Effects of nitrogen forms on nitrate and oxalate accumulation in edible parts of spinach

Nitrate and oxalate are two major quality-related compounds present in high quantities in some vegetables such as spinach and deteriorate the nutritional quality of food.Oxalate could lead to the formation of calcium oxalate kidney stones and reduce the bioavailability of calcium,iron and other minerals.Oxalate forms in vegetables have different(effects) on human health,but previous studies have focused mainly on the total oxalate in vegetables.Little is known about how N source can influence content and accumulation of different oxalate forms in vegetable.A hydroponic experiment was carried out to study the effects of nitrate/ammonium ratio in nutrient solution on accumulation of nitrate and oxalates in the edible parts of spinach shoots.Results showed that biomass of spinach increased firstly as ammonium proportion was increased from 0 to 50% then drastically decreased with the proportion of ammonium further increased.Nitrate content and amounts of nitrate accumulation in leaves and shoots reduced markedly with the increase of the ammonium proportion;Nitrate content in petioles decreased with the proportion of ammonium increased while amounts of nitrate accumulation in petioles elevated firstly then markedly reduced.Leaf was the major oxalate-accumulating tissue of spinach,the amounts of soluble oxalate and total oxalate accumulation in leaves constituted 56.3%—89.8% and 76.6%—87.4% of those in shoots,respectively.Soluble oxalate was the predominant form of oxalate of spinach,which in leaves,petioles and shoots occupied 36.7%—83.5%,79.0%—93.3% and 50.0%～83.0% of total oxalate.The content and accumulation amounts of soluble oxalate,insoluble oxalate and total oxalate in leaves,petioles and shoots decreased drastically when the proportion of ammonium was elevated.The extent of reduction of oxalate content and the amounts of oxalate accumulation in leaves and shoots was higher than that in petioles.These results indicate that higher nutritional quality of spinach with lower nitrate and oxalate content and accumulation in edible parts could be achieved by manipulating the(ratios) of nitrate to ammonium in nutrient solution.Appropriate ratio of nitrate to ammonium is 50/50,at which nitrate and oxalate content and accumulation can be markedly reduced without depressing plant growth.