Water resources pose a considerable limitation in arid and semi-arid regions like Iran, primarily due to the scarcity of rainfall and heightened evaporation rates. Within open hydroponic systems, a significant concern arises from the depletion of nutrient solutions utilized for crop fertigation. This predicament subsequently gives rise to environmental issues of note and a discernible surge in production expenses. Application of humic acid (HA), as a plant biostimulant, enhances plant tolerance to reduced water supply and counterbalances the effects of diluted nutrient solutions. A greenhouse experiment was conducted to investigate the impact of applying HA (600 ppm) in conjunction with three nutrient solution concentrations (complete nutrient solution (NSc), 75% NSc (NS1), and 50% NSc (NS2)), as well as two irrigation levels (100% and 50% of plant water requirement, IL100 and IL50, respectively). The treatment under IL50 was referred to as ½ NSc, ½ NS1, and ½ NS2. The findings showed that almost all of the traits studied were significantly affected by the applied treatments. The highest yield (409.4 g m-2), leaf area (34.67 cm-2 plant), biomass (1.092 g-1 plant), membrane stability index (90.47%), chlorophyll a (0.99 mg g-1 FW), chlorophyll b (0.413 mg g-1 FW), total chlorophyll (1.54 mg g-1 FW), carotenoid (0.437 mg g-1 FW), vitamin C (730.7 mg ascorbic acid kg-1 FW), amino acid (236.8 µg g-1 FW) and nitrate reductase activity (102.8 µg NO2- mg Protein-1 h-1) were observed in NSc + HA. In addition, the highest carbohydrate (46.99 mg g-1 FW), phenol (1129 mg GAE Kg-1 FW), and flavonoid content (1133 mg Catechin Kg-1 FW) and antioxidant activity (74.75%) were observed in ½ NS1 + HA. In addition, the highest relative water content (86.62%) and nitrate content (1106 mg kg-1 FW) were recorded in NS2 + HA and NSc, respectively. The application of HA combined with different concentrations of the nutrient solution under both irrigation levels caused an increase in most of the parameters studied in rocket plants compared to respective treatments without HA application. In addition, the results obtained confirmed that HA could compensate for the stress caused by a shortage of nutrients and water in the nutrient solution in this crop.
