Effects of Nitrogen and Calcium Nutrition on Oxalate Contents, Forms, and Distribution in Spinach

A hydroponic experiment was conducted to study the effects of nitrogen (N) and calcium (Ca) nutrition on oxalate contents of different forms in spinach tissues. Results showed that leaves were the main locations of oxalates in spinach. Total oxalate, soluble oxalate, and insoluble oxalate contents were highest in leaves, followed by petioles and then roots. Soluble oxalate was the dominant form of oxalate in spinach. Nitrogen and Ca2+ (calcium ion) concentrations could markedly affect oxalate contents. Soluble oxalate contents in leaves increased obviously with the increase of N concentration until 8 mmol L-1, above which oxalate content started to decrease. Supplied with the same amount of N, increasing Ca2+ concentration reduced soluble oxalate content in leaves. Total oxalate reached the lowest with 5 mmol L-1 of Ca2+ supply. Leaves and petioles had lower total oxalate and lower proportion of soluble oxalate when N and Ca2+ concentrations were 8 and 5 mmol L-1.