Abstract

The objective of this investigation was to study the effects of NH₄⁺-N/ NO₃^--N ratios on the growth of spinach, the nitrate and organic acids contents of spinach fresh shoot. A completely randomized design was established as 5 treatments, that is, NH₄⁺-N/ NO₃^--N ratio of 100/075/25, 50/50, 25/75 and 0/100, the total N concentration was 12 mmol l^-1. Each treatment had 3 replications, results showed that the spinach biomass significantly (P = 0.05) decreased as the NH₄⁺-N/ NO₃^--N ratio was above 50:50 in nutrient solution. Spinach supplied with NO₃^--N as its sole nitrogen source had the highest biomass, but there was no notable difference (P = 0.05) in dry matter weight between 25: 75 and 0:100 of NH₄⁺-N/ NO₃^--N ratio. A strongly (r = 0.9826) positively linear relationship was found between the nitrate content of spinach fresh shoots and the proportion of NO₃^--N in total nitrogen. When the NH₄⁺-N/ NO₃^--N ratios was decreased from 100:0 to 0:100, the pyruvate, citrate, succinate, fumarate contents were all increased gradually and the malate content was highest at NH₄⁺-N/ NO₃^--N ratio of 25:75. The water soluble oxalate content of the spinach fresh shoot at either NH₄⁺-N or NO₃^--N as sole nitrogen resource, was higher than other combinations of NO₃^--N and NH₄⁺-N and, for example, the soluble oxalate content was significantly (p = 0.05) decreased by 8% when 25% of NO₃^--N was replaced by NH₄⁺-N. In conclusion, some replacement of NO₃^--N by NH₄⁺-N could improve the spinach quality while the dry biomass was not significantly affected.

Key words: Spinach, NH₄⁺-N/ NO₃^- ratio, organic acid, nitrate content, oxalate content.