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I'm a plant physiology researcher, but water potentials are not really my thing, so I don't have a great grasp on the literature. I'm dealing with water potential data right now though, and it seems questionable to me. Some of the measured water potential (solute + metric potentials, after lysing cells to break turgor) are as low as -48 bars (most in -10 to -35 range).

That would, if my math is correct, translate to a solute concentration in the 2000 mOsmol range, which seems extremely high to me. Consequently, I'm trying to determine if there is an analytic issue with the data collection. I've run through the equipment chain, checked calibrations, and everything seems ok. I've also been combing plant physiology lit. for comparable data, and I find some in that range, but I can't find any theoretical work that would help set a 'ceiling' for what constitutes a reasonable solute + metric potential.

So, my questions are these:

  1. What is the theoretical maximum capacity for dissolved ion concentration in water, and what pressure potential would that represent, compared to pure water.

  2. Is anyone aware of plant physiology research that has attempted to establish a minimum in vivo water potential in plants (either total potential, or S+M if it's broken out)?

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  1. Sodium Chloride (which I am here assuming to be the major salt in your solution) has a solubility of 6 Moles/L this corresponds to 12000 mOsmol, so the 2000 mOsmol you are measuring is not entirely implausible. However normally plant cells should have an osmolarity on the order of 300-400 mOsmol. In order to judge if your value was calculated correctly, it would be necessary for you to tell us exactly how you calculated and how you measured.

  2. I am not aware of such literature but I am not a plant physiologist myself.

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  • $\begingroup$ Thanks. Measurements were taken two ways: tissue was measured in the field, with a 'pressure bomb' (en.wikipedia.org/wiki/Pressure_bomb) and additional measurement was done with both needle and woody tissue using chamber psychrometers (a small sealed chamber that holds tissue and contains an electrode, the resistance of which changes depending on humidity, which changes based on the tissue's hydraulic status). The pressure bomb is pretty much foolproof, but the psychometry work depends on calibration, which i've checked, repeatedly, using lab standards. $\endgroup$ – Jas Max May 27 '16 at 14:27
  • $\begingroup$ Also, I'll add that your linked source for osmolarity isn't necessarily troubling for my result--osmolarity increases vertically with plant height (to pull water against gravity) so the lowest osmolarities would be seen in root tissue (tubers) and it would increase as you rise through stem, branches, and leaves. Hard to say though, since tubers aren't 'regular' roots, and plants deliberately manipulate solute concentration in 'storage' tissue, like tubers. I've done a similar teaching lab, and the variance between potato varieties is pretty huge. $\endgroup$ – Jas Max May 27 '16 at 14:31
  • $\begingroup$ Thanks for the clarifications. If you can exclude any other sources of water potential (capillary forces, air pressure, etc.) I guess the 2000 mOsmol are correct. However I must caution you again that I am a biophysicist, not an expert in plant physiology. $\endgroup$ – Thawn May 27 '16 at 14:42

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