[Effects of nitrogen supply on nutritional quality and antioxidative enzyme activities of spinach]

A hydroponic experiment was carried out to study the effects of nitrogen (N) supply on the nutritional quality and antioxidative enzyme activities of spinach. The results showed that when the N supply increased from 4 to 8 mmol x L(-1), the biomass of spinach and its leaf vitamin C (Vc) content increased significantly, but a further increase of the N supply induced an obvious decrease of Vc content. An increasing N supply increased the nitrate content in leaves. The soluble oxalate content in leaves decreased slightly with the N supply from 4 to 8 mmol x L(-1) N, but increased with a further increase of supplied N. Total oxalate content increased firstly, and then decreased with increasing N supply. SOD and POD activities increased with the N level up to 8 mmol x L(-1) N, but drastically decreased with increasing N levels. CAT activity decreased when N level elevated. MDA content decreased firstly and then increased with increasing N levels, while free proline content decreased with increasing N supply. All of these indicated that 8 mmol x L(-1) N was an appropriate N supply level for spinach to improve its biomass, nutritional quality, and resistance.