Due to the remarkable health benefits of celery (Apium graveolens), its consumption has increased over time. A partial substitution of NO3− with NH4+ is recommended to limit the accumulation of NO3− in leafy vegetables. Hence, a factorial experiment with two factors, consisting of six treatments as combinations of three NH4:NO3 ratios (0:100, 20:80 and 40:60) in nutrient solutions and two harvesting times (in the morning and in the evening), was conducted on celery plants in a soilless culture system. The results showed that 100% NO3 as a sole N source significantly increased plant height, leaf number, chlorophyll, fresh weight, N, K, Ca, Mg, Fe, Mn, Zn, protein, dietary fibre, soluble sugars, nitrate, vitamin C, α-carotene, β-carotene and lutein of celery plants compared to either 80 or 60% NO3. However, this increase was not significant compared to 20% NH4:80% NO3 in terms of leaf number, fresh yield, N, Mg, Mn, protein, soluble sugars, vitamin C and α-carotene. Harvesting in the evening significantly increased K, Mg, Fe, soluble sugars, α-carotene and β-carotene, and lowered the nitrate level in celery plants. In conclusion, partial replacement of 20% NO3-N with 20% NH4-N and evening harvesting are recommended for a greater fresh yield, higher quality, and lower nitrate level.
