Ammonium/nitrate ratio affects the growth and glucosinolates content of pakchoi

Nitrogen (N) strongly affects plant growth. However, little is known about the effects of the ammonium/nitrate ratio on pakchoi (Brassica rapa), especially its glucosinolates (GSs) contents which are involved in plant defense and many of them benefit to human health. The aim of this study was to evaluate the effects of a constant N supply (8 mM) but with five ammonium/nitrate ratios (namely 0/8 mM, 2/6 mM, 4/4 mM, 6/2 mM and 8/0 mM) on the growth of pakchoi in a hydroponic system in 2 years. In both years, a higher biomass (dry weight) was in the 4/4 and 2/6 ammonium/nitrate treatments (2.3 and 2.2-fold compared to 8/0, respectively), with no significant difference in biomass between these two treatments. The biomass then decreased with increasing proportions of ammonium in the N supply from more than 10 g/pot to below 5 g/pot. The N and sulfur contents were strongly affected in the sole nitrate or ammonium treatments, as were the contents of chlorophyll, lutein, and beta-carotene. Most of the individual glucosinolates (GSs) detected in pakchoi showed the highest concentrations in the sole ammonium treatment. The ammonium/nitrate ratio also affected the types of GSs. The highest proportion of indolyl GSs and the lowest proportion of aromatic GSs were in the sole ammonium treatment. We concluded that a high ammonium level decreases the biomass but increases the GSs concentrations in pakchoi. Further studies are required to explore the difference in GSs metabolism among plants supplied with different ammonium/nitrate ratios.