Effects of NH4+-N/NO3--N ratios on growth, nitrate uptake and organic acid levels of spinach (Spinacia oleracea L.).

Abstract

The objective of this investigation was to study the effects of NH 4 + -N/ NO 3 - -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, NH 4 + -N/ NO 3 - -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 NH 4 + -N/ NO 3 - -N ratio was above 50:50 in nutrient solution. Spinach supplied with NO 3 - -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 NH 4 + -N/ NO 3 - - N ratio. A strongly (r = 0.9826) positively linear relationship was found between the nitrate content of spinach fresh shoots and the proportion of NO 3 - -N in total nitrogen. When the NH 4 + -N/ NO 3 - -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 NH 4 + -N/ NO 3 - -N ratio of 25:75. The water soluble oxalate content of the spinach fresh shoot at either NH 4 + -N or NO 3 - -N as sole nitrogen resource, was higher than other combinations of NO 3 - -N and NH 4 + -N and, for example, the soluble oxalate content was significantly (p = 0.05) decreased by 8% when 25% of NO 3 - -N was replaced by NH 4 + -N. In conclusion, some replacement of NO 3 - -N by NH 4 + -N could improve the spinach quality while the dry biomass was not significantly affected.