Abstract:Higher plants, accumulate large number of polyphenolic compounds which are believed to act as defence compounds against different environmental constraints. Nitrogen (N) is a critical element for plant growth, absorbed as (NH 4 + ) and (NO 3 -), which affects plant growth and to some extent contributes to secondary metabolites accumulation. Greenhouse experiment was carried out to determine the effect of N forms on growth and phytochemical accumulation in Amaranthus species. Two amaranth varieties; AB6 and AB7 constituted the main plot while three N forms; ammonium, nitrate, ammonium nitrate and control (no N form) represented the subplot. Destructive sampling was done and plant height was recorded. Folin-Ciocalteu's and aluminium trichloride methods were used to determine total phenolic content (TPC) and total flavonoid content (TFC) respectively. DPPH (diphenylpicrylhydrazyl) radical scavenging activity assay was used to obtain total antioxidant activity. variety and TPC accumulation by 23% AB6 and 20% AB7 varieties respectively. Correspondingly, NH 4 + -N form resulted to superior antioxidant DPPH scavenging activity indicated by high scavenging activity and lower IC 50 value (concentration which scavenged 50% of the DPPH radicals). Plant height displayed a significant negative correlation with TFC and TPC accumulation of r= 0.75 and r= 0.81 respectively. The results indicated that ammonium-induced stress enhanced total flavonoids and phenolics accumulation; a salient phytochemical plasticity observed during plant growth and survival trade-off in a vegetable amaranth.