Oxalate synthesis in leaves is associated with root uptake of nitrate and its assimilation in spinach (Spinacia oleracea L.) plants.

Background: Excessive accumulation of oxalate in numerous vegetables adversely affects their quality as food. While it is known that nitrate could effectively stimulate oxalate accumulation in many vegetables, little information is available about the mechanism of nitrate-induced oxalate accumulation. In this study, we examined the association of oxalate synthesis with nitrate uptake and assimilation in two genotypes of spinach (Spinacia oleracea L.), Heizhenzhu and Weilv.

Results: Increasing nitrate levels resulted in enhanced synthesis of oxalate, as well as increased root uptake of nitrate and leaf activities of nitrate reductase (NR) and glutamine synthetase (GS) for both genotypes. Correlation analysis revealed that oxalate accumulation in spinach leaves was positively related with rate of nitrate uptake by roots, as well as leaf activities of NR and GS. Addition of plasmalemma H(+)-ATPase inhibitor sodium vanadate (Na3VO4) significantly decreased leaf oxalate accumulation in both genotypes. Presence of NR or GS inhibitors led to reduction of leaf oxalate contents, GS/NR activities and decreased nitrate uptake rate. Significantly higher levels of nitrate root uptake, leaf NR and GS activities were observed in the high-oxalate genotype Heizhenzhu than in Weilv.

Conclusion: Oxalate synthesis in leaves of spinach is not only positively associated with root uptake of nitrate, but also with its assimilation within the plants.