Effects of Using Plasma-Activated Water as a Nitrate Source on the Growth and Nutritional Quality of Hydroponically Grown Green Oak Lettuces

Abstract

Nitrate is a major source of the inorganic nitrogen taken up by the roots of plants. Nitrate sources are generally derived from inorganic minerals by an energy-consuming chemical process; as a result, the price of chemical fertilizers is gradually increasing year by year. NO3-N, generated from N2 using the plasma technique, is an alternative method of producing nitrate from the air. Therefore, in this research, we aimed to determine the efficiency of generating NO3-N using plasma-activated water (PAW) to replace nitrates from chemical fertilizer in a nutrient solution. Green oak lettuce (Lactuca sativa L.) was grown in a hydroponics system using the double-pot technique. The plants were supplied with three different nutrient solutions (based on Hoagland’s solution), i.e., T1, no nitrate in the nutrient solution (NO3− = 0); T2, using nitrate sourced from a commercial chemical fertilizer (normal nitrate); and T3, using a nitrate source generated using the pinhole plasma jet technique (plasma nitrate). The other macronutrients and micronutrients in each treatment were equally supplied. The results show that, at the harvested stage (21 days after the plants received treatment), the no-nitrate (T1) treatment provided lower growth and yields. Moreover, compared with the normal nitrate (T2) and plasma nitrate (T3), the results indicate that most growth and yields showed no statistical differences. In terms of nitrate accumulation within plants, it was found that the normal nitrate treatment (T2) had the highest levels of nitrate accumulation, in both the underground and aboveground parts of green oak lettuce. These results confirmed that plasma nitrate could be an alternative source of nitrate N which provided a safer way for the environment and human health in terms of nitrate accumulation. In addition, data related to the chemical analysis of free amino acid concentrations in each treatment are discussed in this research.