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Medicinal and aromatic plant (MAP) production is gaining popularity for industrial agriculture, with phytochemical compounds having a significant impact on human health. Plant fertilization must be carefully considered as it is strongly affecting the biochemical profile of MAPs. The present study examined the Sideritis cypria responses to different nitrogen (N: 75, 150, and 300 mg/L), potassium (K: 150, 350, and 550 mg/L), and phosphorus (P: 50, 75, and 100 mg/L) concentration in the nutrient solution (NS) in hydroponics. The NPK levels (150 mg N/L; 75 mg P/L and 350 mg K/L) in the NS, which was regarded an intermediate fertilization scheme, showed a rise in nutritional value with high phenols, flavonoids and antioxidant activity in plants. S. cypria grown in N75 levels revealed a decreased plant fresh weight and chlorophylls content while plants grown in N300 levels revealed increases in mineral accumulation, nutrient and water use efficiency. The NPK and the K550 levels caused oxidative stress as demonstrated by the raised lipid peroxidation and the stimulation of enzymes’ antioxidant activities. The P50 levels in the NS, increased the plant biomass and water use efficiency (WUE) and revealed the lower oxidative stress (malondialdehyde) and increased enzymes antioxidant (superoxide dismutase and peroxidase) activities. As a result, modifying the NS composition in hydroponic culture for S. cypria by using P levels of 50 mg P/L, higher biomass, nutritive value and WUE can be obtained.

The efficient use of water and nutrients in leafy vegetable crops can be improved by effective water management, whereas the form of the supplied nitrogen may affect their quality and productivity. Sowthistle (Sonchus oleraceus L.), a widespread weed collected and consumed as a wild vegetable in the past, currently attracts a high interest for commercial cultivation due to its high nutritional value. The present study evaluated the effects of different nitrogen concentrations: N50: 50 mg L−1; (N 3.6 mmol L−1), N100: 100 mg L−1; (N 7.1 mmol L−1), N200: 200 mg L−1; (N 14.3 mmol L−1), and N300: 300 mg L−1; (N 21.4 mmol L−1) and different ammonium/total-N ratios (Nr: 0.01, 0.05, 0.10, 0.15) on growth, physiological parameters, antioxidant capacity and nutrient accumulation in the different plant parts of sowthistle, grown in Nutrient Film Technique hydroponic system. Total N was ammonium N plus nitrate N. Plant growth, total phenols, and antioxidant capacity were higher at the two intermediate N concentrations, whereas flavonoid and nitrogen content, as well as irrigation water productivity increased only with 200 mg N L−1 (N 14.3 mmol L−1), compared to the other three N concentrations. The accumulation of nitrogen in leaves and roots was lower, while leaf stomatal conductance was enhanced by increased N concentrations in the nutrient solution. Higher Nr ratios than ≥ 0.05 decreased plant dry matter, total phenols, flavonoids, and antioxidant capacity in leaf extracts, and negatively affected nitrogen translocation from roots to leaves. Sowthistle plants treated with a Nr of 0.05 exhibited a less intense oxidative stress, with decreased lipid peroxidation and hydrogen peroxide production, and increased superoxide dismutase and catalase activities, compared to those treated with higher Nr ratios. Increased Nr resulted in the accumulation of phosphorus and magnesium in leaves while the highest irrigation water productivity was obtained in plants grown with a Nr of 0.05. In conclusion, to increase yield, nutritional value and efficiency of water and nitrogen use in sowthistle grown in closed hydroponic systems, a N concentration of 200 mg L−1 of N (N 14.3 mmol L−1) and a Nr of 0.05 are suggested.

Purslane (Portulaca oleracea L.) is a widespread weed, which is greatly appreciated for its high nutritional value. The present work evaluated the effect of different ammonium/total nitrogen ratios (NH4/Total N: Nr 0.01–0.15) on growth, physiological and biochemical parameters, and nutrient accumulation in different plant parts of hydroponically grown purslane, under two growing seasons, spring and autumn. Young seedlings of purslane were transferred to a Nutrient Film Technique (NFT) system and they were exposed to different Nr levels. The pH and the electrical conductivity of the nutrient solution were kept constant at 5.8 and 2.3 mS cm−1, respectively. After the end of the cultivation periods (19 days for spring and 22 days for autumn), a series of assessments (growth parameters, mineral content in different plant organs, antioxidant status of the plant, etc.) were done. Plant height, leaf number, root fresh weight and plant biomass revealed decreased trends at the higher NH4/total N ratios, especially during the autumn growing season. Total phenols, flavonoids and antioxidant capacity appeared increased at Nr ≤ 0.10 during both seasons (autumn and spring), revealing higher nitrogen accumulation rates and increased water and nutrient use efficiency. Purslane plants grown in Nr 0.05–0.10 revealed a less intense oxidative stress, with decreased lipid peroxidation levels that was the result of the activation of both enzymatic (superoxide dismutase, catalase and peroxidase) and non-enzymatic (ascorbic acid) antioxidant capacity of the plant. Increased Nr resulted in the accumulation of potassium, while calcium and magnesium levels in leaves were decreased. Additionally, the greater water use efficiency was measured for plants grown under Nr 0.01–0.05. Therefore, the recommended ammonium/total nitrogen ratio for purslane production of increased yield, improved nutritional value and efficient use of water and nitrogen sources is to employ Nr of 0.05, while additional care should be addressed during autumn periods as plants are subjected to greater impacts of the Nr ratio.