Oxalic acid concentrations in Purslane ( Portulaca oleraceae L.) is altered by the stage of harvest and the nitrate to ammonium ratios in hydroponics

Abstract

Oxalic acid (OA) occurs naturally in a large number of plant species. When present in the human diet, OA may combine with essential minerals such as calcium and iron, to form insoluble salts known as oxalates and hinder their bio-availability. Purslane ( Portulaca oleracea, Portulacaceae) is an excellent source of omega-3 fatty acids and anti-oxidant vitamins. However, its consumption as a green vegetable is limited by high concentrations of OA. In previous studies we reported the influence of nitrate to ammonium ratios in hydroponics on the leaf omega-3 fatty acid concentrations. In this study we determined the NO 3 −-N:NH 4 +-N ratio in hydroponics and the stage of harvest that would minimize the oxalic acid concentrations in purslane leaves. Fully expanded leaves were harvested at 8-true leaf and at 16-true leaf stages from purslane plants grown in hydroponic systems containing nitrogen (200 μg ml −1) as nitrate (NO 3 −) to ammonium (NH 4 +) ratios (1:0, 0.75:0.25, 0.5:0.5 and 0.25:0.75), and analyzed for OA concentrations. Results indicate that at both stages of harvest the OA concentrations were ≈40–50% lower in the leaves grown in solutions containing ammonium compared to the leaves grown with no ammonium. Leaves harvested at 16-true leaf stage had ≈36–45% lower OA concentrations compared to the leaves harvested at 8-true leaf stage. The dry weight (DW), fresh weight (FW) and leaf area (LA) were greater at 16-true leaf stage than at 8-true leaf stage, but were not influenced by the NO 3 −:NH 4 + ratios in hydroponics.