Salinity and Nitrogen Availability Affect Growth, Oxalate Metabolism, and Nutritional Quality in Red Orache Baby Greens

As freshwater resources become increasingly scarce, seawater and brackish water represent alternative sources for crop irrigation, particularly in systems such as saltwater aquaponics. Red orache (<i>Atriplex hortensis</i> var. <i>rubra</i>) is a halophyte with high antioxidant content but also accumulates antinutrients like nitrate (NO₃^-) and oxalate. Oxalate helps plants cope with salinity stress but can cause health issues in humans. This study examined the growth of red orache baby greens in saline and nitrogen-limited hydroponic solutions to assess its adaptability and nutritional quality, focusing on the impact of salinity and reduced nitrogen on antinutrient levels. Four nutrient solutions differing in NaCl (0 or 428 mM) and NO₃^- (10 or 1 mM) were tested. Salinity significantly reduced red orache yield (by 75.5%), pigment levels, antioxidants, and nutrient uptake, while increasing leaf Na and oxalate concentration, ethylene production, and succulence. Salinity decreased NO₃^- concentration and oxalate oxidase (OxO) activity but boosted total ascorbic acid and oxalate accumulation. Low NO₃^- mildly reduced yield (by 25.7%), leaf area, and NO₃^- concentration in leaves, but had no effect on leaf moisture content, succulence, antioxidant capacity, and the concentration of antioxidants, pigments, and total oxalate. In addition, low NO₃^- increased OxO activity, only under non-saline conditions. The high salinity typical of aquaculture effluents strongly reduced red orache baby greens yield and quality to a greater extent than low NO₃^- levels. Both salinity and low NO₃^- reduced NO₃^- concentration in leaves, while salinity increased oxalate concentration, probably due to the reduced activity of OxO.