Effects of Nitrogen on Oxalate Accumulation Mechanisms in Different Spinach Genotypes

From the agricultural and nutritional point of view, oxalate is regarded as a naturally occurring toxin and anti-nutritional factor. Intake of oxalate-rich vegetables may increase the risk of kidney stone production by inducing a significant increase in urinary oxalate excretion. It is commonly accepted that N nutrition is one of the most important factors affecting oxalate accumulation in vegetable production. In this research, two spinach genotypes differing in oxalate accumulation capacity are employed to study the genotypic difference in oxalate accumulation in spinach as affected by N application rates and N fertilizer source. We also focused on the relationships between N uptake, metabolism and oxalate metabolism in the two genotypes by using N uptake kinetics, precursors of oxalate biosynthesis, inhibitors of N uptake and reduction, and inhibitors of photorespiration. The aim of this study is to elucidate the mechanisms of N affecting oxalate metabolism, which could provide the scientific basis for producing nutritional, low oxalate-accumulating spinach by using the combined measures of genetic breeding and nutrient management.

At a N supply concentration of 18 mmol/L, total oxalate and soluble oxalate contents in leaves of 31 spinach cultivars ranged from 9.86 to 21.41 mg/g FW and 9.62 to 13.45 mg/g FW, respectively. Oxalate content in spinach increased with increasing N levels. For instance, when N supply levels increased from 2 to 8 mmol/L, the total oxalate in leaves increased by 8 to 23%. When nitrate supply increased from 2 to 8 mmol/L, total oxalate contents in leaves of spinach increased by 56 to 60%. When the plants were treated with a mixed solution of nitrate:ammonium in 1:1 ratio, total and soluble oxalate content again increased. The oxalate content was positively correlated with nitrate uptake rate, but not ammonium uptake rate. Addition of plasma membrane H⁺-ATPase-inhibitor sodium vanadate (Na₃VO₄) significantly decreased (17 to 29%) oxalate accumulation in spinach leaves. Treatments of Na₃VO₄ also resulted in a substantial inhibition of nitrate uptake, nitrate reductase and glutamine synthetase activity. Results confirmed that oxalate accumulation in different spinach genotypes is positively related with not only root uptake of nitrate, but also its reduction and assimilation within the plants, probably through glycolate metabolic pathway, rather than ascorbic acid metabolic pathway.