[Difference in oxalate content between buckwheat and soybean leaves and its possible cause]

Buckwheat (Fagopyrum esculentum M.) and soybean (Glycine max L.) seedlings with expanded primary leaves were grown in 1/5 Hoagland culture solution for 10 d, then oxalate content was determined in leaves, roots and root exudates. The result showed that its content in buckwheat was much higher than that in soybean, respectively. This clearly indicated that the oxalate content difference in the leaves was due to the metabolic difference rather than to the different rate in transport to the roots and/or in exudation. Activity of oxalate-degrading enzyme oxalate oxidase could be detected in buckwheat, but not in soybean leaves, implicating that buckwheat leaves had some oxalate-degrading capability. This further demonstrated that the oxalate difference was caused by the change in the oxalate biosynthetic process. Buckwheat leaves contained more glyoxylate, and also higher glyoxylate-oxidizing activity of glycolate oxidase (GO) than soybean. GO from buckwheat had a lower Km for glyoxylate than that from soybean. In combination with the higher level of glyoxylate, buckwheat leaves might have higher rate of oxalate formation from glyoxylate. It is suggested that the rate of glyoxylate oxidation by GO may be one of the critical steps which control oxalate accumulation in plants.