Nitrogen concentration and nitrate/ammonium ratio affect yield and change the oxalic acid concentration and fatty acids profile of purslane (Portulaca oleracea L.) grown in soilless culture system

Purslane is an excellent source of omega-3 fatty acids, amino acids, and vitamins. The aim of this research was to study the effect of different nitrogen levels and NO3--N/NH4+-N ratios in the nutrient solution on the yield and on the oxalic acid content and fatty acid profile in purslane grown in a soilless culture system. Two experiments were carried out to test different levels of nitrogen: 1) 8-12-16 mmol L-1; 2) 0-12-24-36 mmol L-1. A third experiment was carried out maintaining the N level fixed (12 mmol L-1) but varying the NO3--N/NH4+-N ratio: 60/40, 40/60, 0/100. The results indicated that plants grew increasing nitrogen level up to 36 mmol L-1. Nitrogen supplied in both forms NO3- and NH4+ produced bigger plants compared to N supplied in NH4+ form only. Nitrogen did not significantly influence the polyunsaturated fatty acid content in the canopy. Significant trends were found for a-linolenic acid (LNA; 18:3 n-3) and linoleic acid (LA; 18:2 n-6) indicating a favourable accumulation of omega-3 fatty acid with increasing N; palmitic acid decreased by increasing N and by switching NO3-/NH4+ ratio toward NH4+ level in the nutrient solution, but using 0/100 NO3-/NH4+ decreased plant quality. Increasing N lowered oxalic acid production and palmitic acid content enhancing purslane nutritional quality. Keeping high N level in the nutrient solution with a 40/60 NO3-/NH4+ ratio would give the best results in terms of yield, oxalic acid concentration and fatty acid profile.