Nitrogen concentration and nitrate/ammonium ratio affect yield and change the oxalic acid concentration and fatty acid profile of purslane (Portulaca oleracea L.) grown in a 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 and (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 with increasing nitrogen level up to 36 mmol L−1. Nitrogen supplied in both NO3− and NH4+ forms produced bigger plants than N supplied in NH4+ form only. Nitrogen did not significantly influence the polyunsaturated fatty acid content in the canopy. Significant trends were found for α-linolenic acid (LNA; 18:3 n-3) and linoleic acid (LA; 18:2 n-6), indicating a favorable accumulation of omega-3 fatty acids with increasing N; palmitic acid decreased by increasing N and by switching the NO3−/NH4+ ratio towards the 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 a 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.