Effect of Nitrate:Ammonium Nitrogen Ratio on Oxalate Levels of Purslane

INTRODUCTION Purslane (Portulaca oleracea L., Portulacaceae) is eaten extensively as a potherb, and added in soups and salads around the Mediterranean and tropical Asian countries (Fig. 1). There is a newer interest in cultivation of purslane as a crop since its identification as one of the best sources in the plant world for omega-3 fatty acid, α-linolenic acid (LNA). LNA is a precursor of a specific group of hormones (prostaglandins) and may offer protection against cardiovascular disease, cancers and a number of chronic diseases and conditions throughout the human life. Research indicates that omega-3 fatty acids will eventually receive the attention and broad recommendations now given to fiber (Philipson et al. 1985). Purslane is also reported as an excellent source of the antioxidant vitamins α-tocopherol, ascorbic acid, and β-carotene as well as glutathione, and the amino acids isoleucine, leucine, lysine, methionine, cystine, phenylalanine, tyrosine, threonine, and valine (Miller et al. 1984). Purslane has been described as a “power food of the future” because of its high nutritive and antioxidant properties (Levey 1993). Purslane has potential as an animal feed (Bosworth et al. 1980), in aquaculture (Simopoulos et al. 1995), and in the food processing industry (Wenzel et al. 1990). Although purslane is listed as a commercially cultivated vegetable of the world (Kays and Dias 1995), in many parts of the world it is still regarded as a “weed with nutritive potential.” Despite its nutritive value in the human diet its acceptance as a green leafy vegetable is limited to a large extent because of reported accumulation of oxalic acid in large amounts, and the harmful health effects associated with oxalic acid in the diet. Studies report that nitrate to ammonium ratios in plant mineral nutrition can have an influence in the oxalate levels in New Zealand spinach (Tetragonia tetragonioides Pallas. Kunz.) (Ahmed and Johnson 2000). In a previous study we reported the influence of nitrate to ammonium ratios in hydroponics on the omega-3 fatty acid concentrations in purslane leaves (Palaniswamy et al. 2000). The objective of this study was to study the effect of NO 3 -N: NH 4 -N ratio on the oxalic acid concentrations and determine the NO 3 -N: NH 4 -N ratio in hydroponics that would minimize the oxalic acid concentrations in purslane leaves.