Rifat Tarik yararbas <[EMAIL PROTECTED]> queried thusly:
What is the upper limit of sodium content orchid tissue culture media?
Other than a few halophytes, there is no demonstrated requirement for sodium in plants. Therefore, any concentration of sodium in plant tissue culture media serves only to raise the osmotic strength of the substrate without providing anything of use to the plant. Even obligate halophytes require sodium at levels around 20 micromolar; back-of-the-envelope calculations which will surely be proven wrong by someone with an RPN calculator at hand show that works out to about 0.46 milligrams per liter.
If this query is in reference to using sodium hydroxide to raise the pH, potassium hydroxide is more useful (providing potassium) and marginally safer. If this is in reference to providing for the mineral nutrition of your charges, then there is probably enough stray sodium in the components in your tissue culture media since sodium is slippery and difficult to remove as an impurity. Worth noting is that the disodium salt of EDTA is normally employed to chelate iron from iron sulfate; "typical" levels of 37.3 mg of disodium EDTA have been inherited from the original Murashige and Skoog 1962 formulation for modern plant TC media. More back-of-the-envelope calculations peg sodium from this source at 4.6 mg/liter when disodium EDTA is used at this concentration. If any plants *were* obligate halophytes, this level would more than satisfy nutritional requirements.
The net upshot is that there is no need to add additional sodium, and that extra will only increase the osmotic strength, which serves no useful purpose for orchids. Very high concentrations of sodium might interfere with uptake of potassium.
-AJHicks
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