Effect of Water Stress on Oxalic Acid Concentrations in Spinach Leaves.

The changes in oxalic acid concentration in response to water stress were monitored to evaluate the contribution of oxalic acid to osmotic adjustment. Potassium and oxalic acid are the predominant solutes in spinach leaves. The addition of polyethylene glycol (PEG) 6000 to the nutrient solution significantly decreases osmotic potential at full turgor (π100) in rapidly growing leaves. The decrease in π100 in response to PEG treatment can be accounted for by the accumulation of potassium, phosphate, nitrate, sugars, and amino acids, but not of oxalic acid. These results indicate that oxalic acid plays a minor role in osmotic adjustment, although potassium oxalate is a major osmoticum in spinach leaves.