forgive me:   · The web site is not claiming, as I read it,  to maintain the soil water at a given content (g water/g or cm3 soil) which was the origin of this long discussion. . What you wrote "...the pots in the drought treatment are gradually reduced to 30% soil water . Record the weight of each pot at the starting point. Weigh the pots every day and add relevant water to maintain a constant pot weight. " is correct tin hat it simply keeps the pot at a constant (more-or-less) weight. BUT THAT DOES NOT MEAN THE SOIL THROUGHOUT THE POT IS THEN KEPT AT 30% SOIL WATER CONTENT. Please see earlier discussion..