Hello Shreejana

The simple answer is that it is not possible!   The basic physics of water movement in soils is the cause. And what does 40% FC mean anyway?  The water potential in the soil is the important point.  I have clarified and  explained the situation in relation to testing of GM plants.  I also explain how to test the effects of  water deficits  on plants and plant  processes in general.  It is a very old problems and the soil physics literature - mentioned in my paper - will give details.  Do not think it is a simple, easy problem - it is not as I point out.  I hope this helps even if you may think it negative.

A  Free full text is available on Research Gate under Lawlor DW or from Journal of Experimental Botany. T.he Abstract is below

Lawlor DW

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.

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Abstract

Fully drought-resistant crop plants would be beneficial, but selection breeding has not produced them. Genetic modification of species by introduction of very many genes is claimed, predominantly, to have given drought resistance. This review analyses the physiological responses of genetically modified (GM) plants to water deficits, the mechanisms, and the consequences. The GM literature neglects physiology and is unspecific in definitions, which are considered here, together with methods of assessment and the type of drought resistance resulting. Experiments in soil with cessation of watering demonstrate drought resistance in GM plants as later stress development than in wild-type (WT) plants. This is caused by slower total water loss from the GM plants which have (or may have-morphology is often poorly defined) smaller total leaf area (LA) and/or decreased stomatal conductance (g (s)), associated with thicker laminae (denser mesophyll and smaller cells). Non-linear soil water characteristics result in extreme stress symptoms in WT before GM plants. Then, WT and GM plants are rewatered: faster and better recovery of GM plants is taken to show their greater drought resistance. Mechanisms targeted in genetic modification are then, incorrectly, considered responsible for the drought resistance. However, this is not valid as the initial conditions in WT and GM plants are not comparable. GM plants exhibit a form of 'drought resistance' for which the term 'delayed stress onset' is introduced. Claims that specific alterations to metabolism give drought resistance [for which the term 'constitutive metabolic dehydration tolerance' (CMDT) is suggested] are not critically demonstrated, and experimental tests are suggested. Small LA and g (s) may not decrease productivity in well-watered plants under laboratory conditions but may in the field. Optimization of GM traits to environment has not been analysed critically and is required in field trials, for example of recently released oilseed rape and maize which show 'drought resistance', probably due to delayed stress onset. Current evidence is that GM plants may not be better able to cope with drought than selection-bred cultivars.

 The complexities mentioned in the previous response are true, but one might be able to address some of these issues in a context dependent manner. I used to work next door to Bruce Bugbee at Utah State where plant growth methods under controlled environment conditions were a big part of his research. Please see the attached and also check the following ref:

Earl, H.J., 2003. A precise gravimetric method for simulating drought stress in pot

experiments. Crop Sci. 43, 1868–1873.

Hello Hormoz

Thanks for the comments and information.  The work in the reference is addressing the problem in a way that may studies have done.  Note the original question was about how to keep 40% WC - which the method described cannot do. Watering to a fixed weight in subsaturated soils actually only wets a small part of the soil to very close to saturation.  The water is not distributed equally within the whole soli volume.

Regards

David  

Thank you David and Hormoz for your kind help in this regard. I found its methodology and it is easy too. Now I am using gravimetric method for this treatment.