Relationship between nitrate reduction and oxalate synthesis in spinach leaves

The relationship between nitrate reduction and oxalate synthesis in spinach (Spinacia oleracea L.) leaves was examined by monitoring changes in the concentrations of oxalate and nitrate in excised leaves with their petioles in an aqueous solution of CaCl2. Nitrate concentration in leaf 4 decreased over the course of 8 h. Nitrate efflux from petioles into a CaCl2 solution was negligible. Oxalate concentration in leaf 4 increased during the first 2 h and remained constant thereafter. In leaves 6 and 8, by contrast, oxalate concentrations did not increase significantly with time despite a decrease in nitrate concentrations. In leaves excised from plants that had been grown in nutrient solutions supplemented with 0.1 mmol . L(-1) tungstate For the previous 3 d, concentrations of nitrate decreased during a 4 h period without any change in oxalate concentration. These results indicate that nitrate reduction is not a prerequisite for oxalate synthesis in spinach leaves.