I think you can do it if you stop the process of photosynthesis.

It´s very difficult that you want to try, I'just was thinking in ABA control, but you dont want alter hormonal process. Also stop photosynthesis alter the normal functionallity of plants.

If you want to measure conductivity under different  water potentials or hydric deficit, just the values you take is the true response of plants. It wiil expected that conductivity decreases with less hydric potential. 

Shouldn't stopping photosynthesis close them instead? That's what happens at night right?

Stomata exist to provide CO2 to photosynthesize. If you stop photosynthesis they will close to stop losing water. They shouldn't remain opened. Right?

some fungal toxin can do this but you shlould be able to try the right one for the app. As example fusicoccin or synthetic depolarizers. http://onlinelibrary.wiley.com/doi/10.1111/j.1469-8137.1974.tb02120.x/pdf

Setting atmospheric CO2 concentration low is known to increase stomata aperture, But this efffect might be too small.

But it seems to me, that you should describe the problem more precise. Whole-plant conductance is an integral measure of all stomata apertures. So why you want to keep it constant? 

Please, could you explain the three lines of your question with more detail? It seems to me that you also want to measure hydraulic conductance in the stem, am I right?

We want to measure conductivity throughout the whole plant. That includes soil, roots, stem and leaves. The major issue here is that stomata will start to close as soon as loss of conductivity occurs preventing us to measure the actual conductivity of the whole system.  One could measure conductivity at each section separately but measuring conductivity in roots is extremely difficult, young stems are also a challenge, and leaves have the stomata anyway.

This problem is what got me thinking if there is a way to keep the stomata open. If I could keep stomata open while I dry down the soil I could measure loss of conductivity through the whole system.

Hormonal alteration is not a good solution since hormones act in several pathways so altering ABA will open stomata but will also alter multiple other physiological pathways that could give me unexpected behaviors and make my whole experiment ecologically meaningless.

Once the conductivity is known, stomatal behaviour can be accounted measuring stomatal conductance to couple both loss of conductivity and stomatal behavior.

The difference between conductivity and conductance is just a matter of standardizing by amount of tissue. Not a big deal.

I hope this clarifies why I want to keep them open.

Hello Gerard

I think Jan Graefe has made the most important point - stomatal closure is part (probably the major plant parameter) of the soil-plant-atmosphere system response to water deficiency in the soil.  So any analysis has to account for it.  Keeping the stomata open is not possible by stopping photosynthesis.  The hormonal treatments would be interesting experimental methods but are likely to have many effects.  There are mutants  in several species (with earlier work on  tomato if I recall properly) which wilt under even in moist soil because of lack of stomatal control over water flux - of course there may be other effects of such mutations, but certainly worth considering as part of a programme of analysis.  

 I published work from a PhD  on the topic for what it is worth: conductance of water in the soil dominates increasingly as soil water content and then potential  decrease (study on grass Lolium perrene) - available in Research Gate

D. W. Lawlor 1972    Growth and Water Use of Lolium perenne. I. Water Transport

Journal of Applied Ecology

Vol. 9, No. 1 (Apr., 1972), pp. 79-98

Published by: British Ecological Society

Article DOI: 10.2307/2402047

Stable URL: http://www.jstor.org/stable/2402047

Page Count: 20

Hope this helps

David Lawlor

Keeping the stomata open is not possible by stopping photosynthesis. Once the conductivity is known, stomatal behaviour can be accounted measuring stomatal conductance to couple both loss of conductivity and stomatal behavior. 

Hormonal alteration is not a good solution since hormones act in several pathways so altering ABA will open stomata but will also alter multiple other physiological pathways that could give me unexpected behaviors and make my whole experiment ecologically meaningless.

I

It looks like a very simple method to keep stomata open when water potential goes down. This is just increasing relative humidity around the plant which would generate a very low vapor pressure deficit. However, the evaporative demand could change and just  only depends on net radiation. 

Thank you Victor, this is a very clever idea which I also thought about. But I got less excited when I found a paper showing how soil moisture also has an effect in stomatal closure at constant VPDs.

Heshmat Soliman has a point, I would like to think a little bit more about his suggestion, let me see if I understood well: Are you saying that I can distinguish between actual loss of conductivity of the system (cavitation) and reduction of transpiration due to stomatal closure just measuring stomatal conductance?

From my point of view it seems like you would get a measure of stomatal behavior and a measure of conductivity but you would not have a way to know when would the plant actually lose a given percent of conductivity if it had had the stomata open.

Doing this is key because that way you can know what is the vulnerability of the whole system (soil, roots, stem, leaves) and how stomata works to prevent the loss of conductivity.

What do you guys think?

The information on the paper below may be useful to you. since it is known that ethylene mediates stomatal closure, you may try silver to block the ethylene action and keep stomata opened. See if that works for you...

http://jxb.oxfordjournals.org/content/62/8/2657.short

This is part of another discussion:

What is the relationship between ABA and ethylene on osmotic plant stress?. Available from: https://www.researchgate.net/post/What_is_the_relationship_between_ABA_and_ethylene_on_osmotic_plant_stress#5581ad4b5cd9e3f89a8b45cf [accessed Jun 17, 2015].

Hi,

I'm not certain to understand totally what you are about to measure? If you are to measure the hydraulic conductance of the soil to leaf pathway (liquid water pathway), namely what you can derive from transpiration and leaf water potential data, then this does not include the stomatal conductance ! You include gs when you measure the conductance of the soil to atmosphere pathway (liquid + vapour pathways) which is mainly determined by gs (or g canopy).

If you try to maintain stomata open during drought, the plants dies by runaway embolism. This is predicted by model simulations and some experiments on plants unable to close their stomata during drought (see Cochard H, Ridolfi M, Dreyer E 1996. Responses to water stress in an ABA-unresponsive hydrid poplar (Populus koreana x trichocarpa cv. Peace) II. Hydraulic properties and xylem embolism. New Phytologist 134: 455-461.)

Hervé