Dear Edgardo,

you could use the following equation of Michel that enable to calculate the required PEG 8000 amount to obtain a specific value of osmotic potential in your solution:

                      [PEG] = [4 - (5,16ψT - 560ψ + 16)0,5] / (2,58T - 280)

Where

Temperature (T) is expressed in Celsius degrees

Osmotic potential (Ψ) is expressed in bar (1 bar=0,1 MPa)

PEG amount, [PEG], is expressed as g of PEG / g of H2O

This equation is the result of a model that considers ψ varying quadratically with concentration and linearly with temperature of solution.

I hope I have been helpful.

 Bye

 Alessandro

Thnaks very much! 

Dear Alessandro,

Can I ask, what would change in the equation for  PEG 4000?

Dear Danny,

I have never had to do with PEG 4000 because it is usually considered to have molecules too low sized and thus, susceptible to be uptaken by roots. However, you could find something interesting in the pubblication I'm attaching you. Briefly, this work elaborates the relationships between the concentration of PEGs with different molecular weight and the osmotic pressure of the solution. You'll find regression equations in order to obtain also the values for PEG 4000. Notice that the reliability of the equations depends on the range of PEG concentration in solution.

Have a good work

Alessandro

Thanks Alessandro, Big Help.

Edgardo Did you utilize the ecuation Alessandro gave to you? here you are a link but I calculated the amounts of PEG8000 in g and is so much. 

http://www.plantstress.com/methods/peg.htm

Yes Susana, the amount calculated seems to be large and you can especially notice it when you put the PEG in water to prepare your solution.

My advice is not to use too much PEG in order to low the pH, because it might be some problems due to the difficulty the oxygen move with into the solution which turns out too viscous. For more information about the PEG-O2 relationships I attach you a useful link. Furthermore, plantlets might have some problems in uptaking nutrients. But notice, this is just my opinion, I haven't read so far scientific articles dealing with this topic.

I hope I helped you a little

All the best

Alessandro       

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC35048/

I prepared osmotic solution of PEG following Michel 1983. my problem is that i am measuring osmolality in freezing point osmometry. can somebody guide me how to convert this osmolality (mosmol/kg) into osmotic pressure in MPa

want to ask you if i want to apply a stress with -1MP for wheat plant using 6000 PEG, how can i prepare it ?

if there is any useful ref, i'll be glad.

thanks to all

I would like to correct Alessandro Romano because the formula is not correct.

Formula for PEG 8000 is:

[PEG] = (4-(5,16 ψ T - 560 ψ + 16)^05)/(2,58T-280)

The 0,5 in the exponent was in a incorrect place, everything else was perfect and thanks to him i find the article.

http://www.plantphysiol.org/content/plantphysiol/72/1/66.full.pdf

(Equation 5)

Dear Sergio,

thank you for your observation of the typo. I have just edited the right formula.

Best regards

Alessandro

Does this formula also apply to PEG 10000?