Effects of nitrogen and calcium supply on the accumulation of oxalate in soybean seeds

Soybean [Glycine max (L.) Merr.] seeds have sufficiently high oxalate concentration that they are not recommended for consumption by individuals prone to formation of kidney stones. One possible precursor of oxalate is glyoxylic acid and this acid is formed in soybean seeds as a product of ureide catabolism. A nodulating soybean variety was compared with its nonnodulating isoline and the lack of nodules resulted in significant decline in ureide supply to developing seeds. However, oxalate concentrations in the seeds of the isoline were not reduced, suggesting that oxalate is not formed via glyoxylate in soybean seeds. Because oxalate and Ca concentrations are often correlated, manipulation of Ca supply to soybean plants was also studied by increasing Ca supply during pod development by 3.5-fold relative to the level in the rooting medium. Surprisingly, there was no significant effect of Ca supply on Ca concentrations in leaves, pod walls, or seeds, indicating that Ca accumulation is stable in spite of Ca supply. The Ca treatment also did not significantly alter oxalate accumulation in developing seeds. In spite of this lack of treatment effect, the correlation of oxalate and Ca concentrations within sampling times was generally significant. Oxalate concentration was 2- to 3-fold higher in developing seeds than in mature seeds. It is concluded that (i) the correlation between Ca and oxalate in developing soybean seeds is probably related to unknown factors influencing oxalate accumulation and the subsequent binding of Ca by the oxalate accumulated, (ii) lowering oxalate in soybean seeds will most likely be achieved by selection of low-oxalate genotypes, and (iii) because oxalate concentrations are higher in immature soybean seeds than in mature seeds, those prone to formation of kidney stones should be cautious about consuming immature soybeans.